Self-biasing class AB input stage

[syn08]

Originally posted by Lumba Ogir
Those LEDs are extremely nonlinear and noisy too (should be avoided alongside with other diodes and ceramic capacitors at all costs).

Perhaps you should be a little more specific and avoid generalizations? The above is your own opinion/preference.

BTW, yes, MOSFETs should be avoided in VAS stages, but your explanation as of why is flat wrong. You don't seem to be prepared to listen to others, so I'll skip the full explanation for the moment. It's enough for you to know that it's not based on listening tests or any other subjective BS and that CBS240 is essentially correct.

[Mr Evil]

Originally posted by Lumba Ogir
Mr Evil,
look, I`m sorry to say but I cannot find much to like about your subwoofer amp...

What sort of designs do you like then? Personally, I am pleased with it. It was a combination of several things that I hadn't used in a power amp before: Complementary JFET input stage (not practical until I found a source of complementary matched pairs - a source which has unfortunately dried up again); Adding gain to current mirrors to allow their use for loading the LTP in a symmetrical folded cascode; and small-signal MOSFETs. I also thought it rather neat the way it ended up requiring no external current or voltage sources for biasing.

Sure, a folded cascode is not the best choice for an audio amp - I could have achieved better measured performance from only common cheap devices if I had used a standard three-stage topology, but I would have learnt nothing from that.

Originally posted by Lumba Ogir
...Those LEDs are extremely nonlinear and noisy too (should be avoided alongside with other diodes and ceramic capacitors at all costs).

They certainly aren't noise-free (I even chose green LEDs for aesthetic purposes rather than the quieter red ones), but then again they aren't used in a position where noise matters. Yes, I used ceramic capacitors for supply bypassing, but I still don't see why that's so bad. The performance of the amp is not lacking in any way.

[Lumba Ogir]

CBS240,
the input impedance should be low. High gain, small reverse transfer capacitance and small conduction losses are further requirements, metaphorically, a medium where voltage swings can move uninhibitedly. The resistive nature of FETs does not promote that. They can impossibly perform both conversions equally accurately, can they? Optimized BJTs are preferable.

Mr Evil,

Complementary JFET input stage (not practical until I found a source of complementary matched pairs - a source which has unfortunately dried up again)

I would not lament excessively.The complementary LTP is far from the best sounding one. The fewer devices carrying the signal waveform the better. Consider a (British-style) single-ended input.

syn08,
diodes and ceramic materials bring about unpleasant distortion, I was not the first to make the observation. Measurement data do not tell the sonic impact of material nonlinearity, without listening test you will never know. For a minimum of distortion, dielectric losses and coloration in capacitors, cables and sockets I recommend Teflon, thank me afterwards.

BTW, yes, MOSFETs should be avoided in VAS stages, but your explanation as of why is flat wrong.

My explanation is deficient, but not wrong.

[Workhorse]

Mr. Evil be polite with members

[CBS240]

CBS240,
the input impedance should be low. High gain, small reverse transfer capacitance and small conduction losses are further requirements, metaphorically, a medium where voltage swings can move uninhibitedly. The resistive nature of FETs does not promote that. They can impossibly perform both conversions equally accurately, can they? Optimized BJTs are preferable.

I agree, this makes sense for the VAS amplifying transistors. If you choose to cascode the VAS, a mosfet could be used as the cascode device if the conditions are right. In my circuit, the VAS output is clamped before the mosfets reach saturation so Crss affects very little. Crss in these mosfets might be 10pf, but is driven by the amplifying transistor not the input stage, or common gate. Separate PS means I can sacrifice a few volts, no big deal. Easier to get rid of them than create them at such relative small currents. I tried both BJT and small signal mosfets, and the difference was small, but with the mosfets, I was able to squeeze out an extra couple hundred KHz or so bandwidth. Lacking minority carriers is one reason they can be faster. The downside is finding a small signal mosfet with higher Vds than 60V. And it would be cool to try depletion mosfets, but the p-type seems to still be made of ‘unobtainium’. Kinda limited there.

I would not lament excessively. The complementary LTP is far from the best sounding one. The fewer devices carrying the signal waveform the better. Consider a (British-style) single-ended input.

IMO, it's not how many devices but how far and where the Q-point varies in each device. This is one advantage of cascode, it allows you to better steer the Q-point of your amplifying transistor to a more linear transition, for a few reason I'm not going into. There is no rule that says you cannot direct couple complete amplifier stages together. Sort of a network like thingy. It all depends on your personal POV and weather or not this is a home project or you have some A-hole boss breakin' your balls over it.

My opinion on cap type, polystyrene, polypropylene, and tiny SMD COG type pf caps for compensation. But you low impedance signal guys don't like them because they do nothing for you.

[Mr Evil]

Originally posted by Lumba Ogir
...I would not lament excessively.The complementary LTP is far from the best sounding one. The fewer devices carrying the signal waveform the better. Consider a (British-style) single-ended input...

The complementary JFET LTP has an appealing simplicity about it. "Best sounding" is a non-issue here because, as with an decent low-distortion amplifier, the differences are way below perception. It is effectively 100% transparent.

Originally posted by Lumba Ogir
...diodes and ceramic materials bring about unpleasant distortion, I was not the first to make the observation. Measurement data do not tell the sonic impact of material nonlinearity, without listening test you will never know. For a minimum of distortion, dielectric losses and coloration in capacitors, cables and sockets I recommend Teflon, thank me afterwards...

I have never seen any double-blind test that showed that anyone could detect different capacitor types except under the most extreme of conditions where the frequency response was affected. And no, I've never heard any differences myself either.

Anyway, this has drifted off-topic now.

[Lumba Ogir]

Mr Evil,
I don`t agree with you at all, but reaching an agreement does not need to be our ambition.