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.

"TonyL" wrote in message
...
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.

Well it all depends on whether you are happy with some kind of limiter or
not, with the sometimes strange effects you can get. That would be one way
to not go over a certain level.
I'm sure its not that hard to do with an op amp and some feedback.
Brian

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My Google mail account
Brian Gaff
"TonyL" wrote in message
...
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.

David Looser wrote:

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.


Thanks for responding.

Yes, that was how I was doing it. However, when I added a second virtual
ground op-amp the audio was killed completely at the outputs of both of the
virtual ground op-amps. Is there something I'm missing here ? Is it possible
to connect two virtual grounds together at a star point ?

Also, is the virtual ground idea going to eliminate the problem of level
changes as the fan-in/fan-out numbers change ?


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.


OK, noted. I haven't got that far yet.

"Mildew Spores" wrote in message
...

Well it all depends on whether you are happy with some kind of limiter or
not, with the sometimes strange effects you can get.

The nice thing about virtual-earth mixers is that, provided the circuit is
competently engineered and used within it's design limits, there are no
"strange effects".

That would be one way to not go over a certain level.

I don't think the OP said anything about not going over a certain level.

David.

"TonyL" wrote in message
...
David Looser wrote:

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.


Thanks for responding.

Yes, that was how I was doing it. However, when I added a second virtual
ground op-amp the audio was killed completely at the outputs of both of
the
virtual ground op-amps. Is there something I'm missing here ? Is it
possible
to connect two virtual grounds together at a star point ?

No it's not. Why on earth were you trying to do that?

Also, is the virtual ground idea going to eliminate the problem of level
changes as the fan-in/fan-out numbers change ?


The beauty of the virtual ground mixer is that it is insensitive to changes
in the number of inputs (what you are calling "fan-in"). If you are getting
level changes as you vary the number of inputs you are doing something wrong
somewhere. What sort of op-amp are you using?, and what value are the input
and feedback resistors?

David.

David Looser wrote:
Is there something I'm missing here ? Is it
possible
to connect two virtual grounds together at a star point ?

No it's not. Why on earth were you trying to do that?


What I meant was two or more...even worse.

The reason is that I need multiple inputs *and* outputs from that star
point. It seems that either:

a) I can have any (reasonable) number of inputs connected to a single
virtual ground summer as the output device. A standard mixer. In other words
a star point with multiple inputs and one output.

OR

b) I can have a single input device with any (reasonable) number of outputs
using op-amps configured as high input impedance buffers. A star point with
one input and multiple outputs.

Both a) and b) work fine seperately. Is there any way to combine both ? I'm
beginning to think not...


The beauty of the virtual ground mixer is that it is insensitive to
changes in the number of inputs (what you are calling "fan-in"). If
you are getting level changes as you vary the number of inputs you
are doing something wrong somewhere.


Sorry, I'm probably not explaining myself very well. With a single virtual
ground mixer it behaves as it should. I'm getting into difficulties when I
try to combine a) and b) above to get several outputs from the same star
point.


What sort of op-amp are you
using?, and what value are the input and feedback resistors?


TL074 with Rf equal to Rin on each input. I used 10K resistors.

David Looser wrote:
"Mildew Spores" wrote in message
...

Well it all depends on whether you are happy with some kind of
limiter or not, with the sometimes strange effects you can get.

The nice thing about virtual-earth mixers is that, provided the
circuit is competently engineered and used within it's design limits,
there are no "strange effects".

That would be one way to not go over a certain level.

I don't think the OP said anything about not going over a certain
level.

I think that what Spores is getting at is to forget virtual earth mixers and
accept the resulting level changes. Then compensate for them using some sort
of soft limiting.

"TonyL"

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 take it you want a mixer whose gain is a function of the number of
inputs actually connected.

So forget using the "virtual earth" summing method.

Feed each input to a high impedance summing point via a suitable resistor
( like 10k ohms) and the job is done.

As each new source is connected, the gain will drop in proportion to the
number.


..... Phil

TonyL wrote:

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.

Absolutely constant ? You need a thing called a hard limiter on the mixer
output.


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.

You need a distribution amplifier for that.

All readily available off the shelf for a price.

Graham