[sdiy] Waveform/source mixers in classic polysynths

[Tom Wiltshire]

==================
       Electric Druid
Synth & Stompbox DIY
==================

> On 6 Oct 2017, at 14:05, Neil Johnson <neil.johnson71 at gmail.com> wrote:
> 
> Hi
> 
> On 6 October 2017 at 13:32,  <mskala at ansuz.sooke.bc.ca> wrote:
>> On Fri, 6 Oct 2017, mark verbos wrote:
>>>> 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.
>>> 
>>> 
>>> Isn’t that a linear current source?
>> 
>> Linear in applied current if you feed it with a current source, but
>> exponential (inverse log) in applied voltage if you feed it with a
>> voltage.
> 
> Having just run up the CA3080 driven by common base transistor circuit
> used in the Oberheim SEM
> page 7 here:
> http://www.synthfool.com/docs/Oberheim/Oberheim_SEM1A/Oberheim_SEM_1A_Schematics.pdf
> 
> in LTSpice I can see it is pretty linear once you get over the Vbe forward drop.
> 
> Neil

Interesting. I’ve seen the same arrangement used in several of the earlier Roland schematics too, 100M era.

Tom