composite amplifiers

[sofaspud]

My source for this circuit is almost 20 years old*, so a forum and web search didn't turn up anything about it. I'm soliciting comments, opinions, and possible mods using other op amps or more powerful chip amps. I'm attracted to its elegant simplicity. And the numbers ain't bad either:

10 watts RMS into 8 ohms ____________ >0.003% THD @ 1kHz

* Charles Kitchin, Scott Wurcer, and Jeff Smith. Composite Audio Power Amplifiers. Electronics Now, Nov 1992: 38-44

[sreten]

Hi,

Unless I'm mistaken very low input impedance circa 1K and very poor gain, x2, I
cannot see the point at all, cannot see it being remotely stable either, very poor.

rgds, sreten.

The below avoids bipolar electrolytics in the signal path, gain is x20 :

[jcx]

the OP circuit "throws away" ~36 dB of gain in the 4k/65 divider - rather inefficient use of potential loop gain - and it still has a region of 2nd order loop gain slope giving ~ 0 phase margin

composite/multiloop amps with op amp added inside global loop with audio power chip amp is difficult given the limitations of achieving composite amp global stability when you may have to assume worst case loop intercept for the chip amp as low as 200 kHz you have limited added loop gain @ 20 kHz

if you stick to Bode's Maximum Loop Gain you only get ~10 dB added gain at 20 kHz, adding more loop gain requires allowing conditionally stable compensation and consequent possible clipping recovery sticking/oscillation

an interesting option is to use Black’s Feedforward to improve higher frequencies while relying on added loop gain at lower frequencies

the resulting complexity may make you wonder why you aren’t just building a discrete amp with 30+ MHz output Q from Cordell, Self books

[geraldfryjr]

There have been a few small discussions on this type of design and they are very hard to find even after doing some major digging on the subject.

I have the original articles published back in that era somewhere.

It is funny that you should bring it up as I was just thinking about such a design today.

Yesterday I had finaly found my copy of a Randall RG300 300watt power amplifier that I have had for 30 years,I had thought that I had lost it forever as web searches had turned up nothing.

I have seen a DIY stereo version of this amp in action as a small PA amp for a band.
It is super simple and very heavy duty as it uses 6 X 2N3773's in the output stage and is rated at 400 watts into 1 ohm.
There are only four transistor types in the whole circuit,TIS97,TIP31c,TIP32c and 2N3773.
So my idea was to use a Composite input setup in order to bring its performance up to date although it performed nicely the way it is from what I remember.

This type of configuration is supposed to make the overall performance of the amplifier as the same as the opamp in the first stage.

jer

[sofaspud]

sreten, what is the source of the circuit you have there? It's roughly the same thing my circuit shows. This article also shows a non-inverting version with the input and feedback resistors removed and the source brought to the + input on a 1M resistor to ground.
There's also 33, 40, and 70 watt versions, repeating this basic "module". I agree that it probably needs a strong input signal and attention to grounding and layout, but with just 5 passives and some decoupling caps this 10W amp just seems too tempting not to give it a shot. If I can't get it to behave, a little more soldering will give me that 3-stage circuit.

[jcx]

sreten's circuit is simply inverting mode for the power chip amp - the only other "improvement" from the op amps is the DC servo

buffering, inversion gives a hi Z positive input

this is essentially the same as Cordell's "Super Gain Clone" - page 540 of his book

I consider it a very feeble attempt at really improving the chip amp audio performance

the only chip amp distortion mechanism addressed is the nonlinear common mode input Z

the bigger thermal coupling distortion is better fixed by the OP circuit with the input op amp inside the global loop but physically separate from the chip amp and its high power, large temperature excursions

also the OP circuit input op amp is adding to the global loop gain below ~50 kHz - this added loop gain reduces all distortions of the chip amp

the input op amp is driving a relatively high 4k Ohm load, and only to a fraction of the output V at audio frequencies - its not working very hard at all so should have low distortion

[sofaspud]

The AD711 is part of the magic with this circuit. According to the article, it seems to be particularly well-suited for this application. Since Mr. Wurcer designed the opamp, if I'm not mistaken, I figure he ought to know.
The output is meant to mirror the performance of the opamp, so the LM1875's datasheet THD of 0.015% gets reduced significantly. My motivation is simply that I have the chip amps on hand and am looking to use 'em. If this puppy needs a good preamp, I have a LME49710-based diy unit ready and waiting.
Thank you for your responses, which I will continue to consider. But don't blow your credibility with talk of low-gain inefficiencies and thermal coupling, please.

[sreten]

Hi,

I cannot see why the gain is so low. If x10ing the values of R2 and R4
it would make more sense to to me. The LM1875 is stable with with
closed loop gains of 10 or greater, x2 = oscillation guaranteed AFAICS.

The usual 1R/0.22uF output loading is missing. No reason the chip-amp
should be run off +/- 18V, +/-25V is better, but too high for the op-amp.

rgds, sreten.

I really can't see the "magic" of the circuit at all .....

[sofaspud]

Don't get me wrong. I am very wary of the circuit's stability. But I'll probably give it a try anyway. The network on the output of the opamp is intended to stabilize the response, and a table is provided showing different values and their effect on phase margin and bandwidth. If things go well, I may try higher ps voltage for the LM1875, but I'd be happy with 10W.

[jcx]

nothing "magic" about the op amp - the AD711 is barley an improvement over LF411, TL070

OPA134 beats it by >2x on noise, GBW

if you want to talk about "magic" jfet op amps for audio you should be thinking OPA627 and the last decade's devices aimed at it like AD8610, ADA4627, OPA827


the OP composite amp circuit isn't particularly sophisticated in compensation, use of the 2 amps worth of gain - Scott Wurcer's name doesn't help its performance at all as far as I can see

Walt Jung, Gerald Graeme, Bob Pease, Jim Williams have all written about various op amp deficiencies and ways to help out



but since SofaSpud can dismiss their ideas so easily as having poor credibility I guess all we can do sit back and await enlightenment