summing mixer impedance

Deepdark · May 20, 2014

Hi all

I was planning to build a summing mixer based around Tim Farrant's design. My goal is to send all my buses from my DAW (Logic pro 9) to the summing mixer and send back to my daw. Pretty simple. The main use of this box will be in stereo balanced. But, where I got confused is at the in and out impedance.

The apogee ensemble's outputs are rated 100ohms, according to the specs and the line inputs are rated 10K.

So, my understanding is that the inputs of the summing mixer has to match or be higher than the ensemble's output, so 100ohms. And, the output of the summing box has to match or be less than the ensemble's input, so 10K.

So, if the transformers inside the summing box are rated 1:1 600ohms, I need a minimum of 100ohms at the input so at 600ohms I'm ok, and I need to be inferior or equal to 10K at the output, so at 600ohms I'm ok.

Am I allright? Thanks folks

ruffrecords · May 21, 2014

In the circuit you posted, the input impedance to the mixer is set by the pairs of 22K resistors. This is plenty high enough for your needs. The output impedance is determined by a number of factors including the op amp , the pair of 75 ohm resistors and the transformer. In any case, it will not be more than a few hundred ohms and will be fine driving the 10K inout of your DAW.

Cheers

Ian

Deepdark · May 21, 2014

Awesome. Thank you

ricardo · May 21, 2014

You could also get rid of the Sowter and connect your Bipolar caps directly to p2 & 3 of the XLR for better THD & noise.

Deepdark · May 22, 2014

I could, but I like to get some iron in my stuff ;D haha But I'm planning putting in some Cinemag instead of Sowter. Maybe I'll make a switch to bypass the transformers.

joaquins · Jun 4, 2014

It wouldn't be floating without the transformer, so if connected to un unbalanced source will short one of the outputs to ground (lot of current, noise, etc) and will work as an unbalanced summing amp, even if the input is balanced your CMRR for the buses will depend on the input and only that, some of which are really poor. I would stick to the tx or another opamp to do the unbalancing properly.

JS