[sdiy] OTA performance (was SSM chip reissue)

[Michael E Caloroso]

> Incidentally, Oberheim managed to make an SVF with CA3080s in the OB-X, and
> they're not matched at all (unless they matched them?).

The OTAs in the SVF used in the Oberheim polySEMs and OB-X are indeed matched.

You're confusing the OTAs in the polyphonic glide circuits in early
OB-X versions, which were not matched.  Those unmatched glide OTAs
resulted in very fat multi voice unison sounds because the glide rates
wasn't the same between voices.  It was considered a desirable enough
"defect" that Oberheim implemented unmatched glide rates in the
digital glide of the OB-SX and subsequent models.

Oberheim never published the matching procedure, but a circuit for
testing OTAs for matching can be found in the Formant construction
guide which is online.  I figured out the matching process and showed
it to Dave Rossum formerly of Emu, whose name appears on the patent
for the Oberheim VCF.  Rossum confirmed the process.

MC

On 4/25/17, Andrew Simper <andy at cytomic.com> wrote:
> It by decent Q you mean Qs that match between voices sure - this is
> true for any filter topology. If by decent you mean high values of Q
> then for SVFs in synths you can get around this with positive feedback
> around the bandpass integrator to push the damping more negative and
> can easily push the circuit into self oscillation, which makes it just
> like any other topology where you can increase the feedback past the
> point of self oscillation with more gain.
>
> Andy
>
> On 25 April 2017 at 18:30, Neil Johnson <neil.johnson71 at gmail.com> wrote:
>>
>> Andre Majorel <aym-htnys at teaser.fr> wrote:
>> > How relevant gain matching within the same chip is to synth
>> > circuits is one of the many things I have no clue about. :-)
>>
>> It's also important in SVFs, where you want good gain matching between
>> the integrators if you want any decent Q.
>>
>> Neil
>> --
>> http://www.njohnson.co.uk
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