Moog VCF (TomG's VCF2B)

[jh]

>Of course I'll use the anti-distortion network (Gilbert). Very often
>this is not used, I think people had problems understanding the data
>sheet (the ca3280 data sheet being worse). I remember an E.N. discussion
>about exactly that fact by S. Thernepin. 

Well yes, I have not yet completely understood why the 13700 works
better with a certain resistance rather than ideal current sources
(like the 3280), but I've learned that that it *is* that way. (Has anybody
figured out *why*, meanwhile ?)


But in 9 out of 10 cases I chose not to use the diodes at all:

>As J. Haible pointed out, the 3080 is ok if you have low dynamic input
>signals, e.g. for waveshapers, filters with vco input, vco mixers.

For me it's not a "just ok" solution. I'm convinced that in most cases
not using the diodes is actually *better*. Let me explain, and start
with an application where I use them, first:

I use the diodes when I want to attenuate CV signals like envelopes.
That's because I want to quech the last bit of SNR out of it, given the fact
that I have input signals of  fixed amplitude. If the envelope goes
up to 10V, I'll make it linear for 12V, for example. So I'm using the
diodes especially for *low* dynamic signals !

>OTOH if you want a filter to deal with any high dynamic input (CD,
>DAT, whatever) you will surely want something better, in terms of S/N
>ratio. I'm not sure if the 13700 is good enough in these situations.

Exactly. For signals with high dynamic range like 80 or 90 dB, the
13700 is simply the wrong device. I once tried to build a dynamic noise
filter for a mixer input with a (linearized) 13700, but mostly it would add
noise rather than reduce it. There are nice VCA chips for these
applications around - no need for a 13700 here.

So far I've covered the 1 out of 10 case, and the cases where I don't use
the chip at all.
What is it good for ? Synth sircuits where you want to live with a certain
amount of distortion, as long as it is the *right*  kind of distortion.
I happen to like how a 3080 (or 13700 without diodes) goes into overdrive,
and I do not like the abrupt clipping that occurs when the diodes are used.

So as a bottom line, my rule is only use the diodes when you're sure that
the input will not clip, and use a non-gilbert input when there is the
slightest chance for overdrive.
And what if you don't like the noise of a 3080 ?  Go for low noise devices,
that still have the same overdrive courve. I think the SSM2024 is (was?) 
of this kind, and a matched pair of discrete transistors is fine, too.

JH.