[sdiy] CVs and passive mixers

[Sowa Roman]

When you connect only one CV out to the mixer, at the mixer
output you'll get exactly the same voltage. If you connect
any device to mix out, you may loose from none to several %
of the voltage due to input resistance. Synth modules usually
have 100k input resistance, so you loose about 5% with every
moudule you connect to mixer output. If not used for VCO
tuning controll it's fairly acceptable.

BUT, when you want to use more mixer INPUTS, the output will
drop dramatically. Assuming all devices connected to your
mixer are standard modules (same output resistance) output
voltage will drop to 50%. In onther words if at one of the two
inputs voltage changes by 2V, at output you'll se only 1V change.
If you use 3 sources, it will drop even more - 33% of
free-run span. 25% with 4 sources etc. That's not acceptable
even for not acuracy-critical controlls like noise colour ;-)
or whatever.

If it's OK with you, simply feed more voltage from sources
to compensate mixer loss.

Roman

__-----Original Message-----
__From: magic_silver_box! [mailto:msb at deletedmail.com]
__Sent: Thursday, May 16, 2002 1:20 AM
__To: synth-diy at dropmix.xs4all.nl
__Subject: [sdiy] CVs and passive mixers
__
__
__Unfortunately I am newbie enough with electronics to have to 
__ask this. I've made a passive mixer of a very simple design, 
__consisting of four RCA ins, each connected via a 4.7K (1/4 
__watt? small ~1cm x 2mm, sorry, retarded) resistor to a small 
__wire that runs along each input to the single RCA output. It 
__works wonderfully for combining audio sources, with no noise 
__and a small drop in amplitude for each source. I am wondering 
__if it can be used to combine a CV from one device and another 
__CV from another device, adding them together. I guess the 
__reason I need to ask someone this is I don't know the voltage 
__and amperage of audio signals and CVs. Would the control 
__voltages overload the resistors, or be reduced too 
__drastically by them, or would there be some other problem? 
__Would I even need resistors? Also, if I send a CV to more 
__than one source, will it be reduced 
__significantly/predictably? Anything else I need to consider? 
__Thank you!
__-Brian
__
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