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Topics - teemuk

Just bumbed to this little whitepaper.

T. Serafini S. Barbati, "A Perceptual Approach on Clipping and Saturation", 2002

Short (4 pages) and written in layman's terms. It basically tries to address the issue that "musical" special effects can't be evaluated with same principles as high fidelity sound reproduction and therefore some pre-established concepts about "good" or "bad" harmonics/clipping need to be reconsidered depending on application. Totally worth reading.
Amplifier Discussion / The ultimate JC-120 thread
July 26, 2011, 01:33:27 PM
...or hopefully something becoming one.

As some of you might know "the JC-120" amplifier is actually a collection of very different kinds of product revisions, all revolving around a design concept of basically featuring

- dual channels, clean + effect
- 2 x 60W stereo output
- stereo chorus or tremolo effect
- reverb and distortion
- uniform cosmetic styling

Everything else but these basic features have been evolving quite a lot since the amplifier was introduced in 1975. That also includes how these basic features were accomplished circuit wise. Since not much is written about this, but the topic seems to come around now and then, I hope we could combine a thread with some info of the different circuit revisions and their features, what S/N's cover those revisions, and hopefully, some user input of how those different revisions seem to have performed...

If anyone's interested, here's a sketch of an effect I've been cooking up lately. Thought it might come handy for those that are keen in experimenting various effects stuff or just needing inspiration for new ideas.

The circuit is basically a simulation of a tube push-pull output stage with distinctive voltage sag. There are two parts: 1) a clipping circuit emulating the operation of a push-pull gain stage and 2) a voltage-controlled regulator driven by a precision rectifier.

The regulator's output provides a DC reference for the clipping diodes, hence controlling their clipping threshold voltage. The regulator's control samples its driving signal from the guitar signal and rectifies it. When the mean amplitude of the signal increases the regulator is driven to decrease its output voltage.

The clipping stage is basically a gain stage and a phase splitter, followed by two shunt diode clippers that clip the peaks of positive half waves. The signals are then combined with a differential amp. When summed, the two differential signals "correct" each other's errors, thus somewhat softening the clipped portions.

The injection point to voltage-controlled regulator was chosen after the clipping stage so that the sagging effect can reduce when clipping decreases the mean signal amplitude.

There you have it in a nutshell.

The following figures show a crude schematic and a waveform capture of a real audio sample driven through the SPICE engine that demonstrates the circuit's operation.

In the latter, the light and dark grey waveforms show the signals at the anodes of the two clipping diodes. (Aside clipping taking place) they are basically invert images of each other. The medium grey DC signal is the regulator's output and the cathode reference of the clipping diodes. Each time this voltage drops circa 600mV below the anode signals the signal is clipped. One can see that the regulator circuit reacts to high signal amplitudes and gradually decreases the reference voltage, thus the initial picking transient is not clipped (though this depends on amount of "sag" currently taking place). Sustained high amplitude signals will result into clipping that increases in magnitude until the signal's amplitude has become low enough to not drive the regulator to introduce more voltage drop. At lower magnitude signal portions the reference voltage "recovers" allowing initial picking transients to be amplified cleanly again.

I deliberately made the effect quite subtle since I most likely intend to use this circuit only as a little extra "spicing". A dedicated clipping circuit can do the heavy lifting – just like the preamp usually does it in modern hi-gain tube amps. The potentiometer in the regulator control can adjust the circuit's sensitivity. The circuit is fine tuned for an input voltage of approximately 1Vpeak but tweaking that should be easy since the circuits are pretty basic.
Schematics and Layouts / Blackstar HT Dual
August 27, 2009, 10:19:41 AM
Took a while to sketch this out but here it is for the viewing pleasure of those who have wondered what's inside those HT things.
Amplifier Discussion / A cool link
April 12, 2009, 06:24:29 PM
Just something that I thought vintage and odd amp freaks like myself could enjoy (Italian vintage guitar amps, effects and other related stuff):  8|
Amplifier Discussion / "Mil-spec" and NASA-spec
October 18, 2008, 10:19:12 AM
The following is an excerption of something I've been working on for adding into the 2nd edition of the amp book. I though it might interest some of you.

When it comes to wiring and construction the issue of using "mil-spec" or "NASA-spec" techniques often comes up. The aforementioned are actually based on to tedious standards issued in various books of standards that typically have anything from 600 to 1000 pages discussing topics such as component materials and construction methods in extreme detail. If one goes through such book, or even just a brief summary of one, it often comes evident that in most cases many amplifiers claiming to employ "mil-spec" construction often actually do not. For example, the popular style of "hanging" components in between turrets, using 90-degree angles (Hiwatt anyone?) in wiring and boosting serviceability by securing components to terminals only by means of solder is not being "mil-spec". (However, that does not mean that these amplifiers weren't built much better than an average one).

The following is a brief summary of topics from NASA's construction standard that relates to generic construction issues one would likely deal with when building guitar amplifiers. If reference to a certain standard is not mentioned, the method is optional but considered as "Best Workmanship Practice" which preferably should be used. One should also bear in mind that this small list of things is just the tip of an iceberg.

Crimped terminations:
-   Crimping of component leads, solid wire or leaded/tinned stranded wire is prohibited. (NASA-STD-8739.4)
-   The crimp termination must have some clearance to wire's insulation. However, this exposed part will be insulated with heatshrink tubing or by another similar method. (NASA-STD-8739.4)
-   Discoloration or charring in wiring, or the use of charred/damaged heatshrink is prohibited. (NASA-STD-8739.4)
-   The crimp must be intact and it is unacceptable to modify one "to fit". (NASA-STD-8739.4)

Solderless wire-wraps:
-   Made by helically wrapping solid, uninsulated wire around a specially designed termination post. The wrap post usually has a rectangular shape.
-   Use of stranded wire is prohibited. Also is prohibited the use of silver underplating. (MIL-STD-1130B)
-   Half to one and a half turns of insulated wire must be in contact with a minimum of three corners of the wrap post. Absence of insulated turns is prohibited. (MIL-STD-1130B)
-   In single termination, "the termination has a specified amount of insulated and uninsulated turns of wire, and is clean and free of foreign material." (MIL-STD-1130B)
-   In multiple terminations, the terminations preferably have a proper amount of spacing. The terminations can overlap but in this case the insulated conductor overwrap does not exceed one turn and is as tight as possible. (MIL-STD-1130B)
-   Insufficient insulation wrap, improper wrapping position, insufficient number of turns in a wrap, "end tails", overwraps, spiral wraps, "open" wraps, improper routing of wires from one post to another, as well as using damaged wrap posts is unacceptable. (MIL-STD-1130B)

Cables and harness:
-   "Cables insulated with materials other than Kapton shall not be bent less than six outer diameters." "Flat and ribbon cables shall not be creased, folded or bent less than three insulated wire diameters (short-term)." (NASA-STD-8739.4). Therefore the 90-degree bend you often see used is unacceptable, at least by NASA's standards.
-   Ribbon cables should not be incorporated to discrete wiring harnesses. Ribbon cables should be routed along flat surfaces. They should not be routed near heat, electric interference or vibration sources. Naturally blocking airflow is prohibited.
-   Splices (meaning joining of two or more electrical conductor wires) must have proper strain relieves and must be located in areas that are not subjected to flexure. The splice connection must follow the standard, being either a crimped splice connector, lash splice or a Western Union/Lineman splice for solid conductors. (NASA-STD-8739.3)

-   Solder joints are clean of dirt and foreign substances, shiny, smooth and concave. Plated-through holes are filled completely with tin. Leads must protrude properly through the holes but not too much. Wire insulation is not allowed to extend to solder joint. Clinched (bent) lead ends shall not be clinched towards an electric conductor or a proper clearance to such must exist.
-   Preferably solder should not extend to stress relief bends but it is acceptable if the topside bend radius is discernable. Solder joint must not extend to component body and clearance of at least one lead diameter is required. (NASA-STD-8739.3)
-   Gold-plated surfaces that become part of a solder joint must be tinned. Gold "intermetallic" (golden stripes in solder) will make the solder joint brittle.
-   Part bodies must not be in contact with soldered terminations.
-   Glass encased parts (e.g. those small diodes) must have a sleeve or must be potted with transparent epoxy. (NASA-STD-8739.3)
-   Heat-producing parts must have sufficient clearing to circuit board.
-   Components that weight more than 7 grams must be mechanically secured on their place. If hot glue (or similar substance) is used for this purpose the mounting must be done with at least four evenly spaced bonds.
-   Eyelets should not be used for interfacial terminations.
-   In lead bending, "the minimum distance from the part body/seal to the start of the bend shall be 2 lead diameters for round leads and 0.5 mm for ribbon leads. The bend radius shall not be less than one lead diameter or ribbon thickness". (NASA-STD-8739.3)
-   The component should be centred between its mounting holes.
-   Splices are not allowed in repairing broken or damaged conductors or part leads.
-   Broken components or tracks, flux splatters and residue, as well as excess solder or discoloured laminates are unacceptable.
-   In case of axially mounted components, use stress relieve whenever possible. Component body should preferably rest in full contact with the mounting surface. Slight gap is acceptable but in case of large caps the component must be mechanically secured on its place. Leads of axially mounted components are allowed to cross exposed conductors, but in this case the leads must be sleeved.

Leads of axial components should be equipped with strain relief "loop" that acts like a spring. The body of the component is firmly mounted on its place (often with glue or cable straps). When axial components are mounted in radial fashion the proper support depends on the board design: Components used in boards that have no plated-through holes rest on their body and are often glued on place as well. The longer leg has two 90-degree bends that make it stiffer. At solder pads, the leads are angled to establish a mechanically sturdier construction. In boards that have plated-through holes the construction is different: The solder joint is mechanically rigid and also needs some clearance at the component side. The longer leg is hence angled to make it spring-like. The component body is left "floating" in air. This method of construction is not very sturdy! Axial components can also be mounted in radial fashion. In this case their leads must have proper strain relief and their bodies should be mechanically secured to the mounting surface.

Discrete wires:
-   Discrete wires are subject to same rules as component leads when it comes to bending, stress relief and soldering requirements.
-   In PC boards, conductor enters the hole perpendicular to board surface. Proper insulation clearances and lead protrusions are used. Excessive insulation gaps (more than two wire diameters) are unacceptable. (NASA-STD-8739.3)
-   No more than one component shall use the same hole.
-   In "lapped termination" (no mounting hole, wire simply soldered to a pad) the termination must run across the longest dimension of the pad. The conductor is not allowed to overhang across the pad. (NASA-STD-8739.3)

Jumper wires also known as "haywires":
-   The jumper wire must be solid, insulated copper conductor with tin/lead plating. Stranded, silver-plated wiring is not allowed. Jumpers less than 25mm in length may be uninsulated if short circuit to adjacent components is impossible.
-   In component side of PC boards, wire route must be the shortest path and does not pass over or under components or over any land or via used as a test point. The jumper wire must have sufficient slack to allow component replacements.
-   In solder side of PC boards, wire route is the shortest path and does not pass over component footprints or lands – except if unavoidable.
-   The wire must be "staked" (securely tied on its place to restrict movement) on specified intervals and in all places where it changes its direction. (NASA-STD-8739.1)
-   Stress relief is used whenever possible. Improper strain relief is unacceptable. (NASA-STD-8739.3)

Solder terminals:
-   Solder is not mechanically sturdy so the wire must first pass through the "eye" (hole) of the terminal and then be secured with hook wrap (minimum of 180-degree wrap), quarter turn wrap (90-degree wrap in contact with terminal face) or a "zig-zag" wrap (two 90-degree wraps in contact with both sides of the terminal face).
-   Soldering the wire to terminal without additional and standardized securing is unacceptable. (NASA-STD-8739.3)
-   Wire runs from one terminal to another must have a strain relief and proper, standardized wrap contact at each terminal. Wire ends must be wrapped 90 or 180 degrees. (NASA-STD-8739.3). Turret board terminals are an exception to the rule and explained later.

Turret board construction:
-   Wires are and component leads are wrapped a minimum of 180 degrees around the turret. Multiple wires or leads may connect the same turret but they must run in parallel, wraps may not overlap and at least one of the wires has to have a contact to terminal base. Wires are soldered on place and they are discernable under the solder joint. The solder joint is naturally shiny and smooth. (NASA-STD-8739.3)
-   Conductor sizes AWG 26 and smaller must be wrapped a minimum of 180 degrees (1/2 turn), but less than one full turn (360°) around the post. Conductor sizes larger than AWG 26 must be wrapped a minimum of 180 degrees, to a maximum of 270 degrees (3/4 turn) around the post. (NASA-STD-8739.3)
-   The wrapping can be either into clockwise or counter-clockwise direction. Important is that the wrap would be tightened against the terminal would the wire become pulled. (NASA-STD-8739.3)
-   In continuous wire runs from one terminal to another, "the wire shall wrap (360 degrees) around each terminal, contact each terminal base, exhibit stress relief, and be terminated to the first and last terminal with a 180° to 270° wrap (depending on wire gauge)." (NASA-STD-8739.3)
-   Mounting components on top of turrets is unacceptable! (NASA-STD-8739.3)
-   Parallel components may be stacked on top of each other but the largest component must be at the bottom and rest against the mounting surface. In this case, all components must be stress-relieved and secured mechanically on place.
Schematics and Layouts / Peavey's steel amps
August 30, 2008, 08:48:43 AM
Here's few traced-out schematics for these old gems (Vegas 400 & Nashville 400).  :tu:
Acoustic Control Corporation:



Award Session:

Carlsbro Sound Equipment:


Dean Markley:




Gallien-Krueger & GMT: (free registration to site required)

Gibson: (Master Service Manual. Warning: 177.5MB!)



Jim Kelley:




Marshall: (free registration to site required)



Music Man:


Peavey: (free registration to site required)






Trace Elliot:

Traynor (Yorkville Sound):


Vox (pre-Korg):

Watkins Electrical Music Ltd. (WEM):
Amplifier Discussion / Design issues (Rant)
March 10, 2008, 05:02:20 PM
While I like designing circuits and seeing my designs come to life I'm really not that enthusiastic about the process of actually building something. ...Well, etching and soldering is actually kind of fun but unfortunately those are usually minor parts of the building process when compared to stuff like mechanical/layout design or hunting for parts. These are such nuisances that they almost make me hate the whole hobby. While the following is sort of a "rant" I hope you will find its content helpful.

At the moment, I'm in the middle of building this amplifier that uses TIP142 and TIP147 (complementary) output devices. These devices used to be pretty "standard choice" few years ago but unfortunately they are not anymore and most component stores seem to have run out of them. Fortunately, they usually can be substituted with TIP141 and TIP146 that are almost identical transistors except for their lower Vce.

So here I am at this component store specifying that I will buy either TIP142 and TIP147 or TIP141 and TIP146 since they are complements. Well, they're run out of TIP142 and 147, which I pretty much could expect since that's been the case for more than half a year. Can I order? No. Fortunately this is no problem since they have TIP141 and 146 in stock. So, the clerk hands me these transistors and when I specifically ask if they are indeed TIP141 and TIP146 (and therefore complementary) I get an answer that they are. I trust the clerk's word and don't bother to check it out.

The plot thickens: I get home, open up the package and find out that they were TIP141 and TIP147 instead! Damn! I don't have use for them alone in the amp project since they are not complements to each other! Obviously the clerk wasn't aware of this - or simply did not care. Anyway, I decide that it was my bad (should have checked it out in the store) and since these things cost nearly nothing and will definitely have future use it's not a big issue – I just have to find few TIP142 or TIP146 transistors as well to get that complementary pair.

So, the next day I'm in this other component store. No TIP142 or TIP146 in there either. Luckily I can order through them – except that I find out that these transistors are no longer in production. Or actually they are, but in another case style that is not specified by their datasheets so I'm afraid to buy them. (I'll get to that later). Long story short, I'm fortunately able to order TIP146 in the correct case and everything looks all right.

(People, don't misunderstand me: The local component stores are doing wonderful job. Unfortunately they can't sell stuff that they don't have - or keep stuff that gives them practically no profit in stock. They are also as helpless with the apparent component variation as anyone else).

So, anyway, what's my gripe about these different case styles? This: When you have a design made for specific power dissipation and PC board you can't just go on and substitute the transistors with ones that have another case style. First of all, the transistor in the alternative case may have different power dissipation or the case may have different pinout or distance between the leads and the mounting hole. This means that the substitute transistor is pretty much unusable.

For example, TIP142/147 are different than 142/147T, which I learned by the hard way few years ago. T comes in TO-218 (I guess - I'll get to this later anyway) while the other one comes in TO-220 (I guess). The other package is significantly smaller and has a power dissipation ratio of only 80W (versus 120W stated by the datasheet of TIP14x). So, it's not a proper substitute in most cases. Unfortunately, there is no standard for the alphabet letter code following the device name so you'll have to figure out what differences are in A, B, C, G, GF, T and et cetera. They also mean different things in different transistors - and I guess in the worst case even among different manufacturers. Have fun browsing through dozens of datasheets. You'll should also pretty much memorize the differences and ALWAYS specify the correct type when you order or shop something – otherwise you could get anything. It's foolish to think that transistors would have some standard form – although that is what most people generally expect.

Another gripe I have is about these TO-218, TO-220 and SOT-93 cases. These seem to make no sense! Run a few searches and you'll find out that they seem to mean the exact same thing – except one of the cases is actually much smaller than the other and naturally has a different power dissipation figure. Unfortunately there's no way to make sense which one of them it is because all sources depicting those cases are conflicting – even datasheets. Run a Google image search with those package names and you'll see what I mean. The only way to know for sure of what you are getting is only at the point when you have the part in your hands. Clerks at electronics stores are also lost with this issue.

It doesn't stop there: Just today I found out that even the specific case styles are not standard: The TIP142/147 and TIP141/146 come in seemingly identical case, except that the TIP142/147 has about 1 mm longer leads and about 1mm longer distance from the bottom of that plastic case to the center of the mounting hole. Everything else is identical. While the difference may seem small, it is actually a very big issue in mechanical layout: If you design based on the "larger" case it may be that the smaller case does not fit to the same layout (the shorter leads simply do not reach to the other side of the PCB). If you design based on the "smaller" case but use the transistors that have the bigger case you'll have to bend leads of them. This means that the solder joints become subject to breaking under thermal stress (the leads should be left sort of "spring-like" so that they have room to "live" when the component warms up or cools down). So basically, you'll have to make unique design for each case style, which means you have less choice of substitutes. It's pretty pathetic considering the fact that the package is supposed to be "standard". You also need to be aware of this issue in the first place.

...Which is another gripe of mine: The datasheet typically shows these physical dimensions but am I the only one who thinks those images are extremely confusing? Usually they depict few possible measures for each dimension coupled with some approximate tolerance value (e.g. 160 – 230). Very helpful indeed... Those are usually in some obscure unit format as well, so you can't really make up whether the dimensions are stated in mm, mil, thou, cm or inches. Comparing different datasheets in this respect is also pretty much impossible. I like to refer to this unit as "FY" because its only function seems to be acting as a nuisance. I don't care to state what the abbreviation means but I'm sure you can figure it out. I never had any help of these pictures but perhaps some people know stuff that I don't.

And it doesn't stop to semiconductors... There are also similar issues with potentiometer and jack bushing diameters that are always different as well. Not to mention the "mounting depth" (if you want to mount these on board and attach the bushing to the chassis). And the issue with all those varying packages of those potentiometers you have! Damn annoying! I don't even want to go to issues with various jack switches... It also seems that today I can't get these insulated jacks anywhere either.

I really truly hate this part of building! If I had to design commercial stuff I'd probably go crazy in a week.  :grr

Do you guys have similar experiences? ...Practical stuff you would like to make people aware of?
Since there seems to be so much confusion about the factors that determine speaker's loudness let's straighten out few parameters:

Speaker wattage = only determines the amount of power the speaker can handle before it gets damaged by overheating the voice coil or exceeding the cone excursion limits. Even though some manufacturers (especially in the auto-HiFi industry) like to market speakers as if their wattage would equal loudness it doesn't, so be careful with vague terms such as "peak watts". The average wattage - often mistakenly referred to as "RMS" - is the only interchangeable and comparable parameter. (RMS rating for watts is not the same thing as average power figure derived from sinusoidal wave that has an amplitude of x V RMS. Read more from here: The "scientifically valid" peak wattage rating is two times higher than the average wattage rating. Other stranger peak watt ratings may be as 10 or 100 times higher but they mean nothing. Since wattage essentially determines how much your speaker can tolerate power you must be able to compare the figure with the output power of your power amp. At this point imaginary and bloated ratings are only annoying and confusing, thus causing a serious risk of destroying the speaker with too much output power. Do also note that "American" wattage = peak rating while "British" wattage = average rating. (I don't know if they use this rule anymore).

Cone size = only determines the approximate efficiency to couple certain frequencies. Large cones can move more air during a single cycle (thus provide a higher sound pressure) but they generally have too much mass that slows them down and prevents them from producing high frequencies. A smaller cone that moves faster (and ultimately moves the same volume of air in numerous cycles) can be equally loud (or louder) at the higher frequencies. However, it can't couple those low frequencies efficiently enough. (In both examples we assumed the amount of "cone travel" is equal). Cone travel means the "distance" of excursion and is essentially the other parameter defining the amount of moved air volume. Naturally, if the cone can travel further it can move more air than a cone with equal area but a smaller travel. Any amplifier can drive any speaker with any cone area/size. It's up to speaker's efficiency how much travel the cone has at the amount of power fed to the speaker by the amplifier. If the speaker is inefficient it will produce less sound pressure with a certain amount of power input than a more efficient speaker would.

Magnet size = Larger magnets can improve the efficiency of the speaker's electrical "motor" - so the magnet size is often sort of comparable to speaker's quality and efficiency. Yet, this is not an absolute rule as the strenght of the magnetic force is only a single parameter in speaker design. A speaker with a big magnet may as well be very inefficient. Do note that AlNiCo, Ceramic and Neodymium materials have different magnetic properties so the magnet sizes are only comparable if magnets are made out of same material. Some manufacturers (especially in the auto-HiFi industry) glue metal pieces to the magnets to make them look bigger and more powerful. Again, about the only rating that can tell anything meaningful about loudness is the SPL.

Be happy to continue the list or discuss...
"Problems that change gradually - usually they decrease or disappear - as the equipment warms up are often due to dried up electrolytic capacitors."

Ity may also be an issue of intermittant signal path. Or a combination of both.
Preamps and Effects / Cabinet simulators
November 10, 2007, 08:33:02 AM
A nice compilation of speaker cabinet emulator circuits and their frequency responses. Too bad I don't understand polish language...
Schematics and Layouts / Baldwin Professional
October 18, 2007, 11:11:40 PM
I've redrawed a schematic from a 1965/69 patent, which - judged by control arrangements - depicts model C1 Professional with "Supersound". I'm not sure how accurate the schematic is (for instance, I had to correct several obvious errors of the patent's schematic) and it's also missing the power supply section and transistor information. Anyway, I haven't seen any Baldwin schematics online so hopefully this one will help somebody. If someone can fill in the blanks the better.

Sorry for small size; I had to keep the attachment size small.
Schematics and Layouts / 60W Power amplifier
July 04, 2007, 09:40:09 PM
I thought I'd share the design I've been cooking up lately since it begins to look extremely promising: If powered from +-32V symmetric rails this can provide about 60W RMS of power to a 4-ohm load. However, ignore the power rating since as configured the amplifier will not give a clean (or linear) output and THD measures are a bit vain. Note that circuit is not yet built and I cannot give a 99% guarantee that it will work. It is "SPICE proven" with simulation containing a realistic speaker load and a realistic power supply so I'm pretty sure that it will also work in real-life - at least with some added compensation here and there. Hopefully I will start building this circuit in the near future.

For those who are more interested in the details: The amplifier utilizes the usual differential topology but the voltage amplifier stage is loaded by two constant current sources instead of one: This will in effect provide more current for the both halves of the output stage but leave VAS running with only the subtraction of output stage currents: As a result the VAS runs cooler, drives the output stage more linearily and has a better slew rate. I've seen this topology used in some Acoustic Control amps from the 70's but they only used simple bootstrap circuits. I decided to give the topology a modern update with active circuitry.  ;) (The "20mA" text refers to LEds and does not mean the stage current - as those with know-how can calculate).

Output stage is basic quasi-complementary circuit with really simple (IMO too simple) current limiter. Bias circuit is simple VBE multiplier. Due to symmetric CCS loading of the output stage the bias servo should not be bypassed with a capacitor (like usually is the case): Under loading where power amp stage might clip there will be no constant DC potential over it and results of this are quite detrimental for performance. If it can be ensured that the circuit always operates in linear state the addition of the capacitor is recommended.

The feedback is a mix of both convenional voltage feedback and current feedback. The current feedback will essentially drive the amplifier in a manner where it reacts to changing load impedance by providing a higher voltage gain to higher loads (in oppose to constant voltage gain). Thus the amplifier provides more magnetic drive (current) for the loudspeaker during resonance and at higher frequencues. Some say this gives a "tube amplifier -like" frequency response. That is at least mostly correct. Note that this is not a current feedback amplifier but a compromise between current and voltage feedback. The following figures might be familiar for some of you and explain what I'm talking about:

Speaker impedance

Excerpt "from Tubes Versus Transistors in Electric Guitar Amplifiers" (

Employing mixed-mode feedback has been a pretty common trick since the 70's but unfortunately it usually introduces a serious limit for the power output since higher load impedance requires higher voltage swing and may therefore result into clipping, which in a power amplifier stage usually sounds extemely awful and may introduce other nasty effects such as rail sticking or parasitic oscillation. If rail clipping is prevented by decreasing gain the amplifier will in turn have a lower nominal output power.

This circuit directs the current feedback to the opamp stage instead. This stage will now respond to the changes in (speaker) load by similarly varying it's gain. However, the stage's voltage swing is limited to about 500 mV. This practically prevents any power amplifier overdrive (except in reasonably long term conditions where supply rails sag enough). Basic diode clipping alone usually isn't very great tone-wise but unless you deliberately overdrive the amplifier hard it will have psychoacoustically cleaner sound. This is because the clipping behaviour is "rounder" and harder to perceive than the harsher rail clipping of conventional SS power amps. Due to limited dynamic range the amp should also sound louder. The idea is basically just to smooth out the transients. Preamps are for providing the more delicate distortion.

The amount of current feedback can be varied with the damping control from reasonably linear response with more output power to a bit "warmer" response with sparkling highs (with the expense of some output power loss though). IMO, listening tests with LTspice have proved this circuit to sound very good. If you remove the opamp pre and the current feedback path (along with the 0.22-ohm sense resistor) you will get a basic, linear 60WRMS to 4 ohms power amp. Anyway, I prefer the configuration with the limited dynamic range.

Power supply is not shown but it should have symmetric 32V rails and at least 4700 uF of capacitance per rail. The minimum VA rating for the power transformer is about 250. Opamps are run from +-15 - 16 V supply that ideally should be regulated.

Comments about the design are welcomed. It is not the final one but I thought it could interest some of you.
I'm pretty soon finished writing my book about SS guitar amplifiers and one of the final chapters I'm writing covers the various circuits that emulate tube amps. Included are some selected circuits under TransTube concept (Peavey), SansAmp (Tech21), Carvin's SX series Classic Tube, FlexWave (Crate), ValveState concept (Marshall) and Valve Reactor (Vox). DSP is out of the topic. Are there any other emulation circuits that interest you people?

...And no: I'm not going to cover Pritchard's circuits. They are far too extreme and IMO in all their complexity pretty much unusable learning material for common DIY:ers.  :)
One of my hobbies besides electronics and music has always been history so I decided to start a thread that would gather some of solid-state amplifiers. As you all likely know, there is a lot written about tube amplifiers and it's a crying shame that so much of solid-state guitar amplifier history has been left undocumented or has been forgotten. Maybe this forum has gathered enough enthusiastic people to find the topic "sexy" enough.

In the future, I'll be updating this post once in a while to summarize and add new information. I'm looking forward to your contributions as the timeline and details are still pretty much "lacking".

The 50's:

  • (Likely) the first transistor radio is unveiled by Intermetall in Düsseldord Radio fair. First commercial transistor radio, the Regency TR-1, is put to sale the next year.


  • Lin introduces quasi-complementary output stage topology (this is output transformerless).
  • Paul Penfield's article "Transistorized Guitar Amplifier" appears in July issue of Radio & Television News magazine.

The 60's:


  • Standel releases first hybrid amplifiers. I know Bob Crooks found Standel, built and designed many of their early tube amplifier models but what about the solid-states? Did he get some design help?


  • Kay's "Vanguard" line-up: The first all-transistor guitar amplifiers? Any info on the designers?
  • First fuzz box, Maestro FZ-1 Fuzz Tone, appears.


  • Leak Stereo 30: First commercial output (and interstage) transformerless transistor (HiFi) amplifier.
  • Hagström introduces Model 1700 - also known as "GA-85". Some people think this is the first all-solid-state guitar amplifier, it's not. (And some sources state that this model was actually introduced as late as in 1965). Any info about the designers?
  • First Burns transistor amplifier. This is likely the "Orbit" model; Around this time it costed more than a new Vox AC-30.
  • Likely the first Gibson transistor amplifiers: "Starfire"-series including TR-1000RVT and TR-1000T. This information is based on to earliest catalog entry (from 1963) I've seen concerning Gibson SS amps.
  • Czechoslovakian company called Jolana introduces the "Big Beat"; a guitar with an integrated SS amplifier and a medium wave radio. The battery-powered circuit is the first reference to a completely transformerless SS guitar amp design I've seen so far.


  • First Vox transistor amplifier (T-60). Any info about the designer? Tom Jennings, Dick Denney?
  • First WEM (Watkins Electric Music) transistor amplifiers emerge. The "Slave" PA system caughts a notable success but their lineup of transistor guitar amplifiers (introduced ca. 1966) can not compete with the new "rock" amplifiers.
  • First commercial digital amplifier (Sinclair's X10 DIY kit).


  • Hartley Peavey founds Peavey. Earliest amplifiers were designed by him and (ex RCA designer) Jack Sondermeyer.
  • Jennings looses control over Thomas Organ Company that switches from supplying imported UK Vox amplifiers to building their own. The transistor models are manufactured and designed at La Sepulvenda laboratories. Any info about the designers?
  • First Carvin solid-state amplifier: T-11. Carvin also introduces other transistor amplifiers such as T-12, T-4-102, T-2-101, T-151 and T-121. (
  • First Standel transistor amplifiers.
  • First Selmer transistor amplifiers. (First one was likely the Taurus 60 that later changed its name and appearance becoming Saturn 60).
  • Likely the first all-solid-state Univox amplifier, BT505 bass, appears. The following years, Univox did produce a lot of hybrids but the all-solid-state guitar amplifier lineups were introduced as late as in 1971.
  • First Baldwin transistor guitar amplifiers. Baldwin had just bought Burns so there were models under both Burns, Baldwin-Burns and Baldwin names. Since Baldwin originally had no expertice in making guitar amps it's needless to say that at first Burns practically just continued to manufacture its existing designs under the Baldwin name. (More information follows later in this thread).
  • Dallas transistor amplifiers appear.
  • Rolling Stones: "Satisfaction". This hit launches a craze for fuzz effects.


  • Bud Ross founds Kustom. Ross was the head designer and founded Road Electronics when Kustom was sold. Road Electronics manufactured high quality transistor guitar amplifiers and later merged with Rickenbacker that produced a series of "Road" amplifiers. Ross has also manufactured police radars and (Ross) guitar pedals.
  • Fender releases their first solid-state amplifiers. These are designed by Bob Rissi and suffer from poor build-quality and field failures.
  • Gibson introduces the transistor GSS-series consisting of models: GSS50 (2x10" combo), GSS100 (head with two 2x10" cabinets) and Plus 50 ("slave"-style 2x10" combo amp). Daughter brand Epiphone introduces the "Maxima" amp, which is a GSS100 copy.
  • First Sears Silvertone transistor amplifiers appear (Models 1464, 1465 and 1466 Bass). These are manufactured by Danelectro. In 1968 the same line-up has inclusion of model 1463.
  • First Jordan transistor amplifiers.
  • Teneyck transistor amplifiers: The G-series. These are designed by Bob Teneyck who also worked for Ampeg (design of Gemini series plus patents for Ampeg's vibrato and tremolo) and designed for Sunn (see 1969 "Orion"). Next year (1967) the T-series of amplifiers is introduced.
  • Mosrite introduces their lineup of transistor amplifiers and fails commercially with the Award BG-500 "The Ventures" model..
  • First solid-state Triumph amplifiers appear.


  • The Popular Electronics magazine introduces many popular and influential kits: i.e. M/M/M Instrument Amplifier.
  • Likely the first transistor Premier amplifiers are introduced. (i.e Model 5530)
  • First all-transistor Magnatone amplifiers appear.
  • Baldwin buys Gretsch.


  • RCA releases application notes describing quasi-complementary and differential input stage topologies. These are highly influential and give a start for numerous small (and bigger) companies manufacturing transistor guitar amplifiers.
  • Transonic line-up from Rickenbacker: These were also designed by Bob Rissi (designer of first SS Fender amps). However, this time most mistakes of Fenders are corrected. Transonic amplifiers are high quality but fail to catch large success; they are endorsed by Steppenwolf and used by Led Zeppelin (US tour) and Jeff Beck. Rissi continues to design and build guitar amplifiers in Risson. Today Risson makes "boutique" tube gear but assumably the first amplifiers (in 70's) were solid state.
  • First Acoustic Control Corporation transistor amplifiers are introduced. Acoustic's designers, employees and founders are fairly well known: Steven Marks and Harvey Gerst, Russ Allee and Roger Smith (the duo later found Amplified Music Products or AMP), Steven Rabe (later found SWR), Gene Cerwinski (later founder of Cerwin-Vega) and Aspen Pittman (sales, later found Groove Tubes) are few of the most famous.
  • GMT 226A, designed by Bob Gallien, is the first instrument amplifier that uses a stacked power transistor configuration ("cascode" or "beanstalk"). The following year Carlos Santana uses the amplifier in Woodstock.
  • Tommy Gumina founds Polytone.


  • First solid-state Sunn amplifier "Orion" is designed by Bob Teneyck. This amplifier is endorsed by Jimi Hendrix but proves to be a commercial disappointment due to many field failures. The following Sunn transistor amplifiers are designed by Dick MacCloud from Tektronics.
  • Fender SS Super Showman. Interesting is the fact that this was designed by Seth Lover, the inventor of humbucking pickup (and the P.A.F.-type as well). The former employee of Gibson was hired by Fender in 1967.
  • Ovation and Lawrence transistor amplifiers appear.
  • Anthony Leo's article in Electronics Australia introduces Playmaster 125 (PM125), another SS guitar amplifier kit.

The 70's:

  • (possibly?) Sears Silvertone "slant control panel" SS guitar amplifier models 1422 (originally tube), 1423, 1425, 1426, 1428 and 1431 are introduced. These are last Sears Silvertone guitar amplifiers, basically bargain bin quality and not manufactured by Danelectro. The Silvertone brand name eventually died after 1972.
  • Kustom establishes a daughter brand of amplifiers called Kasino. Daughter brands such as Krossroad or Woodson are established later. While Plush/Earth Sound Research amplifiers bear cosmetic resemblance to Kustom's products they were actually clones of Fender or Peavey tube amplifiers.



  • First transistor Marshall amplifiers. These are JMP-series: 1994 Slave, 2077 Bass 100 and 2078 Lead 100 Combo. They are followed by more JMP-series amplifiers in 1975/6: 2098 Master Lead, 2099 Bass, 2195 Lead & Bass, 2196 Lead & Bass, 2199 Master Lead Combo, 2200 Lead Combo, 2201 Lead & Bass and 2299 Master Lead Reverb. Any information on who were the designers?


  • GMT 200G is the first channel-switching amplifier
  • Unicord (Univox) introduces "Mobile Ohm" series that are equipped with load impedance selector and E.S.P (Electric Short Proof) short-circuit protection. Models: U-130 Bass, U-130L Lead, U-130PA, U-200L Lead, U-200B Bass and U-600PA.


  • Roland's Jazz Chorus line-up is introduced. During the years, Roland has released at least eight or nine new versions of this amplifier - some completely different from another. Their website hints that the real model name is depicted as JC-120-xxx, where xxx is an obscure letter code not explained anywhere (i.e. JT, U, UT). Anyone has more info?
  • Marshall releases first transistor heads.


  • The concept of rail-switching amplifier (class G) is introduced and used next year by Hitachi.


  • Sunn Beta series is the first commercial product utilizing overdriven Logic IC stages. (Fairchild app notes discussed the concept already in 1973). The amplifier also uses IC switching circuitry instead of FETs. Any info on the designers? The "logic inverter distortion" circuit is later (1978) popularized by an article written by Craig Anderton (before his book) and used in Electro-Harmonix "Hot Tubes" pedal.
  • Unicord (Univox) introduces "Stage" amplifiers (1977 - 1980): Models 25 Lead, 65, 65B, 252 Bass, 450, 400/112, 400/210, 720/115, 720/212, 720/410, 720K (keyboard), 750B (Bass) and 740P (PA). In some cases the second number marked the speaker configuration.
  • Norlin launches a line-up of Lab Series amplifiers and the next year (1980) Lab 2 Series. These are designed by a small group of people from Moog (a division of Norlin). Norlin also releases entry level line-up named "Genesis". One of the co-designers in "Lab Series" team is Dan Pearce who later starts his own company "Pearce Amplifier Systems" that builds high quality transistor amplifiers (i.e. G1 and G2R). See later section of this thread for further details.

The 80's and 90's:

  • This is the real dawn of tube pre - SS power amp -style hybrids. Products like "Legend" amplifiers or Lab Gruppen's "AXE Amp" are preferred by artists such Johnny Winter or ZZ Top.
  • Westbury amplifiers (ca. 1980 - 1982): Westbury was the company that manufactured the late Univox SS amplifier models for the Unicord company. Essentially, Westbury amplifiers just "replaced" the Unicord "Stage" amplifier line-up of the late 70's. Models were W250 Lead, W255/110, W250/115, W550 Lead, W555 Bass, W1000 Lead, W1000-M "Mini-Lead", W1000-MF "Mini-Lead" with Fane speaker, W1005, W1005 Bass and Model 1000 Dual-Voiced Reverb Twin.
  • TUSC "programmable tube amplifiers" (ca. 1981): These were hybrids that had a tube-based power amplifier stage. The interesting thing is that these were likely the first guitar amplifiers with DSP-based preamplifier that was able to store knob positions to switchable patches.
  • Rivera amplifiers: During his career Paul Rivera has done design work for Fender - and not only with tube gear: Transistor amplifiers like Yale, Montreaux and Studio Lead are some of his designs. Rivera has also designed amplifiers for Yamaha (G-100) and Pignose.
  • The dawn of various "tube emulation" circuits introduced by designers such as John Murphy (Carvin), Eric Pritchard (PRS & Pritchard), Sondermeyer (Peavey).
  • Early modelling and DSP amplifiers. Information?
  • Tom Scholz introduces the Walk-man inspired "Rockman" headphone guitar amplifier in 1982.
  • Tech21 introduces SansAmp in 189.

Note: Plenty of the stuff presented above is based on hearsay or to history presented by companies so its accuracy is highly questioned. For example, many companies like to claim they did or invented something first. In most cases this is far from the truth. Please doublecheck all the contributed information (oe at least try to).

I have tried to keep up a detailed list of amp manufacturers up to early 1970's. I consider these companies as sort of "pioneers". In circa 1968 many application notes describing efficient and moderately inexpensive amplifier circuits were released to boost up sales of new transistor models. This caused the amount of SS amplifier manufacturers to skyrocket. Past this point it is pretty difficult to keep any track of the various companies.
Amplifier Discussion / LM3886 Spice model
March 21, 2007, 01:27:21 PM
I have been cooking up this model for LTspice for a while and finally it's on a beta stage... The current model is based on the equivalent schematic presented in the datasheet and is still missing the undervoltage-, VI-limit-, SPiKe- and overvoltage-protection circuitry presented in AN-898 ( All comments on the model are welcome and will be noted - however, as I currently have other priorities do not expect fixes to emerge soon. Enjoy.

.SUBCKT LM3886 posin negin out posrail negrail mute gndpin
* LM3886 spice model by teemuk 21032007 ver.1b
Q1 posrail posin N004 0 NPN
I1 N004 negrail 0.25m
Q2 posrail negin N003 0 NPN
I2 N003 negrail 0.25m
Q3 N011 N003 N010 0 PNP
Q4 N011 N018 N012 0 NPN
R1 N012 negrail 2.2k
I3 N018 negrail 0.1m
Q5 N017 N018 N024 0 NPN
R2 N024 negrail 2.2k
Q6 posrail N011 N018 0 NPN
R3 N009 N010 1.1k
R4 N009 N023 1.1k
Q7 N017 N004 N023 0 PNP
Q8 N017 N013 N016 0 PNP
Q9 N011 N025 N019 0 PNP
R5 N015 N016 4.7k
R6 N015 N019 4.7k
R7 N013 gndpin 10k
D1 N025 gndpin D
D2 gndpin N025 D
R8 out N025 10k
C1 out N017 10p
I4 N026 negrail 1m
Q10 posrail N017 N026 0 NPN
Q11 N030 N026 N031 0 NPN
R9 N031 negrail 800
Q12 N009 N008 N014 0 PNP
Q13 N015 N022 N014 0 PNP
I5 posrail N014 1m
I6 N022 negrail 1m
D3 N021 N022 D
D4 N020 N021 D
R10 posrail N020 1k
D5 N007 N008 D
D6 N006 N007 D
R11 posrail N006 1k
Q14 N008 gndpin N005 0 NPN
R12 N005 N001 1k
D7 N002 mute D
D8 N001 N002 D
D9 N032 N031 D
R13 N034 negrail 150
Q15 out N032 N034 0 NPN
Q16 out N034 N036 0 NPN
R14 N036 negrail 0.45
Q17 N032 N030 out 0 PNP
D10 N029 N030 D
D11 N028 N029 D
D12 N027 N028 D
R15 N033 out 150
Q18 posrail N027 N033 0 NPN
Q19 posrail N033 N035 0 NPN
R16 N035 out 0.45
I7 posrail N027 2.5m
.model D D
.model NPN NPN
.model PNP PNP
.end lm3886
In case someone's interested, here's a link to schematics for AD15 & AD30VT.

It's funny how inconsistent the actual design is with all the "Valve Reactor" hype that Vox website boasts:

Being interested on how Vox actually applied the design of "Valve Reactor" circuit I have been hunting a schem for Valvetronix amp for years. Now that I finally see one: Such a dissappointment! My few favourites hype phrases are:

"Our patented Valve Reactor power amp consists of a valve power amp with an output transformer electronically coupled to a solid-state power circuit..."

Hmm... so where is the output transformer then? ...I know, in another information source Korg mentions that the OT is actually simulated. What a dissapointment: they omitted the component that really could have made some difference. And what about driving those 12AX7s on starved plate voltages... Also, I wonder from where the guys at Korg got the idea that running two low power preamp tubes in push-pull configuration would create a "power amp".  :grr

"The output transformer is connected to this new VariAmp Power Circuit which uses Constant Current design and Reactive Feedback technology."

Ok, the "VariAmp" circuit looks suspiciosly like an opamp stage with variable gain, the only difference being that the control varies the amount of feedback as well. Reactive Feedback = mixed-mode feedback. Constant Current design? Where?

"The VariAmp Power Circuit cannot be overdriven, is totally transparent and can be configured to be 1, 15, 30 or 60 Watts. The VariAmp Power Circuit does not color or change the signal in any way and the resulting output tone is pure."

I'd like to see an amp with high output impedance that is "transparent" and "does not colour or change the signal in any way". Funniest thing is that this is actually the sole point in the whole design and the non-flat frequency response may significantly contribute to overdriving the amp at certain frequencies. This is a very good example of the basic marketing hype scheme: With tubes coloring the sound = good, with transistors = bad. Yeah, right.

It should be noted that in the concerned circuit the tubes actually contribute very little and a high amplitude signal from the preamp will likely overdrive the input stage phase splitter resulting into transistor distortion.

And my personal favourite:

"This new technology actually has the ability to switch automatically between Class "A" and Class "AB" depending on the amplifier it's modeling!"

Just because a part of the preamp can do this doesn't mean the class-AB chip amp in the power amp stage could. In practice, the amplifier now exhibits the worst case behaviour on certain settings creating crossover distortion in both tube and solid state class-AB stages. Mentioning that would not be very good marketing though, (unless one can come up with a fancy name for this phenomenon, of course).

What is so frustrating in the whole thing is that a vast amount of people actually believe all these marketing phrases. I hate the fact that these days it's pretty rare to see an amplifier ad that would be technically accurate or even "correct" by its terminology.  :grr

Anyway, enjoy the schematics.
...and to say something good, I really like the conservative power rating of AD30VT. ;)
Amplifier Discussion / Grounding techniques
August 21, 2006, 05:28:13 AM
This thread was started so that people struggling with humming amps could have an information resource that helps them to understand what's going on. The internet is full of information but it is quite scattered and I hope this thread will develop to collect this information together. There are few resources I can recommend such as...

Epanorama: Tomi Engdahl, Ground loop problems and how to get rid of them ( and Ground loops and equipment design (

Randall Aiken, Star Grounding (

R.G. Keen, Star Grounding in Tube Amplifiers (

I'm sure a more thorough search would reveal even more. From top of my head I have summed up few rules that I regard as most important. Feel free to participate to this thread by comments or presentation of methods that you have found important.

Update 30.01.2007
Some corrections to the terminology. Corrected some spelling issues and added a small description of "loop breaker" circuits.

1) The essential part is to start treating the grounding as a part of the circuit itself: Each time a part of your circuit makes a connection to common it will not have a reference of ideal zero volts. Instead, this connection is just the other end of conductor connected to the only real common point that can exist in the system - a conductor with resistance. Since the circuit stage's current goes through this resistor, then according to U=IR the "common reference" point of the stage must have a voltage potential. It would be better to refer to these points as "common returns" not as commons.

2)  Different common spots and safety ground
Even a typical amplifier circuit can have at least three "grounds": Mains "ground", safety ground and secondary circuit ground, which really is not a ground at all. The first two should be ignored: Mains "ground" or neutral is NOT a ground either and should never be mistaken as one. It has zero volt potential but one should treat it as a conductor carrying mains return currents. Safety ground (of a device) is just an alternative for neutral in the case of faults. Safety ground has a sole purpose of saving you and from the viewpoint of removing hum and noise that should not make any difference whatsoever.

In practically all circuits, the reference used is not ground, it's common. A term common is better also because it indicates that the point might be "floating" and does not necessarily have a zero potential in reference to earth. Any voltage chosen from the secondary side can be used as the common reference: This is also the reason why you can get positive, positive and negative and negative voltage potentials from center-tapped transformers.

In some point of the house/mains wiring, the safety ground is connected to neutral. Usual location to do this is either inside the breaker box (right way) or inside the mains receptacle (works but is a bit unsafe). Having a permanent connection somewhere in housewiring is a safer than trying to make the connection by yourself inside the device since you have 50/50 change of succeeding if you can plug the power cord into the wall outlet in any direction. Since the chassis (safety ground return) and the common are usually tied together they will appear in nearly same potential. If the chassis is insulated from the common the common will float in whatever potential it sees best.

3) Choosing the right common spot
Only one true common point where the returns connect can exist in the circuit and therefore it should be selected with great care. Also, remember that current flows in loops. Consider these points:

a) Diode rectifiers make switching noise that appears as transients. These transients have the worst effect in close proximity to the diodes. You do not want switching noises to affect any other common returns. Ever wondered why commercial devices have those capacitors in parallel with diodes? Well, now you know.
b) Filter capacitors draw current as pulses. A filter capacitor charged by rectified signal never becomes "full" and never stops being charged, this is especially notable when the circuit's current draw is high. Currents created by these charge pulses are high and form a loopfrom the rectifier to the filters. The loop - as always - includes common wiring as well. You want to keep these charge pulses away from any other common returns.
c) Using the center tap as common point and connecting it to chassis most likely prevents you from making any other high current common connections to chassis (ie. speaker jacks).

I would not recommend using either one of the first two as the common point. So far I have found the following arrangements ok:
a) Single supply: Transformer connects the rectifier, rectifier connects the main filter capacitors and capacitors connect the common point. All other common returns connect only the common point. Safety ground may, or may not connect this point directly.
b) Dual-supply: Center tap connects the main filter capacitors and capacitors connect the common point. All other common returns connect only the common point. Safety ground may, or may not connect this point directly.

4) Correct ways to route the common returns
A ground loop is formed if the return currents of one stage inflict the return currents of another stage. Most severe situation is when high current stage (ie. speaker load) return currents share the same route with the return currents of the input stage. Most common example is when a ripple containing supply return shares the return of the input. Since nearly all gain stages use only a common return as a reference, any voltage appearing at this point will be heard in the output. Differential input amplifiers (opamps etc.) are a little more tolerant against this than class-A discrete stages.

The common returns should be separated by their type: audio (signal) / power (supply) and high current / low current returns should never mix up, neither should analog/digital returns. All these will eventually connect each other in the best and correct place, which usually is the common point. If nothing else, you want to at least separate high current and low current returns as well as signal returns from other "dirty" returns. Make it a principle for yourself that connecting these returns together anywhere else than in common point is always a conscious risk of adding a source of hum or noise to the circuit.

a) Star grounding
Ideally, all common returns should be routed separately and connected to common point in a form of "star" in order to prevent the returns from capacitively coupling each other. This method is ultimately effective but very hard to construct in real circuits. Very often the result is just a massive amount of long wires that may catch interference. Many people question the effectiveness of star ground but the reason for this is that they have most likely done something wrong: Either they have very long wires that are highly inductive or run near hum sources, they have bundled a few returns or made a chassis connection. (A true star ground can have only one chassis connection. This connection is made from the common point itself). In latter two arrangements one no longer even has a star ground. I see no reason why a correctly made star ground should hum – ever.

b) Various bundled common returns
Bundling up common returns can make the "grounding" scheme more compact. This is advisable, and properly done will not increase the hum or noise substantially. Usual procedure is to bundle the returns of each stage together. However, this is a pitfall: Many people bundle the supply to the signal return and wonder why their "bundled star ground" hums. The reason is the topology is no longer a star ground and the supply returns are infested by – sometimes – high current ripple or other noise that comes from switching devices etc. Also, bundling up the input stage and chassis (where several other common return connections will be made) together is asking for trouble. A better way is presented next....

c) Ground bus
The signal common returns should be bundled together this way:

Lowest current stage -> higher current stage -> etc -> highest current stage -> common.

The benefit of this arrangement is that the common return currents of the higher current stages will not return to the common point through the lower current stages. Then, the supply wiring returns should be bundled together in the same manner. If, besides these, the returns of power devices and speaker load are routed separately there is a fair chance that one can bundle all preamp/low current stages of power amp returns together with no hum whatsoever. In the end you have only few return wires running to the common point. Even better chance to success is to use bundled returns from multiple stages. Most likely the "grounding" scheme still stays fairly compact. Also, all passive stages returns can be fairly safely bundled to the return of the preceding active stage.

d) Large ground plane
This was a very often used technique in older amplifiers and usually involves connecting a vast amount of common returns to chassis thus forming a large and low resistance path for all return currents. Sometimes this works, sometimes it doesn't and I would not recommend this technique for anyone building their own amplifier unless you are willing to experiment with several prototype boards and layouts. The problem in ground planes is that the return currents choose the least resistive path - not the path we wish - and thus may interfere with each other. The use of either bundled ground or ground bus topologies is far more effective, less confusing and logical. At least with it one can control what way the currents return. For people fixing or tweaking amps ground planes, however, are a necessary evil.

A more common approach nowadays is to use the technique in PCBs, where – sometimes even a complete layer is dedicated to "grounding" purposes only. Properly done this is even more effective than using ground buses. But then again, the whole topology can be ruined by a simple mistake that happens to form a ground loop. (One can, for example, originate from connecting inputs to both PCB and chassis and connecting chassis to common). I have also seen circuits utilizing massive ground planes connected together by a single wire - thus the whole idea of the topology is ruined. In my opinion, a careful following of ground bus topology leaves less room for mistakes than reliying on massive ground planes.

A little more about safety ground...
As said, this is tied into the Neutral wire - usually in the breaker box of the house wiring. The safety ground connection should tie together all conductive parts that the user can touch. Make the connection near the place where the mains wiring connects the chassis and leave the safety ground wire longer than any other mains wiring so that it will be the last one to break loose if tension is applied to the mains cord. Be sure the chassis (or any part of it) will not corrode and break this connection by making it highly resistive. A use of teeth lock washers and lock nuts is advisable. If you ground heat sinks make sure a proper connection is made regardless of the non-conductive anodisized layer they might have. Bear in mind that the safety ground is effective only when it is connected in house wiring as well. All attempts to locate a hum problem should be started by checking the continuity of the safety ground.

Common potential does not neccessarily need a connection to safety ground. However, in guitar amplifiers, where the user can access common potential via the strings of the guitar, this is highly recommended since reference to earth prevents the common point from floating up to high potential. The connection may either be direct or indirect. Since a direct connection to safety ground may introduce ground loops when two devices are connected together the connection is sometimes made through a "loop breaker" circuit: This is usually a 10 ohm resistor in parallel with a 100 nF capacitor. The resistor introduces enough resistance to limit ground loop currents and the capacitor bypasses radio frequencies. Since in fault conditions the circuit can burn "open" it should have parallel high power diodes that pass fault currents through. Diodes usually burn "short" so the protection would remain. Note that the loop breaker circuits are illegal in some countries.