Build A High Quality Audio Amplifier

[dannyf]

Just to add one more point: I would definitely add a small resistor (<1k) in serial with the input capacitor - never let your base / gate see the input signal directly.

[GK]

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;

Resistors will have to be put in series with the emitters, not collectors, of the power output devices to have any significant effect on the bias current stability. Being a CFP "with gain" it's likely not as touchy (loop oscillation local to the power output stage) as a unity gain CFP and those large value (470 ohm) B-E resistors for the power output devices also likely aid stability as they form a compensatory pole (in conjunction with the transistor input capacitance) in the open loop response of the CFP. Quiescent current of the power output devices is essentially governed by beta multiplication of the driver transistor quiescent current. Beta of the power output devices varies significantly with junction temperature, so the dynamic quiescent current stability would be mediocre.

The power output stage also has no implicit protection circuitry. Beta of the power output transistors specified drops of rapidly as Ic increases. This, combined with the weak and under biased driver stage, is what stops this thing from immediately blowing up when the output is shorted - the driver stage just doesn't have enough guts to force the power output transistors to conduct heavily enough to blow up straight away. I do not think that this makes the design a notably sophisticated one however. The open loop linearity of the CFP is in part a product of the (grossly non-linear) power output transistor beta. The low Iq driver stage will not have adequate capability to rapidly switch off the output transistors. The output stage will therefore suffer from significant cross conduction at 20kHz and generally poor HF linearity. A simpler, unity gain "double emitter follower" power output stage would offer much better linearity than the output stage circuitry presented here, but that wouldn't be compatible with the op-amp front-end as the output voltage swing would be limited to that of the op-amp.
Even so 30W into 8 ohms isn't a very demanding specification the THD claims ("well under 0.01%") for the circuit as presented are definitely not credible - particularly so at 30kHz. For THD performance targets that are achievable with CFP power output stages using single pairs of old and slow power transistors like the 2955/3055 pair, the early editions of Douglas Selfs "Power Amplifier Design Handbook" are good references.


 

[Whuffo]

Here's an updated copy of the schematic; the gain control wiring has been corrected, and the diode part number has been corrected. Those were napkin to jpg transcription errors; sorry about that.

I've been reading through the comments posted and I've got to say it's been very informative. I see opinions masquerading as facts being spouted by people who clearly do not understand how this amplifier works.

It is what it is; a very clean amplifier that's simple enough to put together on the kitchen table. Old school parts in places - because they do the job and are cheap. Enough output power for a home stereo system. If you want to substitute the opamp or driver transistors, feel free - the last of these I built used 2N2905 / 2N2907 for the drivers. I'd keep the same output transistors, though - they're very well suited for the job.

For those who have batted the input resistance issue around the block a few times - electrolytic capacitors in the signal path are evil. By keeping the input resistance high, a 0.1uf input coupling capacitor can be used. Does this increase the opamp's input noise? Yes, but it's swamped by the input signal so it's not really a problem at all. And as a bonus, the input capacitor / resistance provides a HPF at the input.

Those who say opamp based audio amplifiers aren't much good are mostly correct. There's been far too many misbegotten examples, but that doesn't mean they're all bad. Sometimes things just work and the whole is greater than the sum of the parts. This is an example of what happens when it goes right. No, it didn't just happen by chance, many hours / days were spent testing various variations and permutations until a stable, minimum cost design was finally arrived at.

[Whuffo]

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.

Sounds like a great choice!

[Whuffo]

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.

Thanks for catching that diode part number error. 1N34 is the right number; I just posted an updated schematic. You might be OK with a TL072; it'd increase the high frequency distortion a bit, though.

[Whuffo]

Just to add one more point: I would definitely add a small resistor (<1k) in serial with the input capacitor - never let your base / gate see the input signal directly.

Look at the updated schematic; there was a transcription error around the input gain control.

[c4757p]

If the volume is turned up all the way, the op amp's input still sees the signal directly. That 100nF capacitor is damn near a short for anything above 20kHz or so. Add a resistor so that ESD on the input can be gently discharged by the input clamp diodes.

[dannyf]

electrolytic capacitors in the signal path are evil.

Poorly designed amplifiers are evil. Electrolytiic capacitors aren't.

Look at the updated schematic;

The same issue persists: you don't seem to understand why it is needed.

As to input impedance, 47K is probably near the upper end: anything above that may introduce more trouble than it is worth.

No, noise isn't the issue: output DC offset is.

[dannyf]

I will give you two examples of beautifully used electrolytic capacitors: the bootstrap capacitors in JLH1969 and AKSA amps. In the case of the former, an ideal capacitor would have meant loads of troubles.

[c4757p]

electrolytic capacitors in the signal path are evil.

Poorly designed amplifiers are evil. Electrolytiic capacitors aren't.

Indeed - in fact, I have found electrolytics in general to be rather handy and decent little things, as long as you understand how they behave. They are, of course, not ideal capacitances, but you're not going to get very far in electronics assuming all your components are physical manifestations of ideal passive properties anyway. And they're just fine for audio apps, again, if you use them correctly. The only real problem that they may cause in the signal path is temperature-dependent leakage, causing bias points to drift - easy to counter in many places by reducing the impedance of the bias point, or even using an active bias servo if you really must.

[Whuffo]

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;

Resistors will have to be put in series with the emitters, not collectors, of the power output devices to have any significant effect on the bias current stability. Being a CFP "with gain" it's likely not as touchy (loop oscillation local to the power output stage) as a unity gain CFP and those large value (470 ohm) B-E resistors for the power output devices also likely aid stability as they form a compensatory pole (in conjunction with the transistor input capacitance) in the open loop response of the CFP. Quiescent current of the power output devices is essentially governed by beta multiplication of the driver transistor quiescent current. Beta of the power output devices varies significantly with junction temperature, so the dynamic quiescent current stability would be mediocre.

The power output stage also has no implicit protection circuitry. Beta of the power output transistors specified drops of rapidly as Ic increases. This, combined with the weak and under biased driver stage, is what stops this thing from immediately blowing up when the output is shorted - the driver stage just doesn't have enough guts to force the power output transistors to conduct heavily enough to blow up straight away. I do not think that this makes the design a notably sophisticated one however. The open loop linearity of the CFP is in part a product of the (grossly non-linear) power output transistor beta. The low Iq driver stage will not have adequate capability to rapidly switch off the output transistors. The output stage will therefore suffer from significant cross conduction at 20kHz and generally poor HF linearity. A simpler, unity gain "double emitter follower" power output stage would offer much better linearity than the output stage circuitry presented here, but that wouldn't be compatible with the op-amp front-end as the output voltage swing would be limited to that of the op-amp.
Even so 30W into 8 ohms isn't a very demanding specification the THD claims ("well under 0.01%") for the circuit as presented are definitely not credible - particularly so at 30kHz. For THD performance targets that are achievable with CFP power output stages using single pairs of old and slow power transistors like the 2955/3055 pair, the early editions of Douglas Selfs "Power Amplifier Design Handbook" are good references.


 

I like Doug Self's work - but there's other ways to do things and he's got his opinions like the rest of us do. Anyway, the "mediocre quiescent current stability" isn't quite as bad as you fear, and the small variations here don't affect the performance of the circuit. My "solution" to this problem was to run a little extra quiescent current so that any possible variations would still be in the linear region of the transistor curve.

I appreciate your respect for the difficulty of CFP designs; you're right, they can be a problem. That is the reason I'm so proud of this little circuit; good power from few and inexpensive parts, stable and clean. And while you say there's no explicit output transistor protection, you follow by describing how they're protected. It's not a bug, it's a feature. For fun, consider how if the amp is pushed to max without adequate cooling, the decrease in beta in the outputs will reduce the power consumption automatically. People do some strange things to their amplifiers; short output terminals, hook up multiple speakers in parallel, etc. A good design should be able to handle these misadventures without letting the magic smoke out. Doing it without added circuitry is a win.

There was a lot of time spent working out how to accomplish all the design goals using as little circuitry as possible. So things like the output protection, bandwidth limiting, etc. aren't obvious - but they are in there.  And it does meet the quoted specs easily - try one and see.

[Whuffo]

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?

Probably so. Hanging capacitors around the outputs and drivers to suppress oscillations isn't the optimum design solution.

[c4757p]

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?

Probably so. Hanging capacitors around the outputs and drivers to suppress oscillations isn't the optimum design solution.

Huh? You're going on about how good it is that you picked slow transistors, etc., because their "inferior" properties make them a better fit for the application, allowing you to make something that works well without putting lots of effort into stability and such. So, how is that any different from adding a capacitor to slow down the transistors and get the same effect?

[Whuffo]

If the volume is turned up all the way, the op amp's input still sees the signal directly. That 100nF capacitor is damn near a short for anything above 20kHz or so. Add a resistor so that ESD on the input can be gently discharged by the input clamp diodes.

R3 on the schematic is strangely suggestive...

[c4757p]

R3 on the schematic is in parallel with the input, and is quite large. An ESD event will fuck the diodes in the op amp and pass right over the resistor. You need one in series.

[Whuffo]

electrolytic capacitors in the signal path are evil.

Poorly designed amplifiers are evil. Electrolytiic capacitors aren't.

Look at the updated schematic;

The same issue persists: you don't seem to understand why it is needed.

As to input impedance, 47K is probably near the upper end: anything above that may introduce more trouble than it is worth.

No, noise isn't the issue: output DC offset is.

Electrolytic capacitors are non-linear devices by design and construction. Leaving them out of the signal path is always a good idea.

And as to input impedance, look at the input circuit again. 100K pot and a 100K resistor - both connected to ground and as you note, C1 is essentially a short at audio frequencies. So the input impedance is 50K; the general design spec for line level circuits specifies a 47K impedance. This is close enough. Also note that as the input frequency drops below the passband, the Xc of C1 increases. This helps keeps "rumble" out of the amp; it's a Pi section RC HPF.

And output DC offset is a non issue. Really; both inputs of the opamp are referenced to ground. The global feedback is also referenced to ground. The only thing that could drive the output DC point off of zero would be a DC input - and those are blocked by C1. Rather than try to force the opamp / bias chain to be perfect, the solution is to let the global feedback correct any small errors. That's proven to be a very successful solution.

[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?

Probably so. Hanging capacitors around the outputs and drivers to suppress oscillations isn't the optimum design solution.

But it was quite obvious where the oscillation started and the scope confirmed that. First I tried to use a cap with the same value as used for the PNP driver (220pF) and it wasn't enough to suppress the oscillation. With 470pF the oscillation was gone and I've also checked if high input frequencies (30-50kHz) pass the amp without major distortions.

[Whuffo]

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?

Probably so. Hanging capacitors around the outputs and drivers to suppress oscillations isn't the optimum design solution.

Huh? You're going on about how good it is that you picked slow transistors, etc., because their "inferior" properties make them a better fit for the application, allowing you to make something that works well without putting lots of effort into stability and such. So, how is that any different from adding a capacitor to slow down the transistors and get the same effect?

The parts were chosen to do the job they are required to do; there's no need to use a sledgehammer to drive tacks. If using those appropriate components means I don't have to deal with problems caused by that curious "bigger / faster is better" mindset, then that's a win. And you have no idea how much effort went into achieving stability - and doing it with a simple and cheap design. That's what real engineering is all about; designs that fulfill the goals and do so as cheaply and simply as possible.

Going back after the design is complete and tacking capacitors in here and there is a sign of lazy design; they should have gotten it right before they went to production.

[Whuffo]

R3 on the schematic is in parallel with the input, and is quite large. An ESD event will fuck the diodes in the op amp and pass right over the resistor. You need one in series.

If you have an ESD event in a line level connection between the preamp / mixer and this amp - then you're going to have much worse problems than a popped opamp. Rather than include meteorite shielding "just in case", remember the old saying about "horses for courses" and try not to overdesign things.

[c4757p]

Why? Never heard of someone touching a connector after walking across the room? And 1k or so in series is hardly overdesigning.

[Whuffo]

I will give you two examples of beautifully used electrolytic capacitors: the bootstrap capacitors in JLH1969 and AKSA amps. In the case of the former, an ideal capacitor would have meant loads of troubles.

I'm not saying that electrolytic caps are useless - and I use them quite often. They have some problems, though - short lifespan, non-linear response, and the tolerances are a bit on the wide side. That said, I'll still avoid putting them into signal paths whenever possible; it's an easy way to improve THD on an amp and cheap to implement. That input cap is in a critical place; any errors there due to an electrolytic cap would not be corrected by feedback. Signal to noise is important, and in this amp design anything in front of the opamp is out of the feedback loop.

[Whuffo]

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?

Probably so. Hanging capacitors around the outputs and drivers to suppress oscillations isn't the optimum design solution.

But it was quite obvious where the oscillation started and the scope confirmed that. First I tried to use a cap with the same value as used for the PNP driver (220pF) and it wasn't enough to suppress the oscillation. With 470pF the oscillation was gone and I've also checked if high input frequencies (30-50kHz) pass the amp without major distortions.

I'm glad you found a solution. And I'm wondering what brand / model that amp is; getting 3055 series transistors to oscillate at 10 Mhz is difficult to do.

[Whuffo]

Why? Never heard of someone touching a connector after walking across the room? And 1k or so in series is hardly overdesigning.

OK, when you build yours you can include that 1K resistor; it's only a penny or two. Then, when someone shuffles their feet across the room, pulls out a cable and zaps the inner conductor you'll - well, nevermind. Line level runs through shielded cables, you know.

If people want to improve the design, then they can. If you build what the schematic shows, it'll work fine. Do that, then try your improvements. You just might find something better. Or you may find that you're happy with the base design.

[madires]

I'm glad you found a solution. And I'm wondering what brand / model that amp is; getting 3055 series transistors to oscillate at 10 Mhz is difficult to do.

Me too It's an old circuit from an electronics magazine (late 80ies). One of both stereo amps was worse than the other, but both channels were always the same.

[dannyf]

the solution is to let the global feedback correct any small errors.

You are so not getting it. Pick up an intro-level book on opamp would be helpful.

And 1k or so in series is hardly overdesigning.

It is less about ESD protection but more about signal conditioning: the serial resistor + input capacitance forms a low-pass filter. I learned it from Pass.

The general design philosophy here is so 1980s: high open loop gain and let negative feedback take care of everything.

We have evolved a couple generations from that.