Eeyore wrote in
:
Raymond Koonce wrote:
Check this out. Ebay 130010957907
Hilarious. Even more hilarious that anyone would think it makes any
difference to a preamp's power supply.
What makes a difference is using solid state rectifiers without snubbers.
Rectifier switching combined with transformer secondary leakage inductance
and interwinding capacitance makes for a nice LC oscillator. Though the DC
filters after the rectifiers would remove it from being conducted to the
load directly, the ringing gets induced capacitively and by radio into
neighboring circuits if the PSU is not shielded from the rest of the amp.
It's at several hundred kHz so out of the audio band, but it's modulated by
the mains frequency. On a scope, I can see the ringing easily near the
extremes of peaks across the secondary, even using soft recovery fast
diodes or schottkies, and an RF probe held nearby also picks it up. Tube
rectifiers never have this problem. For solid state rectifiers, an RC
snubber across the secondary (rather than C's across the rectifiers as one
commonly sees) is the correct solution, and calculations for optimal values
can be seen in
http://www.hagtech.com/pdf/snubber.pdf (measurement of the
transformer parasitics is difficult so I use trial and error and the scope,
though it can be a bit hard to zoom in on the ringing at the AC peaks).