Op amp fan-in then fan out
 

Don Pearce wrote:

Circuit diagram here - it doesn't matter how many inputs you plug in
as long as the op amps are all there. The gain will stay constant.

http://81.174.169.10/odds/multi_cable.pdf


Thanks for the circuit Don, that is pretty much what I have tried and works
with a fixed number of amps. The problem is the "Decide how many" This is
not known in advance and could even vary during operation.

As usual, ignore the Australian.


He he.

But see my response to him detailing what this is all about.

Op amp fan-in then fan out
 

On Tue, 27 Jan 2009 13:28:05 -0000, "David Looser"
wrote:

Don Pearce wrote in message news:498105de.430631890@localhost...

Circuit diagram here - it doesn't matter how many inputs you plug in
as long as the op amps are all there. The gain will stay constant.

http://81.174.169.10/odds/multi_cable.pdf

The best solution to the OPs problem so far. But he does need to know in
advance how many inputs he wants. I got the impression he wanted to be able
to add, or subtract, inputs at will without causing any change in gain,
which, of course, he can't do with that topology.

As usual, ignore the Australian.

Absolutely!

David.


Well, all he needs to decide is the maximum number he could possibly
want, which isn't quite the same thing. I can't think of a topology
that would allow an unlimited number, unfortunately.

Once all the buffers are in place, he can add and subtract inputs
without any change in gain.

d

Op amp fan-in then fan out
 

"TonyL" wrote in message
...

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...


No. As you must be beginning to realise, it's not. You can have a common
star point if it's not a virtual ground star point, but then you get a
variation in gain depending on the number of inputs connected. There's no
other option I'm afraid.


David.

Op amp fan-in then fan out
 

David Looser wrote:


Ah!, you are trying to use the same conductor as an input mixing bus
*and* as an output distribution bus at the same time, you didn't make
that clear.

Sorry. As I said, I'm not an expert, I'm here to learn.

In that case you can't use the virtual-ground method.


As I feared. Comment noted.


I'd forget about using the same conductor for both purposes. Have a
virtual-ground input mixing bus and a separate output bus.


Extra bus is OK. But that would require a separate unit where audio
transfers from the mixer bus to the distribution bus. I wanted to make the
system modular with all units identical. However, it is looking like I need
to provide a "master unit" for my idea to work out.

Your comments/suggestions *are* helpful, BTW.



..

Op amp fan-in then fan out
 

Phil Allison wrote:

" Do they vary in level over time or not ? "


They are speech, so yes. But each person will talk at a reasonably constant
level.

" Are the signals correlated - ie originate from a common source ? "


No, they are not correlated.


Do you expect there to be NO level change when a new source is
connected even when that source is silent at the time ?


I would hope for minimal level change when a speech source is added to the
system.


TELL US what are YOU trying to achieve !!

Then WE can figure out the requirements.


OK. The requirement is for a modular and extensible group intercom system.
I'm exploring the feasibility of daisy chaining a set of identical units,
each one having a microphone input and a headphone output. With two units
connected it would be a simple intercom with one cable connecting them. The
requirement is to then be able to add further units and cable sections up to
a maximum of 20-30 without drastic changes being required to gain settings.

OK ?

Op amp fan-in then fan out
 

In article ,
TonyL wrote:
A variable number of inputs are to be fed *to* the conductor at
different points along its length.

A variable number of outputs are to be taken *from* the conductor at
different points along it's length.

Level changes on the outputs to be zero, or small, as inputs/outputs
added/removed.

How would you do it ?

If you add two identical signals the level doubles. With non identical
ones it could be anything.

To design a system where you add non identical signals with no apparent
change in level sounds like the Holy Grail...

--
*Men are from Earth, women are from Earth. Deal with it.

Dave Plowman London SW
To e-mail, change noise into sound.

Op amp fan-in then fan out
 

David Looser wrote:
Don Pearce wrote in message news:498105de.430631890@localhost...

Circuit diagram here - it doesn't matter how many inputs you plug in
as long as the op amps are all there. The gain will stay constant.

http://81.174.169.10/odds/multi_cable.pdf

The best solution to the OPs problem so far. But he does need to know
in advance how many inputs he wants. I got the impression he wanted
to be able to add, or subtract, inputs at will without causing any
change in gain, which, of course, he can't do with that topology.


Yes.

As usual, ignore the Australian.

Absolutely!


He he. I respond to constructive comments from anyone, including... The rest
I simply ignore.

Op amp fan-in then fan out
 

On Tue, 27 Jan 2009 13:32:16 -0000, "TonyL"
wrote:

Don Pearce wrote:

Circuit diagram here - it doesn't matter how many inputs you plug in
as long as the op amps are all there. The gain will stay constant.

http://81.174.169.10/odds/multi_cable.pdf


Thanks for the circuit Don, that is pretty much what I have tried and works
with a fixed number of amps. The problem is the "Decide how many" This is
not known in advance and could even vary during operation.


Not decide how many - just put an upper limit on how many, and design
for that. You can plug in and out during operation without problem.

d

Op amp fan-in then fan out
 

Don Pearce wrote:
On Tue, 27 Jan 2009 13:28:05 -0000, "David Looser"
wrote:

Don Pearce wrote in message news:498105de.430631890@localhost...

Circuit diagram here - it doesn't matter how many inputs you plug in
as long as the op amps are all there. The gain will stay constant.

http://81.174.169.10/odds/multi_cable.pdf

The best solution to the OPs problem so far. But he does need to
know in advance how many inputs he wants. I got the impression he
wanted to be able to add, or subtract, inputs at will without
causing any change in gain, which, of course, he can't do with that
topology.

As usual, ignore the Australian.

Absolutely!

David.


Well, all he needs to decide is the maximum number he could possibly
want, which isn't quite the same thing. I can't think of a topology
that would allow an unlimited number, unfortunately.


20 - 30 maximum.

Once all the buffers are in place, he can add and subtract inputs
without any change in gain.


Yes, if the buffers are already in place. Which requires a seperate
mixer/distribution unit...as hinted at by David.

Op amp fan-in then fan out
 

David Looser wrote:

No. As you must be beginning to realise, it's not. You can have a
common star point if it's not a virtual ground star point, but then
you get a variation in gain depending on the number of inputs
connected. There's no other option I'm afraid.


Yes, I'm understanding that now. This thread has helped me avoid wasting
time on a concept that can't work without added circuitry to compensate for
the level changes.