Tuesday, 6 April 2010

Stereo to Mono converter - how to do it properly

On occasions you might wish to feed a mono PA input from a small stereo device such as a personal MP3 player or cassette player.

Historical note for youngsters: cassettes are those little plastic boxes full of brown string that your parents keep in cardboard boxes on top of the wardrobe. The de-facto standard of 129 metres of brown string wound on little bobbins is almost but not quite enough to store one whole album. This is what passed for copy-protection in their day. Left in a car for five summers, all cassette recordings of any kind evolved into either Wings or Steeleye Span anyway. This phenomenon has never been fully explained but is thought to be an early experiment in what is now known as consumer subscription expiry.

For example, you may wish to provide some recorded music during the interval between sets at a live gig, or you may be a quizmaster or quizmistress delivering the "music round" at your local pub. Either way, the material is on your personal device and you want it to go front-of-house, through the desk or whatever.

I know what you are thinking. You go to your local electronics store, you get a 3.5mm to 3.5mm stereo extension cable and a 3.5mm stereo to 1/4" mono converter and the job is done. Oh ho ho! No. You risk (at best) distorting your sound or (at worst) frying your kit. That's your decision but here at The Secret Laboratory we like to do things properly.

Here's how to do it properly:

Find a 3.5mm stereo jack plug, a 1/4" mono jack plug, a length of shielded two-core audio cable, two 10K resistors (brown-black-orange) and some insulating tape or heat-shrink tubing.

Solder all the bits together according to the following schematic and following instructions:

On the left, your 3.5mm stereo jack. Source (R) will probably be a red wire - connect it to the "ring" on the jack (you may need a meter to check which tag is the ring but it's usually the tag on the right as you look from the rear of the plug - red ring right is how I remember it). Source (L) might be blue, white or some other colour. Connect it to the "tip" on the jack (usually the left tag as you look from the rear). Connect the shielding (Source G) to the "sleeve" of the jack (that's the BIG tag that also serves to secure the cable when you crimp it). In this particular application it doesn't actually matter if you get the L and the R mixed up but it's good to get it right. Don't mess with the G though, else you'll be pissing signal to ground.

On the right, your 1/4" mono jack going to your PA. Connect the red signal wire (Source R) to a 10K resistor and the other signal wire (Source L) to another 10K resistor, making sure the two wires don't short at this end (that's where the insulating tape or heat-shrink tubing comes in handy). Twist the other ends of the resistors together and connect them to the "tip" (PA Sig) of the mono jack. Connect the shielding (Source G) to the "sleeve" of the jack (PA G) and crimp to secure the cable, just like you did at the other end. Get this right and the whole assembly will fit inside the 1/4" jack housing. Oh, er, now is a REALLY bad time to remind you that you should have made sure the plug housings were already on the cable - I'm sorry I'm sorry I'm sorry...

Right! You have just constructed a simple "audio mixer" - tell your friends! They'll think you're some kind of electronics genius! "Buy an audio mixer? No, I just make my own - it's nothing, really..."

Or: "Yeah Brett, I'd love to come to dinner tonight but I'm, like, building an audio mixer?" Think of the potential to keep 'em hot!!!


I showed my version (above) to Steve Glennie-Smith (Electronics Engineer par excellence) last night and he said "That's quite neat - for a programmer". Praise indeed!

37 comments:

  1. Thanks for this! I have just finished a Single ceiling speaker installation running off a stereo amp and i was getting distortion using a standard Sereo to mono converter. I have just made up a audio lead following your directions and it has work a treat!!!

    Nice 1!!

    Andy

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  2. Andy - I'm thrilled that this worked for you! My original one still works when we have to throw "intermission music" from a stereo MP3 player to a mono PA during the breaks in gigs. I've made several for friends since then.

    Best wishes - M

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  3. God bless ya lad! I need to take an output from my Droid and feed it into an antique German radio, which of coarse has a mono input. This will do exactly what I need. Thanks for posting!
    Steve - Chicago

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  4. Thanks Steve - please let me know how you get on.

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  5. Hi All, Great thread..

    I have a question re summation devices (to connect a STEREO signal such as from a MP3 player, to MONO input/s (such as to my 8 channel powered PA mixer).

    Is there any reason why I CAN NOT take the stereo signal, split it into 2 - 1/4" phone plugs (one left & one right) and plug the RT into say Channel 1, and the LT into say channel 2 of my (mono) powered PA mixer??

    Of course, I realize I would need to set the volume controls of channel 1 & channel 2 the same (say at "5"), and flatten out the channel 1 & 2 EQ, and control the volume and tone with the MASTER CONTROLS.

    Am I overlooking something, or would this be a simple solution to the problem (at least when there are at least 2 unused mono channels available on a multi channel mono unit) ???

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  6. Thank you! Someone who puts it simply. Two 10k resistors, never expected it to be that simple.

    Previous experiences left me with only a single channel and I was wary of connecting the TRS ring straight to a mono tip. Only make 'that' mistake once.

    After nearly an hour of phone calls, web searches and head scratching finally found a great method of driving mono audio circuits from a stereo recording. Whilst keeping both channels.

    Cheers to ya!

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  7. I'm really pleased everyone has found this tip useful! Thanks for your comments. I have replied separately where further questions have been asked. For example, BL's suggestion about feeding to two spare channels sounds perfectly reasonable to me - if you've got channels to spare. However, it does appear a bit extravagant if you're feeding a mono unit anyway. On the other hand, it does mean you can balance your stereo source before the mix-down to mono.

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  8. Hey, I've just finished my converter, following your instructions, but didn't test it yet. But, since I'm a Engineer student, I have to ask "why". So, why we have to use those resistors? What they do with the signal?

    Sorry about the bad english, I'm from Brazil...

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  9. If it's that simple (and, presumably, cheap) why does it cost over £100 to buy a stereo to mono converter 'box' and why does that need to be mains powered?

    I don't really want to spend a load of money if I don't have to!

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  10. I'm guessing that the "box" uses preamps and other stuff to produce a regulated line-level output signal at a given impedance, regardless of the input levels and impedance. Apart from a slight attenuation, my solution doesn't change signal levels or impedance, so you have to be sure that your source is "about right" for the job. £100 seems a bit stiff though!

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  11. EDC - I'm not an electronics engineer, so I'm not 100% certain myself. However, here's how I see it. Suppose the resistors weren't there and, for example, Source Left goes high whilst Source Right goes low. What happens to the signal? You're sinking Left to Right. The resistors stop that from happening and keep the signal voltage on the output side.

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    Replies
    1. How about using diodes (2Nos) instead of resistors?

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    2. Audio is an A/C signal. So you would block the bottom half of the wave of each channel.

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  12. Hey, that is a great solution!
    This may be a stupid question (I don't really know much about this), but which wattage rating shall the resistor have to be optimal for this application?

    Thanks!

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    Replies
    1. We're talking about "line-levels" here (the input side). There's not actually very much current going down a line-level connection. You might measure it in milliamps, and the voltage likewise is low. Therefore, wattage rating is of little consequence - use the titchiest (or any) resistors you can find. They'll cope with line-level signals.

      If you want to try mixing on the power output side however, that's a completely different game. Ask an expert!

      Delete
  13. Previously when I have tried to connect left and right channels directly a curious thing happens quite consistently - I get acappello - The vocals disappear completely, making the output an instrumental version. This could be a handy trick if you want an instrumental. It appears that the vocals are recorded as a positive signal on one channel, and negative on the other so they cancel out. If that is the case using balanced resistors in your design may not solve the problem. - Evan-e-cent

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  14. Hi: Do you think that this one might work properly? (I am not sure): http://cgi.ebay.es/ws/eBayISAPI.dll?ViewItem&item=250757904761#ht_858wt_649

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  15. Hi. I'm trying to connect my electric guitar with a mono output into an fm transmitter that uses a stereo input. Do you think I can use this?

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    Replies
    1. If you're going from mono to stereo you probably don't need the resistors

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  16. Hi. I would like to connect to stereo headphones with mono sound to both ears.Can I
    solder the twisted resisters to both tip and ring on a stereo 1/4 Jack.

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  17. Thanks for the simple instructions! I just made one of these to allow me to play music from my phone through an electric guitar amp.

    The amp had mono line input so this was just what I needed. It works great, I can hear both channels now.

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  18. Hi all i have use this circuit to convert stereo signal into mono, but i can notice some loss in sound quality at the final end.

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  19. Wow, so glad I found this thread. I would like to combine a tranducer pickup and a piezo pickup into a mono signal with a potentiometer on the transducer. This is for an upright bass. Any ideas on if the original schematic still applies? what kind of potentiometer to use? Where would I put the pot in the signal chain? The mono output will go to a 100w tube amp. kudos garage techies!

    thx, sugarfreek

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  20. Hi!

    I need to do this exact same thing but going from stereo signal (2 rca cables) to an XLR (mono) could this be done in the same manner?

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  21. Hi Marcus,

    Thanks for the useful post. I have one suggestion, though: I think it might be better to use 1K resistors rather than 10K. The input impedance of a typical amp should be in the region of 20K. At audio frequencies (where impedance-matching doesn't matter) it is best if the input impedance is at least x10 the output impedance. Assuming an output impedance in the range 100-600 ohms (plausible for an MP3 player headphone output), that would keep the total output impedance below 2K which should result in more signal voltage across the amp's input.

    Best Wishes,
    Eamonn

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  22. Thanks !!!
    Absolutely just what are required to fix the problem I've got connecting a walk man into the gyms PA system that is a bit funny and doesn't really work in the stereorgue thanks 1 million the posting it!

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  23. Thanks and well worth reading all the postings! I too was wondering why this solution was so cheap even compared to the S/M converter at £20!
    I have a poorly PYE Black Box which I am upgrading to a transcription turntabe c/w mag cart and preamp for a line output.Fortunately, the preamp has a spare output so I will be able to feed a signal to the original mono valve amp and listen to how it sounded in '55!
    A black piano finish and an input for your iphone and vintage meets modern - deep joy!

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  25. Why did you use resisters..! I mean .,...It can be connected without resisters ......!

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  26. Why did you use resisters..! I mean .,...It can be connected without resisters ......!

    ReplyDelete
    Replies
    1. why not diodes instead of resistors?

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    2. the sound coming from your headphone jack is AC diodes would rectify half of it and leave the other I tried this before and ended up getting clocking and popping noises due to the diodes in the circuit

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  27. nice of you to put this up on the web.

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  28. Thank you for the great and very useful article. I've just made one of these cables, this is just what I needed to connect an mp3 player stereo headphone output into the mono microphone input on an old portable boom box. Although based on research I ended up using 470ohm resistors. Works a treat with no loss of sound quality.

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  29. Hi. Will this work for my guitar tube amp? I need a stereo to mono to run a Korg mini Kaoss pad KP2S that will be mounted on or in my guitar to my amp. I am now thinking I will need dofferent resistors as well. Which ones should I use? Will this work at all? This is the closest Ive come to an answer for my problem. Thank you!

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