Re: Op amp fan-in then fan out

"TonyL" <TonyLacyRemoveThis@xxxxxxxxxxx> wrote in message
news:Qt-dnSscj4aCYuDUnZ2dnUVZ8vednZ2d@xxxxxxxxx
Here's a problem that is causing me some grief. I'm rusty with electronics
these days, perhaps the experts in here can give me some pointers ?

I want to combine the audio outputs of an unspecified number of op-amps,
say 2-20. A simple mixer would do the trick...except I need the resulting
summed audio level to be constant as outputs are connected to and
disconnected from the system.

I know a virtual ground is the standard method to mix audio but there's
another wrinkle here. I also need to send the summed audio signal to an
unspecified number of outputs, between 2 and 20. In other words, the
common point where all of the audio signals are present needs to be level
insensitive to changes in numbers of audio inputs *and* outputs.

Is this possible ? I've tried some breadboarding and find that each added
input results in a level drop at the "star point". Also, adding a second
virtual ground output completely kills the audio. I can, of course, add as
many outputs as I wish using the output op-amps as high impedance buffers,
which leaves me with the input summing problem.

Note...the electrical "star point" is not a physical point. In practice
the inputs/outputs are distributed along a common cable, of unspecified
length, which can change.

Comments appreciated, other than "design yer own f****** stuff", of course
:-) Just pointers to info would help.


The "virtual ground" arrangement uses an op-amp to null the voltage at the
"star point", the signal voltage at which is more or less immeasurable.
Instead you take the signal from the output of that op-amp. Within reason
you can use as many inputs as you like without any measurable change in
level. And, depending on the drive capability of that op-amp and the load
impedance of each destination, you can drive a large number of destinations.

Simply the star point is connected to the inverting input of the op-amp, the
non-inverting input being at AC ground. A feedback resistor connects the
output of the op-amp to the star point.

There is, however, a potential problem with your distributed "star point".
If the capacitance to ground is too high you may get instability in the
virtual ground op-amp.

David.



.

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