[sdiy] Two ways of mixing modulation routes

Mattias Rickardsson mr at analogue.org
Fri Jan 6 13:32:46 CET 2017

Interesting. I agree it looks a bit strange.

Non-inverting op-amps are often avoided in mixing stages because the
different sources feel different impedances depending on each other,
instead of just feeling a well-known constant impedance to ground.
First I thought they might have used this fact in a clever way to
attenuate the modulation signals a little bit if more than one of them
are switched in to the same destination, giving the sum of modulations
a more similar level regardless of how many modulation sources are
used - but when I look in the schematic I think it seems to be the
other way around: flicking the switch to add another modulation source
seems to make the first modulation a bit stronger instead. Hmm?


On 6 January 2017 at 12:02, Tom Wiltshire <tom at electricdruid.net> wrote:
> Hi all,
> I've been looking at the Sequential Pro-One schematics, specifically the modulation routes:
> http://machines.hyperreal.org/manufacturers/Sequential/Pro-One/schematics/pro1-schematics-a.gif
> (Rotated image, but it's bottom right when it's the right way around).
> I noticed that rather than mix the signals using an inverting op-amp mixer (as I might have expected), instead they do a passive mix, followed by a non-inverting op-amp.
> Is there any benefit to this approach, aside from avoiding the inversion? Is that the reason it was done this way?
> Thanks,
> Tom
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