home-made vca

[Joachim Verghese]

On Thu, 20 Feb 1997, Haible Juergen wrote:

> Sure we would use a 13700 or 3280 nowadays, but this old design may 
> give some hint that linearisation would somewhat work even if the 
> diodes are not on the same chip as the differential pair.

I may be wrong, but I don't think the diodes need to be matched
to the transistor pair unless you want gm to be insensitive to
temperature changes.

> Now the really good thing is that these matched transistors have a much
> better CV rejection than a 3080, 13700 or the like. You still need an
> offset trimpot, but on a 3080, you still have a remaining "thump", as
> the offset compensation only works for one specific current. I don't
> know why, but the discrete transistors are far better here. I can 
> really make the "thump" inaudible by carefully adjusting the trimpot. I 
> don't know what's the reason for this.
> Maybe it's due to the different location of the trimpot (I change the Rc
> balance, like in the Minimoog VCA, not the Vbe (which is the only method
> that works on a 3080). Don't know why (anybody knows?) - but it works.

Could it have something to do with hfe non-linearity - hfe varying
with Ic? If the two transistors in a pair exhibit different hfe-Ic
responses, then there's bound to be problems, especially in current
mirrors, where hfe is an important parameter.

-joachim