Convert stereo to mono? - creating 3 channels, mono, left, right

I have a little problem.... i want to convert Left, Right channels into 3 channels, Left, Right, and Mono is it possible to build this with a couple diodes? Thanks in advance!

Picture of Convert stereo to mono? - creating 3 channels, mono, left, right
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drummerman3 years ago

Here is the circuit to combine two line level outputs into a single mono output without frying your stereo outputs.
forgesmith7 years ago
So you want to merge the signals while preserving the left and right channels? That calls for some level of isolation. Looks like it could be done with transformers or capacitors. For transformers, something low inductance like a ferrite core audio transformer, 1:1 with two coils on primary. Drawing it out to match the phases, left to right (up to down), the connections would be left channel then common (first coil), right channel then common (second coil). Trying to think this thru mentally (and it's late), if you used a center tap with left and right at the outside with common (ground) to the center, with the same sound on both channels they'd try to cancel each other out. With two coils as described, with the same sound both channels would add together, they would both add to the magnetic field. The secondary of course would be the mono output. Now, what you'd have to do to match impedances as needed, I don't know, but that would combine the signals while preserving the original channels. Capacitors are easier to visualize. Join together one leg each of two non-electrolytic capacitors (series connection), then left and right channels each go to one of the remaining legs, mono is tapped where the capacitors are joined, all channels have the same common. Left and right channels are isolated, same sound on both channels (picture only one half of a wavelength) would build the charge at the center point, there'd be addition. The mono signal would be inverted from the added signals (positive charge on adding side of capacitors yields negative charge on mono side), when coming out of speakers there may be some noise cancellation. So add another capacitor in line on the mono side to invert it again. No help on capacitor sizing other than mentioning long-wavelength low-frequency signals will give you clipping of the amplitude with too-small capacitors as they'll fully charge before the peak, likewise on higher volumes, so bigger is likely better. I'm no audiophile, don't know what the sound quality will be. Don't know how much volume you can punch thru, especially with the capacitors, nor how balanced the outputs will sound. This is just basic theory for combining two signals that's really best suited for small signals, pre-amplification. But, at least it's a start.
This is my first time on this site. Even though I took advanved physics in college, I don't understand the explaination; which I have no reason to doubt. My queation is how to convert a common "boom box" radio, Awia@, from a stereo output to mono ?  Each of two speakers are in seperate rooms. I use this in my dental office for backround.  Is there any such "converter box" on the market ??

tone deaf
Sandisk1duo (author)  forgesmith7 years ago
The capacitors clip low frequencices, and i need to hoock up a woofer, which operates primarily on low frequencies, i have to seperate speaker outputs A & B they put out the same thing, like boing connected but not... i currently use only the B channel if i were to make the A channel mono, (after being amplified) how would i do that, i can't just combine both +'s and -'s as it overloads my amp... Looks like this L R (-) (-) (+) (+) Any ideas?
The capacitors clip low frequencices, and i need to hoock up a woofer, which operates primarily on low frequencies,
If the capacitors are large enough there should be no clipping, but it's been a long time since I did the math so I don't know how large is large enough. If you've tried it and are getting clipping try larger capacitors, or build two capacitor banks, each bank is just a bunch of capacitors in parallel, treat as one capacitor for use in the mentioned capacitor circuit.

Now for some clarification.

Can your amp amplify two different things at once, like amplifying a CD player for the A speaker outputs while amplifying a DVD audio signal for the B speaker outputs? 3 things need to be amplified, Left, Right, and Mono. If your amp can amplify 2 different things at once, then essentially you have 4 mono amplifiers to play with. Keep using B for left and right. For A, pick a side, let's say the left one. Attach mono signal to A left input, speaker to A left output. If there are separate balance controls for A and B, and there should be since it can do two things at once, set A balance all the way to the left, no need to irritate the amp by amplifying an A right signal that isn't there.

If your amp can not do two separate things at once, then you can't do it. You'll need another amplifier for the mono signal.

Oh, if you want only low frequencies to go to the woofer, you'll need a filter, as we've been talking will merge everything into the mono signal. Separation of frequency ranges is done with a crossover. This is the Wikipedia article, but you may find this site most helpful. For your application it looks like you'll use a 2-way crossover.

Ideally both left and right signals will have crossovers, use on the amp input side, merge together the two low-end crossover output signals for the mono input. But for your left and right amp inputs you may want to feed them the unfiltered signals, the same ones going to the crossover inputs. Depends on your speakers, if they can do low-end as well as your intended woofer speaker then use unfiltered, your woofer will be a bass boost. If you want the woofer doing all the very-low-end sound, like with a sub-woofer, then the high-end crossover left and right outputs go to the amp.

You could also use just one crossover, on the mono amp output, feed the crossover low-end output to the speaker. This will have the left and right speakers doing the low-end too.
Sandisk1duo (author)  forgesmith7 years ago
i have a 5.1ch amp, Pioneer Vsx-D711 it can enable/disable A/B speakers, meaning that when it is in aux, and both Channels are on, they put out the same thing..
Aux has L and R inputs

I found the sub, there were two of them, but the other one's woofer was taken out, i took a look inside,and i saw a capacitor, a resistor, and a glass-encased inductor, the sub was made by BOSE

i have a 5.1ch amp...




In the specs of the model you listed, at the bottom of "connections: Connector Type" section, it lists "Rear, 1 x Subwoofer output ( RCA phono )." Try hooking up your woofer speaker to that, it's already mono very-low-frequency. If you want more bass response, use the one for the central speaker, that's mono. The parts you describe in the de-woofered sub sounds like a low-pass filter, it's set up to only pass the low frequencies to the woofer, so you're good to go, soon as you get the speaker hooked up to a RCA plug so you can plug it into the amp.
Sandisk1duo (author) 6 years ago


I want to create mono from the output (which is stereo)
i want to do this without directly connecting Both channels to each other
last time i did that, i overloaded my amp, and it shut down!

You said you had this Pioneer 5.1 amp. You wanted to hook up a woofer. The specs that you linked to says you already have a connector for a (sub)woofer, you have no need to make mono because you already have mono. With 5.1 you also have a center speaker connection, which will be mono.

Thus you should already have two places to get mono output, left and right have already been brought together inside the amp to make those mono outputs. You have no need to make mono, you already have mono.

If that is not the case, then you don't have 5.1, and you don't have that amp.
Sandisk1duo (author)  forgesmith6 years ago
it has plugs(in) for woofer and center, i'll have to try the sub(out) connection
gmoon7 years ago
You need a simple audio mixer. Here's one. Just google "simple mixer" for more circuits.

You're just adding inline resistors to reduce any crosstalk. You might need to add resistance values for each line (R,L,R+L) to correctly balance the output (depends on the next stage.) It's not a perfect solution, and would be inferior to an active mixer... but might be more than adequate for your needs..

Such a passive mixer is altering the output impedance somewhat, which theoretically acts like a series resistor anyway. It works because the input impedance of the next stage (amp, etc.) is generally much higher than the output impedance of the previous stage.
Sandisk1duo (author)  gmoon7 years ago
that seems like an easy build...
Yeah. But now reading the rest of the thread, you're trying to separate power output channels (not input, like from an ipod.) Other than the crossover already described, I wouldn't recommend doing this between the "power amp" and the speaker.

Channels are best separated / combined early, when you're not dealing with so much current. To get a decent mix of two signals, you're guaranteed to attenuate the signal. Why do that at the end, when it can't be recovered? (plus signal attenuation here generates heat...)

Mix the signals first, and use another amp for the mono.
Sandisk1duo (author)  gmoon7 years ago
ya, i'm working on it now..
LinuxH4x0r7 years ago
What do you need mono for?
Sandisk1duo (author)  LinuxH4x0r7 years ago
Woofer... i don't have an amp for it, so i want to run it off the the other outputs, so basically i'm making a 3.1ch system
use an amp or two coils. Otherwise everything becomes mono
Sandisk1duo (author)  LinuxH4x0r7 years ago
Any links you want to show me?
ummm, remind me tomorrow
Sandisk1duo (author)  LinuxH4x0r7 years ago
Just use a 1:1 coil
Sandisk1duo (author)  LinuxH4x0r7 years ago
can i strip the wiring off an inductor, and re-wind it in 1:1 ratio?
not sure..... Try it
Sandisk1duo (author)  LinuxH4x0r7 years ago
how many winding do it need? seperate lengths of wire?
Sandisk1duo (author)  LinuxH4x0r7 years ago
Fine... i shall