First time I saw the circuit was in Audio Amatuer magazine over 20 years ago. Jung wrote an article about it ref the LH0002 buffer. You might check that datasheet. It is on line.
Is that AN227 "Applications of Wide-band Buffer Amplifiers"? I just had a quick look at that one. It doesn't help me much.
Also you might check this more modern article by Jung. It is a much more complex version than the original. http://waltjung.org/PDFs/WTnT_Op_Amp_Audio_2.pdf
Yes, I read that one already. It doesn't show the capacitor between the bases of the output transistors! Nor does AN227.
That's disappointing, because that capacitor is very helpful - it allows the drive stage to actually provide significant base current to the output transistors. Without it, the driver transistors can only suck current away from their respective output transistors; with the capacitor, each driver transistor is an active current source for the base of the opposite output transistor. I'm not surprised that the capacitor makes a significant improvement, especially when operating into a significant load.
Three points. DC stability is very important if you are biasing in Class A. You may want to operate the buffer outside a feedback loop. Thermal runaway is not controlled without the bias trans and output trans in thermal contact. So why not get it stable in all respects before deciding to apply feedback or not?
Agreed, and I wasn't suggesting no thermal connection between the drivers and the output transistors. I was saying that any thermal connection - from top output to top driver and bottom output to bottom driver, or vice versa - should prevent thermal runaway. And I agree that it's helpful to have a circuit that starts off clean without feedback - that's why I suggested that option, and the dual emitter follower design is the obvious choice for a low-distortion buffer.
I'm not sure I agree with you about whether a DC output offset is a major problem though. I wonder if 10 mV or 20 mV of DC offset will cause any actual problems with headphones - apart from a click when you plug them in and unplug them.
Assuming it really IS a problem, surely a DC blocking capacitor of a few hundred uF isn't a big issue?
And if you can't have the blocking capacitor, you could stabilise the DC output voltage by applying DC-only feedback without losing the apparently magical audiophile qualities of a no-feedback signal path...
The second cap seems to have little effect in Class A region. But does help at crossover and beyond into Class B. Found it here: http://www.tubecad.com/2012/09/blog0244.htm
That's an interesting blog. Nothing much new for the basic diamond buffer, but he does show the use of one or two Schottky diodes to increase the output stage bias and current. Have you considered that idea? I'm not suggesting it, just wondering.
He mentions that adding the base-to-base capacitor improves distortion from 1% to 0.1%. Those figures are far higher than the claims for the LH0002. Perhaps that's because the LH0002 is trimmed in some way? What distortion figures do you get with your SPICE analysis?
I am not an engineer. Indeed, that is one of the questions I was seeking guidance on. But it seems to me that the transistors do not need to be at the same temp before thermal feedback for the feedback to be beneficial. For examp. say the two transistors were very different in Tj when separated and then they were thermally connected and heatsunk. The cooler trans (driver) would be heated up and deliver less drive while the hotter trans would cool and dissipate less. At some point, homeostasis would occur and thermal runaway would be held in check. I assume that would be at a temp in excess of the driver trans Tj when on its own. It might take some fiddling to get the output to bias properly in Class A, but thermal runaway should be avoided. At least that is the way it seems to me.
Yes of course, you're right.
That is correct. I do not have a URL for that cause it made no sense to me. But here is one where the PNPs are thermally connected and the NPNs are thermally connected (no sinks). It too makes no sense to me: http://phonoclone.com/diy-sapp.html
I wouldn't take much notice of that site. I read the text. He talks about avoiding IC regulators in the power supply, and recommends a zener regulator, R-C filter, and emitter follower and says "It is efficient, effective, and sounds very good." Even before I read that, I was concluding that his explanations are not very rational. So he has little credibility with me. The nail in the coffin is the lack of even the base-to-base capacitor.
DC offsets are important if you want to DC couple and avoid large and expensive film caps.
I answered this before, but... film caps? Are electrolytics "dirty" in some way? Do they colour the sound? Sorry to sound cynical, but I've learnt enough about perception in the brain to know that all of the claims of so-called golden-eared audiophiles can be very convincingly explained as psychological, and unrelated to reality. These people often make claims like "vinyl sounds better" or "digital sounds bad" which really beggar belief.
As I've said before, any of these claims COULD be supported by properly conducted double-blind testing, but the people who make them seem to be allergic to the whole idea. So I have to try to apply skepticism and refuse to accept their claims until they can demonstrate their validity. This would include the idea that electrolytics are somehow bad for sound quality.
[... snip ...]
No, mine are 32 Ohm. But the current required for lower impedance phones is higher and that is where these little guys tend to fall down if they are capable of swinging the necessary V for higher impedance phones. Indeed, that is one of the problems in trying to cover 30 ohms to 300 ohms. If you have the voltage capability to handle 300 and the current capability to handle 30 total possible power at low impedance is dangerously high without some sort of current limitation.
I agree. Perhaps a headphone amplifier could be designed to measure the actual load resistance (weak DC current into load; measure voltage across it) and adjust its characteristics accordingly... Using some kind of very soft clipping to limit the output power without colouring the sound unnecessarily the way a compressor/limiter would... sounds like an interesting project that I would never be bothered with!
Well, I have responded above. But my real response is that perhaps it is time to buy the bricks to build a house rather than make the bricks to build a house. By that I mean that perhaps I should go the OPA route rather than discrete.
Oh, that would be a shame! I like the diamond buffer idea, with the base-to-base capacitor. By all means, use an op-amp in the input stage, and within the feedback loop, but don't you want to use the diamond buffer as well?
I started this thread by asking how to cut back the gain on a 2030. It was suggested that a power amp should not be used for phones. I disagree. Phones need a power amp. But it should be a little one with limited current capability. The buffers we are discussing are in effect the same as a power amp output stage.
No, I definitely disagree. A headphone amplifier is not a power amplifier. It's part way between a signal amplifier and a power amplifier.
In fact, they can supply enough current to fry headphones and your ears. So even these circuits need to be used with care.
Sure. But that's not the only reason I said you shouldn't use a power amplifier with headphones. My first reason was that an amplifier designed to driver a loudspeaker will produce too much noise for direct connection to headphones. That and the fact that a power amplifier could literally melt the voice coils of a pair of headphones and set them on fire! And the fact that it's simply huge overkill and totally inappropriate! So I stand firmly by my opinion that you should not drive headphones from a power amp!
A possible solution is to use OPAs that current limit. For example the venerable 5532 current limits at about 40 mA. But the current limit is hard and it gets ugly long before it limits. So paralleling two or three of them might do the trick. Worst case the phones will see 80-120 mA and that should be survivable by low impedance phones (not your ears).
Yeah, I might do that for simplicity, but I would rather have a clean output stage and do the limiting in a controlled way by soft clipping the voltage rather than current limiting.
I don't see the point in using a whole lot of op-amps in parallel when you could just use a standard output stage such as the two we've been discussing. It has some curiosity value but that's all, IMO.
Those are not the only way. See this article by an engineer on a mission. Read the whole page including the measurements on the bottom. His writing style is not the best, but I bet you will end up reading all the pages.
That might well be the way to go. In fact I am almost fininshed with a PCB for that topology.
Good luck with that!
Of course simpler is easier. So I left out the battery option and protection circuit. But how about on-off transients? I am hoping that switching the DC side of the supply while leaving the AC side always active might help. Also, having a snubber across the DC switch might help moderate pops and clicks. Got any simple ideas that do not involve relays, complex protective circuits, etc. that will minimize on-off thumps and clicks?
Yeah. Unplug the headphones when you're turning it ON and OFF!
Not really. Perhaps you could arrange for the supply rails to come up gradually using an R-C circuit driving an emitter follower in series with each supply rail. That might work assuming the op-amps don't do anything strange while their supply voltages are low.