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This seems like it should be a really easy one, but I'm having trouble finding a clear reference or explanation for it.

According to Wikipedia, line level has a peak around 0.5V, but I can't tell if that's GND to 0.5V, or -0.5V to 0.5V, or something else entirely. Which is it? (EDIT: Nevermind, it's now clear that it's the second case. Here's a scope capture linked from the Wikipedia page.)

The reason is that for switching ICs (mux, pot, etc), you need to have a V+..V- which exceeds the range of the analog signal, eg from MCP42XXX datasheet p15:

For linear operation, the analog input and output signals must be in the range of VSS to VDD for the potentiometer...

Since the line level audio goes below GND, I assume I need a charge pump or virtual ground arrangement to use an IC such as the MCP42XXX. Is this so? Which is the recommended approach?


Edit: I discovered the answer to the first part of my question, but the second part remains relevant.

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You don't need a charge pump for this. Charge pumps are used to move energy, here you're only interested in a signal (voltage). BTW, a charge pump would leave nothing left of your signal. Adding a voltage can be as simple as a resistive adder, or, more sophisticated, you can use an opamp summing amplifier.

enter image description here

The advantage of the opamp circuit is that, because of the virtual ground, input signals (your audio signal and the offset voltage) don't influence each other.

Note: if you want to use the digital potmeter to create a volume control, that's not a good idea. Volume control potmeters are logarithmic, this one is linear. For a volume control the CS3310 has excellent specs, like click-free switching, though it's not cheap.

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  • \$\begingroup\$ Okay, this is really helpful, thanks. Is there any other reason (apart from it being linear) not to use a simple digital pot as a volume control? If I don't need a lot of resolution in the end application, can I just do the log conversion in software? (especially if I put a 1M and 5K unit in series) That might not be cheaper than an $8 IC, but I just want to consider my options. \$\endgroup\$ – mikepurvis Apr 14 '12 at 15:27
  • \$\begingroup\$ @mikepurvis - I guess you can approximate a logarithmic function with the two in series, though I think you'll end up with a lot less usable steps. Have you looked for a logarithmic potmeter, like this one. You'll find that they also have a limited number of steps. Another possibility is to make a linear pot more or less logarithmic by adding a resistor between the wiper and one end. \$\endgroup\$ – stevenvh Apr 14 '12 at 15:36

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