Can you see why this amp hums?

[paulspencer]

My chip amp works, but has quite a noisy hum. It is an ESP design. I built the power supply in a separate box as I thought it might make it all simpler as far as hum goes. It didn't seem to work that way.

I'm hoping that someone can point out something small that I've missed or misunderstood that will just fix it!

This is a prototype, although if I get rid of the hum, can't say if I will actually make a nice box as it's probably going to be hidden away in a cabinet.

Here it is:



Power supply input +, -, and GND are not connected here. Inputs are a simple and straight run. I've tried to keep it all as simple as it can be.

Now the underside:



As you can see, a simple and straight run from the inputs, using shielded interconnect cable. The shield is connected at both ends, not shown in this image.

Now the top side:



For the intputs, I've connected the shield to the GND on the PCB using a short piece of green hookup wire. The shield is connected at both ends of this input wire.

The power supply connections are not hooked up in the photo, but in use they were - +, -, and GND. -ve Speaker outputs were both connected to the GND of the power supply input. This "star earth" was then connected to the chasis firmly.

By my (limited) understanding of these things, it should be fairly quiet. It's not. The hum is quite loud and objectionable, such that normal dialogue level in a movie or TV isn't loud enough to drown it out. When I turn it up loud enough to not hear the hum, the amp is otherwise nice sounding.

Can anyone pinpoint the problem? Can anyone suggest what I might do to fix it?

[owen]

Easy.

The ground point on the PCB is not connected to the ground on the chassis, and they in turn are not grounded to a single point in the PSU

Ok?


Owen

[paulspencer]

Hi Owen and thanks for the response. I've shown it with the PS disconnected, but in use I had the ground pin connected to the chasis and to the ground in the PS. I probably should have connected it all before taking those photos, but I'm connecting it all up now.

[bobhayes]

Does it hum with no input connected?

Is the hum 50 or 100Hz?

I don't want to state the obvious, but is the chip amp meant to be running single ended (speaker - to ground)?

[paulspencer]

I'm onto it now and at the moment I'm not getting ANYTHING at all - no hum, no sound, no nothing!

To state the obvious? .... To me not much is obvious!

Regarding speaker output -ve to earth - I can't see any other way to connect it - there are no dedicated pins for it that I can find, and arn't all earth connections joined anyway?

[paulspencer]

Wooohoooo

problem solved!

*dances around room

Hum has gone compeletly, not sure what I did different this time. It's a pity that my other amp isn't working!

http://www.diyaudio.com/forums/showt...25#post1150125