On Wed, 18 Apr 2007 06:09:30 +0100, Eeyore
wrote:



John Larkin wrote:

Eeyore wrote:

I'm helping to sort out a piece of alleged studio recording gear that was
somewhat thrown together by a bunch of useless idiots whose company has now
ceased trading. It's a quasi 'replica' of some historic equipment that once
belonged to Air Studios in Monserrat.

The latest joy (of many!) is pickup from the PSU. I was concerned about these
spikes (see pics) that the mic amp delights in reproducing. I've never seen
anything as bad as this before but I don't use mic transformers myself and this
unit does, which appears to be a problem.

The ac line frequency rectifiers are 1N5404s. I simply tacked 0.1uF across them
to see if it helped at all but it didn't. I have some UF4004s I could use
instead of them (their 1 A rating is fine in this application) but I'm open to
other advice.

The top trace is the output of the mic amp on max gain (70dB).
The bottom trace is the ripple voltage on the (half wave) rectified positive
supply (only on 2nd pic).

I can't figure why the spikes seem to be displaced but it's that rotten TDS210
again and I have next to zero faith in it. Sorry about the pic quality btw, I
didn't take them.


Try either fast-recovery or soft-recovery diodes. Or lower voltage
rectifiers. The higher-voltage 1N4004+ and 1N5404+ series parts are
actually p-i-n structures that like to behave like step-recovery
diodes. The bitch is that they will make a wafer of, say, 1N4007's and
sell them as 4001's, 4002's, and so on, so they may all be pin's.

I see. But it's the higher voltage process that's to blame ?


Or use schottkies; they don't snap at all.

Yes I found the 11DQ10 and MBR1100 which could do the job. Vrrm needs to be ~70V.
Would schottkies still benefit from snubbing ?

Graham


It wouldn't hurt. It wouldn't make much difference to emi, but some
schottkies are fragile in the reverse direction, so some 0.1 uF or so
caps could help protect them from line transients. People are making
silicon schottkies up to about 150 volts (IR for example), and up to
about 600 volts in SiC (Infineon and Cree.)

There are soft-recovery rectifiers, too, designed to not snap.

John