[sdiy] Waveform/source mixers in classic polysynths

[Michael E Caloroso]

On the old Oberheims in the voice card summing circuit, the log
conversion is done by the common base transistor feeding the iABC pin
of the OTA.  The common base configuration also happens to have a high
output impedance which is precisely what a current source is.

Oberheim used a predistortion bridge diode on the OTA input, which was
MANDATORY for voice card audio summers.

On polysynths with CEM3360 VCAs, the 3360 includes a log converter on
the chip for linear CVs.

MC

On 10/5/17, Mattias Rickardsson <mr at analogue.org> wrote:
> Audio mixing takes signals of very different input levels and set them to
> quite different output levels. A huge gain range is needed.
>
> Synth waveform mixers, on the other hand, take signals of a well-known and
> constant amplitude and very often set them to almost the same amplitude
> into a filter. A small gain range is needed, and it's even good to have the
> higher gains taking up most of the potentiometer range.
>
> I guess that's why they differ in this regard. :-)
>
> /mr
>
>
>
>
> Den 5 okt. 2017 11:16 em skrev "Richie Burnett" <
> rburnett at richieburnett.co.uk>:
>
>> I've noticed this too Tom, and am interested to hear people's comments.
>> I
>> have no idea why linear pots are used in synths to mix oscillator volumes
>> instead of the more usual log taper for audio mixing!
>>
>> -Richie,
>>
>>
>>
>> -----Original Message----- From: Tom Wiltshire
>> Sent: Thursday, October 05, 2017 9:29 PM
>> To: *SYNTH DIY
>> Subject: [sdiy] Waveform/source mixers in classic polysynths
>>
>> The accepted wisdom is that audio mixers should use a log response rather
>> than linear response. Audio mixers are usually equipped with log pots for
>> this reason.
>>
>> However, many polysynths have a waveform or oscillator/noise source mixer
>> which usually uses linear control. Is there any reason for this?
>>
>> Is it just that it was simplest when the obvious VCA to control a
>> source’s
>> level ahead of the filter was based on an OTA, which has a linear
>> response?
>>
>> Are there any examples of classic synths where this is *not* the case,
>> and
>> the waveform/source mixing is done with an audio/log response?
>>
>> Background: I’ve been looking at old schematics again, for the Prophet 5
>> (learning from the masters!). I suppose there’s an outside chance they
>> did
>> a Lin/log conversion in the software before they sent the CV out, but I
>> seriously doubt it.
>>
>> Thanks,
>> Tom
>>
>> ==================
>>       Electric Druid
>> Synth & Stompbox DIY
>> ==================
>>
>>
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>