[sdiy] ASM1 VCO input question

[Happy Harry]

Hi Roman.... (et al)

imho the difference between 'summing' and 'mixing' is that
in a summer the signals do not interact, load each other etc...
as in inverting summer.

'mixer' just combines signals in an acceptable manner... they
might interact but you don't care.

The resistors described in the start of the thread constitute a
'mixer' but are not a summer.

Those CV inputs that go direct into the expo tranny are really
a mixer... to remain in scale you MUST terminate them in a low
impedance (ground unused, etc).

As you pointed out... the summer does not have this 'feature" (bug).
I got burned on this one in my theremin synth... I had to drive
the VCo from buffer amps so I could unpatch signals without frequency
shift.

It was small, but not acceptable. I ended up using more buffers than
I would have with using inverting summer from the start.

Mixers may also have desirable characteristics... there are some
waveform mixers that use the attenuation of the passive mixer to
reduce signal level as more waves are selected... so the combo of
saw and square might be no louder than either one alone...

H^) harry


>From: Sowa Roman <Roman.Sowa at upc.com.pl>
>To: "'buchi at takeonetech.de'" <buchi at takeonetech.de>,   
>synth-diy at dropmix.xs4all.nl
>Subject: RE: [sdiy] ASM1 VCO input question
>Date: Thu, 16 May 2002 15:15:34 +0200
>
>If you don't want to use them at all, don't even mount
>relating 100k resistors. If you connect them to panel
>jack, they can be left open, but if you are purist,
>use ground switching jacks.
>
>It will not affect 1V/oct trimming in any way no matter
>if they are left open or tied to GND. It's a summing
>node at voltage close to 0V (2-3mV, ideally 0V), so
>any resistance you put between this point and GND
>can't cause any significant current flow, hence, no
>influence on rest of the circuit.
>
>Roman
>
>PS, I haven't slept much last night and maybe that's why I
>don't get it - what is the difference between summing
>and mixing?
>
>
>__-----Original Message-----
>__From: Michael Buchstaller [mailto:buchi at takeonetech.de]
>__Sent: Thursday, May 16, 2002 1:43 PM
>__To: synth-diy at dropmix.xs4all.nl
>__Subject: [sdiy] ASM1 VCO input question
>__
>__
>__Hallo Group,
>__
>__a simple question for the Gurus within us:
>__
>__A friend has 2 ASM-1 boards, and i am helping him to build
>__it. He has made
>__a front panel similar to the one made by Chris List, that has
>__several CV inputs
>__for the exponential CV.
>__
>__Now, in the schematic i see that there are actually 7 inputs
>__with 100K resistors
>__as a summing node. The opamp U1-A has enough gain to make
>__this acutally
>__summing and not mixing.
>__Now my question: What will i have to do with the unused CV
>__inputs ? Will i have to
>__tie them go GND (maybe via a switching jack), or can i just
>__leave them open ?
>__I am under the impression that the 1V/oct trimming will be
>__affected if the number of used
>__input changes - or am i missing something ?
>__
>__TIA !
>__
>__
>__-Michael Buchstaller
>__
>__
>__---------------------------------------------------------
>__This message has been scanned for viruses by MIMESweeper.
>__
>__Wiadomosc zostala sprawdzona pod katem wirusow przez
>__oprogramowanie MIMESweeper.
>__
>
>
>---------------------------------------------------------
>This message has been scanned for viruses by MIMESweeper.
>
>Wiadomosc zostala sprawdzona pod katem wirusow przez
>oprogramowanie MIMESweeper.
>




_________________________________________________________________
Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp.