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Combine a mono and a stereo audio signal to stereo headphones?

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neoflame

New Member
Hi, Let me just start by saying that I don't have much experience with electronics so I'm sorry if my post seems basic or illogical.

I'm trying to build a small unit that accepts a stereo audio input (via phono sockets) and also a mono audio input (wired connection from an xbox 360 controller). It should then split the mono input so that a copy of the signal can be combined with each channel of the stereo input. The result is then outputted to headphones via a 3.5mm jack socket.

My problem is that my stereo input also becomes mono as the signals merge where I have tried to split the mono input. I thought I would be able to use diodes to stop this happening but I wasn't aware that I'm dealing with a voltage far below the forward conduction threshold of the diode I was using (1N4148).

I have measured the voltage of the signals from my both my stereo and my mono sources:
Stereo source: 0.4mV
Mono source: 1.4mV

As I understand it, if I use a diode with a voltage this small it will just block both directions. Is it possible to get diodes that operate at much lower voltages? Or should I be attempting this a different way?

Any help would be greatly appreciated :)
 

BrownOut

Banned
Why not just use a small switch to disconnect the "mono" input when using stereo. Simple.
 

crutschow

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Most Helpful Member
Diodes are rectifiers and allow current to flow in only one direction. They can't be used to isolate AC signals. AC signals have to flow in both directions.

You need to use a resistive divider to combine the signals. Try 10kΩ resistors at each stereo output to the amp. Connect two 10kΩ resistors from the mono output, one to each amp input also. That should combine the two signals without interference.

You may need to adjust the resistor values up or down if one signal is too loud with respect to the other. For example, the mono source appears to have a higher signal level then the stereo source, so you would increase the mono resistor values.

If you wanted the relative volume between the two signals to be adjustable, you could use a stereo potentiometer instead of fixed resistors for the mono signal.
 

neoflame

New Member
Why not just use a small switch to disconnect the "mono" input when using stereo. Simple.
Because I'm trying to make it so that I can hear both sources at the same time through my headphones, not just the one.
 
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neoflame

New Member
Diodes are rectifiers and allow current to flow in only one direction. They can't be used to isolate AC signals. AC signals have to flow in both directions.

You need to use a resistive divider to combine the signals. Try 10kΩ resistors at each stereo output to the amp. Connect two 10kΩ resistors from the mono output, one to each amp input also. That should combine the two signals without interference.

You may need to adjust the resistor values up or down if one signal is too loud with respect to the other. For example, the mono source appears to have a higher signal level then the stereo source, so you would increase the mono resistor values.

If you wanted the relative volume between the two signals to be adjustable, you could use a stereo potentiometer instead of fixed resistors for the mono signal.
Thanks for your reply, but if I split the mono input with resistors will this not cause the stereo input to be combined?

To clarify my problem, I am trying to keep the left and right channels from the stereo input separate but overlay a copy of the mono input onto both channels. These two channels can then be output to my stereo headphones.

I have attached a diagram of what I am attempting to achieve, hopefully this will convey the setup better than my description can.
 

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crutschow

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It would work but you need much smaller resistors then I originally stated. I was thinking the signals would be going into an amp, but they're going to low impedance headphones.

I assume that you have tried this and each works ok separately with headphones? That is, the outputs are capable of driving a headphone and are not just low level line signals.

If so, try about 10Ω resistors in series with each line out from the XBox and one resistor each in series with the two lines breaking out from the Controller. The XBox and Controller have low impedance outputs, so the resistors will isolate the stereo signals from being mixed.

You will lose some volume that way, but that's the only way unless you want to add an external amp for the headphones, and use my original resistor values into the amp input.
 

neoflame

New Member
It would work but you need much smaller resistors then I originally stated. I was thinking the signals would be going into an amp, but they're going to low impedance headphones.

I assume that you have tried this and each works ok separately with headphones? That is, the outputs are capable of driving a headphone and are not just low level line signals.

If so, try about 10Ω resistors in series with each line out from the XBox and one resistor each in series with the two lines breaking out from the Controller. The XBox and Controller have low impedance outputs, so the resistors will isolate the stereo signals from being mixed.

You will lose some volume that way, but that's the only way unless you want to add an external amp for the headphones, and use my original resistor values into the amp input.
Thanks for replying again :)

Yes, the headphones work with both the outputs individually. I was trying to do it without an amp if possible so I'll try what you suggested with the 10Ω Resistors.

Just to clarify their position, would I be correct if I wired them as shown in the new diagram I have attached to this post?

Thanks again!
 

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crutschow

Well-Known Member
Most Helpful Member
That's the correct connections.

You might have to play with the values to get proper operation.
 

Kodiak

New Member
Hi,
I've stumbled across this page thanks to Google finally giving me a result.
I was just wondering what the outcome of this was as I'm wanting to do something similar with my own setup. Only, with the setup I have planned, it would have two dual-gang potentiometers wired up to act as two independent volume controls for the stereo volume and the game chat volume.

Do you think this might work as well? I'm guessing I wouldn't need the resistors because I'm using some pots...
 

audioguru

Well-Known Member
Most Helpful Member
The resistors reduce the stereo separation and reduce the volume of all signals.
 

MikeMl

Well-Known Member
Most Helpful Member
I do this exact same thing in my airplane. All of the aircraft audio sources (various COM, NAV, GPS receivers, and aircraft intercom) are mono. My noise-canceling headsets are stereo, as are my entertainment sources (CD player, XM radio receiver, MP3 player). I built a stereo summing amplifier to drive the headphones.

Each channel is a summing amplifier, with two inputs, capable of driving about 5Vp-p into four 150Ω headphones. One input comes from the mono aircraft source; the other input comes from the right/left channel of the stereo signal, respectively.

My system has another requirement, where the mono signal is about 3Vp-p, while the stereo signal is only about 1Vp-p, so the summing amplifier needs to have a gain of 1 for the aircraft levels, but a gain of 3 for the entertainment inputs.

I used two LM386s for the amplifiers. If you are willing to build something this complex, write back and I will see if I can find a schematic. Otherwise, I recently posted a schematic of how I am building a mono summing amplifier for a different aircraft application in this thread. The circuit you would build would have two channels, not one.
 
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Kodiak

New Member
Hi Mike ML,

It's been a while since I've thought about this, and now that I'm back on it I realise that I probably need to make myself a summing circuit with a couple of op-amps.
I realise that I'm going to need two so that I can add the mono signal to both the left and right channels for the headphones, but would it work if I put some volume controls/potentiometers in the way to individually control the mono and stereo signals?
Also, can this be done as a passive circuit rather than having an active amplifier as well?
 
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