Passive audio mixer, stereo to mono switch per input.

Discussion in 'The Projects Forum' started by Zatnikitelman, Dec 21, 2012.

  1. Zatnikitelman

    Thread Starter Active Member

    Jul 17, 2005
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    Hello all. I'm attempting to build a passive audio mixer from multiple sources to a single stereo headset. It's based off of this design here: http://www.epanorama.net/circuits/linemix.gif
    I had originally intended to include the stereo potentiometers, but their price is such that I'll just live with controlling the volume at the source. I'll have much more than two inputs. However, I'm a ham radio operator, and all of my rigs output audio in mono rather than stereo. I'd like to hear their sound through both ears so I plan to include a stereo to mono switch on some of my inputs so when the radios are plugged in, I can throw the switch and get audio out of both speakers on the headset. However, I'm stuck on how exactly to wire the switch in. I understand that I should have some "isolation" resistors to isolate the various inputs from each other. I also figure that I can't just short the Left and Right channels together without bad things happening. Here's a rough MS Paint schematic of a single input and its relationship to the output: http://i226.photobucket.com/albums/dd230/Zatnikitelman/monostereooptions_zpse1afaf37.jpg

    The left diagram has a 5.6k resistor in series with a SPST switch before the 10k isolation resistors. The right diagram (should say input "stage" not "state") excludes the resistor. The other design I'm considering, are those two designs depicted, but the switch and/or series resistor are after the isolation resistors, but "before" the connection from the other inputs. This last solution has been suggested to me, but it doesn't seem right. Wouldn't that bridge all the inputs and make the entire unit output mono audio? I'd like to have the radios output to my headphones through both speakers, while still playing a game on my computer where having stereo audio is imperative. What's the better solution here? Or is there another solution entirely? I've seen designs using active op-amps and such, but I'd like to keep this as simple as possible for now and not need to get power to this.

    In my various searchings, I've found the recommended "Why Not Wye" link that others have recommended for similar problems, but the circuit that looks most similar to my situation (Figure 3), looks like it's for either mono or stereo, but that everything would go from stereo to mono, and not selectively like I need.

    Thanks for any advice you can provide on what my solution needs to be.

    Matt
     
  2. BillO

    Well-Known Member

    Nov 24, 2008
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    136
    All it requires, to mono everything, is that the left and right summing networks be connected together. Nothing more complicated than that is needed.

    See the attached diagram. When SW1 is closed, the summed mono signal can be taken from either the left or right output.

    (common {ground/sheild} connections are not shown)

    To do selective mono switching, or to have both stereo and mono available at the same time and maintain separation would require buffer amplifiers to isolate the channels.
     
    Last edited: Dec 21, 2012
  3. Zatnikitelman

    Thread Starter Active Member

    Jul 17, 2005
    31
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    Why do I need buffer amplifiers? Isn't a mono input on both lines just like a stereo input that just happens to have identical signals on the left and right channels?
     
  4. BillO

    Well-Known Member

    Nov 24, 2008
    985
    136
    To keep the separation of the stereo channels.

    If we think of the system as having two passive networks, a stereo network, and a mono network. Then...

    The stereo network is actually two physically separate mono networks and separation is maintained by having no connection between them. So adding a stereo input to it is a simple matter of adding the left input to the left network through a mixing resistor (value of R) and doing the same on the right.

    Now, to add that same stereo input to the mono network, we need to attach both the left and right inputs to the network through mixing resistors (value of R). The left and right channels of this input are now essentially connected together by a resistance of 2R.

    If that input is still connect to the stereo network, we now have the left and right channels of the entire stereo network connected to each other by a resistance of 4R and separation is no longer maintained and all signals will cross-talk. The more inputs you add to the mono network, the more crosstalk you will have.

    See the attached.

    If all the channels supplying inputs were ideal (zero impedance) this might not be a real problem, but the real world being what it is, there will be considerable cross-tlak
     
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  5. Zatnikitelman

    Thread Starter Active Member

    Jul 17, 2005
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    Thanks for the replies, and I apologize for the lateness in mine. I went ahead and built the device as I drew it, but using 47 ohm resistors, and a 25 ohm resistor for the mono/stereo switch. It works well and there isn't much of an issue when switching from stereo to mono. You are right of course (I had no reason to doubt you, Bill) and there is some crosstalk when I have the switch set to stereo (bridging the left and right channels). For my purposes, this isn't too much of a problem right now, but I would like to know more about what I need to build a properly isolated network. Do you have more information that I could use to build a device which doesn't allow any crosstalk?
    Thanks!
     
  6. BillO

    Well-Known Member

    Nov 24, 2008
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    136
    Ideally, you'd need a pair of amplifiers (one for mono, one for stereo) for each channel of each input. That would be 4 amplifiers for each stereo input. It's not actually as bad as it sounds a you can get all 4 amplifiers on a single chip. Something like the TL074 is only about $0.50 each. While there are lower noise and lower distortion op-amps available, this will probably meet your needs very nicely.
     
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  7. Zatnikitelman

    Thread Starter Active Member

    Jul 17, 2005
    31
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    Ok, so basically, I'd need a DPDT switch just after the jack. Then, when in the stereo position, the left and right channels would just go into the amplifiers as normal. But when in the mono position, they would be bridged like I currently have it, then each side would go into their own amplifiers like (but separate from) the stereo input's amplifiers?
     
  8. BillO

    Well-Known Member

    Nov 24, 2008
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    136
    Have a look at this. This should do the job.

    Note the feedback resistors on the output op-amps. They are n10R for the stereo and n20R for the mono where n is the number of inputs. This sets sufficient gain to compensate for the mixing/summing losses to each network.
     
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  9. stevegraff

    New Member

    Sep 29, 2013
    3
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    Hi all, I know this thread is pretty much dead, but I came across it in a search, and was hoping to get a little advice.

    I'm trying to do something a bit similar. I am building an adapter for an aviation headset, and I'm trying to mix a mono signal (from the airplane radio) with a stereo signal (from an MP3 player) without getting crosstalk from the two stereo signals being hooked up to the same mono source.

    I had been looking at buffer amps, but I would really like this to be passive, if possible, perhaps a little network with some Schottky diodes. I figured I'd run this idea by you all before I warmed up the soldering iron. Any takers?

    Thanks,
    Steve
     
  10. #12

    Expert

    Nov 30, 2010
    16,278
    6,789
    No such thing as a passive network with no crosstalk, except for some strange directional transformers used by the telephone companies. You had better decide which you want, buffers or no crosstalk.
     
  11. stevegraff

    New Member

    Sep 29, 2013
    3
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    That's kind of what I thought, but figured I'd ask. Thanks for the reply.

    SG
     
  12. stevegraff

    New Member

    Sep 29, 2013
    3
    0
    That's what I thought. Thanks for the reply.

    Any more info on those?

    SG
     
  13. #12

    Expert

    Nov 30, 2010
    16,278
    6,789
    I'd have to google for a while to find anything about them.
     
  14. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    Welcome to AAC!

    A thread belongs to the OP (original poster). Trying to take over someone elses thread is called hijacking, which is not allowed at All About Circuits. I have therefore given you a thread of your very own.

    This was split from Passive audio mixer, stereo to mono switch per input.

    It is possible to duplicate the telephone transformer using several normal tranformers. I've never done it, but I used to work for a company that did telephone stuff.
     
    Last edited: Sep 30, 2013
  15. #12

    Expert

    Nov 30, 2010
    16,278
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    It's called a hybrid transformer.
     
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