Balanced audio summing mixer

[rampantandroid]

Hi!

I'm looking for a way to mix two balanced audio stereo signals. I've got two PCs right now, and I'd like to be able to connect and listen to both at the same time from a single amp without switching between inputs.

I can find plenty of passive SE summing mixers, but I can find none for balanced audio that also have volume control built in.

Before I wandered over to the DIY section, I wanted to see if anyone here knew of existing solutions that would work with no extra DIY work.

Thanks!

 

[cabirio]

There are plenty of small mixers out there that would work, but generally you're going to be paying for lots of extra stuff such as mic/instrument preamps, more than two inputs, USB, etc. For something like this I'd go DIY, it's pretty straightforward and you can get exactly what you want much more cheaply. All you need silicon-wise is two INA2137, one dual opamp and two DRV134 (I'm assuming stereo), I'd probably do it in a small proto-board, you don't even have to have a PCB made.

Edit: above I assume you also want a balanced out. If not, even easier: you can drop the two DRV134.

 

[Johno]

Convert each of the two differential signal pairs to two single ended signals and sum these at the input of your pre-amp (with its volume control). PITA with discrete circuits but fairly easy with 2 quad op amps and a bunch of matched resistors.

 

[rampantandroid]

There are plenty of small mixers out there that would work, but generally you're going to be paying for lots of extra stuff such as mic/instrument preamps, more than two inputs, USB, etc. For something like this I'd go DIY, it's pretty straightforward and you can get exactly what you want much more cheaply. All you need silicon-wise is two INA2137, one dual opamp and two DRV134 (I'm assuming stereo), I'd probably do it in a small proto-board, you don't even have to have a PCB made.

Edit: above I assume you also want a balanced out. If not, even easier: you can drop the two DRV134.

I assume you mean that I should be taking only the V+ of each input into the INA2137 correct? I found this on the subject:

When it becomes necessary to control gain in a
balanced system, designers are often tempted to keep
the signal balanced and use two Voltage Controlled
Amplifiers (VCAs) to control the gain on each half of
the balanced signal. Unfortunately, this can result in
common-mode to differential-mode conversion
(degrading CMRR) when there are even slight differ-
ences in gain between the VCAs. A better approach is
to convert the signal to single-ended, alter the gain,
and then convert back to balanced.

What purpose do the two opamps you reference serve?

How would I control volume of each input - would a simple Alps RK27 on each V+ L/R before the INA2137 be sufficient for this? And would a DPST switch be sufficient for muting - the INA2137 datasheet doesn't say what happens when one of the summed inputs is open.

Thank you for the reply! I can draw up a circuit later to have reviewed (either here or in an appropriate subforum.)

 

[cabirio]

The INA2137 is a dual balanced to single ended converter, so each would take one balanced channel (i.e. In A Left and In B Left to one INA2137, In A Right and In B Right to the other) and convert it to single ended. Now you take the two left, single ended signals and sum them with one side of the dual opamp, same thing for the two right, single ended signals and the other side of the opamp.

Attached is the basic idea (some ancillary stuff like input and output DC-blocking caps not shown for clarity), duplicate everything for the right channel. R4-R5 would be one gang of the Input A pot, R6-R7 one gang of the Input B pot. Here you can use 50k linear pots, which loaded by the 10k input impedance of the summing amp will give you roughly a log taper, with the advantage of linear pots being generally better balanced between gangs than log pots. Note that I show the input sources flipped (+ terminal to - input, - terminal to + input) to compensate for the summing amp being inverting, so the overall phase isn't affected. The NE5532 is one of many dual opamps that would work fine for this.

Note also that I suggested the INA2137 for convenience, but it's very easy to implement a balanced to single ended converter with opamps if you already have some in stock, see e.g. here.

 

[cabirio]

...and the two DRV134 I suggested, if you also need balanced outputs, would go at the outputs of each channel of the opamp.

 

[Mark Tillotson]

I can find plenty of passive SE summing mixers, but I can find none for balanced audio that also have volume control built in.

Perhaps something like: