Merging left and right channels into one

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Does anyone know of a way to take left and right channels from a stereo amplifier and turn them into one mono channel? I had a computer sub amplifier go out and I want to use the driver for a very small subwoofer. This project is not big enough to justify the purchase of a separate amplifier and I know there has to be a way to merge the two channels into one. This is being used with the center part of an old boom box, 15 watts tops, but I don't want to risk destroying it like I did with an old receiver.
Its pretty simple. You connect the two signal grounds together and then connect a resistor to each signal live and then connect those two together. The value of the resistors should be about 5k I think. I think you can also just connect the lives straight together without resistors, but I wouldn't suggest it. You might lose a little bit of quality depending on the type of resistor you use. I'm not sure what would be best. I use normal 5% carbon film ones (1/4 watt) and they work and sound fine.
I think Rory was referring to using two amplifier channels to drive a single speaker. While you can probably do it with resistors, 5K won't be the right value. If you drive a stereo amplifier with a mono source (possibly using 5K resistors to combine the two channels before the amplifier), then use low-value resistors (maybe 5 ohms or so?) to combine the two amplifier outputs to drive a single speaker, it should work.
Is that what you meant, Rory? And if so,why not just use one channel?

[Edited by paulb on 07-26-2001 at 07:58 AM]
What I'm trying to do

I have a small reciever. I want to have the left channel going to a small 3" full-range speaker, and the right channel going to another small 3" full-range speaker. But this needs some strengthening in the bass region. I had a computer speaker system's amplifier go out on me yesterday, so I figured, why not use the driver from the little subwoofer? It is a 4" high-excursion woofer, 4 ohms. The 3" speakers are 4 ohms. I could just use a crossover and use the 4" to drive an 8" passive radiator that I have. That would serve me quite nicely. But I need to find a way to take the stereo-sound output from the back of the reciever (each channel is throwing out something different here) and merge that into one channel for the woofer unit. So that's what I'm trying to do.
Okay, now I understand. What you want is a passive crossover that shares a common woofer between two channels. It should be doable, but I don't think there is a way to isolate the two amplifier outputs from one another and drive a common speaker without losing power in the isolation network.
The simplest solution would probably be just an inductor and resistor for each channel. This assumes a common ground between the channels (true unless it uses bridged amplifiers per channel, normally done only for automotive).


The bad news: because the original speakers aren't attenuated, the new woofer will be much quieter than them You probably won't be able to notice that the woofer is even there. You could build a separate mono amp for the woofer (like the National LM1875 or Overture series)?
So you've got three 4-ohm speakers connected in parallel to two amplifier channels wired in parallel? I'd be worried about damage to the amplifier channels, they will be fighting each other trying to drive the 1.33 ohm load. If you haven't already done so, I suggest you switch the receiver to mono. And I still think there should be some resistance, at least, between the two outputs so they share the load properly.
Well, I scrapped my plan to try and have a single-voice-coil sub in favor of a 6th-order BP sub with two drivers (one driver deiven off of each channel). I used a 6th-order bandpass box instead of a 4th-order bandpass box so that the two cones wouldn't interfere with each other that much.
sub to fronts merging, 6.1 -> 6.0

i have a 6.1 receiver w/ 2 front monitor speakers (420w RMS). is it possible to down mix my LFE channel back into my 2 front channels after i amplify it. (my receiver's setup doesn't have the option for large front speakers.)

after searching Google, not finding help, i constructed this.
An externally hosted image should be here but it was not working when we last tested it.

will it be able to handle the wattages? with the minimalist distortion?

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