Mono from two balanced line signals

[Othello]

I want to fiddle with a subwoofer and try to get a mono signal from my stereo console, which happens to have balanced line outputs.

How do I combine these two channels for a mono signal?

Uwe

 

[Gayan Soyza]

A simple Method........

Only need just two A & B resistors.

Give left & right signals to A & B resistors and the other side of the resistors combined as one & give to the sub woofer AMP input.

 

[Sceadwian]

You should probably be using a crossover circuit to do this as if you're not filtering the audio at the source the sub is going to be fed frequencies it's not supposed to be producing. You could just add a properly valued capacitor to ground just after the resistors to bleed off the unwanted frequencies. It's a basic low pass filter and the simplest kind of crossover you could create.

 

[Hero999]

If you make sure both the left and right channels are the same then you could get away with using 100m balancing resistors on the output.

 

[Nigel Goodwin]

100m balancing resistors.

So what's '100m' - or did you perhaps hit m instead of k?.

 

[Hero999]

No, 100 mili-ohms.

Connect one to each output, then connect the outputs together.

If tthe left and right inputs are also paralleled then the output from each amplifier shouldn't differ too much.

 

[Nigel Goodwin]

No, 100 mili-ohms.

That's completely pointless?, you may as well just short them together as use 100 milli-ohm resistors!.

It's a standard requirement to feed a sub-woofer amplifier, and it's normal to use resistors in the kilo-ohm region to avoid overloading the driving preamp circuit (and not to mention shorting out the existing signals for it).

 

[Othello]

I have to admit I have a hard time following these answers.

If I had unbalanced lines my guess would have been to simply connect left ground to right ground and left hot wire to right hot wire.

That would have been my guess, but I don't know if this would result in cancellations etc.

Now I have two balanced lines with a total of 6 wires and I wonder how I produce a mono signal out of all this. I wonder because the two hot wires of a balanced line do not carry exactly the same information (when viewed on a scope).

I would appreciate a touch more detail in the responses, so I can understand the reason behind the recommendations

Thanks Uwe

 

[Hero999]

That's completely pointless?, you may as well just short them together as use 100 milli-ohm resistors!.

It's a standard requirement to feed a sub-woofer amplifier, and it's normal to use resistors in the kilo-ohm region to avoid overloading the driving preamp circuit (and not to mention shorting out the existing signals for it).

I'm confused, I thought he was talking about paralleling the left and right channels and 100m is totally different to a complete short which would be in the region of 1/100 of that value.

 

[Nigel Goodwin]

I'm confused, I thought he was talking about paralleling the left and right channels and 100m is totally different to a complete short which would be in the region of 1/100 of that value.

In comparison to the 600 ohm (presumably) output of the balanced preamp it's exactly the same as a piece of wire! - and assuming this was a power amplifier (which it's not) two 100 milli-ohm resistors connected across the L & R outputs would destroy the amplifier.

What he needs to do is MIX the outputs of the two channels, to feed through a low-pass filter and a mono sub-woofer amplifier.

Check **broken link removed** for an example, notice it uses 33K resistors to avoid disturbing the preamp outputs.

 

[audioguru]

If I had unbalanced lines my guess would have been to simply connect left ground to right ground and left hot wire to right hot wire.

Then you are shorting the stereo channels together. When the signals on the two channels are trying to be different then their outputs mght be damaged or they might produce severe distortion.

You want to use series resistors to mix the two channels together without loading them too much.

Balanced signals have one wire in-phase and the other wire out-of-phase. Select a wire from each channel that has the same phase as the other.

 

[Hero999]

In comparison to the 600 ohm (presumably) output of the balanced preamp it's exactly the same as a piece of wire! - and assuming this was a power amplifier (which it's not) two 100 milli-ohm resistors connected across the L & R outputs would destroy the amplifier.

Oh sorry, I missed the balanced bit.