Build A High Quality Audio Amplifier

[Whuffo]

Audio amplifiers aren't as easy as they look; they have to handle a wide frequency range and wide dynamic range and do it without adding or subtracting anything while driving a load that has varying reactance. Not to mention the human beings that use and abuse them.

So here's the schematic and parts list for one you'd be proud to own. All you need to add is a +/- 24 VDC power supply; I'd recommend a 36VAC center tapped transformer, a large bridge rectifier and some big filter caps. At least 5.000 uf per rail; double that won't hurt. If you're going to run multiples of this circuit, plan on supplying 2 amps per amplifier.  Heat sinks are critical; the driver transistors will be happy with finned hats, but the outputs need to be thermally coupled (and electrically isolated) from a heat sink of at least 60 square inches per amp circuit. Fins make this easier; using smaller heat sinks is risky. The power dissipation depends upon the output power, and at worst case it'll need all of that heat sink.

It's an elegant circuit; some of the design may be unfamiliar but it's been carefully optimized and if you build it exactly as designed, I guarantee it'll work great. Maybe some of the bright folks here can find ways to squeeze a little more out of the circuit? If you have questions or comments, I'd love to hear them.

Sorry about the rough looking diagram; I only had it on paper and didn't spend much time redrawing it to post here.

EDIT: See updated schematic on page two

[Tube_Dude]

Hi

- First of all, your input pot wiring is incorrect. The way you have it, when the pot is at minimum the input source will see a dead short, and it will not enjoy it. The center pin of the pot must be connected to the C1 input capacitor, and the point that was connected to C1 will become the input node.

- A 1KOhm resistor placed between C1 and the op-amp + input will help to not occur oscillations when the pot is at minimum, or maximum (in this last case with a cable connected at the input but not connected to the source).

- A electrolytic capacitor, say 100uf, across the biasing arrangement (between the bases of Q1 and Q2) will help the driver transistors (Q1/Q2) to have the same signal at each base.

- A electrolytic capacitor, say 100uf, in series with R1 will make the offset voltage lower, without loosing anything, in the usual audio band.

Cheers

[Whuffo]

Hi

- First of all, your input pot wiring is incorrect. The way you have it, when the pot is at minimum the input source will see a dead short, and it will not enjoy it. The center pot must be connected to the C1 input capacitor, and the point that is now connected to C1 will become the input node.

- A 1KOhm resistor placed between C1 and the op-amp + input will help to not occur oscillations when the pot is at minimum, or maximum (in this last case with a cable connected at the input but not connected to the source).

- A electrolytic capacitor, say 100uf, across the biasing arrangement (between the bases of Q1 and Q2) will help the driver transistors (Q1/Q2) to have the same signal at each base.

- A electrolytic capacitor, say 100uf, in series with R1 will make the offset voltage lower, without loosing anything, in the usual audio band.

Cheers

You're absolutely correct about the gain control, it should be connected as you say (and as it is on my paper drawing). I was in too much of a hurry when I transcribed the schematic. Swap the wiper and the high end of the pot and you're golden. Actually, for most purposes deleting everything before C1 would be fine.

The resistor to ground at C1 is not 1K, it's 100K - and it's there to properly set the differential input to the opamp without loading the input too much. It also acts as part of the input voltage divider.

Adding an electrolytic to the biasing chain is valueless; the diodes in the chain are in saturation at all times. They're used to develop the correct bias voltage and have no effect on the AC performance. And as far as the electrolytic in series with R1 goes (make the offset voltage lower), you're overlooking R2, which guarantees zero offset as well as setting the gain for the amplifier. The desire to add reactive components (like capacitors) to the signal chain is very common, but it's not advisable.  This design has one and only one capacitor in the signal chain - the input coupling capacitor. It's there because it's impossible to guarantee that the input will average zero volts DC. Beyond that, the amp is DC coupled all the way through. The fuse is there to protect the speaker - just in case.

I tested one of these on a Rhode and Schwartz distortion analyzer. At any output level, the R&S showed nothing more than its residual noise level. I've also (inadvertently) applied a dead short across the output before the fuse - at full output or several minutes. No damage resulted.

The design has been optimized many times; many of the components serve multiple purposes. Since no component is perfect, purity is best served by keeping the number of components to a minimum. The "fancy" biasing arrangement is needed to compensate for manufacturing variations in the transistors; there's no "one size fits all" value here. Once it's set, you won't need to touch it again unless you replace parts.

The circuit is a bit subtle; there's not one part in there that isn't absolutely required. Take another look, and try building one - hide it behind some glowing tubes and you'll fool the audiophools every time.

[ElectroIrradiator]

A few additional comments:

- VR1 should probably be 10K log. Otherwise the presence of R3 will skew the potentiometer curve to a noticeable degree.

- No high frequency compensation? Maybe a tiny cap across R2, or use a NE5534. The latter would also offer a convenient location for an output null adjustment, if needed. I would worry about potential high frequency oscillations at certain loads without one.

- C4 should be two 0.1 uF caps, one from each rail to ground, in parallel with the 5 (4.7?) uF 'lytics. As C4 is currently located, it will not be able to help supply high frequency current spikes to the output. (Or rather, it will draw them through the 'lytic on the opposing power rail).

- Would using the classic 'active' BJT bias network in place of D1-D3 + VR2 reduce thermal drift of the DC operating point?

[Tube_Dude]

- A 1KOhm resistor placed between C1 and the op-amp + input will help to not occur oscillations when the pot is at minimum, or maximum (in this last case with a cable connected at the input but not connected to the source).

The resistor to ground at C1 is not 1K, it's 100K - and it's there to properly set the differential input to the opamp without loading the input too much. It also acts as part of the input voltage divider.

Please reread what I have typed about the 1k resistor. It have nothing to do with the 100k biasing input resistor.

[[ Adding an electrolytic to the biasing chain is valueless; the diodes in the chain are in saturation at all times. They're used to develop the correct bias voltage and have no effect on the AC performance]]

You are forgetting the variable resistor in series with the diodes. Obviously it will work the way you have it, the cap will just help the symmetry of the output stage.

[[  And as far as the electrolytic in series with R1 goes (make the offset voltage lower), you're overlooking R2, which guarantees zero offset as well as setting the gain for the amplifier. The desire to add reactive components (like capacitors) to the signal chain is very common, but it's not advisable. ]]

What determine the DC offset voltage of the amplifier is the relation between R1//R2 and R3 (being R1 much lower than R2, is R1 the dominant between the two).

Curious to know how many mV offset voltage you get with your values... 

[Gall]

The quality of this circuit depends solely upon the quality of the opamp. It compensates everything except for high frequency phase delay that may cause RF oscillations if not compensated properly. A small capacitor in parallel to R1 will help if the circuit oscillates. Try to get the best existing opamp. Especially important are open-loop gain and good common mode and power rejection ratio. A large common-mode input voltage would make the circuit more RF stable. In any case, do not try to make the gain ((R1+R2):R1) less than 100. Anything like 1k:100k should be fine. An extra resistor between the opamp output and its inverting input will increase stability. This should be in order of 100xR2 or more, resulting in open-loop gain of 10000.

[Whuffo]

- A 1KOhm resistor placed between C1 and the op-amp + input will help to not occur oscillations when the pot is at minimum, or maximum (in this last case with a cable connected at the input but not connected to the source).

The resistor to ground at C1 is not 1K, it's 100K - and it's there to properly set the differential input to the opamp without loading the input too much. It also acts as part of the input voltage divider.

Please reread what I have typed about the 1k resistor. It have nothing to do with the 100k biasing input resistor.

[[ Adding an electrolytic to the biasing chain is valueless; the diodes in the chain are in saturation at all times. They're used to develop the correct bias voltage and have no effect on the AC performance]]

You are forgetting the variable resistor in series with the diodes. Obviously it will work the way you have it, the cap will just help the symmetry of the output stage.

[[  And as far as the electrolytic in series with R1 goes (make the offset voltage lower), you're overlooking R2, which guarantees zero offset as well as setting the gain for the amplifier. The desire to add reactive components (like capacitors) to the signal chain is very common, but it's not advisable. ]]

What determine the DC offset voltage of the amplifier is the relation between R1//R2 and R3 (being R1 much lower than R2, is R1 the dominant between the two).

Curious to know how many mV offset voltage you get with your values...

The DC offset is indeed determined by those resistors as well as the quality of the opamp. Note that R1 and R3 are connected to ground; the feedback resistor R2 nulls the output offset to zero. And that's what I've measured - this circuit works fine.

The variable resistor in series with the diodes is a fine tuning adjustment for the bias current; the bulk of the bias voltage is developed across those germanium diodes and the additional amount across the potentiometer is minimal. There's 48 volts across the bias chain, and less than 200 millivolts of it is across that potentiometer. Is the drive perfectly symmetrical? Not quite - but notice that the entire amp is enclosed in the feedback loop. The feedback takes care of any tiny errors here.

And while I understand what you're saying about that 1K resistor - it's not necessary here. This amp has been tested with both 535 and 5535 opamps and was found to be completely stable. There's some subtle bandwidth limiting in the circuit - it's not obvious, but it keeps the amp from doing strange things at frequencies we're not trying to amplify. Build one and give it a try; you'll be impressed.

[Whuffo]

A few additional comments:

- VR1 should probably be 10K log. Otherwise the presence of R3 will skew the potentiometer curve to a noticeable degree.

- No high frequency compensation? Maybe a tiny cap across R2, or use a NE5534. The latter would also offer a convenient location for an output null adjustment, if needed. I would worry about potential high frequency oscillations at certain loads without one.

- C4 should be two 0.1 uF caps, one from each rail to ground, in parallel with the 5 (4.7?) uF 'lytics. As C4 is currently located, it will not be able to help supply high frequency current spikes to the output. (Or rather, it will draw them through the 'lytic on the opposing power rail).

- Would using the classic 'active' BJT bias network in place of D1-D3 + VR2 reduce thermal drift of the DC operating point?

The selection of VR1 and associated input components is part of the bandwidth limiting and the component values are correct for their intended purpose. Is the curve of the pot skewed? A little, but it's a "set and forget" control in most applications.

Yes, there's no compensation capacitor. It's not necessary here due to bandwidth limiting. And C4 is intentional; it's there to stabilize the power inputs to the opamp. During operation, the current input varies dymamically and individually; C4 keeps the DC level on the output stable during these excursions. Supply of peak power to the opamp comes from the electrolytics - but keep in mind that the opamp doesn't supply much output power, that's developed in the transistor stages. If you want to add capacitors to ground, that won't hurt. But don't delete C4; it's needed to keep the amp stable.

The bias network is intended to provide very fine control of the idling current in the output devices in a simple / inexpensive way. As far as thermal drift of the DC operating point goes, please note that the feedback network through R2 controls this voltage and keeps it at ground potential. The "classic active" biasing solution is good for mass produced items - my design does require that each amp be individually "tuned". But this biasing method provides far lower distortion than those mass produced amps.

Regarding high frequency stability; consider the phase shift from input to output on this amp. Then take a look at the Zoebel network on the output and you'll see how it's stabilized. That's not according to "the book" but it's very effective.

[Whuffo]

The quality of this circuit depends solely upon the quality of the opamp. It compensates everything except for high frequency phase delay that may cause RF oscillations if not compensated properly. A small capacitor in parallel to R1 will help if the circuit oscillates. Try to get the best existing opamp. Especially important are open-loop gain and good common mode and power rejection ratio. A large common-mode input voltage would make the circuit more RF stable. In any case, do not try to make the gain ((R1+R2):R1) less than 100. Anything like 1k:100k should be fine. An extra resistor between the opamp output and its inverting input will increase stability. This should be in order of 100xR2 or more, resulting in open-loop gain of 10000.

I'd also recommend looking for a high slew rate in your opamps intended for audio amplification.

Moving on to the other comments about stability: the extra resistor between the opamp output and its inverting input would probably make this amp unstable. Think of the entire amp as a big power opamp. If you do that, then the feedback resistor and other features will make better sense. Stability is guaranteed by bandwidth limiting; if you build this amplifier as per the schematic, it will not oscillate.

Your comments about minimum gain are appreciated, but they don't work well for this application. Taking a "line level" audio signal and boosting it up to 30 watts puts some real limits on the possibilities. The difference between the 0.707 volt AC input and the 48 volt AC output doesn't allow for the kind of opamp gains you suggest. And if it did, affordable opamps can't produce that kind of output level.

For what it's worth, this isn't some partly baked idea - this is a valid and proven design. Many people are listening to good music being amplified by amps built from this same circuit. If some wish to "roll their own" I wish them the best of luck and hope they're successful. For those who just want to build an amplifier that works very well, here's your design.

[dannyf]

if you build this amplifier as per the schematic, it will not oscillate.

I am not quite sure.

The use of a CFP pair here is necessary due to the opamp front end: you need additional voltage gain in the ops. Unfortunately, CFP ops is difficult to stablize, especially with a global feedback loop. Your saving grace here is the output devices used - they are very slow and that helped with the overall stability.

If someone had built it with a modern output device (1943/5200 or 15P/N), it sure will oscillate.

To be safe, I would put some miller capacitance there or follow the suggestions from one of the posters by using a 5534.

[Whuffo]

if you build this amplifier as per the schematic, it will not oscillate.

I am not quite sure.

The use of a CFP pair here is necessary due to the opamp front end: you need additional voltage gain in the ops. Unfortunately, CFP ops is difficult to stablize, especially with a global feedback loop. Your saving grace here is the output devices used - they are very slow and that helped with the overall stability.

If someone had built it with a modern output device (1943/5200 or 15P/N), it sure will oscillate.

To be safe, I would put some miller capacitance there or follow the suggestions from one of the posters by using a 5534.

That's OK, you can doubt all you wish - it won't make me unhappy or invalidate this design. The components weren't chosen because they're old, they're chosen because they're cheap. And it was designed to use a minimum number of inexpensive components; this is what good engineering is all about. The output devices were chosen on purpose; yes, they're slow and have low gain - and that isn't always a bad thing. Each component was chosen not for it's obvious characteristic, but also for the "less well known" characteristics. The whole is more than the sum of its parts.

Saying that it would certainly oscillate with a "more modern" output device only indicates that you didn't really consider the schematic. There's more going on than a quick glance would indicate. I made my living doing this kind of stuff, and this is one of my favorite designs. I was sufficiently successful at these things to retire at 50 to a tropical island. Build one; it'll only take a few hours. Then test it - after you've done this we can discuss the performance. Until that happens, there's an imbalance here; I know it works, and you're just speculating. Build one, get the experience, then let's talk about it.

[ElectroIrradiator]

Which opamp will substitute for the 5535? They don't seem to be available new any longer...

[c4757p]

It's a decent op amp but not particularly special. It does have quite a low input noise current. How about OPA2134? (That's a dual package, just like the 5535, but if you only need one there is the OPA134) Once you wade through the audiophool crap (where's the barfing emoticon?) it's pretty decent. Noise voltage is a bit high, but still less than the 5535, and the noise current is two orders of magnitude lower.

[dannyf]

Until that happens, there's an imbalance here; I know it works, and you're just speculating.

I think that's something that we can agree on: there is an imbalance: you claim that it works; and you have shown zero verifiable evidence that it works.

[c4757p]

It doesn't look particularly unstable to me. The voltage gain in the output section is small and controlled (about 2-4 depending on load). It might not care for a capacitive load, but I'm sure it can handle resistive and probably inductive as well.

I don't have the time to build the circuit right now, but I just tried it in SPICE with some quite fast parts (2N2222, 2N2907, D45H11, D44H11 and LM7171) and it had no problems. There are no anomalies in the frequency response, no hints of instability, nothing. Obviously YMMV in real life.

For noise purposes, you might want to consider reducing the 330k/10k divider to 33k/1k.

For (our) sanity purposes, you might want to consider putting component values in the schematic rather than off to the side.

[Gall]

Moving on to the other comments about stability: the extra resistor between the opamp output and its inverting input would probably make this amp unstable. Think of the entire amp as a big power opamp. If you do that, then the feedback resistor and other features will make better sense. Stability is guaranteed by bandwidth limiting; if you build this amplifier as per the schematic, it will not oscillate.

In most cases such a resistor increases stability. The idea behind this is simple: decrease the overall phase shift by compensating the opamp phase shift with a local feedback. This moves the pole on Bode plot away from working area. If the entire circuit is a "big opamp", the extra resistor is like an internal compensation circuit of every modern opamp.

Your comments about minimum gain are appreciated, but they don't work well for this application. Taking a "line level" audio signal and boosting it up to 30 watts puts some real limits on the possibilities. The difference between the 0.707 volt AC input and the 48 volt AC output doesn't allow for the kind of opamp gains you suggest. And if it did, affordable opamps can't produce that kind of output level.

Then it's better to attenuate the input. The usual stability problem is caused by negative feedback being too heavy, resulting in high common-mode signal at the opamp input. At higher frequencies due to phase shifts this turns into a positive feedback. The simpliest recipe against it is to make the gain higher. This also makes the opamp work at smaller signal and improves the overall linearity. An opamp-like circuit is likely to oscillate at gain=1. Many circuits are stable only at gain>=10. Since 0.7V is high enough, the noise is not an issue even if the input signal is attenuated.

If you want to build this circuit "as is", then DO NOT change the opamp. Using a "better" opamp without changing anything else may be a very very bad idea. Its open-loop gain may be too high and so the bandwidth limit may be not enough for a certain phase shift. Also its internal bandwidth limit will not help at all, modern opamps are way too fast. This may have catastrophic result.

[Whuffo]

It doesn't look particularly unstable to me. The voltage gain in the output section is small and controlled (about 2-4 depending on load). It might not care for a capacitive load, but I'm sure it can handle resistive and probably inductive as well.

I don't have the time to build the circuit right now, but I just tried it in SPICE with some quite fast parts (2N2222, 2N2907, D45H11, D44H11 and LM7171) and it had no problems. There are no anomalies in the frequency response, no hints of instability, nothing. Obviously YMMV in real life.

For noise purposes, you might want to consider reducing the 330k/10k divider to 33k/1k.

For (our) sanity purposes, you might want to consider putting component values in the schematic rather than off to the side.

The capabilities of the program I used to generate that schematic weren't up to putting component values on the diagram. For what it's worth, it was done in Visio; not the most appropriate tool, but it was close at hand. Many schematics in my files are on paper (often on the back of a Denny's placemat) and I need to find a better way to put them online.

The selection of the 330K/10K resistances wasn't arbitrary - and if you use metal film resistors per the schematic the noise will be largely minimized. In the last one I built, I used 2N2905 and 2N2907 transistors for the drivers; 2N2222 / 2N2219 would run a little hotter than I'd like here. There's opportunity for different semiconductors and opamps here; as long as they're reasonably compatible the circuit will work as designed. Global feedback is a wonderful thing, you know.

I just hope that the people here who are interested in audio amplifiers give this circuit a chance. The performance is better than they think, and 30 watts per channel is enough for most home stereo systems. I used a stereo version of this at home for many years; these days I use a 100 watt version - but that design is encumbered and can't be shared. I've got seven channels of the 100w version - plus a subwoofer powered by the same amp in my home theater system.

[c4757p]

The capabilities of the program I used to generate that schematic weren't up to putting component values on the diagram. For what it's worth, it was done in Visio; not the most appropriate tool, but it was close at hand. Many schematics in my files are on paper (often on the back of a Denny's placemat) and I need to find a better way to put them online.

If you're going to continue to use this program, why can't you just use the same feature you used to do the "Test Point" label?

Your schematics are probably the tastiest thing to ever grace a Denny's placemat.

The selection of the 330K/10K resistances wasn't arbitrary - and if you use metal film resistors per the schematic the noise will be largely minimized.

Thermal noise from the resistors, yes, but the op amp's input current noise will be amplified by the high resistance. I figured this was a concern to you since you chose an op amp with such a low current noise. Nearby EM fields (think: mains hum) will also be picked up more by high impedance nodes, though that's pretty easy to avoid, so not much of a concern here if you do the layout right.

And if it wasn't arbitrary, then why was it chosen? The only reason I can see would be to match the impedance at the noninverting input, but you're not even close to having matched that (and using a modern FET-input op amp makes that largely pointless at low frequency anyway).

[Whuffo]

Moving on to the other comments about stability: the extra resistor between the opamp output and its inverting input would probably make this amp unstable. Think of the entire amp as a big power opamp. If you do that, then the feedback resistor and other features will make better sense. Stability is guaranteed by bandwidth limiting; if you build this amplifier as per the schematic, it will not oscillate.

In most cases such a resistor increases stability. The idea behind this is simple: decrease the overall phase shift by compensating the opamp phase shift with a local feedback. This moves the pole on Bode plot away from working area. If the entire circuit is a "big opamp", the extra resistor is like an internal compensation circuit of every modern opamp.

Your comments about minimum gain are appreciated, but they don't work well for this application. Taking a "line level" audio signal and boosting it up to 30 watts puts some real limits on the possibilities. The difference between the 0.707 volt AC input and the 48 volt AC output doesn't allow for the kind of opamp gains you suggest. And if it did, affordable opamps can't produce that kind of output level.

Then it's better to attenuate the input. The usual stability problem is caused by negative feedback being too heavy, resulting in high common-mode signal at the opamp input. At higher frequencies due to phase shifts this turns into a positive feedback. The simpliest recipe against it is to make the gain higher. This also makes the opamp work at smaller signal and improves the overall linearity. An opamp-like circuit is likely to oscillate at gain=1. Many circuits are stable only at gain>=10. Since 0.7V is high enough, the noise is not an issue even if the input signal is attenuated.

If you want to build this circuit "as is", then DO NOT change the opamp. Using a "better" opamp without changing anything else may be a very very bad idea. Its open-loop gain may be too high and so the bandwidth limit may be not enough for a certain phase shift. Also its internal bandwidth limit will not help at all, modern opamps are way too fast. This may have catastrophic result.

If you want to try the additional feedback, then go for it. I didn't find it useful, but maybe your implementation will be a win. Regarding gain; there's a fixed amount of input noise at the opamp. If you attenuate the input to keep the gain down, you'll be seeing more of that input noise in the final signal. Best to keep the input hot and the gain down. You can play with this if you want, but there's nothing to gain here.

And as far as changing opamps - as long as they're internally compensated then they'll work fine. The bandwidth limiting comes from the surrounding circuitry, and the gain is set by the feedback network. Make sure you've got a high slew rate opamp and that the voltages are compatible and you'll be fine. I've used 535 and 5535 opamps because they're cheap and readily available; if you want to spend more and get the very best it'll be OK. It might not improve the performance, but it'll be OK.

[Lightages]

As far as the schematics go, a very simple easy to use and free one is:
http://sourceforge.net/apps/mediawiki/tinycad/index.php?title=TinyCAD
It is easier to learn than the full blown schematic/circuit board program.

[c4757p]

I've used 535 and 5535 opamps because they're cheap and readily available

Do you have a large bin of NE5535 in your garage - that's what "cheap and readily available" means? Because from here, they look to be many years obsolete. The only place I could find them was eBay.

[Whuffo]

I've used 535 and 5535 opamps because they're cheap and readily available

Do you have a large bin of NE5535 in your garage - that's what "cheap and readily available" means? Because from here, they look to be many years obsolete. The only place I could find them was eBay.

Yeah, I've got a good supply of them in my parts cabinets. But don't despair; any old high slew rate opamp which is internally compensated will work. Low noise is a plus, but keep the slew rate high. It could be worse - I could have put a 741 on the schematic.

[ElectroIrradiator]

Do you have a large bin of NE5535 in your garage - that's what "cheap and readily available" means? Because from here, they look to be many years obsolete. The only place I could find them was eBay.

...and those are from our friends in the far east, who by magic are able to supply every obsolete components you could possibly want.

Hmm, doesn't look like I have any opamps, which beats a 5535 in every way. A TL072 is close, though the slew rate is slightly lower.

Edit: 1N914 are not Germanium diodes, just plain Silicon.

[madires]

Yesterday I fixed two old amps (40W with 2N3055s) with some bad caps and RF oscillation on the negative side. The oscillation was about 10MHz and started at a specific input level. The driver for the positive side got a small ceramic cap between base and collector of the end stage driver PNP but there was none for the negative side. A 470pF ceramic between the base of the lower end stage and negative rail fixed the oscillation problem. Must have been a design error? 

[dannyf]

I don't have the time to build the circuit right now, but I just tried it in SPICE with some quite fast parts (2N2222, 2N2907, D45H11, D44H11 and LM7171) and it had no problems. There are no anomalies in the frequency response, no hints of instability, nothing. Obviously YMMV in real life.

I sim'd it in LTSpice (and orcad) and both showed instability - the gain peaked at around 500Khz. With square wave output, considerable pinging on the output (slew rate limited to about 4.5v/us, but that's dependent on the devices used). A 220ohm/22p pair around the opamp cures that.

For noise purposes, you might want to consider reducing the 330k/10k divider to 33k/1k.

thermal noise really isn't an issue for audio amplifiers (line-level). 330k/10k is in my view the right combination as it allows the use of a 10k resistor from the non-inverting end to ground (the original schematic used a 100k resistor and that's clearly wrong).

I would point out a few inconsistencies (to be polite) in the design:

1) I would avoid the use of CFP at all costs. They are very difficult to handle, even for seasoned designers. A follower OPS is much easier in my view;
2) I would put a small resistor on the collectors of the output devices to provide some thermal stability, particularly when you are using diodes to set the bias. Adjust VR2 to drop about 25mv on that resistor;
3) The drivers idle at about 1.5ma - way too low for such devices, particularly if you are driving 3055/2955 at high current levels - those guys' hFE can droop to 10x or so. I would use at least BD139/140 or some Motorolla devices for that - but then they have much higher fT and that exposes the instability issue;
4) the thermal compensation portion is poorly designed: the output devices want the drivers idle at 1.5ma; The diodes want the drivers idle at 3ma at least, with VR2 shorted. The end result is that they idle current on the output devices will be pushed to a very high level to balance out the two. R9/R10 should be at least 1/2 of R8/R11;
5) R6/R7 drop considerable current and I would use 1/2 w resistors to be safe.
6) the peak current from the opamp can be as high as 15ma -> that limits your choices of opamps used here.
7) the max output power from one pair of output devices is ~30w into an 8ohm load - assuming no SOA issues with the ops. Because of the cfp configuration, it is harder to parallel output devices.
at the max current output, considerable voltage drop happens on R4/R5 (1.5k now), not counting the current needed for the zeners. I would use no more than 110ohm there.
9) if i were to design it, I would use the same transistors as Q1/Q2 for D1/D2 to match the thermal characteristics.
10) idling the OPS at 100ma is on the high side in my view and runs the risk of gm doubling. I would do no more than 75ma, corresponding to a resistor of 0.22 - 0.47ohm on the collectors of the output devices.

I think you will generally find that opamp based audio amps don't perform well.

and those are from our friends in the far east, who by magic are able to supply every obsolete components you could possibly want.

Those are typically remasked TL072 or 5532.

The oscillation was about 10MHz and started at a specific input level.

When clipping takes place, ...

3055's are quite helpful in those cases due to their poor fT. A 1943/5200 would have done some serious damage.