DC blocking capacitor value - DPA high voltage PS

[Handcrafted Tone]

Does anyone have a formula or a general rule for finding the capacitor value when blocking DC?

I built a high voltage power supply to power my DPA 4004 mics. 125VDC goes into the mic, and 54VDC is present at the output. I tried a 100u 100V cap there to block the DC on the output, but that only brought it down to 30VDC.

Any suggestions?

BTW, The PS works great. I plugged the mic output into a P.O.S. behringer mixer I keep around just for this purpose (abusing it with DC on the inputs). The PS was surprisingly quiet considering the transformer is close to the unbalanced output of the DPA mic.

If anyone wants to build this PS, let me know. I made the traces and drill holes big for an easy home etch.

 

[Guest]

Use resistors to ground on your outputs after capacitors. Your voltmeter has too high resistance.
Also, values of your capacitors seems to be high.
What output resistance has the amp in your mic? What resistance are you going to load it on?

 

[NewYorkDave]

You're reading it with a high input impedance DMM, aren't you? Try a tiedown resistor on the "cold" side of the cap.

Good rule of thumb for coupling cap voltage rating (space/budget permitting) is 1.5 or 2 X the normal operating voltage.

 

[Guest]

[quote author="NewYorkDave"]
Good rule of thumb for coupling cap voltage rating (space/budget permitting) is 1.5 or 2 X the normal operating voltage.[/quote]

I prefer max possible voltage during any possible processes such as overload or cold/dead tubes/transistors/whatever.

 

[Handcrafted Tone]

thanks for the replies, it makes sense now.

So far I can't find info on the mic's output impedance. I'm planning to use this PS to interface my HV mics with API type preamps I've built.

I've designed a transformer-less preamp that will plug the unbalanced output of the mic directly into a AD797, with THAT 1646 output. I'm not going to home etch that one.

 

[NewYorkDave]

yes, by "normal operating voltage" I meant the full value of B+. I should have been more clear.

As an example, for a tube preamp with a B+ of 300V, I'd usually go with a 400V or higher cap.