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

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

Originally Posted by crutschow

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.

Originally Posted by crutschow

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!