[sdiy] MS-20 Filter Clone Queries

[Antti Huovilainen]

On Tue, 2 Dec 2008, Aaron Lanterman wrote:

> A roll-your-own 4-BJT OTA expects a control current to be sucked out of 
> it, so in places you see that - like the SSM2040 clones and the Buchla 
> VCO triangle cores - you will see NPNs in their converters.

Beware that with just 4 BJTs, the transfer function will be quite 
dependent on output voltage. This may or may not be what you want. If you 
connect the output to an inverting opamp (acting as current to voltage 
converter), this problem largely goes away and you're just left with 
some asymmetric nonlinearity which can be a benefit for the sound.

>> Can you drive the filter's input without blowing anything up, and if so, 
>> how..?
>
> As far as the audio goes, the most that will happen (pretty much) is that you 
> can saturate the OTA inputs. They have a tanh style nonlinearity that starts 
> to get its business on in the millivolt range, which is why you see the 
> massive 220R/47K divide down ladder in front.

The transfer function is Ic*tanh(V/2Vt) (IIRC). There's also feedback 
operating for frequencies much below the cutoff which also reduces the 
distortion (the distortion is only applied to the difference between input 
and the stage capacitor).

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova