Mains DC blocker advice


Yeah, but no buzz when the pre is disconnected reasons …


I’d rather have the hum.


I have one of Sodders original design models, you are welcome to try. It is quite substantial, so postage will be £11-12.


Thanks for the kind offer Gregg.

I just need to find out if the learned 'toirists still think that is the problem, with the new info, before trying another.


Does it do it when the pre is disconnected, not just turned off?

Edit: just read your post properly. So you have a mains hum loop. Fuckers to fix.


Is there a best order of attack Adam ?


Do you have an interconnect with the shield disconnected at one end? That might break the ground loop if used between the pre and main. I’m assuming that you use unbalanced (RCA) interconnects. I’m also assuming that both pre and main amp have proper safety grounding!


I don’t have an interconnect with the shielding disconnected at one end Dave. Being an older Quad pre, it uses the Quad 4 pin DIN to RCA lead. Both amps are properly earthed, as far as I am aware.


Sadly being ‘properly earthed’ is what tends to cause the problem.

Does it make any difference if you unplug just one of the RCA inputs to the amp (you might need to short that now open input to ground to stop a different sort of hum from that channel drowning out any remaining hum in the still-connected channel) ?

Sometimes you can reduce ground loop hum by plugging the two mains leads into adjacent sockets and then running them as close as possible to one another on their way to the amp and pre-amp.

More reliable solutions are either to fit a ‘signal ground lifter’ circuit into either the pre or power amp, or to put a pair of isolating transformers in the interconnect leads. The isolating transformers will be more expensive and some people object to them on religious grounds (no pun intended). The signal ground lifter should be cheaper but it comes with safety implications, so needs to be wired in by someone competent. If your equipment is new enough to be CE marked then making any electronic changes to it at all, e.g. lifting the signal ground, is liable formally to invalidate its CE status. You may then be on shaky ground (there I go again) when you come to sell it.

As far as Dave’s suggestion about the disconnected interconnect shield goes, you may be able to simulate this by pulling the RCA plugs out of the power amp very gradually until the outer connector just separates but the plug’s centre pin is still touching the centre contact inside the socket. If you are lucky then when you do this for one or both of the leads the hum will go away.



Hi Graeme,

Thanks for your input.

Unplugging either RCA lead just stops the hum in that speaker.

Removing either RCA slowly has no effect, either, unfortunately.

Looks like a signal ground lifter it wil need to be :disappointed:


Sorry folks; I’ve just realised that I’ve left this thread in the air.

Would it be better to fit a ground lift to the pre or power amp and how easy a job is it ?

Anybody local’ish happy to fit it for the appropriate amount of shekels ?


I recently built this circuit:

And made this case for it:

I had tried a smaller blocker before and found it made no difference. However, rather to my surprise, this larger unit proved to be quite different - it completely silenced my transformers (there are three 500VA ones and a number of smaller units) and, most surprising of all, has actually improved the sound!


That’s a handsome looking case.


Thanks :). I must admit, it has come out well. I made it in the same style as the Hydra mains lead I had previously done. I think I primarily did the build to distract me from doing my turntable ;).


Tell us more about the turntable! (start a topic if you feel appropriate)


Hi Pete :), I thought you were hence, hence I have joined. You will be amazed to know that I am actually making it - will be doing some more today! I’ve been promising to do this project for as long as I can remember, at least fifteen years!

Should I start a topic? I think there is a DIY section - will check.


Yes please, would love to know more!

Welcome onboard, Mark. The locals can be grumpy, but they’re just frustrated middle aged men. :rofl:


Who are you calling middle-aged ? I’ve met some old people on here !



Have you calculated how many platters it needs?


I’ve calculated it doesn’t need a pair of contra-rotating nonsense if that’s what you mean ;).