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Author Topic: 60W Power amplifier  (Read 35846 times)

teemuk

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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" (http://milbert.com/articles/TvsT/tvtiega.html)

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.
« Last Edit: July 04, 2007, 09:41:21 PM by teemuk »

syndromet

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Re: 60W Power amplifier
« Reply #1 on: July 06, 2007, 01:51:13 AM »
Wow! :o
Amazing.....

jfetter

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Re: 60W Power amplifier
« Reply #2 on: July 06, 2007, 10:49:15 AM »
teemuck,
 you might try my method, build the amp using conventional methods ie feedback, spice model etc but then use a output "isolator". The best ss amps i have ever heard were always capacitor output coupled, then add to that a series air core high resistance inductor. You end up with a bandpass output that is somewhat isolated from the load. use a hefty zobel. use a simple first order model for the inductor, 8ohm load .3 mH cutoff at 5khz(or less!), let the speaker gen harmonics. clipping sounds great again.

its all counterintuitive but try it. if you have an existing amp add the network and enjoy.
I'm designing a high power class d amp and will use this concept again. Keeping the jfet preamp. Air core toroidals are much better but not too available..

http://www.partsexpress.com/pe/pshowdetl.cfm?&PartNumber=255-218&DID=7


teemuk

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Re: 60W Power amplifier
« Reply #3 on: July 06, 2007, 01:10:35 PM »
Nice ideas jfetter,

Personally I have found that it's somewhat "difficult" to implement the capacitor coupled output to a symmetric amp since the capacitor must be non-polar. It also has to be fairly high quality and high-capacitance not to distort the signal (assuming you want clean output) and practically a capacitor that matches those clauses does not work as a hi-pass filter for frequency range of a guitar: 1000uF, which is about the lowest "standard" and accepted value, has corner frequency of 40 Hz at 4 ohms. That will be even lower with a real speaker, more like 25 - 30 Hz.

I'm more interested in this inductive output filter configuration. I assume you have to damp the coil somehow, similarly to the inductive Zobel filters with a coil of approximately 0.3uH and a 10-ohm parallel resistor (cutoff above hearing range). The conventional RC Zobel is before the inductor not to cause a resonance peak, right? If I understood this configuration correctly the intention is to filter out the high order harmonics caused by overdriving the power amp and therefore you can't just put a similar low-pass filter to the preamp stage (where it would be a lot less bulky and waste less power).
« Last Edit: July 06, 2007, 01:52:42 PM by teemuk »

R.G.

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Re: 60W Power amplifier
« Reply #4 on: July 23, 2007, 06:31:49 PM »
Just curious - why didn't you just use an LM3886 for the power amp? That's what I've done when running current feedback setups. It works fine. In this iteration of the current feedback where you're using the current feedback to in effect predistort the signal into the power amp you should have even less problems than normal with it.

It is very hard to come up with good reasons for making amps under 60W out of other than the LM3876 or 3886.

LJ King

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Re: 60W Power amplifier
« Reply #5 on: July 23, 2007, 08:44:34 PM »
Just curious - why didn't you just use an LM3886 for the power amp? That's what I've done when running current feedback setups. It works fine. In this iteration of the current feedback where you're using the current feedback to in effect predistort the signal into the power amp you should have even less problems than normal with it.

It is very hard to come up with good reasons for making amps under 60W out of other than the LM3876 or 3886.

Other than "why the heck would anyone use a chip power amp?" - probably no reason at all.

I don't know if you would consider it a good reason to not use a chip power amp for under 60watts, but my reason is very simple - no short circuit protection, no current limiting.

I know engineers will tell me I need these things, but heck, I have been overdriving SS poweramps without them for over 35 years with great results - so nothing is going to convince me I need their tone defeating qualities this late in the game.

Teemuk's design is way too complicated for a 60 watt amp. I can build one with only 5 transistors and get great sound. Personally this design may be way too - lean towards tube emulation - for me.

teemuk

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Re: 60W Power amplifier
« Reply #6 on: July 24, 2007, 04:32:55 AM »
I agree with all of you: It's a pretty complex design for only 60W but also a product of experiment. I could build a 60W (push-pull) amp with three transistors but simplicity wasn't my design objective. However, if you take the basic Lin design, add another current source to the VAS and replace the passive current sources of LTP and VAS with active ones you'll get seemingly complex design similar to this. Anyway, since chip amps were mentioned I'd say the internals of LM3886 are a lot better example of a complex design.

BTW, one can easily squeeze out more power from this design (or other similar ones) by adding more transistors in parallel and increasing rail voltage. At that point the "complexity" begins to matter; it's more predictable.

Like R.G. mentioned, this circuit will work as well with a chip amp power section. Might be lot more easier and cheaper to build as well. Likely modifying some existing amps to try this is also very easy thing to do. Why I don't like to use chip amps: First of all, it's a black box and therefore it's usually not very interesting or educative. You can't modify the internals of an IC. Chip is prone to suffer from temperature distortion; generally the chip also dissipates heat a lot poorer than discrete transistors. After 20 years one can probably fix my design (or any discrete one) but not the chip amp since the chip may have gone obsolete. Replacing few blown discrete parts is easier than replacing the whole chip. Troubleshooting is "easier" as well: With IC you can merely find out whether it's dead or not.

joecool85

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Re: 60W Power amplifier
« Reply #7 on: July 24, 2007, 08:28:01 AM »
I agree with all of you: It's a pretty complex design for only 60W but also a product of experiment. I could build a 60W (push-pull) amp with three transistors...

Could you?  I'd love to see it.  I'm interested in building a simple transistor based power amp.
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teemuk

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Re: 60W Power amplifier
« Reply #8 on: July 24, 2007, 01:42:19 PM »
Quote
Could you?

Well... Yes, at least by "cheating" with Darlington output transistors. Finding the power transistors that have enough gain to be driven by a single common emitter alone - and that still have adequate current and voltage ratings - is the most difficult task in the whole process. Using MOSFETs might be another nice choice since they need a very low gate current.

Here's a low power circuit from Bill Bowden, the basic topology can lend itself to some mods:


For example, increase rail voltage to 65 V, replace the output transistors with TIP142/TIP147 (Darlington) or MSK1058/MSJ162 (MOSFET) - the voltage amplifier transistor with BD139, decrease emitter resistors to 0.47 ohms or lower, in case of Darlingtons add few more biasing diodes, replace 22-ohm emitter resistor with 220-ohms and bypass it with 100 uF, replace 470-ohm resistor with a bootstrap circuit of 2 x 2.2kohm & 47uF, decrease load impedance to 4 ohms, scale other component values so that the output node between the emitter resistors is biased to half supply and that you get the desired input sensitivity. i.e. 5.6K -> 52K, 1.5K -> 4.7K, fit 470-ohm (series) input resistor. I tested the circuit with SPICE and let's just say that it... well... works.


But seriously, why would you want to build an amp like that?

I can find plenty of reasons why you should NOT build this kind of stuff:

- The topology practically forces you to run the amp from a single supply, which in turn forces AC coupling, which in turn requires non-linear, bulky and expensive output capacitor.
- It also has to be inverting amp, which results to reduction in bandwidth and SNR.
- The single common emitter VAS is very poor in error correction, thus the linearity is poorer than in conventional, more complex designs.
- The CMRR and PSRR are reduced and you need to filter the supply better than in the case of dual supply circuit that uses error correcting input stage (ie differential amp). Since rail voltage is also higher you need to use caps with higher voltage rating that, in turn, cost more.
- The voltage amplifier stage also needs more current to work properly so in turn it will have to handle and dissipate more power: You are forced to use a transistor that has poorer high-frequency characteristics and slew rate than a lower power one. Again, more distortion is introduced

It doesn't end here...

- You are forced to track temperature with diodes since VBE-multiplier would require an extra transistor. This usually results to compromised reliability and at least to increase in crossover distortion
- The topology with only few transistors can't likely house an effective protection scheme so you likely blow up the amp quite soon - likely when you plug the speaker cab in and out while the amp is powered, thus momentarily shorting the output.
- The passive constant current source of the VAS, also known as bootstrap, is very non-linear. We can live with it but generally, the performance of all passive constant current sources is very non-linear and dictated by the changes in supply voltage. Effectively they are not that constant after all. If you want reliable performance at higher power levels (and better PSRR) the predictable and linear operation of these circuits becomes more and more important.
- Likely (to enhance simplicity) all HF compensation is left to shoulders of "Miller" Cdom, which leads to poorer slew rate and increase of distortion at higher frequencies
- At worst, all these non-linearities are compensated with a large amount of NFB that makes the clipping very harsh


Hmm... This all "surprisingly" sounds like stuff that caused SS amps to have bad rep in the first place. Anyway, if you want to build a simple discrete amp then you can find plenty of examples of them from the era between 1965 - 1975. Not that many good SS amps were manufactured back then.

My advice is that if you want to build a decent SS amp with at least medium power then FORGET the simplicity. You can't find a good, reliable design that wouldn't use at least 9 transistors. Because 99% of circuits tend to follow the Lin topology it's basically all about the component values but here's an example of what I consider the minimum circuit for a medium power application: http://sound.westhost.com/p27-f2.gif, I'm pretty sure most of you are familiar with it already. To my eyes it looks pretty simple, especially when compared to this:


...and it doesn't even include the protection and CCS circuits  ;D
« Last Edit: July 24, 2007, 01:46:57 PM by teemuk »

LJ King

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Re: 60W Power amplifier
« Reply #9 on: July 24, 2007, 07:46:38 PM »

I have some rather basic questions... and I guess my answers. I guess the umbrella question here is "why am I wrong in thinking what I think"?

o Why are specification related aspects like linearity, slew rate and bandwidth even being considered for a guitar amp?

Sure, if I were building a hi-fi amp, these specs would become more important. The purpose of guitar amps is to produce a sound - not to reproduce an existing sound accurately. I've never heard of these specs being advertised for tube amps, so I think they can safely be discarded for solid state amps.

o Protection circuits are necessary for the ignorant consumer and manufacturers that cater to them. Professional level equipment do not need them.

I guess this is more of a statement than a question. But as an example, take short circuit protection. When is it really necessary? When the output is shorted. Ok, what kind of idiot would short the output? An ignorant consumer.

Most professionals follow a simple rule that makes short circuit protection totally unnecessary: "Never power on an amp without a speaker load attached." Catering to people that ignore this rule (that pertains equally to solid state and tube amplifiers) isn't necessary for yet another reason: output transistors are cheap. Tubes on the other hand are at least an order of magnitude more expensive, not to mention the output transformer - but I've yet to see a tube amp with "open circuit protection" which is more damaging to a tube amp than a short circuit (other than low wattage plate and screen resistors acting as fuses).

So maybe the real question is: why use circuits designed to protect a device that is cheap to replace? Especially when they can adversely affect the sound.

o SS amps do not have a bad rep due to amps produced between 1965-1975. At least not the professional level SS amps produced during this time frame.

This is part of the myth of better sound coming from tubes. Look at what manufacturers were producing professional SS equipment during this timeframe: Standel, Thomas/Vox, Baldwin, Kustom, Acoustic, Sunn - probably more, but these were the larger players. Fender tried but failed due to manufacturing problems, not related to the design of the amp's circuits.

Pick one that contributed to the bad SS rep.

All of their power amp designs were simple, used very little feedback, were probably non-linear as heck, and all but Kustom (at least up to the introduction of the K200B in 1968) didn't use any short circuit protection.

What I think contributed to a bad SS rep - the sheer expense of purchasing an SS amp during this timeframe!!

Go to your record collection, and pull out ZZTop's Tejas. The guitar on the entire album was recorded through a Thomas/Vox Super Beatle. FACT! I remember a review in some magazine when the album came out - proclaiming the great tube sound of ZZTop does it again.


teemuk

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Re: 60W Power amplifier
« Reply #10 on: July 25, 2007, 12:10:54 AM »
You have a valid point about protections and ignorant consumers. Unfortunately, my experience is that majority of the people using guitar amplifiers are these "ignorant consumers". Every week I can read about someone who asks "ohms law questions", who asks can he connect the output of his 50W tube amplifier to his 100W solid state amp, who asks can he use two amps to drive one speaker cabinet, who asks what the "gain" or "tone" controls do etc. The list is endless. The variety of stupid things these people are capable of doing is amazing. Very often these folks also ask help because they have blown their equipment. I know some of these folks and from my experience I can tell that no matter how talented musicians they are, they are also reluctant to learn anything new; i.e. how to treat amplifiers properly. My personal standard today is to build stuff with a principle that I dare to loan it to these folks with an expectation to get it back in a working order.

I don't expect people to take care of their amps when experience has shown that majority don't even know how and abuses the hell out of them. The worst are people who think they know enough about electronics and who therefore make silly experiments when they really should just keep their hands off the equipment.

Sure. Solid state amps are "cheap" to fix - assuming you do it yourself. Still, it's pretty onerous task and sometimes inflicts damage for the internals such as PC boards. Take the amp to a tech and he will replace the cheap parts and charge at least ten times more than what the parts costed for the labour. Goodbye to seeming "cheapness".

Anyway, my point is: If you can prevent the amp from blowing up in some cases by installing some extremely cheap circuits, that should be transparent under normal operating conditions anyway, why not do it?

Speaking about that, making the short circuit protection to track SOA curve accurately is not extremely difficult but will require a more complex circuit than what is seen in majority of amplifiers. So far I have seen no guitar amp that would have more sophisticated method of tracking than 3-slope one. That is the reason why those protections also get audible. Anyway, I rather listen to ugly sound of short circuit protection kicking in than the silence of a blown amplifier. I guess that answers your question about the protection circuits. This is how I see the issue.


Linearity, slew rate, bandwidth? Discarded? No way! Those are pretty important parameters if you want to have a transparent amplifier. Most tube amps are not and you can definitely hear it. (Maybe that's why they do not bother to mention those parameters). Most "clean" tube amplifiers are nothing but clean while certain solid state amplifiers shine in that respect. It's a matter of taste but I'm not that big fan of the tube sound myself. So yes: I think those design parameters ARE important, especially when it comes to linearity issues that relate to crossover distortion or slewing.

If you want to introduce non-linearities intentionally then that's a different issue. For the record, I prefer to have an option to turn those kinds of effects off if I want to. Can't do that with a design that is inherently non-linear. Anyway, this is a subjective issue and each one has his/her own opinions about it.

I have noticed that if the design is done properly and doesn't exhibit some erratic clipping behaviour, that, say, creates oscillation or rail sticking, then there isn't much difference between the overdriven tones of simple and complex solid state amps. They clip quite similarly. In fact, most linearizing circuits have no audible effect in that domain. However, they become important in acquiring good CMRR, PSRR and in minimizing crossover distortion - or if the design needs to mend itself for higher output powers. For me it's therefore an obvious choice to use circuits that are more linear.

Nice sounding clipping? That's another issue, which likely has more to do with careful and dynamic EQ:ing than the "softness" of the clipping.


I hope you do not have an impression that I have something against solid-state sound or old transistor amps in general...?

You list some manufacturers who made amps that were so good and nice sounding that we bother to remember them. Those were just the tip of the iceberg. But still, considering the bad rep of SS, even from that list of yours I can pick at least Thomas Vox (notorious for producing very unreliable amps), Standel (mistake to use epoxy coated preamp modules) and Sunn (the Orion failure, which was even endorsed by Jimi Hendrix).

Funny though: None of the companies you listed actually survived although they had pretty nice products: Standel exists but is practically re-found and now builds the same tube amp design that initially started the whole company, Thomas Organ was resurrected to build digital keyboards, Baldwin discontinued amp production at least two decades ago, Kustom was sold and its founder ventured to other projects (which were unfortunately less successful) - the newer Kustom makes (if I'm correct) mainly tube amps, Acoustic is no more either (but it did spawn a lot of people who become famous in the guitar amp industry), Sunn was sold to Fender, which practically meant its downfall.


It's pretty difficult to make any generalizations about the circuits: Thomas Vox amps were practically using that totem pole topology with interstage transformers: Very non-linear, quite unreliable as well but had nice preamps and pretty decent speakers. Some people truly hate the tone of these amps - some people love it. In contrast, the later designs of Acoustic Control were actually quite "Hi-Fi". OTL Lin topology, lots and lots of negative feedback all over the circuit. I believe that their speaker systems should not be underestimated either. Their amps were good but the speaker cabinets were awesome - especially those W-bins. Never seen schematics or insides of Kustom or Baldwin amps from that era. Seeing those designs really interests me...


What you mention about the price issue is basically correct. The first SS amps were very expensive  - but that didn't last very long. At the time when SS amps had gotten rid of their worst problems they had become a lot more cheaper. You should know that the major fixes were inclusion of short circuit protection and getting rid of the transformers in the signal path so that NFB could be increased in order to obtain more linearity. These were all inventions popularized by the influential RCA papers that appeared at the late 60's. And by the way, your statement about the lack of short circuit protection is generally misleading: Most of the companies in your list incorporated the circuit as soon as they managed to get rid of the interstage transformer: Kustom, Standel, Acoustic, Sunn. They all started to employ the VI-limiter at some point - typically in the late 60's when the circuit was devised anyway. That leaves only Baldwin (of which I'm unsure of) and Thomas Organ Company. The latter didn't have the circuit and guess what: The output devices of those amps blew very often.

Of course the aforementioned was a pretty rough generalization: The designers of those Vox amps had to resort into using speaker connectors that couldn't be shorted while plugging the coord in and out, so shorts weren't the main issue compromising reliability: Output devices of the early "TV" amps were germanium and heatsinking and thermal management ridiculous if judged by the modern standards. Luckily those things are an issue that designers are more knowlegeable of these days.


Of course there has been very good and nice sounding solid state amps. For the most parts they have been a lot more innovative than tube amps from the era as well. For example: Mesa is falsely proclaimed as the developer of first channel switching amplifier although this feature had been implemented to a SS amp years before it appeared in any "major brand" professional tube amps. Many good SS amplifiers have been left unnoticed due to hype about tube tone.

I have noticed as well that people like to proclaim that "tube tone" thing everywhere. IMO Santana sounded best at Woodstock where he used a solid state GMT 226A (amp that gave birth to Gallien-Krueger company). The amp was also heard on his first record. The rest of the records he did using Mesa Boogies and whatever essentially mimic that SS tone. Zappa (and who knows how many other musicians) used Pignose in the studio. Beatles used a lot of solid state stuff. Darrell Abbott of Pantera used Randall amps (and while I'm not a fan of Pantera those amps are IMO about best ones I've heard so far when it comes to both clean and distortion tones. Way better than Marshalls or Mesa Boogies). The brilliant clean tones on Metallica's Master of Puppets were created with JC-120. I guess one could find countless similar examples.

I have some ZZ top records but their overall sound (not just the guitars) has always sounded too "synthesized" to my ears. I guess we have different taste in music. Do you know what they used on "Afterburner"?

LJ King

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Re: 60W Power amplifier
« Reply #11 on: July 25, 2007, 01:38:30 PM »

No, don't have any idea what was used on Afterburner. For what it's worth, a Legend was used on Eliminator - I'm not familiar with them, but it's supposed to be a hybrid, tube preamp and 50w SS power amp.

The ZZTop CD's are unfortunately remixed versions - not copies of the master tapes used to produce the vinyl recordings copied to CD. That's a shame because they really screwed it up. Supposedly the "six pack" CD collection were all remixed in only three days - "specifically for CD" which is a crock. Some are way too bright and others are very dull sounding.

Yes, I agree - all those companies I mentioned are defunct - but I seriously doubt that more complex circuits, hi-fi designs and additional protections would have had any bearing on their demise - other than possibly make it happen sooner.

Seems to me that there is a life-cycle that eventually affects most corporations. A company starts up, produces innovative products... then stagnates into bureaucracy, starts using "market research" and focus groups to tell it what to do, drives out all of the geniuses (aka mavericks) that gave the world those innovative products and settles into "business as usual" waiting for the coffin to close. Sad in a way. The only companies that survive this cycle are the ones that are "reborn" and get a chance to start the cycle over again at the beginning - Fender is a good example, although they are now back in stagnant mode.

Acoustic started out with some "state of the art" designs. The 361 does have differential inputs and short circuit protection. Fortunately they left that stuff out of the 260. I don't know about it's 270 successor. I lost interest in Acoustic around 74-75 - if that's when they went hi-fi, maybe that is the reason. I didn't like the sound although I can't put my finger on a reason, although I still love the sound of a 260. I'm surprised you didn't say they "de-evolved"... IIRC, they used a transformer driven totem pole design in the 150s?

I don't disagree that the Thomas/Vox Beatles were unreliable. But I don't believe that is just a natural consequence of the transformer driven totem pole design - there are too many reliable examples using this design out there. However, ANY DESIGN, if overextended, will transition into unreliability. I consider the Beatle power amp way overextended - the Royal Guardsman is probably on the edge.

I don't consider the Buckingham to be overextended. 35 watts, 8 ohm minimum load, three transistor transformer driven totem pole power amp. I don't believe they were ever converted to silicon outputs. I've had one for almost 20 years, overdrive it constantly (within the range of the preamp limiter which I have cranked up for as little limiting as possible - but I believe it only has a 6db range of adjustment, if I estimated correctly). I'd never do that to a Beatle, but this little amp just loves it. I do expect the outputs to give up eventually - probably due to age and thermal cycling more than anything else.

The Orion. I hated when Sunn discontinued it - and it's brother Aquarius which I have never seen outside of the 68 catalog - it was probably never released. Power amp overheating problems as I recall. I've never seen the power amp - there are rumors that it had germanium outputs and/or it was too enclosed without a chance of cooling air circulation.

How about the Sunn Concert? The earlier ones used a transformer driven "dual" totem pole in a "grounded bridge" configuration with a floating power supply for the outputs. Used with a minimum 4 ohm load, these amps have proven their reliability. What usually happens is that someone sees that the 1975 and later version is rated for 200 watts into 2 ohms and thinks they can run the earlier Concert into 2 ohms. The later design - trying to get 200 watts out of four 2N3055s into 2 ohms, even though the bridge configuration was dropped in favor of parallel output transistors, is what I would consider the edge of overextension. These later versions are just not as reliable as the early ones.

The Baldwin power amp has proven reliable. In the C1 Custom, 45 watts into 2 ohms (the 2 12" speakers are 4 ohm units connected in parallel - the extension speaker jack is in series). Also a transformer driven totem pole. A voltage amp (NPN silicon) to mix and boost the two channels, directly coupled to the Class A (PNP germanium) transformer driver. The secondary side uses six PNP germaniums, same type as the driver, paralleled three per side. Feedback is from the output (after the coupling cap - it's single supply) to the emitter of the transformer driver, and through a 2.2K resistor to the base of the "grounded" side of the output string only. Unlike the Thomas/Vox, there is no limiting - with the possible exception of the eventual clipping of the class A transformer driver. The emitter resistors of the outputs are .47 ohm fusible links however.

The Kustom K200B power amp is dual supply +/-39.5 volts, no output cap. It has differential inputs, typical class A VAS. One unsual aspect is each output transistor has a separate driver transistor. The short circuit protection doesn't seem to affect the sound even when driven to clipping, unlike other more hi-fi power amps. A single transistor on each side is used to shunt the drive signal at the base of the driver transistors through a diode. The control signal is derived across a 1 ohm emitter resistor to ground on both of the output transistors on that side, and is summed to the base of the shunt transistor through separate 1K resistors.

The earlier Kustom K200A series didn't have short circuit protection and are in general regarded as the better sounding of the two. The K200A is can have overheating problems which were solved in the K200B by an increase in chassis size, better mounting and more airflow over the output transistors. John Fogerty stuck with his K200A-4 - probably more for the additional advantage of having all effects available on one channel and not split between two channels like the K200B-4 design - but you will notice, he always had a backup ready to go if necessary!!

My favorite "tube tone that's not" story is Frank Morino of Mahogany Rush. An interview came out sometime during the early 1970s and one of the questions was "What amps do you use". His response was two Acoustic 260's. The next question from the obviously clueless interviewer was "Do you use any special tubes to help you get your sound?" I remember exactly how the response was written: "<pause> Uh... no. The 260's are solid state amps. Tubes are too unpredictable for me." Priceless!!

When did Mesa get credit for channel switching? I hadn't heard that one. I know Mesa had a patent on "mode switching" which isn't the same thing, and the primary cause of their suit against Acoustic.

teemuk

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Re: 60W Power amplifier
« Reply #12 on: July 25, 2007, 02:40:52 PM »
Seems to me that there is a life-cycle that eventually affects most corporations... ...Fender is a good example, although they are now back in stagnant mode.

Let's mention Peavey as well. Anyway, those were pretty hectic times in guitar amp industry - companies merged or changed owners frequently and produced guitars and amps for each other. It's pretty difficut to track down any of that history. Some companies just seized their production for no particular reason. I have an impression that Kustom was put out simply because Bud Ross got bored to running it.

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How about the Sunn Concert? The earlier ones used a transformer driven "dual" totem pole in a "grounded bridge" configuration with a floating power supply for the outputs.

That is one of my favourite totem pole circuits and in fact, it served as my introduction to floating rail amps.

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When did Mesa get credit for channel switching? I hadn't heard that one. I know Mesa had a patent on "mode switching" which isn't the same thing, and the primary cause of their suit against Acoustic.

Well they have never really been credited for that in true sense but usually people mention Mesa (Mark II if I remeber right) as an example of the early channel switching amps. That amp came out in the early 80's, so did Fender's channel switching amps. As far as I know GMT 200G was the first amp with channel switching feature. It was introduced in 1974 so talking about Mesa, which came out half a decade later is a bit strange. Anyway, in the eyes of a common consumer Mesa is more famous, which likely explains this sort of stuff.

My favourite pick concerning tube tone myth would be Russell O. Hamm's famous "Tubes vs. Transistors" convention paper from 1974. Even today people like to refer to it as gosbel although most of it's content has been debunked or otherwise corrected.

R.G.

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Re: 60W Power amplifier
« Reply #13 on: August 06, 2007, 03:51:27 PM »
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I know engineers will tell me I need these things, but heck, I have been overdriving SS poweramps without them for over 35 years with great results - so nothing is going to convince me I need their tone defeating qualities this late in the game.
I think you're right - nothing will convince you. There is nothing as absolute as a closed mind.

Protection in overdrive is not what current and power limits are about. They're about avoiding repair bills. You ever have a repair bill?  :)

Before you get all up in arms, the Thomas Vox Beatle is one of my favorite amps. I have a section of my web page devoted to how to fix them and make them more reliable.
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o Protection circuits are necessary for the ignorant consumer and manufacturers that cater to them. Professional level equipment do not need them.
You didn't play with some of the folks that were in my bands, I guess. :)
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I guess this is more of a statement than a question. But as an example, take short circuit protection. When is it really necessary? When the output is shorted. Ok, what kind of idiot would short the output? An ignorant consumer.
Or my old bass player. Or the guy who ran a metal wheeled cart over my speaker cable.
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Most professionals follow a simple rule that makes short circuit protection totally unnecessary: "Never power on an amp without a speaker load attached."
SS amps in general don't give a fig whether there's a load between rated and infinity (open circuit) attached.  And attaching a speaker to a SS amp does not protect it if there's a shorted cable, or jack, or if you get a beer spill.
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Catering to people that ignore this rule (that pertains equally to solid state and tube amplifiers)

Actually, it's a tube amp only rule. Many tube amps have internal resistors that are auto connected as a protection against turning on without a speaker plugged in. Is that a tone sucker, too? Remember it only acts when the speaker is NOT plugged in.  ;)
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isn't necessary for yet another reason: output transistors are cheap. Tubes on the other hand are at least an order of magnitude more expensive, not to mention the output transformer - but I've yet to see a tube amp with "open circuit protection" which is more damaging to a tube amp than a short circuit (other than low wattage plate and screen resistors acting as fuses).
I've seen a number of tube amps with open circuit protectors in them. Usually this takes the form of a 100-270 ohm 2W resistor soldered onto the output jack, but there are others.

And here's a more important issue. Transistors are cheap these days. But leaving your amp with a tech will cost you a minimum of $50 plus parts, could easily go higher, and it will also cost you the amp not being yours to play with for a week or two while being fixed. Protection circuits may make the amp sound odd if you make it do things it should not be doing, but at least when you quit abusing it, it will start playing again without the week in the shop.

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So maybe the real question is: why use circuits designed to protect a device that is cheap to replace? Especially when they can adversely affect the sound.
Because it's expensive to have any professional equipment fail, both in terms of repairs and time, and neither of those are directly related to parts cost.  :)

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My favourite pick concerning tube tone myth would be Russell O. Hamm's famous "Tubes vs. Transistors" convention paper from 1974. Even today people like to refer to it as gosbel although most of it's content has been debunked or otherwise corrected.

Could you supply pointers to the debunking and corrections, please?

LJ King

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Re: 60W Power amplifier
« Reply #14 on: August 06, 2007, 08:09:57 PM »
R.G. - Have you always this paranoid about SS amps? Obviously your logic has a strong bias towards tube amps.

Why pretend that a "no plug in jack" resistor is the same as "open circuit protection". For example, isn't it just as likely to have your tube amp speaker cord run over by a guy with a metal wheeled cart? And wouldn't that accident have roughly the same if not higher chance of producing an open as a short?

Why would you accept the odd sound of protection circuits? Because when they start operating it means I'm abusing the amp? Sorry, but that definition of abuse rests with the designer that didn't know his subject - Amps of any type are routinely "abused" by overdriving them since the sixtys.

Since tube amps don't have protection circuits that corrupt the sound, does that imply that tube amps are not abused when overdriven?

If a blown tube can be accepted in persuit of tone, no reason that a blown transistor can't be equally accepted.

What's a tech? Oh, you mean those delusional self appointed guru's of tone that won't even look at an SS amp.

Teemuk - thanks for mentioning Peavey. I didn't know they had been around that long. I've read through some of the articles on the site. I especially enjoyed the transtube article by Hartley. I really loved the part about the double blind "guess which amp is SS" test!!