[sdiy] phantom power: how to avoid electrolytics?

[Ingo Debus]

Czech Martin wrote:
 > As far as I remember, the output impedance of studio
 > mics is about 100-500 Ohm. The amplifier should be
 > in the same area (was this noise impedance matching?
 > could someone point out for my poor brain what the
 > optimisation idea was?)
 >
 > So the caps impedance should be way lower even @ 20Hz.
 > Assuming 200 Ohm, this gives about <= 160uF or so
 > as a ball-park figure. Quite a lot for foil cpacitors.
 > with 2000 Ohm impedance things would be manageable.

AFAIK the input impedance of typical microphone preamps is much
higher. I have an Analog Devices app circuit here (for the SSM-2017)
that uses a pair of 47uF caps and a pair of 10k resistors at the input.

Ingo