How to properly convert a balanced (line level) audio signal to unbalanced?

Question

I want to connect a balanced line output of a pro grade mixer to a consumer grade power amp that only accepts unbalanced RCA aux in connectors. The mixer has balanced TRS (tip-ring-sleeve) 1/4 inch output sockets, one for each channel.

If I connect a mono/unbalanced TS (tip-sleeve) 1/4 inch to RCA jack converter to the balanced mixer output, the negative/cold signal will be shorted to the sleeve (ground). I know that the simplest way to convert balanced to unbalanced is to short the negative/cold signal of a balanced to ground, BUT... I'm not sure if it's going to work fine with line level signal, especially for long term use.

Such shorting, if done on a higher impedance mic level signal won't likely affect anything. I've seen people do that on line level signal, but I'm not sure if they're aware about any long term effects that might exist.

That leads to another question: What would happen if I do that at a line level balanced signal? Does the short between the negative signal to the ground will result in any sort of equipment failures or damages in the long run? (Forget about any short term effects such as noise issues).

Answer 1

The "proper" way to do this, as you asked, is to use an audio transformer per channel. Pricey though.

The way we've got round this on a PA installation I operate is to use unbalanced auxiliary outputs to drive the power amp. The main outputs on our mixer (a Yamaha MX12/4) are balanced XLRs so we use two of the unbalanced 1/4" group outputs to drive the (stereo) power amp, along with some careful selection of signal routing to get the correct signal out.

Edit: I see you have a Yamaha mg12xu mixer. The manuals can be found here.

The technical specifications(PDF) show that the Stereo Out connections are balanced (and available on both XLR and 1/4" TRS jacks).

The Monitor Out, Group Out and Aux Send outputs (all pairs) are described, in the Analog Output Characteristics section, as "Impedance Balanced" jacks. These are all unbalanced outputs with the single-ended signal applied to the tip of the jack. The sleeve is grounded. The ring is connected to ground via a resistor equal to the source impedance of the output amplifier (stated to be 150 Ω).

The arrangement is shown in the block diagram on page 2 of the technical specification and enlarged in the owner's manual(PDF) e.g. page 26.

This produces an unbalanced output that can be used to feed an unbalanced input via either a TS or TRS plug. It can also be used to feed a balanced cable and input via a TRS plug. The impedance balancing means that the balanced interference rejection on that circuit is retained. (However, as the return line isn't driven you lose 6 dB of signal and the circuit could cause more cross-talk to adjacent circuits as we are not sending equal magnitude but opposite polarity signals on the cable pair.)

The Aux Send outputs bypass the main group and stereo faders so it is probably more useful for you to use either the Monitor Out or Group Out outlets to drive your amplifier.

Which you use will affect how you setup and use the mixer. I suggest you study the overall block diagram in the technical specifications, and the more detailed sections in the owner's manual, to see which switches to set and to work out which outlets will better suit your needs. (I find it useful to keep a copy of the overall block diagram by my mixer to sort out those awkward "where did the signal go" moments during bouts of finger trouble or when visitors have used the mixer and changed the settings.)

Answer 2

(Added) It depends if sleeve = Pin 1 is a supply, shared or floating Gnd.

If unsure;

Use either Pin 2 or 3 for each L/R channel but use same pin # to be in-phase.

Do not short pin 2-3 on XLR, otherwise audio will mute (cancel)

Answer 3

It depends if you have a true balanced output. If you do, grounding the "cold" line is OK. If you don't, Tony's answer is better because it avoids the short on the "cold" (ring) to gnd.

If the mixer only has jack (not XLR) outputs, I'd doubt it's to full professional standards, so read its manual carefully. (OK it does have XLR ... still, its manual should help.

If you have an oscilloscope or audio level metering you can easily test for a true balanced output.

Measure the level on the "hot" leg (ideally, from a test tone) with both legs driving the same impedance (e.g. 600R) to ground. Then momentarily short the cold leg to ground. A true balanced output will double the amplitude ( +6dB ) on the hot leg; a differential output will not.

Then you know which way to proceed.