Unbalanced Stereo to Isolated Unbalanced Mono Converter

Stereo to Mono Conversions

At North Ridge Community Church in our Worship Center we have a mono system.  Mono meaning that there is no left or right, there is just the one point source that we hear audio from.  Our room doesn’t have a good shape for everyone to get a good stereo feed, so mono it is.

Almost every CD player, computer, MP3 player or media player is in stereo.  On a sound board, to get these media devices connected, you have to use a stereo channel or two mono channels (one for left and one for right).  With a mono audio system you combine the left and right channels inside the sound board to send to your speakers thus eliminating any stereo feed and summing them into a single mono feed.

Most churches require a lot of media devices to play from.  At NRCC we have a computer, iPhone/MP3 player, CD Player, and DVD player.  If you add that up that is 8 different connections into the board for 4 devices!  If your church has a smaller format audio console, those add up quick and take up your channels!

One way to fix this is by using a stereo to mono summing circuit.  This isolates the left and right signals and then joins them together to form one output that you can send to the sound board.

Stereo to Mono Conversion

Stereo to Mono Conversion - Unbalanced Stereo to Unbalanced Mono Converter

As we can see in Figure 1, we have our left and right channels.  The easiest way to think of this is, think of an RCA cable.  So you have your red and your white cables.  The pin that sticks out of the RCA is the positive, the part on the outside is the ground or negative.  This circuit takes the positives of both the left and right, puts them through a 1,000 ohm resistor, joins them together and then puts it into one single RCA jack.  This circuit works well, you may loose a bit of volume using this, but it’s nothing a gain adjustment can’t fix!

Stereo to Mono Conversion – with Isolation

One way to improve on this is to add an audio isolation transformer to isolate the audio ground.  By isolating the audio ground you remove any chance of ground loops (humming/buzz).

Stereo to Mono Conversion - Unbalanced Stereo to Unbalanced Mono Converter

As we see in Figure 2, there is a transformer added into the mix.  This transformer, in a simple way of thinking about it, copies what is on the left side and pastes it on the right side while keeping both sides electrically isolated from each other.


Stereo to Mono Conversion - Unbalanced Stereo to Mono Converter with Isolation Box

In the photograph above is a old broken “Live Wire Solutions Direct Box SPDI” in which I removed the transformer.  I separated the two 1/4 inch jacks and connected them together via 1k resistors, this goes to the -20dB pad switch, then goes into a 1:1 audio isolation transformer and then gets connected into the XLR connector.  Pin 3 and Pin 1 are connected together on the XLR connector.  Positive is connected to Pin 2.  This box is now converting my stereo feed into a mono feed.

By making cables or small converter boxes you can “sum” the stereo to mono before your mixer and save yourself some channels.  If you have any questions feel free to post below!


I have made a follow-up post about correctly interfacing external audio devices to the sound board, check it out here: Connecting External Devices to the X32.

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15 Responses

  1. Hi. I tried the schematics in Figure 1 and it didn’t work for me. It outputs some noise i can barely hear and if i increase the source volume to maximum i’m still not getting any eligible sound. I don’t know how to fix this.

  2. Can you give a specific as to what transformer to buy to do this above (unbalanced stereo to isolated unbalanced mono converter)-what do I buy from say Radio shack? Thank you,

  3. I tested this schematic, and it did not isolate the channels. Any thoughts? I am capable of wiring something incorrectly – would love to see if you have tested the bleed of signal back to the source. -Bruce

  4. Hello there. Great article. I was looking for something like this for my guitar fx system. Quick question: is there a way to do this but with a switch? to be able to switch between stereo out when you’re feeding a stereo system and mono out (summed) when you need so.
    I’m a little new on this so if you can do a drawing or pic it would be great.
    Thanks a lot and god bless you!

  5. Hi, thank you for your post. I just purchased a Blackstar HT studio 20 guitar amp. I also have a Boss Eband JS8. As the latter is stereo, I want to know whether it is necessarry to add resisters to the RCA cable when I join the 2 channels and the 2 grounds, to connect to the mono jack.

    1. Hi Pierre, the resistors are not required but suggested as they provide isolation between the two inputs. Thanks so much for your comment!

      Drew Brashler

  6. Drew, Thanks for the article. What were the specs on the transformer? Seems the wrong one might serve as a low pass filter and block the brightness of the signal.

  7. Hi Drew, thanks for this article. I’m looking to use this in “reverse”, just wondering if that would work or whether the wiring would need to change? By reverse, I mean I want to take a mono signal and sum it to stereo (actually, dual mono, but both sides of a set of IEM).
    The Aux 3/4 outputs on a QSC TouchMix-8 can be output on TRS, which would do one stereo IEM feed. I want to mix Aux 3 and 4 separately, but still send them out on the TRS output, to two different sets of wired IEM. So I need to take the mono feed from each aux, and make it appear on both sides of the IEM.
    Is this stereo to mono circuit able to do this?

  8. Hey, thank for your idea.
    I’m wondering why there is 1k ohm resistor used. Would u explain about it?

  9. Circuit works well, but had to push the gain to get good signal on mixer.
    Is it possible use lower value resistors, how is the 1k ohm calculated. What if i replace with 470ohm or lower?

  10. Hey there, you need at least 2 transformers to get a mono summing. The outputs of the trafo´s are put in parallel. You could achieve this with 2 Di boxes. For correct application one can use 3 trafo´s and than make up gain for the loss.

  11. Hello , my question is by using figure 2 , does the original signal remain stereo ? ( I am trying to give signal to a subwoofer while stull using the original stereo signal for the other frequencies in another stereo amp )

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