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.


Graham

On Wed, 18 Apr 2007 05:21:44 +0100, 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.


Graham

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.

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

John

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

"Eeyore" wrote in message
...
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.


Graham


Hard to see with the picture quality but do you have full wave? (Xfrmr
secondaries may be out of balance.)
Try 1nf across the Xfrmr secondaries as well. John Larkins suggestions as to
rectifier selection is on the money.

"Daniel A. Thomas" wrote in message
...

"Eeyore" wrote in message
...
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.


Graham


Hard to see with the picture quality but do you have full wave? (Xfrmr
secondaries may be out of balance.)
Try 1nf across the Xfrmr secondaries as well. John Larkins suggestions as
to rectifier selection is on the money.

That should have been 10nf!
Dan

On Tue, 17 Apr 2007 21:50:43 -0700, John Larkin
Gave us:

use schottkies; they don't snap at all.

Proper response.

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

On Wed, 18 Apr 2007 00:08:58 -0700, MassiveProng
wrote:

On Tue, 17 Apr 2007 21:50:43 -0700, John Larkin
Gave us:

use schottkies; they don't snap at all.

Proper response.

Thank you kindly.

John

"Eeyore" wrote in message
...


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


Some of my experiences with SB diodes from my monitor repairing days might
be informative.

First you should note the ESD warnings most manufacturers put on SB diodes,
they are very sensitive to excessive reverse voltage. Some monitor
manufacturers used SB diodes to protect the gate in the SMPSU MOSFET as it
doesn't suffer junction storage delay if ringing drives it into forward
conduction and they reliably fail S/C if the PIV is exceeded, on some blown
PSUs in which the MOSFET channel had been destroyed the 1N5817 had
successfully protected the O/P of the UC3842 long enough for the fuse to
break.

My experience with snubbing SB diodes is probably not all that relevant,
some monitors were badly designed with underrun heaters - the cure was to
upgrade the heater rectifier to an SB type, as the SMPSU was invariably a
flyback type the peak reverse voltage was often many times the rectified
voltage, which made it harder to find SB diodes with adequate PIV. The cure
was to snub the flyback peak with a diode, capacitor and resistor, the diode
and capacitor forming an extra O/P with opposite polarity to the heater
supply and the resistor to load it and pull down the flyback peak.

As SB diodes seem somewhat less robust against excessive reverse voltage,
and fail S/C more reliably. It is important to consider carefully before
replacing regular diodes with SBs, what are the chances of spikes exceeding
the PIV? And what other damage will happen if the rectifiers fail S/C?