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I am designing an audio circuit that operates at 5 volts. Is it reasonable to buy a wall adapter that outputs 5V, or should I buy a wall adapter that outputs a higher voltage and step the voltage down at the PCB with a linear or switching regulator?

The audio circuit operates at 5V to achieve a volume level. The audio signal source will come from a microcontroller regulated at 3.3V.

The wall adapter I am considering is medical grade; part # MDS-030AAC05 AB.

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  • \$\begingroup\$ I'd suggest it depends on the supply voltage sensitivity of your solution (I assume it's a power amplifier of some type). If you are not doing high fidelity, using a 5 Volt supply would seem quite adequate, especially since I doubt you are anywhere near the 3 A rating. \$\endgroup\$ Apr 4, 2017 at 16:47
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    \$\begingroup\$ What microcontroller are you using? Some microcontrollers have 5 volt supply rails. I think that would be better so that you don't have to worry about tripping over a chord or something. \$\endgroup\$
    – user103380
    Apr 4, 2017 at 16:48
  • \$\begingroup\$ The main thing is to have sufficient filtering. I don't trust most wall adapters to get the hum/noise level down to levels adequate for audio or other sensitive uses. \$\endgroup\$
    – Hot Licks
    Apr 4, 2017 at 22:18

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Yes you can, match the voltage of the rail and make sure that the total current of the circuit (and IC's) you are using is less than the current rating on the supply.

Realize each voltage source carries noise. Be aware that for analog circuits the PSRR rating will tell you how much noise will transfer from the power supply rails to your signal (this number is usually well over 80dB).

If you have a direct pathway to the rail (like a pull up resistor or a DC path to the power rail) then this could inject noise in directly from the rail.

Measure the noise on the power supply with a volt meter set on AC mode, this will tell you the RMS value of the ripple or noise, if its acceptable for your application then you can use it. If the noise is to high for your application, find a new supply, use an LC filter or a precision linear regulator.

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The main distinguishing factor of audio circuits is the low noise level that must be maintained. While powering a audio circuit from some wall wart will work, it may, and quite possibly will, result in unacceptably high noise.

Most wall warts today are switchers. The switching frequency is usually well above the audible range. However, depending on the control algorithm, there may be sub-harmonics in the audio range.

With a sufficiently strong and low impedance input signal, a little extra noise from the power supply might not matter.

You haven't said anything about the circuit you are powering. The circuit may have some power supply rejection. If the circuit is opamp-based, then this is probably true. However, active power supply rejection only works up to some frequency. Above that, the active circuit can't keep up with, and therefore compensate for, the power supply noise. The switching frequency of a common wall wart may well be above that range. You can't hear the 1 MHz or whatever switching frequency, but such high frequency noise inside a chip can mess up its normal operation on much lower frequencies. The noise could get AM-demodulated and added to your signal, for example.

All in all, I think it's a bad idea for "audio" without knowing something more specific about the input signal, output signal, and the circuit.

If your circuit contains a power stage, then the worst power supply noise could be from the changing current demand of the power stage. If this is not dealt with properly, then your circuit could oscillate.

It would be prudent to filter the supply to any sensitive front end circuit. You can use the raw unfiltered supply for a power stage. Note that sensitive front end circuitry generally requires little power current, so a few ferrite chip inductors followed by capacitors to ground won't get in the way.

Medical grade or not has no bearing on audio use. Medical grade means extra low leakage between power input and output, and possibly extra high isolation voltage. It has nothing to do with noise on the output.

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  • \$\begingroup\$ I've looked at many 5V USB chargers and the power quality varies greatly. 60 Hz ripple could be a problem for audio applications and that's something I'd worry about if you use a cheap power supply. I've seen 1V of ripple on a 5V output from low cost chargers. \$\endgroup\$ Apr 4, 2017 at 21:55
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It just happens that I tried to power an audio transmitter last night using a few separate USB adapters. Results varied but were never good, too much noise. I ended up picking one that was more or less tolerable. I think this can be improved by having a filter/conditioner but I didn't have time.

In summary, if quality matters, you should invest in building a simple power source for your audio that would be free of these problems.

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