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I'm working on an audio project and was planning to power it off of a +/- 12V wall-wart power supply, of the type commonly used to power modular synthesis racks. I've noticed that these supplies only have positive and negative voltage connections, and no connection for common mode or ground.

If I am using this type of power supply, what do I use as my ground connection?

Specifically the issue I am concerned about is that I am using a step-down regulator (https://cdn-shop.adafruit.com/datasheets/tsr1.pdf) to power a microcontroller I am using in the project, and want to make sure this is properly grounded.

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  • \$\begingroup\$ That's a little hard to answer.. Is that wallwart DC or AC out?, Do you need +12V and -12V rails. Do you want the logic biased at zero volts or at -12V, what else is going on in this box, do you need a virtual ground for other reasons... etc. \$\endgroup\$ – Trevor_G Apr 10 '17 at 19:54
  • \$\begingroup\$ You need a negative voltage regulator. There are some which also transform +12V into -12V or similar, but if you want to draw power from such a circuit, it's simpler to begin with a transformer with middle tap. \$\endgroup\$ – Janka Apr 10 '17 at 19:55
  • \$\begingroup\$ I have tried using a negative voltage regulator (LMC7660), but I was having problems with the switching noise getting into the audio path. When I power the preamp using the LMC7660 I get a very audible high frequency tone on the output. \$\endgroup\$ – Emmett P Apr 11 '17 at 15:44
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Short Answer:

Because the input range of the switching regulator is up to 36V, you can use the (-) terminal as your "ground."

Longer Answer:

The wall-wart (or whatever PSU you use) likely has an internal transformer, which isolates the system from the mains power. As a result, there is no official ground, and the voltage on the secondary side (available to you) is the direct result of whatever AC component is going through the primary. Without a center tap, you don't really have a ±12VDC PSU. Instead, you've got a 24VDC PSU where you can define an arbitrary ground point. You could make a simply voltage divider between the two rails and call that a ground. It would work, but it would be a relatively high impedance (or a high power loss) reference.

Instead, if you insist on a unit without a ground reference, just use the (-) terminal as your ground. The SMPS will work fine, albeit less efficiently, because it's still within the input range. This will mean, however, that you can't get negative voltages from that PSU because you have defined the (-) rail to be your ground.

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  • \$\begingroup\$ Okay that makes sense. Thanks for your reply. If I want to create a virtual ground at zero volts is there another method I can use which is lower impedance? \$\endgroup\$ – Emmett P Apr 12 '17 at 3:14
  • \$\begingroup\$ Because you only have a (+) and (-) terminal available, call one the virtual ground and leave it at that. That gives you +24VDC or -24VDC available to use. Otherwise, you'll want different equipment. \$\endgroup\$ – Hari Ganti Apr 12 '17 at 20:18
  • \$\begingroup\$ Alternatively, you can use an op-amp to create a buffered virtual ground at the halfway point. You'd use a voltage divider to set output voltage and you'd have a low-power consumption virtual ground, but with lower impedance (up to the op-amp limit). \$\endgroup\$ – Hari Ganti Apr 16 '17 at 18:50

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