tube rectifiers, HexFreds, and the Big Deal

[Tony Allgood]

Bill said;
What this means to me, is that the guidelines for filter cap size in
power
design may need to be re-thougt, on the basis of 'bigger is not better'.
How would you like to find out that that ultra-quiet 10X overfiltered
supply you built is doing more harm to the overall sound of your system,
than good for the device it is powering! Obviously the extreme case, but
the logical end nonetheless.


Hi,

Put a large inductor in the line to the capacitor. The current surges
are minimised... but the voltage across the filter cap will now be the
average value of the rectified sine ( not the peak) but you do need a
minimum draw for this to happen. The calculations are quite easy, I
don't have to hand at the moment, but the one big draw back is that the
inductor has to be physically large for 50/60Hz systems, almost as big
as the transformer in fact. However, you do get 2pole filtering as well
as a good power factor. Interesting the current taken from the supply is
actually a square wave, the max value at the RMS current drawn by the
load. However, this is significantly better than the seven times RMS
current spikes you get from a zero inductance filter. Sometimes you will
notice this effect from the inductance of the power transformer, but
this normally is considered a problem because of supply regulation. What
you really need is a mains supply running at 44KHz, then you could make
your caps and inductors really small... somehow I don't think it will
ever catch on. :-)

Regards,

Tony Allgood, Cumbria, UK

e-mail: oakley at techrepairs.freeserve.co.uk

Rack mounted Moog VCF module. Details to be found at...

http://aupe.phys.andrews.edu/diy_archive/schematics/effects/filter.html