[sdiy] A few Memorymoog questions

[Tim Parkhurst]

Hi Don,

On 4/14/07, Don Tillman <don at till.com> wrote:
>   > Date: Sat, 14 Apr 2007 14:49:14 -0700
>   > From: "Tim Parkhurst" <tim.parkhurst at gmail.com>
>   >
>   > A few questions about the Memorymoog voice card:
>
>
>For VCO1 and VCO2, when you press more than one waveform
> button the waveforms average, but for VCO3, when your press more than
> one waveform button the waveforms arithmetically add.
>
Ah, now I've noted that the "summing" amps (CEM3360 VCAs) for VCO1 and
VCO2 are non-inverting. Now this is a new one for me: It looks like
the summing node for the 3360 is a non-inverting pin, and no feedback
is used (at least, no external feedback). Could the same configuration
be used to acheive averaging of several signals with a regular op-amp
(without CV control of gain, of course)?

> Also note that VCO1 and VCO2 are AC coupled while VCO3 is not.
>
Right, I saw that. There are two distinct signal paths for the VCO3
signal (each with their own 3360 VCA), and it's a little curious that
even the audio signal path is not AC coupled. Well, curious to my eyes
at least.

> The summing of the VCO3 waveforms adds an extra opamp inversion into
> that path, so when the VCO's are summed together after their
> individual VCA stages, that extra inversion is accounted for in that
> audio summing stage that you noticed.
>
Again, more interesting stuff. This probably means that the sound from
VCO3 is slightly different from 1 and 2, especially as more than one
wave is selected.

>   > Is there a noticeable (audible) difference between a standard
>   > inverting summer and a difference summer?
>
> Yes, there are several differences.  One is that a standard inverting
> summer isolates the inputs from each other, while with the difference
> summer the noninverting input interact with each other and with the
> inverting inputs.
>
Interesting. If I can ever get this thing up and running, I'd like to
run a test where I can switch between a standard summing amp for all
three VCOs and the "stock" difference amp setup. Again, I'm told that
the Memorymoog gets a little more distortion and thickness than the
Mini when all three VCOs are used, and the internal mixer was set up
this way on purpose. Still, this voice card is absolutely packed with
extinct chips (3x 3340, 2x 3360, 2x 3080), so I'm going to be very
careful with it. I also wouldn't want to modify it in any permanent
way, in case it's needed to ressurect a Memorymoog some day.

>
>   > And finally: I'm watching "The Mummy" and I'm noticing that
>   > Rachel Weisz is a very beautiful woman... sorry, got distracted
>   > there. Guess I drifted OT.
>
> That depends, could there be a MemoryMoog in the soundtrack?
>
No, I'm pretty sure they used a MummoryMoog.


Tim (you know I had to say it) Servo
-- 
"Imagination is more important than knowledge." - Albert Einstein