BJT, MOSFET or JFET? Which sound better in class-A

[neazoi]

This is a very general question which depends on many factors of course but in general which one should sound better in a single ended class-A stage, BJT, MOSFET or JFET?

[CBS240]

MMMmmmm J-fets..

Unfortunately power J-fets are a thing of the past.

[AKSA]

Try the new SemiSouth power JFETs. Aside from a stiff gate current, they are reputed to be very good by Nelson Pass.

Hugh

[ingenieus]

I've seen more MOSFETS in single-ended class A amps than BJTs.

Must be because they require less circuitry as far as pre-drivers, temperature compensation e.a. go, therefore they fit in better with the minimalist philosophy of single-ended class A.

[ingenieus]

Originally Posted by AKSA

Try the new SemiSouth power JFETs. Aside from a stiff gate current, they are reputed to be very good by Nelson Pass.

That would be the SJEP120R100A and SJEP120R063ASiC JFETs for audio

SiC Transistors | Semisouth Laboratories, Inc.

It would be nice if they published SPICE models...

[neazoi]

I can see a tension towards audio morfets of new audio jfets. What about bjt.
constructions like these have remarkable sound from my listening impression.
Le monstre

[AndrewT]

MosFETs in ClassAB do not seem to respond to optimal bias voltage/current in reducing crossover distortion. This makes ClassAB for BJT & FETs somewhat different.

Most knowledgeable commentators draw attention to the FACT that mosFETs seem to get better the more the output bias is increased. I recall Borbely mentioned 500mA of output bias as minimum recommendation for every mosFET amplifier.

It is only a small leap of faith to move from >=500mA of bias to full ClassA and then there can be no crossover distortion to attenuate.

To me, that seems to explain why mosFETs predominate in small ClassA amplifiers.

[AKSA]

Andrew,

You may not need that much. Here is the (simulated) crossover event depicted by the source resistor currents on a mosfet AB output stage. The pmos (R18) is rather more linear than the nmos (R17), but you can see the transition is well controlled with no sign of ringing or instability. This is into 8R//100nF, a difficult load, at 1KHz.

Quiescent current is just over 150mA.

I feel that modern mosfets do not lose out too much to bipolars these days.

Hugh

[AndrewT]

Hugh,
I do not think that your example is a good guide to what modern FETs can achieve in terms of reducing crossover distortion and doing so at lower bias currents.

[toprepairman]

If properly designed they won't sound different at all, not even a tiny bit.