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Old 23rd June 2014, 07:11 PM   #1
ChiDave is offline ChiDave  United States
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Question Diodes instead of input selector (insane newbie question)?

Hi,
I'm planning on a preamp as my first DIY audio project. Is it workable to put diodes between all inputs and my volume control instead of a source selector? Obviously, mayhem will ensue if more than one source is on at the same time. That won't be a problem for me.
Other than source management, would this work? The diodes are needed to isolate the preamp from the components that aren't on/sending a signal, right?

Thanks,
Dave
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Old 23rd June 2014, 07:18 PM   #2
Elvee is offline Elvee  Belgium
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Diode gates were widely used in the sixties for almost any purpose, and are still in use today for SHF, but what you seem to envision is a sort of audio OR gate, and this wouldn't work: this kind of gate doesn't work with AC signals.
A diode gate requires a DC selection level to operate.
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Old 23rd June 2014, 07:25 PM   #3
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It could work if you put a coupling capacitor from each input to its diode and one from the diode to the volume control, and connect resistors to each diode such that you can forward and reverse bias it. You then keep all diodes except one reverse biased with a bias voltage that is well above twice the expected peak signal voltage. The diode that must conduct has to be forward biased at a current well above the expected peak signal current.

Distortion-wise, it will probably be inferior to just about any other solution, so I'm not sure why you would want this.

If you just want to add all input signals while ensuring that no signal current will flow from one source to the other, you could simply give each input a resistor to a virtual ground node (for example, the negative input of an op-amp connected as an inverting amplifier).
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