[sdiy] [OT] Analog synths with 2 pole filters

[Tom Wiltshire]

On 21 Feb 2013, at 20:58, KA4HJH <ka4hjh at gmail.com> wrote:

> 
> On Feb 21, 2013, at 2:34 PM, Gordon JC Pearce wrote:
> 
>> On 21/02/13 18:26, KA4HJH wrote:
>>> 
>>> On Feb 21, 2013, at 11:36 AM, cheater cheater wrote:
>>> 
>>>> It's not like linearity is even desirable in a synth filter.
>>>> 
>>>> Many synths have diode clippers in the feedback path in order to
>>>> change the loudness of the resonance vs the rest of the response, for
>>>> one thing. It's very practical.
>>> 
>>> For example, the Oberheim SEM.
>> 
>> The Transcendent 2000 has a Zener diode in a diode bridge, in the filter feedback path.
>> 
> 
> The SEM a resistor network with two 1N4148s paralleled.

The SH101 is another example of this trick, and in my view it's a key part of the sound of that filter. I've not analysed it closely, but it sounds like there's some distortion in there that fattens the sound up when you apply a good dose of resonance.

Going back to Scott Nordlund's comment, 2-pole filters definitely self-oscillate. After all, the typical "resonance" control on a SVF filter is actually a "damping" control, and if you  remove it, the filter can easily become an oscillator.

Tom