[sdiy] Module power - regulated or filtered (passively)?

[Neil Johnson]

Hi,

Spiros wrote:
> Opamps generally don't require much in terms of supply filtering because they have power supply rejection (called PSRR- power supply rejection ratio).

The PSRR of opamps is pretty good at DC to a couple of hundred Hz, but
after that it drops at a steady 20dB/decade.  For example, the TL072
PSRR curve is estimated here:
http://e2e.ti.com/support/amplifiers/f/14/t/648449
At 10kHz the lowly TL072 has a PSRR of about 45dB.  Maybe not a
problem if your power rails come from a linear supply, but a lot of
high frequency noise (switcher PSU, LED switching, sawtooth reset
pulses, etc) will pass right through the power pins into the audio
path.

Also note that quite often the PSRR of the negative power pin is
different to that of the positive power pin.  For example, the OPA2134
(pretty good audio op-amp) has a difference of about 35dB between the
PSRRs of the two power pins.

Cheers,
Neil