AW: VCO-1C

[tomg]

On 04-Jan-99, BJ wrote:
>jhaible wrote:
>> 
>> > Talking about old Korgs, is there any one besides me who like
>> > and use the MS20, JX3p VCA design?
>> 
>> I still think that it's responsible for the rather bad SNR of many Korg
>> Synths. I never have made tests on a MS-20 (and here the VCF probably
>> contributes a lot of noise, too), but I once replaced the VCA in a
>> Mono/Poly with a SSM2020 VCA, and it was a *big* difference.
>> No more annoying hiss on low bass notes.

>Actually i dont find the MS20 noisy it is in fact rater silent,
>but on the other hand the Monopoly is wery noisy, i have owned both.

>The one transistor VCA is  used in the JX3P and JX8P and they are
>not particular noisy i think!
> 
>> BTW, there's a 1-transistor and a 2-transistor version of this Korg-VCA,
>> and they have very different transfer courves when overdriven.
>> (1-transistor version is only clipping one side of the signal)
>> 
>> Interesting for sure, but you'd rather have the *option* of a low noise
>> VCA, at least.

>Yes, if one would overdrive the one transistor version at some 
>levels one get a sinus wave ot of a triangle wave.

>BJ

This is not really a VCA it's more of an attenuator. I call it
an emitter pass attenuator after the power supply regulator
of the same configuration. 

It drags the signal very close to ground. That's why it's 
followed by a op-amp set at a gain of 100,000 - 220,000. 
I think it kills the balls on the siginal. It's like turning
a level pot up to 5% then amping the hell out of it. Depending 
on the op-amp it can add a ton of noise set for that much gain. 
It's pretty close to fuzz-tone levels.

It works, it's simple and it's cheep, not bad but not 
really very good.

-tg