AW: Re: discrete SSM2040-style filter

Joachim Verghese jocke at
Wed Jan 8 12:34:36 CET 1997

On Tue, 7 Jan 1997, Haible_Juergen#Tel2743 wrote:

> Wow, you did the same thing! Must have missed it - last thing I remember
> is the stuff with clipping diodes on existing otas ...

I think it was as far back as April last year...

> CV rejection is not a problem in my filter. This was the great (positive)
> surprise, that the total offset is nearly independend of control current!

I noticed that too. That's if you use high gain PNPs for the gain cell
mirrors. The EP2015 arrays I tried have a beta of around 100 only, and
this makes the gain cells unsymmetrical, which in turn leads to CVR
problems, I think.

> It's only temperature gradients that make problems. So I guess I'd just
> have to use the same heat-conductive epoxy stuff ARP and Emu used
> back then (??)

Yes, the good old method. Or you could use silicone rubber which is
easier to peel off...

> Another ideat that haunts me is using the Wasp design for 2pole 
> responses. It doesn't have opamps at all, but unbuffered CMOS inverters
> as integrators and amplifiers. Unfortunately, component values are very 
> unreadable on my wasp schematics copy.

Oh, the Wasp design is rather interesting. I'll check to see whether
my copy is clearer (doubt it).

> I used the Mix/Drive concept on my JH-2 synth, but I did not like it very
> much for one reason: A single VCO with a certain amplitude always sounds
> louder that a mix of two VCOs with half amplitude. So turning the knob
> from single VCO to dual VCO gave the impression of *removing* something
> instead of adding something. But of course this is just a matter of taste,
> similar to the "feeling" of loudness of different waveforms, versus their
> amplitude (which will be affected by clipping).

I've given this a lot of thought recently. The problem (for me) with
the independent-volume-per-VCO mixer is that you tend to ignore
single-VCO patches simply because they don't sound as loud as two-VCO
ones (the opposite of the problem you just described :). There are lots
of interesting sounds you can make with a single VCO if you have a good
VCF and VCA (and versatile control paths), which is why I opted for the
single mix control.

Actually, since all level controls are under uC control, I could tweak
the response curves so that I'll have constant-power (or whatever
sounds good) mixing.

> PS.: I'd love to see the schematics of your all-discrete synth!

I'll try to come up with something soon.


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