Ground loop misery

[Filterlab]

:(

I nabbed an Audiolab 8000A from the 'Bay a couple of days ago. It was delivered yesterday and it's a cracking example, needed a clean up of the connections but other than that it's superb. The idea is to get it modded (to the same spec as my 8000P) and use it to power my Revel C52 centre.

Anyway, wanted to give it a trial run last night, so hooked it up to my processor's centre pre-out and bzzzzzzzzzzzzzzzz, major humming. So, I disconnected it and moved the amp to a different rack, a different mains socket and changed the interconnect. Same thing again. So I tried it with my DAC, used the same input and speaker; perfect. Popped the processor centre pre-out back in and bzzzzzzzzzzzz. So, I tried the centre pre-out fed into my 8000P and perfect. Seems like it's an issue solely between the centre pre-out on the processor and the 8000A - very weird.

I noticed that both the 8000A and the processor have a grounding point (which I've never had to use on anything) so I hooked them together expecting it to make a difference and it did - it made it worse! Other than using the 8000A for my front channels / 2 channel and the 8000P for the centre (something I don't want to do), I can't see how I can sort it out.

So all you electronic engineers out there, what things should I try please? Thanks.

[Filterlab]

A diagram may help.

When the centre (C) goes to either the 8000P or the Tag 60i, there's no hum.

When the L & R or the RL & RR go to the 8000A, there's no hum.

When the C goes to the 8000A, there's loads of hum!

http://i192.photobucket.com/albums/z121/Filterlab/Layout.jpg

[Dave Cawley]

Have you tried the 8000A on anything else?

Dave

[Filterlab]

Yep, directly from my Beresford DAC (perfect) and from the L&R and RL&RR outputs (perfect again).

[Themis]

Try to connect a (simple) cable between the RCA ground left (the outer part of the RCA) and the RCA ground center of the processor outputs, while having the 8000A connected to the center pre-out.
Does this stop the hum ?

[Dave Cawley]

And try the 800A on a diffrent output?

Dave

[Filterlab]

Try to connect a (simple) cable between the RCA ground left (the outer part of the RCA) and the RCA ground center of the processor outputs, while having the 8000A connected to the center pre-out.
Does this stop the hum ?

I'll give that a whirl when I get home, cheers.


And try the 800A on a diffrent output?

Dave

I've tried the 8000A on all the inputs (except phono obviously) and all four speaker outputs, I've also tried the power amp section directly too. Always the same; hum from the centre pre-out but perfect with any other output / source component. The oddest thing for me is that when the centre pre-out is connected to my 8000P, there's no hum at all. It seems to be only when the centre pre-out goes to the 8000A.

My first port of call will be checking the mains plugs on both components, would a loose earth connection in the plug cause a ground loop?

Sorry for the seemingly daft questions, but this isn't something I've much experience with. :)

[Filterlab]

I've just thought; when I tried the centre pre-out on my 8000P, the other channel was still hooked up to the right pre-out, which, if Dimitri's theory is correct, would close the grounding and stop any hum.

Hmmmm, this'll be fun to rectify.

[Ali Tait]

Earth lift resistor and cap in the amp?

[Filterlab]

Earth lift resistor and cap in the amp?

Would an issue like that cause an intermittent problem or a problem all the time?

With other sources it's fine, it's just this one connection that's causing an issue.

[Ali Tait]

Sorry Rob,I meant fit an earth lift resistor and cap in the amp if it doesn't already have them.

[Filterlab]

Sorry Rob,I meant fit an earth lift resistor and cap in the amp if it doesn't already have them.

Oh I see, okey dokey. I'll check cables / common grounds etc first and if it yields no result I'll try that (after some instructions have been located ;)).

Cheers. :)

[Filterlab]

Now sorted. A reroute of the mains earth was in order. Now rerouted and sounding good, will sound better after modding of course. :)

[Ali Tait]

Nice one Rob,good you got it sorted.I have a similar problem.I have a WAD 5881 PP on loan.The amp is silent until I plug in the phonos from the pre2.Huuuummmmm.I haven't got a circuit for the power supply,but looking in the amp there doesn't appear to be an earth lift resistor and cap on the phono ground,so I guess I'll need to fit them.Hopefully will cure the hum.

[Filterlab]

Good luck with that mate, hum is so frustrating!

[Jonboy]

I had a bout of mains hum which was identified as a earth loop, so according to my man that can you only need to earth the pre amp (via the plug) the rest don't need to be earthed becuase the interconnect between each item of kit earths all the rest.

It worked for me anyway.

[Filterlab]

I didn't know that Jon, but as the interconnects are grounded to the chassis then I suppose it's obvious. I guess that's why connecting the 8000A to the processor caused the loop - the processor has one or two connections to say the least. :)

I ran my new earth to a spare pre-amp I have sitting on the rack (unused), I still won't use it but it's now plugged in to return the earth.

[Ali Tait]

The idea with the resistor is to separate the chassis and signal earths.At the moment they are just connected together in this amp.I know it can cure hum,but I'd not rely on an I/C to earth a chassis of an amp,especially a valve amp with it's high voltages!

[Filterlab]

Yes indeed, valve amps tend to be a little more endowed in the voltage area.

[The Grand Wazoo]

The next time you get an earth hum, play Little Wing from 'Hendrix in the West' really loud. It sounds fab - honest!

[Rare Bird]

Is it a british '8000A' as these had major problems with the Phono sockets, theres a guy on e-bay selling them, tells you more about the problem, half my connector were knacked when i had mine.

http://cgi.ebay.co.uk/AUDIOLAB-TAG-MCLAREN-GOLD-PLATED-QUAD-RCA-CONNECTORS_W0QQitemZ330385548937QQcmdZViewItemQQpt ZLH_DefaultDomain_0?hash=item4cec836689

[Filterlab]

It is a British 8000A, but all the connectors are fine. I've had a problems in the past but they are quite easy to swap. In this case it was just the earth.

[Primalsea]

Yeah, Dont go earthing your equipment via its interconnect grounds only! Its a potential hazard (pun intended). If there's a problem with your amp it will be grounded until you decide for a change and remove the IC and then the chassis will raise to mains voltage!

Some hifi were classed as double insulated, identified by a small logo of 2 squares, one inside the other. The idea here is that any part you can touch can never be at mains potential in the event of a fault. These are only earthed via the IC and should not cause an earth loop unless there is a problem inside such as a poor earth connection inside the chassis.

I had huge problems with earth loops on my active system that consisted of 3 power amps, a preamp, active crossover, CD player, TT, and phono stage. The problem was traced to the 3 power amps. I had to open them up and separate the signal grounds from the mains grounds with a 10 ohm resistor and 100n capacitor. Also had to do something with the speaker grounds but cant remember what.

I the past I have also inserted an earth breaker inside a mains plug which works really well. I wont give details as I have discovered that they may not strictly be legal but I'm not sure on this. IMO they are fine as long as your equipment is fused properly, I think the problem is that hardly anyone actually uses the correct rated fuses in their mains plugs. I'm fairly certain that many have put the wrong rated fuse in there equipment also!

[Ali Tait]

Yep good advice there Paul,even more so where valve amps are concerned.I have one that has 1000v volts floating around in it,so could be seriously lethal! Regarding my current poblem,in the amp the signal and chassis earths are tied together to a push-on crimp on the IEC socket.Someone had obviously had the same problem,as they had disconnected it,so the amp wasn't earthed at all.Yikes!

[Primalsea]

Its amazing what people do sometimes!!

[Filterlab]

Yeah, Dont go earthing your equipment via its interconnect grounds only!

I'm glad to say that I haven't done that.

[Ali Tait]

Its amazing what people do sometimes!!

Indeed,especially when you consider this is a WAD amp,so presumably was built by someone who knows what they're doing.