In article , Jim Lesurf jcgl@st-
and.demon.co.uk writes
In article , Rich Wilson
wrote:


What I want to know is how exactly the RFI gets through all the
smoothing capacitors and so on in the rectifier. I've got a little
headphone amp here with enough capacitance to not notice, say, a
2-second cutout in its power supply, so I fail to see how any audible
frequency could get through.

Alas, if only it were that easy. :-)

Two examples of snags the designer should have dealt with:

1) Large electolytic caps (as normally used as reservoirs in a linear PSU)
will have self inductance and resistance that means they stop acting as a
capacitor - probably somewhere below 10MHz. Hence for RF 10MHz they may
do nowt.

2) The RF may enter via leads, then radiate around inside the box, neatly
bypassing any filtering. :-)

In each case the RF "might" then be rectified by a device inside the amp,
or combine nonlinearly to down-convert into the audible range.

Its almost always caused by unwanted demodulation, and that can also
cause the DC voltages to shift causing further upset!...


The point, though, is that the designer should know about such things, and
know how to deal with them. The normal result being no audible effects to
bother the user. This was hard many years ago, but devices and techniques
developed, so that even 20 years ago, it was largely tractable.

It was..


Slainte,

Jim


--
Tony Sayer