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Full disclosure: when it comes to electronics, I'm the type that knows enough to be dangerous. And that's kinda why I'm here - I took an electronics course in high school in 2005, but it's been a while so I've forgotten a lot...

I have two computers hooked up via a KVM switch, but I didn't like how audio was also exclusive when I switched inputs. I came across the Rane note "Why not Wye?" and built a balanced summing box so I could hear both computers at once.

Works great, except that when both computers are on, the output volume attenuates regardless of whether or not either line has sound. I notice this a lot, especially when one computer sleeps and thus cuts the audio and suddenly the music on my running PC is louder.

My circuit is the one on figure 4 of that article:

Balanced Summing Box schematic

Is there a way I can make sure the output volume is consistent regardless of whether or not both inputs are present?

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  • \$\begingroup\$ You will need amplification to keep the volume the same. See op-amp summing amplifier. \$\endgroup\$
    – skvery
    Commented Feb 8, 2017 at 20:06

3 Answers 3

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You can get most of the way to what you want with resistors, as long as you have enough spare level at the source, or gain following.

At the moment, your input resistors are small, and the load resistance is high. This means that when one computer is low impedance, it is responsible for all the attenuation of the signal, with equal input resistors, that's bout 6dB. When it goes high impedance, the output level jumps up 6dB, which you find objectionable.

If the 'wye' had more attenuation built in, say 10k resistors to replace the 470s, and a 2k instead of the 20k, you would lose a lot of level. This is why this method will only work if you have some gain in hand. The attenuation to the signal when the other computer is on is due to the 10k resistor and the 2k, about 1.7k. When the other computer goes high impedance, that attenuating resistor becomes 2k, and there is only a 1.5dB change in level.

The smaller you make the resistor that replaces the 2k, the less the level will change, but the more level you'll lose.

What a proper summing amplifier does is make that 2k resistor position effectively a short circuit, it presents a virtual ground to the summing junction, which totally isolates one signal source from the other, and provides gain so you don't lose level. It's the better solution, but it requires amplifier and power source.

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The resistor summing circuit that you are using is a lossey circuit. What that means is that the resistors consume some power and end up reducing the net amount of voltage that can be delivered to the load.

When both computers are ON some signal from one computer can actually feed back into the resistors and output of the other computer. This is what causes the sound level decrease that you noticed. When the second computer is OFF or has gone to sleep the audio output driver from the chipset inside the computer goes to a high impedance and thus removes the extra current path back into the output.

One simple thing you can try is to add a capacitor in series with the series summing resistor on the signal output from both computers. This will block the DC current path and eliminate some of the signal level shifting. You could start by trying a 1uF non-polarized capacitor. (Note that a capacitor will be required on both the left and right signal paths of each input - so for your circuit total of 4 capacitors).

Another choice is to build or buy an audio mixer device that buffers the two audio outputs through opamp or transistor gain stages.

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  1. Buffer the inputs individually before mixing them together.
  2. Use a jack that would short the summing resistor to ground if not used.
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