Moog VCF Transistor Matching Question

[Fahl, Romeo]

Surprisingly Doepfer's service manual calls for matching all pairs in the
ladder in their A120 VCF.  

-----Original Message-----
From: Mike Granger [mailto:mgranger at greenville.infi.net]
Sent: Wednesday, October 25, 2000 9:25 AM
To: metamatic at webtv.net
Cc: synth-diy at node12b53.a2000.nl
Subject: Re: Moog VCF Transistor Matching Question


I've heard that Moog resorted to just matching the upper and lowermost pairs
as pairs, not pair to pair. I can't verify this, it is just what I was told
many years ago. The optimum would be to match all the transistors in the
ladder to each other, and match the ladder caps also, so that the response
poles would almost exactly align. I did this on my filter, and it sounds
very good to my ears.

I have the benefit of a semiconductor curve tracer for my scope, which I use
to curve match batches of transistors. I then select from a specific batch
transistors of equal Vbe. This works very well for my purposes.

Another trick I use is to superglue the flat faces of the paired transistors
together for thermal contact, or use heat sink compound and a small cable
tie to keep them in contact. Not as good as all being on one chip, but it
works okay.

metamatic at webtv.net wrote:

>  I've just finished building the transistor matching circuit from the
> Moog modular service manual, and I have what is very likely a numbskull
> question: Is it necessary to match all the transistors in the ladder, or
> just the pairs?
>
>  Thanks,
>
> Richard Hess
>
> metamatic at webtv.net