Small value cap across large filter caps?

[Axtman]

I have seen this on a few amplifiers. Why do they put a small value capacitor across the large value filter capacitors?

[Enzo]

The huge caps smoth ripple - low frequency noise. But they respond poorly to high frequency noise. The small caps however respond well to that, shunting noise off the rails.

[Axtman]

Thanks. Is this the same reason why I see small value caps across the individual four diodes (full wave rectifier) in high powered Fender tube amps?

[Enzo]

yes.

[Helmholtz]

For low distortion (and sometimes stability) at high output currents it is essential that filter caps have very low impedance at all audio frequencies.
Older ecaps often had increased impedance (composed of ESR and inductance) at HF.
Paralleling foil caps lowers HF impedance.

Modern ecaps suitable for SMPS application have very low impedance even at HF, so adding foil caps gives no benefit.

[tedmich]

Thanks. Is this the same reason why I see small value caps across the individual four diodes (full wave rectifier) in high powered Fender tube amps?

Those are snubbers like mentioned previously, more commonly I've seen a single cap across the AC inputs. The cap value is based on load and should be the right type of cap (there is no "'one sized fits all" snubber) they are chosen to deal with a specific type of noise thats encountered in a specific circuit. proof of this is the fact that they are quite rare now a days. Maybe our diodes are better?

There is a article with some decent math by Jim Hagerman on calculating their values, available by googling "calculating optimum snubbers hagerman"

[mikeydee77]

Good question Axtman thanks for asking...

I am not surprised by the answer but didn't really know for sure. It may help with a noisy WEM I am dealing with.

[Helmholtz]

Always waiting/hoping for inquiring questions .

[Enzo]

Why is there air?

Should I walk to school or carry my lunch?

[Helmholtz]

Why is there air?

Should I walk to school or carry my lunch?

Thanks for your questions.
The answers don't lie within my core competency.

One can always lead a good intent ad absurdum .

[mozz]

Often those snubbers are also need to stop noise from flowing back into the AC line. UL/CE Certification.

[eschertron]

Why is there air?

Air is to blow up footballs with. C'mon, you knew that

[stokes]

Why is there air?

Should I walk to school or carry my lunch?

Theres air because we cant breath underwater.Definately take the bus to school and carry your lunch.

[gui_tarzan]

Air is to blow up footballs with. C'mon, you knew that

Unless you're with the Patriots.

[Enzo]

He He He ...