tube rectifiers, HexFreds, and the Big Deal

[Bill Layer]

Hi Larry, All,

>I was having trouble following the logic that a linear power supply using
>solid state diodes would cause distortion of this type.  Then the obvious
>hit me like a brick --  the diode does not start conducting until the
>voltage magnitude of the sine wave overcomes the voltage required to cause
>the diode to connect.  So, at each zero crossing, you have a shut off
>period on both sides where neither diode is connecting.  

This is true, but the zero-crossing is not always the ugliest hour for the
SS rectfier. Imagine a simple single-wave rectified supply, with a single
diode feeding a single filter cap. Assume that this supply is unloaded, the
voltage across the capacitor now rises to the peak of the AC waveform,
about 1.414 x RMS. Assuming that no current is drawn (not the case, for a
real cap) the voltage remains constant AND the diode never conducts again.

But when current is drawn, something else ocurrs. The voltage across the
cap falls, and now the diode wants to conduct, BUT ONLY at the peak of the
waveform. Now, the filter cap tends oppose that change in voltage (the
bigger the cap - the bigger the opposition... read on), and when that diode
does switch on - look out. The AC waveform is at the near peak of it's
power cycle, and when the diode switches on, the instantanious current is
large. If the supply voltage now falls to 1.2 X peak, this means that for
over 10% of the waveform, the transformer and rectifier are pushing against
a brick wall.

To an extent, this also relates to the fact that given two otherwise
identical power supplies, more current may be drawn from an inductive 1st
filter than a capacative 1st filter design. Peak currents are lower in the
inductive filter. 

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.

Any thoughts?





 

 
Bill Layer
Sales Technician
<b.layer at vikingelectronics.com>

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