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  • Stereo to mono summing circuit?

    I've been looking for a safe way to combine a stereo signal into a single mono signal. I've heard it can be done by combining the signals with a 1k resistor between each input and the point they combine (http://brashleraudio.wordpress.com/2...o-conversions/) but I've also seen more complicated circuits.

    Anyone know a legit circuit for this, or have one they've used and trust? I've been doing the 1k circuit and it all works for me (at least nothing has broken that I am aware of), but I'd like to make something I could give to friends and family and not worry about it frying their electronics.

    My ultimate goal is to create a small but powerful mono speaker thats easy to carry around.
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  • #2
    Re: Stereo to mono summing circuit?

    i run an inverter on left channel.
    For stereo hook the left speaker reverse polarity.
    For mono (or single VC subwoofer) then connect right to positive terminal and left to negative.

    mathematically, Right - (-Left) = Right + Left

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    • #3
      Re: Stereo to mono summing circuit?

      Originally posted by moron#99 View Post
      i run an inverter on left channel.
      For stereo hook the left speaker reverse polarity.
      For mono (or single VC subwoofer) then connect right to positive terminal and left to negative.

      mathematically, Right - (-Left) = Right + Left
      His diagram is showing the input side not the speaker side.

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      • #4
        Re: Stereo to mono summing circuit?

        Originally posted by bpmoose View Post
        I've been looking for a safe way to combine a stereo signal into a single mono signal. I've heard it can be done by combining the signals with a 1k resistor between each input and the point they combine (http://brashleraudio.wordpress.com/2...o-conversions/) but I've also seen more complicated circuits.

        Anyone know a legit circuit for this, or have one they've used and trust? I've been doing the 1k circuit and it all works for me (at least nothing has broken that I am aware of), but I'd like to make something I could give to friends and family and not worry about it frying their electronics.

        My ultimate goal is to create a small but powerful mono speaker thats easy to carry around.
        That will work for most line level sources.
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        • #5
          Re: Stereo to mono summing circuit?

          Originally posted by killa View Post
          His diagram is showing the input side not the speaker side.
          yes. a line level op amp inverter. You can build opamp circuits for under $10 using wall adapters and radio shack parts. If you have a breadboard, no circuit board or soldering is required.
          By using an inverter you get three outputs from a stereo amplifier. If you have a mono amp then I guess it would have to be a summer circuit. you can turn the summer into a simple mixer by making the input resistors a 5K in series with a 100K variable (and adding more inputs). you can make it an inverter by having only one input

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          • #6
            Re: Stereo to mono summing circuit?

            Originally posted by moron#99 View Post
            yes. a line level op amp inverter. You can build opamp circuits for under $10 using wall adapters and radio shack parts. If you have a breadboard, no circuit board or soldering is required.
            By using an inverter you get three outputs from a stereo amplifier. If you have a mono amp then I guess it would have to be a summer circuit. you can turn the summer into a simple mixer by making the input resistors a 5K in series with a 100K variable (and adding more inputs). you can make it an inverter by having only one input

            [ATTACH=CONFIG]44054[/ATTACH]
            what type of op amp could be used in the above configuration? Is that necessary for line level inputs? Do you know if the original configuration I posted with only the 1k resistors could ever damage any components?
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            • #7
              Re: Stereo to mono summing circuit?

              I am told op amps are pretty much still the same. I'd hit the surplus circuits for the cheapest split rail power supply I could find and then pick an op amp designed for that voltage.

              opamp gain is determined by the two resistors, R1 and Rf. If you double them both, then nothing changes.

              On one hand, the higher the better. Your source would not be affected by tapping into it with 10gazillion ohm resistors.
              On the other hand, if you want to buy precision resistors or variable stuff then 100K(ish) is usually easy to find and cheap. Plus, anything over 10K is usually not noticed by your electronics.
              So ... use whatever fits.

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              • #8
                Re: Stereo to mono summing circuit?

                What's the difference between using two resistors to sum a stereo signal, then feeding that sum into an amp... and using two resistors to sum a stereo signal, then feeding that sum into an op-amp, and then into an amp?

                In this case, I really don't think there's any benefit to the active electronics unless you need gain above and beyond what the amp offers.

                I would, however, advise higher resistance values than 1k. 4.7k to 47k should be fine. 100k is probably overkill that won't do anything but add a touch of thermal noise.

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                • #9
                  Re: Stereo to mono summing circuit?

                  Originally posted by SirNickity View Post
                  What's the difference between using two resistors to sum a stereo signal, then feeding that sum into an amp... and using two resistors to sum a stereo signal, then feeding that sum into an op-amp, and then into an amp?

                  In this case, I really don't think there's any benefit to the active electronics unless you need gain above and beyond what the amp offers.

                  I would, however, advise higher resistance values than 1k. 4.7k to 47k should be fine. 100k is probably overkill that won't do anything but add a touch of thermal noise.
                  if V1 = V2 = 0.1V then the resistor output is still 0.1V
                  in an active summer, Vo = V1 + V2 = 0.2V

                  i have no idea which is musically more or less accurate.

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                  • #10
                    Re: Stereo to mono summing circuit?

                    I have found this reference also.
                    http://www.rane.com/note109.html
                    It recommends the following circuit, which seems similar to the op-amp circuit minus the op-amp.
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                    • #11
                      Re: Stereo to mono summing circuit?

                      As usual, the Rane Note is a well-written and thorough review of this topic, and brings up a good point about the resistor values chosen in the different approaches above:

                      With a passive summing box, you get to pick between good channel separation and the ability to drive long cables. My suggestion leans more toward better channel separation at the expense of high output impedance, while the schematic posted by bpmoose favors less channel separation, but lower loss over long cables.

                      To get the best of both worlds, you'll need to buffer both inputs with op-amps for better isolation, then sum them, and finally buffer the output with an op-amp for low output impedance. Of course, this introduces a new compromise: Complexity, cost, and higher noise (depending greatly on the quality of the op-amps and PSU.)

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                      • #12
                        Re: Stereo to mono summing circuit?

                        Originally posted by SirNickity View Post
                        With a passive summing box, you get to pick between good channel separation and the ability to drive long cables. My suggestion leans more toward better channel separation at the expense of high output impedance, while the schematic posted by bpmoose favors less channel separation, but lower loss over long cables.
                        )
                        That is very interesting! what do you mean by 'better channel separation'?

                        It seems that you're saying if I only need to run a combined mono signal a few inches from the stereo input to a mono amplifier, two 10k resistors (or something between 4.7k and 47k), one on L and one on R signal, would be adequate? And there'd be no need for a resistor between the combined positive and ground signal?
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                        • #13
                          Re: Stereo to mono summing circuit?

                          Channel separation (as you probably understand) is the absence of bleed-over between L and R channels. If you take a stereo line-level output, combine that to a mono signal, and only use that for a subwoofer input, hey no harm done. However, let's say you're taking that stereo line out and splitting it two ways -- one goes to a stereo amp (to power your mains), and the other goes to the passive summing circuit, and then to your subwoofer... then having the mono sum resistors in place will contribute to cross-talk between the left and right channels as seen by the stereo amp's input.

                          So, if you use large-value resistors in the summing circuit, you reduce the amount of bleed between channels. In exchange, you increase the impedance of the summing circuit's output. Use the shortest (reasonable) cables you can to the input of the mono amp. (You're fine with a couple feet. Just don't run it across your house, and don't use this to run PA amps at the stage from the mixing desk in the rear of the venue.)

                          Conversely, if you use low-value resistors, you increase the bleed between channels (slightly less stereo width), but you can afford to increase the cable length after the summing box. IMO, if you have long enough cables that it's an issue, you should go active. And balanced.

                          As to why Rane uses a resistor to ground in addition to the ones on the L and R hot, I'm not 100% sure. Maybe just to ensure that the input to the mono amp is (weakly) grounded in the case that nothing is plugged in to the inputs of the summing box? An actual signal would override this weak ground, but in the absence it could help prevent noise.

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                          • #14
                            Re: Stereo to mono summing circuit?

                            ok, so I'm thinking for my purposes (creating a portable mono speaker), two large-value resistors will be fine. do you think 10k, 1/2w, 1% would be adequate?
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                            • #15
                              Re: Stereo to mono summing circuit?

                              I think that would be overkill. ;) Line level signals don't produce much current. A typical op-amp will be stressed by a load of 20mA. 1/4W or 1/8W resistors will be fine -- whichever you have at your disposal.

                              You don't need to be concerned with 1% tolerance either, unless that's just what you have. (It's about all I keep on-hand, since it doesn't save me any money to buy 5% parts.) If you were working on balanced signals, then it would be worth trying to match parts a little better.

                              10k is good.

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