| 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 |
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| 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? |
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| 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 |
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| 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! |
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| 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 |
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| 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 |
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| 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. |
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| 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? |
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| pjanda1 |
Thanks for your help thus far.
| quote: | 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?
| quote: | 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.
| quote: | 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. |
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| 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 |
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| AndrewT |
Hi,
it sure sounds like an earth grounding loop, as said by others, but why should it change?
Check resistance between safety earth and audio ground.
and between safety earth and RCA ground.
Could the neutral and the earth in the mains plug top or socket outlet be swapped? But neutral should be isolated from all except live input. |
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| MikeBettinger |
| The ground to the input is missing. The input signal does not have a ground reference. |
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| AndrewT |
Hi,
the RCA ground must be attached to something.
When he touches the RCA ground he hears the hum. |
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| MikeBettinger |
| He should not get any sound touching ground if it is a proper part of the amps circuit. This just isoltates the problem to whatever path supplies the connection to the ground side of the jack. A check between the jack and the power supply ground will answer this. |
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| poobah |
Yep... sure sounds like something is floating.
:xeye: |
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| pjanda1 |
This is exactly what I was hoping to hear!! I will check with a DMM when I get home.
Paul |
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| pjanda1 |
As I was pulling out the DMM, I finally realized what has changed. We got cable television. I seem to be getting a ground loop, as the cable is grounded and connected to the TV, the TV is connected (by component video cables) to the disc player, the disc player is connected to the chip amp, and the chip amp is grounded. When I disconnect any link in that loop, the hum goes away. (when I unhook the cable, or the component video cables or the IC from the player)
Now, what do I do? If I insert a resistor between power ground and chassis ground will it solve this? Should I just detach the PG from the chassis ground?
Sorry it took me so long to figure this out,
Paul |
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| poobah |
Yes... it may fix the problem. Try a 100 Ohm resistor...
:) |
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| pjanda1 |
I'll get to Radio Shack when I can. I assume the 10W is preferred for safety reasons.
Paul |
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| poobah |
Yes but, don't spend a fortune... you could try it with a low wattage value first. Let's see if this fixes the problem first.
I don't think you really need a 10 watt either... unless you have heard something otherwise from someone that does NOT have powdered rhino horn speaker cables.
;)
EDIT: this resistor goes between the cicuit board ground and chassis ground. THE GROUND CONNECTION FROM THE POWER CORD TO THE CHASSIS MUST BE DIRECT. |
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| MikeBettinger |
Curious, the symptoms changed then. The fix doesn't resolve the hum when touching the input shield. But who's to argue with success.
Regards. |
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| pjanda1 |
How did the symptoms change? Refer back to my description, "touching the input shield" was with an RCA connected to a source. Either source in this case is attached to the cable TV. Now, I don't fully understand how the positive pin to the ground ring can be part of a ground loop. But then I don't know much. Let me know if there is something I'm overlooking.
Paul |
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| MikeBettinger |
If ground is truely ground than touching the shield at any point should not inject any signal into the amplification path.
I read your symptoms as: the amp quiet until the input cable was connected, and then it hummed; The amp was quiet with no source connected, but touching the input shield at this time would cause a hum. If this was the case my suggestion would be correct.
I guess I didn't read the symptom carefully enough. I should know better but it seemed straight forward at the time.
Anyway it's solved. Regards. |
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| AndrewT |
Hi,
these last few contributions have got me confused.
Can you summarise the problem and the solution?
I think there is still a hidden problem, two problems, is more likely.| quote: | | Now, what do I do? If I insert a resistor between power ground and chassis ground will it solve this? Should I just detach the PG from the chassis ground? |
Confusion reigns, I thought you had already confirmed that the safety earth was NOT connected to the audio ground. |
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| pjanda1 |
The 100 ohm resistor between the star ground on the PS and the chassis ground seems to have done the trick. Sorry it took me a bit to find the obvious problem.
thanks,
Paul |
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| AndrewT |
Hi,| quote: | | star ground on the PS |
in the words of John Mac "you can't be serious". |
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| pjanda1 |
I indeed did misspeak. I should have posted this link in the first post when I referred to the CarlosFM grounding scheme:
http://www.diyaudio.com/forums/show...5710#post645710
I placed the resistor between the power supply and the illustrated star (not star ground) on this drawing. I had initially tried the grounding scheme the boards were intended for (everything connects on the PS board), but I had a little hum. It wasn't too bad, but since I was taking the whole thing appart to address some RF issues, I changed to this grounding as well.
In any case, it is again quiet (no hum, just a little hiss with my ear in the whizzer) and sounding wonderful. Now if I only had some better looking knobs. . .
Paul |
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| classd4sure |
| Consider the possibility of breaking the ground loop where it began, you should be able to pick up a little "ground loop isolator" or whatever fancy name they have for a small signal transformer for pretty cheap. |
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