Anyone following the Gainclone Heaven thread may have noticed that I had some hum "issues" after completing the build of the bi amped bridged gainclone amp - well maybe not that bad (I thought) but still there was some hum with ear right next to LF drive units and I mean right on the cone of the drive unit (92db sensitivity). OK not that bad because the sound quality was good and the bridged bi amped GC sounded great. And then I had all sorts of problems at home and then a project to build three pairs of speakers (uurrgh). So the hum problem got sidetracked and forgotten - BUT actually not quite forgotten.
Those that read this thread
http://www.phasure.com/index.php?topic=1575.msg26747#msg26747 will know what I am talking about.
So always in the back of my mind was the thought that hum MUST be eliminated - (from experience of my first GainClone build when Peter badgered me to get noise to the absolute minimum and when I succeeded wow what a difference).
If you check out the thread above the thinking was that the hum was possibly due to much bigger transformers in the new bridged GC amp and maybe linked to some EM interference. BUT to modify my amp by moving the transformers well away from the amp (to reduce EM problems) is a very very big job. So today it seemed like a good idea to make sure that all possible earth loop problems were looked at before taking the VERY big step of ripping the amp apart. So I tried every possible combination of earthing and I found a combination that really made a difference as follows:
Amp - electronic ground NOT connected to PE
NOS1 - as above but that is the way NOS1 is built anyway
Amp & NOS1 electronic grounds connected together (but not to PE) via the balanced i/c pin 1 connection.
But this is where it gets weird. Previously the PE of the amp and NOS were connected to the PE of the balanced power transformer which in turn was connected to an earth rod in the garden. So the metal boxes
only of the amp and NOS1 were earthed via the supply to the main earth in the garden. Nothing wrong with that Eh?
Well today I changed the earthing by connecting the PE of the amp and NOS1 with separate wires to the earth rod in the garden. So I ran separate earth wires from the boxes of NOS1 and the amp to the earth rod. A proper star earth arrangement. My PC remains connected to the earth of the balanced transformer which in turn is connected to the earth rod. So now I have three earth wires connected to the earth rod in the garden instead of one. Now this could not possibly make any difference could it? (because amp and NOS1 PE's are only connected to the boxes and not the electronic grounds). Wow what a difference!!!. With this earth arrangement detail has increased to a new level, imaging is much better, depth of sound stage is much better strings sound much sweeter.
The above is not something I would have ever thought possible until reading Peter's musings about grounding. This seems to move away from simple ground loops to RF earthing problems - how else can connecting the boxes only of amp and NOS1 separately to the earth rod make that difference?
BUT I can still hear a very very low level of hum (MUCH reduced from before). OK there has to be total quiet in the house and ear right on the cone of the LF unit to hear it but it is there JUST. But I now know that even that HAS TO GO. So I am going to have to find some way of looking at the noise on the earth rail.
Maybe Nick will be kind enough to help me with his super duper scope when he comes next? - who knows but now I am absolutely determined to reduced noise to the ABSOLUTE minimum.
So probably a rebuild of the amp will go ahead anyway - but for other reasons as well as hum and that is I need bigger heat sinking.
Of course my amp has no volume control as I do not have a pre amp so it is on full blast all the time. So for those that have pre amps and no control over grounding arrangements how much noise do you hear when the volume is turned right up to maximum? it would be interesting to know because if you do have some noise (any at all) then there is the potential to improve your system by getting rid of it.
By the way why is it that all the Sage's of the hi fi world say that balanced connection gets rid of earthing problems - I found it much easier to get noise levels down with single ended RCA connected i/c's than balanced connected. But there again my experience could be a one off.
All the best
Paul
