Line Level Mono Matrix Mixer

[metal_head_82]

Hey there!

I'm currently working on a line level matrix mixer with 8 mono inputs and 4 mono outputs - of course all balanced.

This is what I got so far. I simulated the circuit but simulations use ideal compnents... so I need to if I got something wrong before I burn my equipment. The circuit is attached below.
The inputs will be summed through 10k resistors to the 4 output busses. As OP-amps I will use TL-072 or TL-074.
Another use could be an active 1-to-4-splitter... So what do you guys think?

Thx in advance...

EDIT: Updated schematics!!!

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[kkissinger]

10K is somewhat low -- most circuits that I've worked with use 100K values.

The main thing is that you don't want your input impedance so low that it pulls too much current from (i.e., shorts out) the input signal.

[blue hell]

I think you should consider using two opamps for the output stage, one as the bus mixer and the other as the balanced output driver. Your output driver now has two very different impedances for + and - outs, which unbalances things.

The mixing may not be well defined either now, mixing into a non-zero impedance ... there should be resistors then in series with the capacitors as well (like in the usual CV mixer).

[metal_head_82]

@Blue Hell:

Yes, thought about that too this morning. I'll add a second non-inverting OP amp to the outputs.


@kkissinger:

You're right. Just read through some stuff an hour ago.


I'll add the changes and update the first post. I'll also add the resistors to the caps that go to the 4 busses to the drawing.

Thx for your help!

[metal_head_82]

Just updated the schematics.
I added your changes to the circuits.
I think this could really work.

Again Thx guys

[Sound]

Hello Metal,

It is possible add the gain at the first stage -the differential amplifier- making the “feedback resistor'' and the “to ground resistor” = 200K (for 100K resistor inputs)
So, the signal to process in next stages will be more big, so the ratio noise/signal, lower.
Its important that those 4 resistors be matched at 0.1%
This differential amplifier, with same 100K resistors is showed as a "Basic differential amplifier for audio use" in the book "audio IC op-amp applications" by Walter G. Jung with the addition of blocking DC capacitors in its inputs.
However also is described a line receiver in a balanced transmission system with input resistors = 6K and compensation with 100 pf capacitors in parallel with the “feedback resistor” and the "to ground resistor" that are 2K (gain)

I don't see on the schematics attached the inverting summing amplifier for sum the signals.
You could use its output and another inverting amplifier to achieve the balanced driver. It could be the most simple way.

For inverting amplifier with TL07X op-amps the non-inverting input should be tied to ground directly without any resistor.
As a resistor output I have seen values from 50Ω to 600Ω, so the 4K7 value seems so big. It is needed very low impedance on the outputs.

There are chips dedicated for balance and unbalance signals, see for example documentation of the THAT1203 and the THAT1646. Maybe it is more expensive but it is easy to implement. In any case to read their data-sheet is interesting.
http://www.thatcorp.com/datashts/THAT_1200-Series_Datasheet.pdf
http://www.thatcorp.com/datashts/THAT_1606-1646_Datasheet.pdf
Also interesting -for build balanced devices- is the RANE notes
http://www.rane.com/library.html see the notes in the folder “Wiring, Interconnection & Grounding”
And! Elliott site: http://sound.westhost.com/project87.htm