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

[Neil Johnson]

cheater cheater wrote:
> It's not like linearity is even desirable in a synth filter.

I think that's a matter of individual taste.

> 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.

No, it's a cheap brute-force way of controlling the "oscillator"
amplitude.  It results in significant distortion, which probably
accounts for the characteristics that some people like in certain filters.

A good sinewave oscillator (e.g., a filter with resonance turned up to
cause self-oscillation) needs just the right amount of feedback - the
lamp in Wien bridge circuits being a classic example.  Too little and 
the oscillations will decay; too much and the oscillator will happily 
distort into its rails.

The late Jim Williams put considerable effort into amplitude control in
his low-distortion sinewave oscillators, compared to the relative
simplicity of the oscillator core itself.  Some oscillators have
multiple amplitude control loops to achieve the required performance 
characteristics (distortion, step response, etc).

Cheers,
Neil
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