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Old 13th June 2011, 06:10 PM   #1
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Default Isolate an Audio Signal Quesion....

I built a backplane that has 4 individual connectors where in each connector, a module plugs in. Each module produces a different sound effect.

There is also a 5-way, 2-pole mechanical switch that routes the input signal and power to each of the modules. Therefore, only one module is powered up at a time with the appropriate input signal.

However, the output signal is sharing the same trace in the backplane to the cable which goes to an amplifier, since I thought if the other modules are not powered up, they would have no effect on the output.

Here is the problem, if I plug in more than one module into the backplane there is no output signal or it is extremely low. If I plug a single module in, it works great. My error, I think I inadvertently put all the the modules in parallel.

So, how do I isolated the output signal such that it does not try to go through the 3 unpowered modules?

I thought about maybe adding a diode to each of the outputs or someone mentioned perhaps using an audio trandformer on each of the output lines to block the output signal to the 3 unpowered modules.

Any suggestions will be great.
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Old 13th June 2011, 06:46 PM   #2
DF96 is offline DF96  England
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You need to design the modules so that they each have a high output impedance and just emit some current, or at least have a high impedance when switched off. Wire them in parallel (as you have done) and they won't interact.

Alternatively, make them have very low output impedance and wire them in series. Harder to do, would probably need transformers.
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Old 13th June 2011, 06:59 PM   #3
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You could also feed audio to all of the modules (in parallel) and put the switch on the output of the modules so that only one module is feeding the output.
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Old 13th June 2011, 07:01 PM   #4
macboy is offline macboy  Canada
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Do you really need to power down the unused modules? Try using the 2 pole switch to select the input and output signals, but leave all modules powered on.
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Old 13th June 2011, 07:16 PM   #5
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why not use an active summing node at the backplane and large resistors say 10k at the modules outputs?
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Old 13th June 2011, 08:48 PM   #6
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Quote:
Originally Posted by macboy View Post
Do you really need to power down the unused modules? Try using the 2 pole switch to select the input and output signals, but leave all modules powered on.
I thought about that, I would have to see what effect that has on the life of the 9V battery.
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Old 14th June 2011, 06:53 AM   #7
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Switch power and output using one rotary switch.

Gajanan Phadte
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Old 14th June 2011, 02:10 PM   #8
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Quote:
Originally Posted by gmphadte View Post
Switch power and output using one rotary switch.

Gajanan Phadte
So you mean to use 1 pole for power, the other pole for the outputs and the input signal to be common to all 4 modules?
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Old 14th June 2011, 05:51 PM   #9
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Here is a portion of the schematic where I connected the battery to the input. Is this a possibility since the battery is DC and the input AC?

Click the image to open in full size.
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