Rod Elliot P3A amp noises

[tktran]

Hello all,

I have built my first amp- the Rod Elliot P3A.

However there exists a small amount of hum. I
have yet to build a preamp so at the moment I have a
pot to control volume, and whilst the hum is soft it
seems to be loudest at min volume and softest at ~ 1/3
max volume. Culd it be the pot that's causing the 50Hz hum?

Is there a way to test the source of the hum? I have
access to multimeters and a CRO.

In other designs eg. commercial, the transformer is
usually situated at the far left/right of the case,
the PCB on the other side and an internal heatsink in
the middle, which also acts to shield the PCB/input
from the supply.

I made the mistake of putting the transformer at the
front centre of the case, and PCB at centre rear,
connected to an external heatsink. I did this because
I wanted the amplifier weight to be evenly distributed
(for carrying safety) however now there is nothing
shielding the transformer from the pcb.

Is it worth shielding the transformer? If so, how?
Or should I relocate the transformer?

Thanks.

[paulb]

Hum can be induced into your input wires. Try shorting the input right at the PCB to see if that helps. Although if it's loudest at minimum volume, it's doubtful this would help.
I used an external power supply for my P3A for exactly this reason; I had problems with a previous amp. With the previoius amp, I was able to move the input wires around to minimize the hum. Maybe you can do some wiring (input, power, or output) rearrangement to reduce your hum.

[Cradle22]

Hi!

But most of the time you got hum in an amp like P3A because of improper grounding.

Use a real star ground, with no connection to chassis (all connections to ground go to that star ground)

Earth the chassis via the earth of the mains cable, and then make a connection from star ground to chassis via a resistor (about 100 Ohms), bypassed by a capacitor (with mains voltage rating, better more), for example Wima MKP10 0.47 uF 400V AC.

Today I fired up the testing version of my P3A (both channels), in somewhat the same layout you used (but not in metal chassis, have yet to gather enough money for chassis... ), and no hum was audible!

Actually the first amp I build that worked from the start on (and seems to be real nice...)

You can also try using shielded cables from input connectors to amp.

Bye,

Arndt