[isw]

In article ,
"Mogens V." wrote:

Too many of my (guitar) rack devices use simple 78/79 regulators.

I've seen some with a transistor supposed to shunt noise to ground, but
seems more like a phase inverter, depending on accurate resistors to
match the emitter resistance. Seemed to also depend on how many, and
size of, capacitors dropped around the board.
Another one used two high speed swithing transistors.

Would be nice merely replacing the 78/79 and lifting the output leg to
hook in a filter.

Anyone have links to simple add-on filters, so I don't have to invent a
commen wheel?

Those are actually pretty good regulators, and the problem may be
somewhere else. I'd suggest you go back and check out the way the power
circuitry is wired. It's very common for that to get done wrong, and
cause all sorts of hum problems.

No current flows from the transformer during most of the power cycle.
Only when the voltage there is greater than the voltage in the filter
caps will any current flow at all, and when it does flow, the capacitor
looks like a short circuit and the current can be very high. The voltage
drops from those high currents, if not isolated, can couple directly
into the output of the regulator -- and even a perfect regulator would
not help because the hum has cleverly bypassed it entirely.
Specifically, the resistance of even a short wire can be sufficient to
cause problems. Also, there can be large magnetic fields due to the
large currents, and those must be isolated too.

The wiring should be like this (in order):

From the transformer to the diodes; from the diodes to the terminals of
the filter capacitor; from the capacitor terminals to the input and
common of the regulator. Do not connect the regulator common to any
other place than the appropriate terminal of the capacitor --
specifically not to any other place on the common lead, say nearer the
diodes or transformer. To minimize the magnetic field problem, use
twisted pair -- from the transformer to the diodes, and from there to
the terminals of the capacitor.

Take the power from the output and common of the regulator to the
circuitry. Make no ground connections anywhere else. The common pin of
the regulator should be the ONLY place the input common and output
common are connected. It should be a "zero-length" lead.

Isaac