QSC TouchMix 30 output topology?

[Shanks]

Hi, I've been a member here only for a short time so feel free to give me any feedback if this post is inappropriate or in the wrong forum.

I'm trying to find out the output topology of my QSC Touchmix 30 mixer.

So I'm guessing the two most likely topologies are those described in the diagram above. (Please ignore the value of the 1k resistors - I realised afterwards they're unlikely to be so high.) The top one is a pair of op-amps, one driving the positive, the other the negative. The bottom one is a single op-amp driving the positive, and pin 3 tied to ground via a resistor that makes the output impedance on pin 3 match that on pin 1.

Which topology is used matters a little to me. I want to connect (for training purposes only) a speaker with an unbalanced input. If the output uses two op amps, bridging pins 1 & 3 on the XLR plug means the op-amp driving pin 3 will have to work very hard, leading to heat and potentially damage.

(It's possible, by the way, that the mixer's Aux outputs have a different topology to the Main L/R outputs, as I heard somewhere that while there's two electrolytic capacitors for each of the main outputs, there's only one for each of the auxes.)

 

[AnalogSteph]

I would assume that a 32-channel mixer that costs over 2 grand would have the budget for some "proper" balanced output circuitry. You can get a pretty good idea which one it is by using an adapter to TRS (for easier probing) and checking the AC signal amplitudes of both hot and cold vs. shield with a half-decent multimeter (sporting a low voltage AC range) while playing a constant test tone. The impedance-balanced variety would have nothing of note on the cold, obviously.

 

[Shanks]

I would assume that a 32-channel mixer that costs over 2 grand would have the budget for some "proper" balanced output circuitry. You can get a pretty good idea which one it is by using an adapter to TRS (for easier probing) and checking the AC signal amplitudes of both hot and cold vs. shield with a half-decent multimeter (sporting a low voltage AC range) while playing a constant test tone. The impedance-balanced variety would have nothing of note on the cold, obviously.

I did the measurement. With a healthy signal coming from an Aux output, a multimeter read 0V AC between cold and ground. So there should be no problem using a bal to unbal cable with cold shorted to ground at the XLR end. Thanks for the suggestion!