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Leak Stereo 20 versus current valve amplifiers?

If it's a normal interconnect (earthed at both ends... as they should be) then it's nothing to do with the interconnect. FWIW I sometimes have to lift the earth on my own St20 to get rid of hum, depending on what it's connected to and whether or not an earth loop is formed etc.... at my own risk (slight as it is). To do it at YOUR own risk simply open the mains plug and disconnect the earth wire then reassemble the plug so the earth wire is hanging out and the plug top clamps it there... or insulate it using insulating tape... but make damn sure the earth can't move about and touch the live!! Usually one of the two conditions (earthed/not earthed) will be silent in any circumstance of particular other items of equipment in use. A St20 in proper working order is silent in use.
 
If it's a normal interconnect (earthed at both ends... as they should be) then it's nothing to do with the interconnect. FWIW I sometimes have to lift the earth on my own St20 to get rid of hum, depending on what it's connected to and whether or not an earth loop is formed etc.... at my own risk (slight as it is). To do it at YOUR own risk simply open the mains plug and disconnect the earth wire then reassemble the plug so the earth wire is hanging out and the plug top clamps it there... or insulate it using insulating tape... but make damn sure the earth can't move about and touch the live!! Usually one of the two conditions (earthed/not earthed) will be silent in any circumstance of particular other items of equipment in use. A St20 in proper working order is silent in use.

Thanks for this Jez, now that you mention it my friend’s Leak had a yellow/green earth wire sticking out of the bulgin plug when I visited the other day. He was using a Croft pre too. Out of curiosity I just tried the 20 with my Quad 34 and got the same hum, but less loud than with the Croft. Just curious why my 34/303 combo doesn’t have this problem when they both use 3-pin mains leads, whereas 34/Leak does hum?

I might try disassembling the mains plug as you suggest but I’m not sure about the risks.
 
FWIW I’d leave the Leak fully earthed via its mains plug and lift the earth on whatever lower voltage source component that is causing the buzz. Best to ground the hot old thing with the really high voltages IMHO! I’d also not lift any mains earth anywhere in a system unless I had a multimeter to hand and could confirm I still had earth continuity (i.e. the component was still finding earth via its interconnects).

PS Obviously this is, in accordance with the pfm AUP, is only personal opinion (as is that of Jez and others) as neither of us are qualified electricians prepared to financially underwrite any suggestions given. pfm takes zero responsibility for any such “advice”.
 
The high voltage HT of the Leak makes diddly squat difference at the end of the day! It's MAINS LIVE that is the potential (ha) problem. Other earths could be lifted yes but non moulded on plug required of course.

Bulgin plugs are dodgy enough without doing it at that end!
 
Right, I’m going to remove the earth wire from the mains plug so it’s sticking out. Should do the trick.

Just another question though, if in the future I got hold of a passive pre would I need to reattach the earth wire before switching everything on again?
 
Right, I’m going to remove the earth wire from the mains plug so it’s sticking out. Should do the trick.

Just another question though, if in the future I got hold of a passive pre would I need to reattach the earth wire before switching everything on again?

Yes... or no... depending on what else is earthed and how and where etc! All that will happen is you will get the mains hum or not.
 
Just another question though, if in the future I got hold of a passive pre would I need to reattach the earth wire before switching everything on again?

Yes.

PS As stated my advice is to leave the Leak earthed in all scenarios. I don’t care in the slightest if I’m at odds with Jez here (there are countless things we disagree on), but I’d not want a high-voltage metal-chassis valve amp only having a path to earth via the interconnect screens. The Leaks can be totally silent when earthed correctly (I’m listening to my TL12 Plus monos now, both mains earthed, and not a hint of buzz, passive pre and SACD upstream).
 
I just unscrewed the earth wire and clamped the plug back together. Does that seem ok? I assume there’ll be no issue if I touch the earth wire by mistake?

32xWIbd.jpg
 
Again my advice is not to. As it stands that is not an electrically safe device. Under UK electrical safety legislation metal chassis equipment such as the Leak needs to have an earth wire. Ok, there is no way a Stereo 20 would pass a PAT test as it has no bottom panel, but best not to go full nutter here!
 
Right, so does that mean I’m legally not allowed to use a Croft with a Leak because they’re both metal and need earthing? What’s the right way to do this, or is there no right/safe way?

Ian my friend was given a mains lead by his Leak restorer where the bulgin plug had the earth wire sticking out.
 
Right, so does that mean I’m legally not allowed to use a Croft with a Leak because they’re both metal and need earthing? What’s the right way to do this, or is there no right/safe way?

To the best of my understanding of UK electrical safety regulations, which I admit is sketchy as I am obviously not a qualified/certified electrician, it is not legal and it would jeopardise any insurance claim should something go very wrong, leave you liable should you harm anyone etc. As website owner/publisher this has to be my stance here.

I have in the past used a gain-reduced Croft 25R with my Stereo 20 and I can’t recall any mains hum at all (there was some hiss from the Croft, but I was using high-efficiency speakers) and I certainly never lifted the mains earth on the Stereo 20. Do you still get the earth buzz with no source components connected to the Croft? As stated up thread I do have the earth lifted on my DPA DAC as that is the problem component in my system. My call, my risks, not offered as advice. Everything else is mains earthed correctly via its mains plug.

I’d be inclined to contact Glen Croft, he likes Leaks and a lot of his preamps end up in this context so none of this will be new to him.
 
I just unscrewed the earth wire and clamped the plug back together. Does that seem ok? I assume there’ll be no issue if I touch the earth wire by mistake?

32xWIbd.jpg

Spot on. Just DO be sure the top is well screwed on as you don't want it coming off while the rest is still in a 13A socket.

Is the Croft earthed? It should be and so you should still have a connection to earth via that.

As Tony says LEGALLY etc... don't do it... but I do all the time and so do plenty of others... YOUR risk. If you die don't come running to me:D
 
Oh, just a thought... have you checked that the earth is connected at the Bulgin plug and that the earth from the inside of the Bulgin socket is connected? You may have a lack of an earth rather than one too many!
 
When checking the above please be aware that many of the capacitors hold a charge for quite some time after power-off and some have 300V+ sitting in them. Basically look, don’t touch!
 
I’ve put the earth wire back in it’s place in the mains plug and just put the Quad CDP back as source again. Too much of a worry for someone who doesn’t know the first thing about electrical safety.

Hi-fi is a real PITA sometimes (often?). I suspect I’m back to just trying out different preamps in the hope they don’t cause hum.

Thanks for the advice chaps. I’m taking it all into account.
 
This note on Ground Loops from Pass Labs is interesting (though much of the electrickery goes over my head - I'm still not really clear on the difference between signal ground and safety ground...).

Just posting in case a custom interconnect might a low risk option to try rather than messing with 240v.

If you are running unbalanced cables, always use two conductor shielded wire. Using the more common single conductor inside a shield mandates that you combine signal return and shielding to the same wire; thus breaking the preferred protocol.

Shields are to keep stray noise out of component inputs; the common or signal return is part of the signal path, two opposed tasks. Because of these separate tasks your cables should be directional, and the shields should be part of the input circuit only. Shield ground should not be connected on the source end of the wire, only at the input component end; label them and don’t forget! This will of course mean that at the input component end of cable the shield and ground signal conductors will bond together.


https://www.passlabs.com/technical_article/ground-loops/
 
I'm still not really clear on the difference between signal ground and safety ground...
Signal ground is part of the audio circuitry, safety ground isn’t - it’s just there to stop you coming into contact with high voltages if a fault arises. In any given piece of equipment the two may be connected together. Or not. Sometimes connecting piece of equipment A to piece of equipment B will have the effect of connecting the two grounds together in one of them when they were not intended to be. Sometimes (to quote FZ) there is trouble.
 
Signal ground is part of the audio circuitry, safety ground isn’t - it’s just there to stop you coming into contact with high voltages if a fault arises. In any given piece of equipment the two may be connected together. Or not. Sometimes connecting piece of equipment A to piece of equipment B will have the effect of connecting the two grounds together in one of them when they were not intended to be. Sometimes (to quote FZ) there is trouble.

Thanks. So I guess when equipment has a ground lift it's breaking the safety ground?

I've probably missed a trick but wouldn't it be better to just design the equipment so the earth ground and safety ground are kept separate?
 
Sometimes connecting piece of equipment A to piece of equipment B will have the effect of connecting the two grounds together in one of them when they were not intended to be.

In Leaks there is full continuity between mains earth and signal return both at the inputs and outputs. All of the grounds are bolted to one of the eyelets that hold one of the big screening cans in place on the chassis and everything beeps continuity wise. This is why I’m convinced that this kit should have a proper earthed three-core mains lead. I can’t predict every potential fault condition that can occur in a vintage valve amp, but given the Leak’s are somewhat under-protected electrically (no plate fuse, Harold’s magic green resistor etc) I’d want to be absolutely sure there was zero potential for the chassis going live. A properly earthed mains lead provides that security.

I’m also very aware of how Leaks were designed to be used. The preamp was powered from the power amp, the power amp was always earthed. This being conceptually different to say a Quad 303 which took its earth via the 4 pin pre/power link to the 33, in this case the 33 is the system earth.
 
Thanks. So I guess when equipment has a ground lift it's breaking the safety ground?

I've probably missed a trick but wouldn't it be better to just design the equipment so the earth ground and safety ground are kept separate?
Yes, that’s right - or at least can be in a non-approved way (see 13-amp plug pic above.) If it’s built in to a piece of equipment it’s more likely to be something like a switch breaking the link between signal and safety earth.

For the second part, rather than imagining me waving my arms about while trying to explain you could have a look at this!

https://forum.allaboutcircuits.com/threads/signal-ground-chassis-ground-theory-vs-pratice.121451/
 
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