Hmmmmm, my chip amp hummms more than I do!

[paulspencer]

I built my first chip amp and it *works* except for the loud hum

It's based on ESP's P19 chip amp

I have done some searching but haven't yet found what appears to be the answer I'm looking for.

At first I did not use shielded cables - which cables should be shielded?

Should I shield the transformer as well?

I doubt the problem can be purely from lack of shielding, I suspect it is about ground loops. When I tested the amp, I ran a signal from a CD player with no earth on the mains plug and I used the headphone output so there is no separate preamp involved. Hence I'm not sure how I could get a ground loop at all icon_confused.gif

Can anyone suggest what it might be and how to solve?

Do I need a loop breaker circuit?

[aditaquil]

At first I did not use shielded cables - which cables should be shielded?

Yes you should! especialy for input.

Should I shield the transformer as well?

Depending location of trans, if to close, yes you must shield.
BTW check again cabling between PSU and amp, input stage and amp.

On my GC I used 25V IE trans 250W and 10.000uF cap on rail. I had hum free for my ear (day or night). My GC was P2P wiring, sorry I haven't digital cam so no pic of my GC. Soon i'll post it.

[jaudio]

Connect the ground point from r5and input ground together,then connect the two with a single wire(signal ground) to power ground. It possible that moving pin7 to signal ground might help


Is your amp in a case

[paulspencer]

I have no case for the amp yet. Wanted to get it working first, as I plan to build the case from sheet metal.

So I need to shield input wires. Any others?

[jaudio]

I also start by just working on the amp but when i put it in a case it is alway less hum,so maybe you'll want to put it in a metal case(just to test )

No just the input cable

Try braiding your supply wire

[jeff mai]

Originally posted by paulspencer
I built my first chip amp and it *works* except for the loud hum

My guess is that you've got a ground loop within the amp. My chip amp was very sensitive to how the ground returns were routed to the power supply.

The two channels need to return to exactly the same spot. When I first wired it up, I had one return wired to the earth of each bridge and the bridge earths wired together with about 2cm of heavy wire. BIG HUMMMM! I moved one return wire so the returns went to the same point - all silent.

This is a kit/pcb though isn't it? I'd double-check all my solder connections. A cold joint can cause this sort of thing, too.

[paulspencer]

Are you Jeff Mai from MAC who used to run the DIY group? If so we have met. I have been meaning to get back to MAC, is the DIY section running still?

[jaudio]

Paulspencer

Have you change anything yet?

[jeff mai]

Originally posted by paulspencer
Are you Jeff Mai from MAC who used to run the DIY group? If so we have met. I have been meaning to get back to MAC, is the DIY section running still?

Yes, I am he.

The DIY section is not running at the moment because no one has volunteered to lead the group this year. Pretty sad because it's a popular group with the members.

[paulspencer]

Originally posted by jaudio
Paulspencer

Have you change anything yet?

Not yet, I've got a lot of diy things happening, but its high up on my list. I'll post when I have got some more progress.

Originally posted by jeff mai


Yes, I am he.

The DIY section is not running at the moment because no one has volunteered to lead the group this year. Pretty sad because it's a popular group with the members.

That's a shame. My main interest in the club was meeting diyers. I organised a diy speaker event at my place a few months ago with a good turnout from this forum, and if I had been with the club a bit longer, I might have been up for running the diy section.