[sdiy] CVs and passive mixers

[harry]

Guilty. I said 10K  (less load on the driving circuits)

Yes there is a limit... how much loss can you stand. If you wanted to mix a
bunch of +/-10V signals and get a few millivolts out you could use a LOT of
inputs.

Even the opamp summing circuits have limits...eventually the poor opamp runs
out of current and can't do any more. Then you increase the impedances (less
load
but potentially higher noise) or use several opamps to mix groups of
inputs...than another to mix the outputs of those opamps...

like four, 4 input opamps stages driving one final summer would do 16
inputs....
maybe that would be too much for a signle opamp.

SAD to say, the way to predict this is usually applying Ohms law to figure
out the currents involved... If you know where the signals are comming from,
and where they are going we could chat offlist  (or on-list... maybe some
others want to play
too).  We could work through a simple example.

If you're not real familiar with Ohms law its time to get acquainted. Its not
really hard
and can solve a whole lot of electronic problems directly...and give a good
wild- at ss
guess at most of the others...

H^) harry

Terrence McWee wrote:

> At the risk of sounding like the complete newbie that I am, am I correct
> in assuming that the passive mixer outlined below (basically X number of
> inputs connect to a common out via a 4.7K resistor) could be expanded
> more or less indefinitely?  What's the max number of inputs that could
> be mixed in this way?  And what governs the resistor choice (I seem to
> recall someone saying they'd favor 10k over 4.7k in this application)?
>
> Sorry to be somewhat lame; still learning (and learning a great deal
> from lurking on this list)
>
> Thanks Much,
> T.McWee
>
> On Wednesday, May 15, 2002, at 07:20  PM, magic_silver_box! wrote:
>
> > 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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