Best VAS?

[bcarso]

Originally Posted by tvrgeek

If you look at what this is, you will see it is exactly the single FET, self biased with a resistor. I have not imported their model. I hope they picked a very fast FET. If so, it could be a winner. Looking at the data sheet does not give me a lot of hope.

They do not use fast FETs, because those have short channels for a high transconductance to capacitance ratio. They use long-channel devices for low output (drain) conductance. Because they are trying to make a good (i.e., high impedance at low-medium frequencies) current source/limiter.

[dirkwright]

Originally Posted by tvrgeek

If you look at what this is, you will see it is exactly the single FET, self biased with a resistor. I have not imported their model. I hope they picked a very fast FET. If so, it could be a winner. Looking at the data sheet does not give me a lot of hope.

Well, yes that is in fact the case. I know that. Just using a CRD alone was really good, as I posted earlier. The lower current values of CRD have higher impedance.

[dirkwright]

Originally Posted by Lazy Cat

Hi dirkwright

Please proceed, I'm just searching freq flat CCS. Can you please sim attached CCS?

Sorry I have not simulated this one yet. Can you provide some part numbers please?

[Lazy Cat]

Hi dirkwright

Yes of course, maybe it would be better to test these two CCS-s.

[nelsonvandal]

I think the AD797 bootstrapped current mirror VAS sounds close to perfect, and I think cascoded VAS has an unnatural softness. I hope the question "best VAS" means best sounding VAS.

[MiiB]

nelson do you have some schematics to look at..??

[nelsonvandal]

Compared to a normal common base VAS, i think the A797 VAS sounds "a lot" better. It doesn't alter "the tone" of the amp like some other tweaks do, it just sounds clearer and better.



I've messed around with LIN-type amps with and without VAS cascode, not as thorough as with my small headphone folded cascode amps, but I think cascoded VAS softens the sound in an unnatural way.

[tvrgeek]

Nelson,
Hmmm, several things here I have not seen. Feeding a current mirror from a ccs. Guess it does not matter which end the ccs is on. I wonder in a power amp VAS if it would eat too much of the available rail voltage. May I assume that the power for the above is well regulated?

I can't figure out that Q4 is for. I am quite new at this, so I may as a lot of simple questions. I am not questioning you, just my education.

[tvrgeek]

Nelson,
Where did you get models of those transistors?

[nelsonvandal]

Originally Posted by tvrgeek

Nelson,
Where did you get models of those transistors?

Such a long time ago, I can't remember. SK170 and SK246 models are probably from this forum. I don't really use SA970/SC2240. I use the better soudning SA1016/SC2362 from Sanyo but I can't find proper models for them.

I have almost no electronic knowledge, so it's impossible for me to explain the function of Q4. There's an explanation in the datasheet and there are several threads here about AD797 and clones. I do know I can measure the reduction in distortion and hear a cleaner and more natural sound.