adding a bypass cap works up to a point. you seem to have removed all the
ripple a cap could remove.
From the waveform, it sounds like a bad isolation issue: adding caps will
help but you'll need much more than nanofarads to get that slow ripple out.
Use bigger caps if you want to keep it to simple cap patching (something
like 1100uF or 2200uF is nice.
Personally, I prefer the capacitor array method to the simple addition of
single poles.
Power supply would go as such:
XFO - DIODE BRIDGE(tube rect for purists..)- Pi Filter (Shunt cap +choke +
shunt cap)
At equal main's frequency attenuation, the tradeoff is the filter's
inductance vs instant current draw. I usually lift off that limit by using
bigger chokes and a few (BIG) tank capacitors right after the pi filter (I
like arrays of 1100uF,2x2200u,3300u...you can make a nice little square with
1-sided pcb routing if you put 2 together...little footprint, nice instant
draw compensation).
You got a schematic somewhere of that PSU? Tweaking the pi filter should get
rid of the problem
GLM
"Serge Auckland" wrote in message
...
"Fred" wrote in message
...
"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.
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