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

Neil Johnson neil.johnson97 at ntlworld.com
Fri Feb 22 00:57:28 CET 2013

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).

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