Summing a stereo signal to mono

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  • Mazeroth
    Senior Member
    • Nov 2004
    • 422

    Summing a stereo signal to mono

    I purchased some in-ceiling speakers for my whole house audio install a while back and bought single voice coil speakers with a single tweeter with the intention of summing the stereo output to mono, into an amp, and just running a single speaker per room. My question is, how much signal degradation takes place if you simply use a stereo to mono RCA adapter? By no means is this going to be used for high fidelity listening, rather, just for background music.
  • Kal Rubinson
    Super Senior Member
    • Mar 2006
    • 2109

    #2
    Not ideal and, more importantly, simply shorting the L and R signals at the source will make it impossible for you to have stereo anywhere, unless those signals are buffered. So, where are you getting them from?
    Kal Rubinson
    _______________________________
    "Music in the Round"
    Senior Contributing Editor, Stereophile
    http://forum.stereophile.com/category/music-round

    Comment

    • Mazeroth
      Senior Member
      • Nov 2004
      • 422

      #3
      I have a 17" in-wall touchscreen computer monitor that's going in my kitchen wall and will have a picture frame border. It will be hooked up to an extremely efficient Intel Atom based computer running 24x7 in my basement that will serve as a music server. The 1/8" mini out from the computer will feed into an efficient 2-channel digital amplifier that I've yet to purchase. To minimize holes in the ceiling I decided to buy single channel in-ceiling speakers as I don't care about stereo imaging from my ceiling and the volume will be low to moderate in level. I've had the ceiling speakers for a while and if I had to do it over again I would get the ones with dual voice coils and two tweeters but it's too late for that. I even have the CL rated 2 conductor wire, and I would need 4 conductor (or 2 runs) if I had the other type of speakers.

      So, my option now is to go stereo to mono and then mono into the amplifier. From the amplifier the speaker runs will go to the 6 volume controls throughout the house, and each volume control will have a single speaker hooked up to it.

      The computer in the basement will also have access to the internet, traffic and weather, a front porch camera, an OTA HDTV tuner which will be fed by a DIY UHF+VHF antenna in my attic and anything else I can think of. I'm very excited to get this project up and going and my Dad is coming down for Thanksgiving to give me a hand for a few days. I am just curious as to the technical regards of going stereo to mono with a simple adapter. I am using iTunes to control the music because I have an iPod Touch that will allow me to change songs throughout the house or even outside (outside speakers to be installed next spring). iTunes does not allow you to change to mono output like VLC does. If only it did I would be golden.

      Comment

      • Dennis H
        Ultra Senior Member
        • Aug 2002
        • 3791

        #4
        I'd at least use some resistors so one preamp output isn't feeding directly into the other.

        Code:
        L---10K---|
                  |----Mono
        R---10K---|

        Comment

        • BobEllis
          Super Senior Member
          • Dec 2005
          • 1609

          #5
          That will get you some crosstalk between L & R channels, but preserve some stereo for the rest of the system. If driving the mono system is the only thing you are using that particular output for, it will be fine.

          Follow Dennis' circuit with an inverting op amp buffer and you'll preserve stereo due to the buffer's input being a ground equivalent. Use a 10K feedback resistor. Not as quick and dirty, but lets you keep stereo elsewhere.

          Comment

          • mjb
            Super Senior Member
            • Mar 2005
            • 1483

            #6
            mmm, lots of answers, the best way is with a zero gain amp though. If you're not a constructor it might not be easy. Otherwise use a 741 op-amp (or better) soemthing like this:



            Google for "741 summing amp" or similar for more idea's!!
            - Mike

            Main System:
            B&W 802D, HTM2D, SCMS
            Classé SSP-800, CA-2200, CA-5100

            Comment

            • Mazeroth
              Senior Member
              • Nov 2004
              • 422

              #7
              This will only be used for a single output to the amplifier so I don't need to retain the stereo signal at all. So, a stereo to mono adapter should do the trick, right? :B

              Comment

              • Dennis H
                Ultra Senior Member
                • Aug 2002
                • 3791

                #8
                Originally posted by Mazeroth
                This will only be used for a single output to the amplifier so I don't need to retain the stereo signal at all. So, a stereo to mono adapter should do the trick, right? :B
                It might work or it might fry your preamp.

                Comment

                • Saurav
                  Super Senior Member
                  • Dec 2004
                  • 1166

                  #9
                  It'll probably hurt the preamp. Each preamp output will see the amp's input impedance, in parallel with the other preamp channel's *output* impedance. The combination will be a very low impedance load that the preamp is probably not designed to drive. At least put the two 10K resistors in series like Dennis suggested, you can pick them up at RadioShack if you don't care about fancy parts quality.

                  Comment

                  • Curt C
                    Senior Member
                    • Feb 2005
                    • 791

                    #10
                    Looks to me like the optimal solution would be to purchase 2 amps: One for normal stereo use, and the other to be used for the mono summed output. Use an amp that features a stereo/mono switch, or a separate preamp. The output of the preamp could be summed to mono using Dennis's 2 resistor circuit if it doesn't have an onboard mono switch.

                    C
                    Curt's Speaker Design Works

                    Comment

                    • mazurek
                      Senior Member
                      • Mar 2006
                      • 204

                      #11
                      My sound card will let me pan each channel to center in software, which would effectively make both the left and the right channel outputs mono. Its got a custom mixer, so this might not be standard for generic windows driver sound cards.

                      Comment

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