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