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How to choose VAS current
How to choose VAS current
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Old 4th December 2018, 02:31 PM   #1
JeffYoung is offline JeffYoung  Ireland
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How to choose VAS current
Default How to choose VAS current

Say for something like the BA2: LTP input stage followed by single-ended VAS. How does one choose the current to give the VAS stage?
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Old 4th December 2018, 03:51 PM   #2
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How to choose VAS current
there is adequate calculus , which includes input capacitances and input impedances .... taking rise time in account as main thing ..... but I used it so long ago , that I simply forgot it


always used rule of thumb in meantime , later resorted to thumb of rule ...... just crank it
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Old 4th December 2018, 06:38 PM   #3
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Generally it wants much-more than the drive requirement of the Output stage (which you did not mention).
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Old 4th December 2018, 07:29 PM   #4
JeffYoung is offline JeffYoung  Ireland
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How to choose VAS current
I'm looking for general guidelines.

But let's use this for an example to start with:

How to choose VAS current-ba-2ff-jpeg
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Old 4th December 2018, 07:31 PM   #5
Itsmee is offline Itsmee  England
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How to choose VAS current
Just look at Papas circuits (BA2 15mA, Sony Vfet 30mA), don't read too much tech stuff or... you'll see.
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Old 4th December 2018, 08:15 PM   #6
JeffYoung is offline JeffYoung  Ireland
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How to choose VAS current
Thanks for that link, Itsmee. Now I understand PRR's question, as it appears the answer has much more to do with what the VAS is driving than with the VAS itself.

The output stage in the example is a 4-pair BA2, which I think was one of its original incarnations (though the current one is 6-pairs). So the BA2 number is certainly relevant.

The article suggests I want as much current as possible to counteract the common source inductance and the Miller effect charging the capacitance of the gate. Is there a downside to supplying more current?
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Old 4th December 2018, 08:27 PM   #7
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How to choose VAS current
No but everything is a compromise, you just have to pick and choose which ones you want.
Depends on how far you go, main one is heat, component dissipation, geting rid of it, powering it in the first place - bigger psu...
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Old 4th December 2018, 08:41 PM   #8
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How to choose VAS current
Sorry for jacking your thread...

This is a couple of ideas I've been kicking around for a while, haven't gotten round to trying them yet, it's in the queue - too many project, they might not work.

Preserving the jfet gain is a trade off in reduced drive, seems to me that the best way would be to use a current mirror; the first leg monitors the JFET current, the second leg replicates the first leg (no load to drag it down), legs A, B, C are copies (current multiplier). Only Q7 & Q8,A,B,C need to be mached.

If you look at the normal LTP, the feedback JFET is only used as a sorce follower, seems to be a bit of a waste; a unity gain non inverting opamp could do that, it's only going to see a volt or two.
It shouldn't change the sound too much.

EDIT think i need to flip opamp to inverting???
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Last edited by Itsmee; 4th December 2018 at 08:57 PM.
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Old 4th December 2018, 09:47 PM   #9
JeffYoung is offline JeffYoung  Ireland
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How to choose VAS current
Can't you get the current mirror to multiply the current just by varying the resistance between the first leg and the second? Why do you need the other legs?

For what it's worth, I think the HPA-1 uses a current mirror in a similar fashion, only with a complementary (F5-ish) front-end and current mirrors on both sides. (It also uses an op-amp, but as a DC servo. The feedback mechanism is F5-ish.)
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Old 4th December 2018, 10:07 PM   #10
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How to choose VAS current
Quote:
Originally Posted by JeffYoung View Post
Can't you get the current mirror to multiply the current...
Yes, most people do it that way, if you do, to me it's not a true mirror, the transistors aren't operating at the same point.

Quote:
...It also uses an op-amp, but as a DC servo...
Isn't that what the feedback jfet's job is? or did I misunderstand.
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