Noise suppression bypass caps on Si bridge

[Steerpike]

Yo all,
4 small caps to bypass each of the 4 diodes in a bridge rectifier...
some commercial amplifiers have them, some (even costly high end ones) don't.
Do they make any audible difference?

(my particular application would be a 300VA transformer, and 2x 22000uF, for a 150W mono amp)

[Mooly]

They can indeed - a lot depends on other factors as well such as wiring and layout. I first came across this effect years ago with an old AM/FM Hitachi radio. It always buzzed on mains (through the speaker) , quite a harsh buzz really, and it was this very problem. As the diodes come into an out of conduction they can emit a "burst" of RF energy, I always call it comutation noise but that's not strictly correct. It's an interaction between the diodes switching and the inductance of the transformer windings. Even putting a finger on the bridge stopped it and in this case a single 0.01 across the secondary was a complete cure. A lot of folk's use a series resistor and cap as a snubber rather than a direct cap across the windings.
Yes is the answer, I used four 0.1Mfd/250 Vac caps ( Do watch the rating) in my amp.

[Steerpike]

Ah, well thanks for that Mooly. I do wonder why they are so seldom used then, even on quite costly equipment.
Mostly I see them on older equipment - maybe modern hi-fi doesn't include AM receivers as much as it used to.
I'm too lazy to redo the PCBs I just completed - so I'll be soldering the caps on the foil side - and hope no-one looks!