New Hum

[pjanda1]

I'm sad to return to the chip amp forum under such circumstances.

As some may recall, I've got a unsnubberized NI3875 BrianGT kit with a 25k pot and selector switch. I had some demons, but switching to the CarlosFM grounding scheme, adding LP filters on the inputs and cleaning up the wiring solved them. About a month ago, I began experiancing a whole new problem (no change precipitated this, one day it sat quietly, the next it didn't). I finally got annoyed enough to take a look today, and I have no idea what the problem is.

It hums (60hz) whenever I have a source connected, or even when the positive pin from a cable connected to a source touches the ground barrel of the RCA jack. The hum doesn't change regardless of volume pot position or the input selected. The source does not need to be turned on. Either channel or both connected will create the problem, but the left channel is slightly worse. The amp is still very quiet with either the inputs shorted, or no source connected.

I've poked around a bit and can find nothing amiss.

Any ideas?

thanks,

Paul

[poobah]

Could something outside the amp have changed?

A new lamp?

A light dimmer?

New toy?

These may not be "at fault"... they may be exposing a flaw that went unnoticed...

Loose wire?

[pjanda1]

I don't think I've got anything new. . .

If I think of something, I'll unplug it. I'm assuming it's a cold solder joint or something, but I can't find it by looking and jiggling things. I was hoping the specific symptoms would point to a problem area I could focus on. I don't really want to take it all apart and put it back together again.

pj

[Sherman]

It sounds suspiciously like a ground loop between your source(s) and the amp.

By any chance have you changed the outlets that your amp and sources are plugged into? Example- added a multi-outlet surge supressor or extension cord, or just changed which wall socket your amp is plugged into?

Also, Poobah could well be on to something by saying that the fault has been there but has not been rearing its ugly head until now. I purchased a new flourescent shop light to put over my electronics bench in the basement. Suddenly I couldn't use my FM tuner and one of my amps buzzed. I plugged the light into a different circuit and the buzzing stopped (still some FM interference though so moving my antenna helped that).

One last thing- don't give up! Things like this can be frustrating as heck. Just take it one thing at a time, check all your solder joints, check the way things are plugged into your outlets, check the connection between your input jacks and the amp and the jacks and your grounding point, etc.

Good luck!

[pjanda1]

This is annoying me to no end. I unplugged most everything in the house and it didn't help a thing. I've poked around the inside and I can't find anything even remotely suspicious. I know I should probably just start reheating joints, I was just hoping the symptoms might indicate the best place to start.

pj

[John Medvich]

pjanda1

I had/have a similar problem with a chip amp. I think that I have tracked the problem to a bad cap. One of the 1500uF caps took dive. Like your problem the hum was there all of the time, but got worse when hooked up to a source. I checked and double checked the grounding. I tried all of the suggestions on the web, but no fix was in order. I finally decieded to pull the amp apart and check all the pieces one by one. What I found was a bad cap. I just recieved the new caps today and since I had to work I was never able to put the new cap in, but if you think about it this is sound reasoning (no pun intended) as the cap sits on the v+ and pg+ pads. The cap should block any hum, but if it is not working properly, the hum comes through. If anybody thinks this is bad reasoning, please let me know.

John

[peranders]

The amp _has_ been silent and now hums?

If yes, if you turn down the volume and has nothing connected, does hum then?

If you have the volume turned down _and_ connects something, does it hum now?

If the answer is yes you will problably have a ground loop problem. If the hum is there with nothing connected the problem is within the amp.

[AndrewT]

Hi,
two questions and then guess where I am going to next.

Have you isolated the input RCAs from the chassis?

Have you isolated the safety earth from the audio ground?

[pjanda1]

Thanks for your help thus far.

Originally posted by John Medvich
pjanda1
I think that I have tracked the problem to a bad cap.

I finally decieded to pull the amp apart and check all the pieces one by one. John

A cap failure occured to me to. I'm hoping to really narrow it down before I commit to a rebuild. The last one was a pain. Also, would a cap failure cause the hum under such specific circumstances?

Originally posted by peranders
The amp _has_ been silent and now hums?

If yes, if you turn down the volume and has nothing connected, does hum then?

If you have the volume turned down _and_ connects something, does it hum now?

If the answer is yes you will problably have a ground loop problem. If the hum is there with nothing connected the problem is within the amp.

No, Yes

It seems like a ground loop to me too. The problem is that nothing in the system changed to create it. In addition, no other component in the system is grounded. When I get home tonight I'll spend more time unplugging various components and things.

Originally posted by AndrewT
Hi,
two questions and then guess where I am going to next.

Have you isolated the input RCAs from the chassis?

Have you isolated the safety earth from the audio ground?

Yes, Yes. I will look the RCA's over again. I checked to make sure a connection wasn't touching the chassis, but I should look the isolation washers over again. The safety earth is connected directly to the chassis (no resistor as some us), but it wasn't a problem before.

[pjanda1]

I know the first post was a little rambling. To clarify:

Does not hum:

When no source is connected
Regardless of source selected
Regardless of volume control position

When no source is connected, I can hear background hiss only with my ear in the 95db sensitive whizzer. I can not discern any 60 or 120hz component in this noise.

Hums:

When a source is connected (or just the + pin touches the ground barrel of an RCA)
Regardless of source selected (if any source is conected to any input and any is selected)
Regardless of volume position (doesn't change whether all the way up or down)
Even if source is not powered up
Regardless of IC's used
When source is connected to either or both channels
Left channel hums louder (very slightly)

When a source is connected, the hum is clearly audible 10ft away in the listening chair.


Any other thoughts?

Paul