Nick Gorham wrote:
Ian Thompson-Bell wrote:
Nick Gorham wrote:

Ian Thompson-Bell wrote:

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


Maybe some snubbers or LCR filtering on the secondaries.


Yes, I was thinking maybe small series inductors for each diode - is
that the sort of thing?


There is a nice article in one of the old Sound Practice issues, that
describes using tuned parallel LCR blocks in series with the TX out to
the bridge (both leads), and the a series LCR across the output of the
bridge. In both cases tuned to the TX's resonant frequency. I guess you
can work out the details


Is it available on line?

Looking at the pulses on a scope they do look like a decaying sinusoid
with aperiod of about 20uS which I guess implies the TX resonant
frequency is around 50kHz.


Are you using DC heaters? you may find the spikes are from the heater
supply


Definitely not heaters - at present they are supplied by a battery.


Ok, I ask, as I have had cases where B+ switching pulses have been
coupled back via the heater supply.


That's a good point. Although the heaters are dc their source impedance
might not be very low - I'll try some decoupling to see if that helps -
that said if you turn off the HT the spikes go and there's enough energy
in the 470uF HT caps to operate the circuit for over a minute.

Thanks for the input.

Ian