[sdiy] SSM2164 Phaser - another way?

Mattias Rickardsson mr at analogue.org
Thu Feb 20 21:13:42 CET 2014


On 20 February 2014 18:59, Neil Johnson <neil.johnson71 at gmail.com> wrote:
> (remember: datasheets are marketing documents,
> their first job is to sell the chip, so make it look great!).

Oh... I've never thought about it that way. Kind of depressing, haha! :-)

> For example, if you're concerned about audio quality (e.g., in an
> audio VCA) then you'd want to keep the 2164 around the 0dBu region for
> most of the time.  And as I said, in a professional audio product such
> as a mixing desk where internal levels are likely to be +4dBu, you
> would need to either attenuate by -4dB before the 2164, OR use 47k
> input resistors (and adjust the output stage as well).  Or in the case
> of a consumer product which might be at -10dBV you would need to do
> some other scaling as appropriate.  In the case of Eurorack modules
> which seem to operate at a nominal +13dBu (5V peak, 3.5V rms) the
> input resistor should be higher still, around 137k (130k NPV).

Also the audio signals are very different in general audio compared to
in synths, even in different scenarios in a synth. Both the need of
headroom and the dynamic range of the input signal can vary from
nothing to a whole lot, which affects the design choice.
* General audio: unknown levels, occasional strong peaks, many quiet
passages. Both enough headroom and low noise needed, with the expected
typical signal somewhere in between, hopefully near the optimal levels
of the VCA.
* A volume control for a constantly sounding VCO: always the same
volume, never quiet. Keep it at optimal level, input noise is not even
an issue.
* A VCA at the end of a synth voice: various levels, occasional strong
peaks from resonating filters etc. Maybe it's not important to have
the optimal sound quality in those peaks? Lowest possible THD is not
important for a single voice either, so I usually let the loudest
peaks *almost* clip the VCA just to keep the overall noise down.
* ... etc ...

All other design ideas & experiences welcome. :-)

/mr


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