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The Case of the Disappearing Ground Loop

Mike Hanson

Mike Hanson

Trying to understand...
Just before I went on a short vacation, my main system suddenly developed (while playing) what sounded like a ground loop in both channels. Before doing that I noticed (for a few minutes) a slight bit of spitty noise from one channel, whereas background noise is usually virtually non-existent from this system.

I have a Pi connected via USB to the DAC, which is connected to my integrated with RCA cables. The amp is a Cary SLI-100 (tubes).

It has additional stereo RCA outs, which feed a GoldenEar SuperSub X. Those outputs are full range, connected to a secondary wiper on the amp's volume pot, and doesn't involve the tubes.

The hum was coming from my main speakers, and continued regardless of the amp's input selection. I can't recall whether it changed with volume changes.

I turned the amp off, then realized the hum was still coming from the subwoofer, until it automatically shutoff as well.

Upon returning from my trip, I endeavored to investigate, only to discover the suspected ground loop was gone. Everything sounded perfectly amazing again. :eek:

I've never experienced an intermittent ground loop before. This is my first tube amp, and I'm wondering if this is a symptom of a failing tube. The amp is mostly dual mono, so it seems odd that the hum was in both channels. It had been paying for a few hours when this occurred. And after turning off the amp, the sub (with its internal amp) was still humming.

Any thoughts?
 
I should mention that I purchased this amp used. One of the input tubes hissed a bit, so I replaced the pair with something better, after which it's sounded great. I suspect the seller put in some old input stage tubes when he sold it to me. Perhaps he did it with other tubes as well. If so, is that likely to exhibit a ground loop behavior, after being turned on for a while? (I'm really a tube noob.)
 
davidsrsb said:
More likely a dodgy earth connection or bad RCA socket. With the volume lowish, prod cables
Click to expand...
I suspected that, and if it returns, that's certainly what I'll do. It's just odd that it came and went like that, without touching anything.
 
Check all of the mains plugs and pull the sockets off the wall and check the wires on the back of them too.

One of the first things I did when we moved into this house was pull all the sockets and light switches to check the connections. I found a lot of lose wires. Partly because the guys throwing the wiring together are more concerned about speed than quality but also because they can come lose over time. I know this for a fact as just last week my wife said that the bedroom light switch crackled and when I checked it the live wire was totally lose. Just sitting in the hole. And I know for a fact I tightened that up twenty years ago.

If it's not been touched for a long time it's well worth checking your sockets. Remember that in a UK house with a ring circuit the electricity is going through connections in every socket on the ring.
 
Mr Pig said:
Check all of the mains plugs and pull the sockets off the wall and check the wires on the back of them too.

One of the first things I did when we moved into this house was pull all the sockets and light switches to check the connections. I found a lot of lose wires. Partly because the guys throwing the wiring together are more concerned about speed than quality but also because they can come lose over time. I know this for a fact as just last week my wife said that the bedroom light switch crackled and when I checked it the live wire was totally lose. Just sitting in the hole. And I know for a fact I tightened that up twenty years ago.

If it's not been touched for a long time it's well worth checking your sockets. Remember that in a UK house with a ring circuit the electricity is going through connections in every socket on the ring.
Click to expand...
Good suggestion, and I've also been planning to replace those receptacles with something better regardless.

BTW, I'm in Canada. :)
 
Test all your valves. if ye don't have a tester, get a mate or a shop to test them. Sounds like ye have a dying valve somewhere..especially with the "spitting" .:cool:
 
John said:
So this problem with your electrical infrastructure is revealed with a tube amp?
Click to expand...
No. My infrastructure is likely fine, but his suggestion of potential receptacle problems reminded me that I've been meaning to upgrade those. Kill two birds with one stone. ;)

FYI, the hum was coming from both the main speakers (connected to the integrated) and from the sub. Those are actually in two different plugs on two different walls, so there's a chance that there's a loop happening between them. The fact that the hum continued from the sub after turning off the integrated is intriguing.

The loop hum problem occurred only once, preceded by the slight spitty distortion in one channel. I had been listening for a long time that day, so it could theoretically be heat related.

I still have my money on a bad cable or connector, but all options are possible at this point.

But but for the moment it hasn't reoccurred, so I'm just pondering, determining mitigation strategies upon its likely return.
 
Is there anything else you had connected or plugged in before you went away, that is now not plugged in ? Just wondering could it be something else in the system
 
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