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Topics - teemuk

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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
« on: 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...

Preamps and Effects / Dynamic clipping threshold control with regulator
« on: November 25, 2009, 03:40:46 PM »
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
« on: 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
« on: 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
« on: 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
« on: August 30, 2008, 08:48:43 AM »
Here's few traced-out schematics for these old gems (Vegas 400 & Nashville 400).  :tu:

Schematics and Layouts / Schematic sites for different brands of amps
« on: June 28, 2008, 04:11:12 AM »
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)
« on: 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?

The Newcomer's Forum / Loudspeaker parameters and loudness
« on: February 15, 2008, 06:30:43 AM »
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
« on: 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
« on: 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
« on: 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.

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