.1 uF in B+ ...... what's it fer?

[mook]

I've seen a few amps (hi-fi and guitar) that have a .1uF somewhere in the B+ rail. Somtimes in parallel with another cap (say a 10uF), or perhaps at the negative end of a full-wave bridge recto, or sometimes when elevating DC voltage for heaters. Why use a .1uF?? what does this do?


Mook

[andrew_k]

Generally used to reduce noise. Bypassing rectifier diodes to reduce hash, ensuring high frequency noise in the power supply gets filtered (big electros aren't real great in the high frequencies)

[Gaz]

Or perhaps you're referring to the .1 caps sometimes seen in parallel with B+ filter caps to help with the relatively low ESR of electrolytic. The rule of thumb is 100 times smaller than the cap it's paralelling - a 22uf electrolytic/.22 poly, for example. I remember trying this in a guitar amp (rather than HI-FI), and I could not hear a difference, but perhaps I would have with cheaper electrolytic.

[imaradiostar]

I don't know if it helps with guitar or not but it's common to see a small film or ceramic cap near the power supply pins of an IC though their may be much larger caps further upstream. Usually they help with oscillation caused by the resistance of the PCB traces interacting with the IC.

I would imagine it would do similar things in a guitar amp, especially in a circuit that may already be close to oscillating on its own.

jamie