New Hum

[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.

 

[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

 

[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.

 

[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.

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.

 

[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

 

[AndrewT]

Hi,

star ground on the PS

in the words of John Mac "you can't be serious".

 

[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/showthread.php?postid=645710#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

 

[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.