The Virtual Engineer - Broadcast Engineering Forums

How to properly convert the balanced stereo output from a mixer to a balanced mono signal ?

I've been having a discussion with another guy about this topic...

He suggested a 3-resistor circuit...

+Left -------------- +Mono
|
R1
|
+Right---
|
R2
|
-Left----
|
R3
|
-Right ------------- -Mono

All resistors are 600 ohms.

My concern is that without knowing the particulars of the driving circuits... and without knowing the particulars of the circuit we will be driving... a simple passive circuit could modify the frequency response and/or distortion of the audio.

In our case the circuit will be driven by the output of a stereo mixer and the output will be connected to the EAS system and then to the input of our audio processor.

My suggestion was to treat the two signals as you would treat any two separate audio sources and put them through a simple two channel mixer that would mix the two balanced audio signals into a single balanced audio signal.

Of course if you assume the driving circuit is a low impedance circuit like a complimentary pair in the output of an op-amp... and that the circuit the restive "combiner" was driving was a high impedance input... then the resistors may work... but assumptions have always made me worry

What do you guys think? How do you do this?

Thanks for any comments.

Make of mixer? Make of audio processor?

If I understand your question, you wish to mix a stereo feed down to mono in order to feed a mono processor?

I suspect you probably can just use a "Y" connector without much effect on the frequency response. Most modern equipment has a fairly low output impedance, while many processors have a bridging input. I assume the EAS just switches audio for alerts, otherwise it is essentially a straight thru path.

Something looks wrong here. Particularly the resistor between + Right and - Left.

How about this?

@Shane: I have the same circuit shared on one of my sites and that looks like a circuit for unbalanced audio.

This tech note from Radio Systems regarding the use of the Lattice Splitting Pad (Adobe PDF) sounds more appropriate.

And that appears to be the equivalent of the originally proposed circuit! So I learned something today.

An RDL STA1 with the input wired stereo and the output tied together is what I typically use. An RDL MXx mixer works well as well.