JFET input, MOSFET VAS, LATERAL output = Perfect!!

[mikelm]

Many here are driving O/P devices directly from the VAS with 10-15mA so the VAS is looking at the comparatively high o/p device i/p C - about 500pF in my case.

My VAS is driving into about 40pF which I believe is much lower than the devices you suggest and as has been commented on in this thread several times, lateral mosfets are very easy to drive - bottom line is that there was a perceivable improvement in sound quality when I added the drivers

mike

Actually I tried the 2sj 2sk devices you mentioned in the middle of my oscillation crisis and things just got worse.

[kees52]

I did mean the zvn into powerfets, it is enough for one pair, and some limited bandwith, have you simulate that?

I like a little current and low impedance for mosfets, but I do most quasy.

Simple is your design, Hugh like that, I wil in future also try that.

happy designing.

[keantoken]

I think Roender's RMI-FC100 would be a great starting design to convert to CFB, in order to achieve a CFB+folded cascode design.

[keantoken]

It would be difficult or impossible to do a CFB+FC design because the signal has to be inverted in the VAS. So you would need an extra inverting stage, and that might moot any benefits.

[GregH2]

Originally Posted by mikelm

Well, since I posted my last DC version here I've been travelling and doing other stuff.

As soon as I increased the voltage, the parasitic oscillations set it. I can hear Greg laughing . . . . I had to really work at it from several approaches to finally fix it.

mike

Haha, actually no I'm not. I feel your pain, though these days I understand how to deal with oscillation a lot more and it's not so much of a problem.

The problem with this amplifier topology, for me at least, is square wave overshoot. Specifically, correcting it without using an overly large output choke, which does remove a little urgency to the sound.

This is of course asssuming that you care what your square wave performance is like... even with bad overshoot it sounds pretty good!

[keantoken]

One good way to get rid of overshoot is to make open-loop gain very low. To many that is not an attractive option.

[mikelm]

Nearly every mod I did was to aimed at reducing ringing on square waves - the others were to reduce other types of noise

I tried every measure I could think of to keep the amp's rise time fast & minimising ringing.

Driving from a low impedance - less than 50R - and making the o/p device gate stoppers as small as possible are both useful measures in this regard and a good tight layout helps a lot.

The only other measure which works in theory is a cap across the feedback R this works wonders for rise time vs ringing - but I'm not sure I like the sound of it - I'll check again.

I do love the sound of this amp, when I returned home I was delighted just how good it sounded - but I suspect there may be others that just as good or better

[mikelm]

Originally Posted by keantoken

One good way to get rid of overshoot is to make open-loop gain very low. To many that is not an attractive option.

Yeah, so far I always found I prefer the sound of fast, high feedback amps so long as they are very stable in the critical region.

[tessier]

Originally Posted by AKSA

Yes, you should drop approx 200mV across this resistor.

Quite by chance, I discovered this about 15 years ago working on a cascoded VAS, I was amazed, but could never explain it.

Hugh

Hi Sir

Cascoded VAS, 200mV drop across a resistor in that VAS?

You was amazed, that's interesting.

Can you say more, any drawings?

Thanx

Paul

[keantoken]

Mikel, a miller cap or a series feedback cap will decrease overshoot only by decreasing gain at overshoot frequencies; it will only give the appearance of a better behaved amplifier. It does not change the cause of the overshoot which is the resonance at GBP when undercompensated.

I think you should try TMC.