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I have a backplane where 4 individual modules plug into it and provide various sound effects. To control which module is powered and to route the input signal to it, there is a 2-pole, 5-way mechanical switch so the power and signal are sent to the desired module. However, the output signal from each module shares the same trace going to the amplifier.

This is a problem because when more than 1 module is plugged, in the output signal goes south. Individual modules work great, but with more than one module installed, not so good. I thought that since the other 3 modules were not powered they would have no effect on the active module, well, I was wrong.

So I was trying to think of a way to route the input signal and power to the appropriate module and separate the output signals so they are not in parallel. That led me to see if I can use one of the poles of the switch for both power and input signal and the other pole for the output signal.

Two other thoughts were to add a diode to each of the output traces or an audio transformer. I just don't know, I already have hardware built so I am stuck trying to make what I have to work.

Do you think this will work or have any suggestions about the idea of adding the power to the input signal?

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The cheapest solution would be to use a 3 pole 5 position switch and use the 3rd pole to switch the outputs. It is possible to combine the input and DC voltage into one line, but that brings up new complications. The input signal could be coupled to the DC with a capacitor, then extracted by using a second capacitor on the amp end. Problems to overcome with that would be power supply ripple would need to be very small, and you need to deal with impedance matching on both ends. A third solution could be to use individual relays at each amp module location. The switch would energize a 2 pole relay that which would switch the input & output.

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  • \$\begingroup\$ That would work except I have no more space for a larger switch. \$\endgroup\$
    – Joe C.
    Jun 15, 2011 at 19:57
  • \$\begingroup\$ Link for explanation of how that signal coupling would work? \$\endgroup\$
    – NickHalden
    Jun 16, 2011 at 15:11
  • \$\begingroup\$ Actually, I found a switch that would work - looking a little further into it now. \$\endgroup\$
    – Joe C.
    Jun 17, 2011 at 12:57
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I don't see why 9v power has anything to do with this, other than it's on the switch and backplane.

There are two reasons I see as to why the audio signal is going bad: 1. The ESD protection diodes on the output of each card is getting powered down and so clamping the audio to GND. 2. Something else on each card is loading the signal, causing it to have a low level.

The solution could be to make a passive audio mixer using some resistors. This is easy, since all you have to do is add a series resistor to the output of each card. The volume of this mixed audio will go down, dependent on the # of cards, but that could be compensated for by increasing the gain downstream. The value of the resistors really depends on what you have downstream, but something in the 500-10,000 ohm range is reasonable.

I should mention that if your problem is ESD protection diodes then the passive mixer could introduce some noise-- although for your application that may be acceptable. Again, it depends on your circuit.

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  • \$\begingroup\$ Thanks for your input, so it may be as simple as adding a single resistor in series to the output signal? \$\endgroup\$
    – Joe C.
    Jun 16, 2011 at 14:49
  • \$\begingroup\$ @Joe C. A single resistor per card, and bumping up the gain in whatever this is feeding. \$\endgroup\$
    – user3624
    Jun 16, 2011 at 15:48
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This thing is already in use, if I understood the question right:
http://en.wikipedia.org/wiki/Phantom_power

Lines going to audio will need at least a capacitor on the 'hot' line, preferably another one on the ground in case the connections gets reversed.

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Why do you need to mux the power and inputs at all? Why can't all the modules always be on creating their outputs, then you selectively mix or select the output you want. Your existing 2 pole 5 posisiton switch could still be used but with 1 pole left over.

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  • \$\begingroup\$ Yes, that is a possibility, I would need to see the effects this would have on the life of the 9V battery. I am not sure how to calculate the amount of power consumption of the circuits. I guess I could just try and time the battery life. \$\endgroup\$
    – Joe C.
    Jun 15, 2011 at 19:39
  • \$\begingroup\$ Then maybe you can use the existing switch for the power and output of each module and keep the inputs permanently wired. You'd have to make sure there is enough resistance in front of each module so that it won't load down the master input signal when the module is off. \$\endgroup\$
    – Olin Lathrop
    Jun 15, 2011 at 21:13
  • \$\begingroup\$ Yes, that is another approach Olin, and perhaps easy enough to implement, the traces of the inputs are already separate from the rest of the circuit. I would just have to cut the output traces on the backplane to break the parallelism and solder a wire to each of the connectors output terminals. I would post a schematic, but I don't have enough points yet. Thank so much for your help!!! \$\endgroup\$
    – Joe C.
    Jun 15, 2011 at 23:34

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