"Fred" wrote in message
...

"Serge Auckland" wrote in message
...
"Fred" wrote in message
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"Ian Thompson-Bell" wrote in message
...
West wrote:
"Ian Thompson-Bell" wrote in message
...
As I have mentioned I am designing a 6AU6 based mic preamp. Thanks to
Patrick et al I now have hum free HT rails. However, I still find
some
50Hz hum and 10mS period PSU spikes in the output. By shorting the
grid
of the first stage these both disappear so that is where they are
getting in. By turning off the HT they also both disappear so it
seems
the source is the PSU. So the question is what should I do to stop
it. I
tried grounding the mains transformer metal clamp and this helped a
bit
but there is no connection of this transformer for a screen of any
kind.
I am also using fast recovery diodes in the HT supply because I have
read they produce less spike interference. Any help appreciated.

For a not very good picture of the waveform at the preamp output look
he

http://s103.photobucket.com/albums/m...6CFopnoise.jpg
Cheers

Ian

I would suspect the spikes are from switching transients from those
fast
recovery diodes (are they hex-freds?). Try a snubbing cap across each
diode
and I'll bet you a dollar to a cup of coffee that you will see a
difference.
Keeping my fingers crossed for you.

west



I tried a 0.1uF straight across the HT transformer secondary and that
did the trick.

Ian

A few years ago someone here asked if bypassing fast recovery rectifiers
like
hexfreds was necessary. Nelson Pass surprised everyone when he dropped
in
and responded with the advice to snub them with a .05 cap in series with
a 100
ohm 1/2W resistor. It just makes things even quieter, in a way that can
only be
appreciated when heard.

In a mic preamp, every possible source of noise needs to be attacked.
That's a
big part of what differentiates the $1000 per channel professional mic
pre's from
everything else. They don't always measure all that much better, but
you can
hear the difference in a heartbeat.

Fred

Sorry Fred, but if they sound better, then you should be able to measure
what the difference is. If you can't, then they don't.

S.

You can sometimes measure some differences with the right test equipment
(Audio Precision or a computer with the right (expensive) hardware &
software),
but not necessarily with your run of the mill THD+noise measurements. The
spectrum of the distortion and the noise is more different than the
amount,
so which spectrum is going to sound better? Measurements won't tell you
that.

But your ears will tell you, like I said, in a heartbeat.

Check with the guys over at RAP and see if they use measurements to tell
the difference between a great mic pre and a pretty good one (they don't;
they
use their ears, just like you and I do).

Fred

You may Fred, I prefer to put my trust in Audio Precision.

S.


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