[alfalover]

Dear all, this is actually a repost from 2017. I don't think anyone had any ideas at the time, so thought I'd have another try. I am new to this forum and was hoping that someone could help me with a problem I seem to be having with my Little Dot MkIII headphone amp. I notice a few historical threads talking about a humming noise generated from this amp and the causes of this, but none of the threads seem to cover my problem exactly (at least I dont think they do!). So hopefully Im not going over old ground and If I am - sorry! I have some grado sr125's and a Little Dot MkIII amp (replaced stock tubes with Voshkod 6ZH1P-EV.) The Little dot is hooked up to the tape out input on my cyrus straight line amp. I have no speakers and only use the Cyrus to allow me to swap easily between sources.I recently purchased a Denon Dp300f turntable (with Audiotechnica at95e cartridge). I was initially using the internal phono amp in the turntable and all was good. I found an old Musical Fidelity XLP phono stage. I disabled the internal phono stage of my turntable and added the external phono stage. Here is it where it started to go wrong. I got a very very loud electrical humming noise coming out of my headphones which got massively louder when I increased the volume on the Little Dot. I switched back to the internal phono stage in the turntable and all was good again. I then tried another phono stage, the Cambridge Audio CP1, again disabling the internal phono stage on the turntable. Again exactly the same problem. Richer sounds checked out my turntable and phono stages in the store and they were functioning fine. They tested out the turntable with the external phono stages using an in house amp and some floor standing speakers, and all worked like a dream (they didnt have my little dot to check). So I guess it looks like there is some odd compatability issue with the Little Dot and the external phono stages that is creating some electrical interference problems. The problem is, when it comes to the internal workings of electrical components I know about as much as my 3 year old boy. Does anyone have any idea about what may be going on? Any advice would be much welcomed

 

[tomb]

1. Are you using the phono ground wire from the turntable? If not, this can cause huge hum issues that would not show up using the internal phono pre-amp of the turntable, because the ground is likely built-in with the turntable's internal phono pre-amp.

2. This could be a sign of the dreaded ground loop phenomenon. It's essentially caused by a secondary loop to ground that effectively short-circuits the ground needed by the signal source (turntable). You might try dis-connecting the ground on one of the devices you have in your music sequence. Most likely (just a guess), the ground loop could be in your Cyrus that you're using to switch between sources. I would suspect that before suspecting the Little Dot. A way to do this is by using a 3-wire to 2-wire plug convertor on your wall outlet.

 

[alfalover]

Hi Tomb,
Thanks a lot for your time replying - much appreciated. Actually there is no obvious ability to attach a ground wire from the turntable. There is a switch under the platter that you can activate if using an external phono amp, which I did change when using the external phono amp. I will have a look at your second point re the ground loop, this may well be the explanation. Thanks again.