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Old 5th December 2007, 10:52 PM   #1
diyAudio Member
Join Date: Dec 2007
Question stereo to mono network/circuit

I have recently purchased a 5 channel amp (Memphis Belle) and the sub channel only has one RCA input. The manual says to NOT use a Y-Adapter (Splitter) as a means of summing the Left and Right inputs to provide a single mono input as needed. Instead they offer a list of products they produce to solve the situation they have created. I've done a little research and have found their claim to have merit. I found the following article titled "Why Not Wye?"

This article describes various ways to solve the problem of summing two channels to one. The only thing is that this article and its solutions are intended for home audio. What I'm looking for are resistor values that will work for my particular components, or an equation or ratio to use to get those values. I have three possible scenarios.

Scenario 1: Stereo Head to Mono Amp (No in-line equipment)
Head Output 1K ohm - Amp Input 22K ohm

Scenario 2: Stereo Head to Mono EQ
Head Output 1K ohm - EQ Input 20K ohm

Scenario 3: Stereo EQ to Mono Amp
EQ Output 150 ohm - Amp Input 22K ohm

The circuit I was planning on using from the article is the one with a resistor (R1) on each output tied together (after resistors) then with a resistor (R2) after that tying that to the common.

Also... Is the resistor (R2) necessary?
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Old 5th December 2007, 11:40 PM   #2
gootee is offline gootee  United States
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Location: Indiana
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As the article mentions, the resistor values can be varied over a wide range, without changing much. The only thing that might matter is the matching of the values of the two summing resistors.

They also suggest multiplying all three of the resistance values by a factor ten, to get better channel separation, at the cost of "only" being able to drive cables that are less than about 60 feet long. That was for the case where you still want to also use the original stereo signals for other inputs, which you probably do. And your vehicle is probably less than 60 feet long.

Regarding the resistor to common, after the two summing resistors: Why do you want to leave it out? Keep it. But, if you are driving an amp with the new mono output, which I assume you are, and if you could guarantee that that amp had a similar resistor to common, right at its input, then you might not really need the resistor. But, if you multiply all of the resistance values by ten, anyway, then keeping it shouldn't hurt anything (and probably wouldn't hurt anything even if you didn't 10x the R values).

So you should probably use two 4.7k 1% resistors to sum the signals and then have a 100k or 200k 1% resistor back to common, or to both commons if both channels' cables have a common wire.
The electrolytic capacitors ARE the signal path:
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Old 5th December 2007, 11:52 PM   #3
jol50 is offline jol50  United States
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Join Date: Nov 2007
Just when you think you have something good....and they hose you. Seems like you could just get a cheap used 2-way crossover that monos and you would be good. I would try to hack the amp if room and mount twin RCA with a deal like you linked, if possible, but that is just me. I have a nakamichi crossover I ran for many years (many yr ago) with an old RCA I actually cut and wire nutted together the positives and negatives....don't tell anyone.....but it worked just fine, not that you should do that. For all I know the crossover had already made it mono I don't know. That was only used on 2ch amps not bridged.
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