How to mix few balanced output?

[Steinberg]

As I wrote in the other post, I build small mixer...


FIRST QUESTION:

If all inputs are balanced & the outputs of each SSM2019 is unbalanced,
can I mix those unbalanced signals & than make it balanced with SSM2142 as master output?
My solution for mixing with R/pot & C is good? Any other solutions?

http://www.diyaudio.com/forums/attachment.php?attachmentid=194792&stc=1&d=1288710011

or maybe there is any solution for mix balanced signals?


SECOND QUESTION:

I want to make input mic & line as the same connector.
So where I have to place attenuator with resistors?
At balanced input stage or after SSM2019 at unbalanced stage?
What solution is better for audio quality?
If I use 'attenuator with resistors' or 'gain with pot' (at the input of SSM2019), these amp will rise the noise of this elements; Am I right?
So, where is the right place for attenuator?

The idea of set the gain with resistors of SSM2019 is good/best?
Then I leave the balanced input as it is & the mixing stage
I will release with pot between SSM2019 & SSM2142?

Thank You so much!
Chris

 

[ChristianThomas]

Mixing is usually done on the summing (inverting, negative) input. Yes, you can sum on this input in the same way as in your diagram. To vary the attenuation of summed signal, put a potentiometer in the feedback loop (best to do it in parallel with a fixed resistor). You can vary the level of the individual inputs by varying the input resistors.

 

[Steinberg]

So, I should to put between SSM2019 & SSM2142 another amp & then vary the level of each signal by setting gain/attenuation of each amp. Am I right?

 

[ChristianThomas]

Sorry to reply late. You have probably got it by now (I've been forced off line by a BT fault which they took from Monday to this morning to fix) but the answer is 'yes'. All the outputs meet on the inverting (negative) input. Put a preset in series with these on the way in and a fixed R in the feedback loop - generally quite low in value for both - depending on the op amp. Gain is Rfb/Rin for each of the channels. Put a fixed R in series with the preset to limit the gain available (remembering that they sum.

Incidentally you can do this with an SSM2019 in this position as well, though it's a little less obvious, but might give you some extra options for the future.

PS When I got back online there was an email from BT saying "Visit BT.com to keep track of the progress of your fault." Yeah, thanks.