[sdiy] OTA performance (was SSM chip reissue)

[Tom Wiltshire]

On 25 Apr 2017, at 10:20, Andre Majorel <aym-htnys at teaser.fr> wrote:

> On 2017-04-24 23:22 +0100, Neil Johnson wrote:
>> Tom Wiltshire wrote:
>> 
>>> What, too embarrassed to say that the matching is a bit
>>> worse than a quarter decibel out at -40dB?!
>>> What's the smallest decibel difference people can detect?
>>> About 3dB, isn't it? Quite a bit more than 0.25dB.
>> 
>> Gain matching is important in stereo recording otherwise the stereo
>> image drifts away from where you want it.
> 
> Right. Gain matching between channels is why serious mic preamps
> have stepped gain controls instead of pots which would be much
> cheaper.

That article Neil posted suggests that if the Stereo Mic outputs aren't well matched, you can boost the gain at the preamp to compensate. Easy. So you can have your stereo image "as good as your nulling technique", as they put it. So who cares about a 1/4dB in the VCA at this point?

> How relevant gain matching within the same chip is to synth
> circuits is one of the many things I have no clue about. :-)
> 
> A patch using FM in a polyphonic synth can be completely ruined
> by mismatched VCA gains across voices. Before you dismiss it as
> a simple matter of having the CPU calibrate the VCAs, try it.
> Because I've seen it done in the Alesis Andromeda and it was not
> enough, not by a long stretch.

FM is a particularly demanding application for analog, as we know. I wonder how close the between-voice matching was in the Andromeda. Maybe it was much worse than 0.25dB, or maybe it was better.

Tom