Little Dot MK4 and S300 hum - help!

I’m running a little dot MK4 as a preamplifier for my s300. After using a rothwell line attenuator to fix the gain mismatch, the results are sonically excellent with one problem I can’t resolve. There’s a constant low level hum which my ears say is a 60hrz hum. It’s not affected by the volume and is equal in both speakers. The kicker is it’s present when the little dot is switched off as well as on. It’s definitely the little dot because when I plug other pre amps into the system it’s silent - the hum only comes back the moment the LD is connected - even powered off - the connections are the same - I just remove them from the LD and plug them into an alternative device.

I’m non technical, but I’ve tried any number of isolators and socket switching assuming this might be a grounding problem, but nothing works.

It’s low level, it’s not really noticeable from listening position but audible next to the speakers. Because you know it’s there, it’s bothering me more than it should. In all other respects the LD is superb looking and sounding when partnered with the S300 - my view is you so need the rothwells in the chain as well.

Any suggestions to fix this?

Welcome, ollyS

My immediate thought is as yours, it is a ground loop problem, but it appears you have already taken steps to eliminate a ground issue.

I have had luck connecting a thins wire to the offending unit and touching the end to potential grounds to determine the offending ground path. If the hums disappears you know where the conflict is.

This is going to sound like the dumbest questions, but where should I attach a wire too?

Is it literally sticking a wire onto the chassis and grounding it elsewhere or is it via a specific point on the LD unit?

Not dumb at all. :slight_smile:

Yes, just touch the wire in the LD chassis, a chassis screw is great as it will make a good ground connection.

All we are trying to do is to connect the offending grounds thereby eliminating any voltage potential as a way to diagnose what is going on.

Does the LD use a 3 prong IEC? If so, definitely try a cheater plug and see if you can isolate the hum.

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I think the answer is yes. I’m in the UK so all our stuff that plugs in is 3 prong. The power input in LD is standard 3 pin socket found in kettles and audio gear the world over.

I really have only the technical knowledge I can get from a YouTube video, so mostly this goes over my head, but can you get a cheater plug that works in UK?

Ooh sorry, I assumed you were here in the states. I’m not certain about cheater plugs in the UK. I wouldn’t be terribly surprised if they are highly discouraged if offered at all.

FIXED?? So as a postcript to this story, i noticed a slight variation in hum volume between the left and right and swapped out a cable - that wasn’t humming via another pre-amp, but possibly more sensitive via the LD output (is that possible?). I used a pro audio DI box which was my last throw of die and the hum was almost eliminated.

Power up the system this morning 24 hours later, and the hum had vanished, or at least receded to a level that i my ears are not really picking up. The DI box is passive, is it possible that it has gizmos inside that take a while to charge up or something, because it makes no sense to me that it could fix itself after 24 hours.

I think i was having some sort of ground loop or transformer hum issues through the system, that a combination of new cables and the ART DI box seems to have fixed by magic. I tried about three other variants of isolators and read a review of the ART box on a pro audio site that made me think it was worth a try.

I can tell you that hum in the system is the most frustrating, mind boggling, WTF hassle i’ve had to experience. It takes about a day to wire and re-wire the system to get it fitting in its rack and many cramped up fingers squeezing in behind various components and i’ve wired and re-wired it 4 times at least to try to isolate the problem.