Thank you very much to you both for your help.

I earlier removed a module from the 6U case and put my ear has close to the transformer has I dare, and yes, there is a slight hum being emitted, only slight and barely noticeable. I have had this case for some time and have never had a problem with it at all.
I received this new 9U case only last week and it is empty but for a few modules - perhaps the larger size and the fact that it is empty, is just making it seem to amplify the normal operational sound of the transformer.
I have had it on for some period and there is no abnormal operation with the modules - certainly no static, etc coming from the module face plates... the only thing beside the sound that I have noticed is, that when switched off the bus board led's take a few seconds to dim to off, whereas with the 6U they dim instantly when the case is powered off.

Reading elsewhere, I have noticed that others have experienced similar and the solution was to perhaps mount the PSU on rubber washers and the like - but, it really is not such a problem for me, it does not interfere with the normal operation of sound creation. I was only concerned that it may have been an electrical fault.

I am very grateful for all replies.

Kind regards and best wishes

--- In Doepfer_a100@yahoogroups.com , analogmonster@... wrote:
>
> I agree to some extent, but he does state the frequency changes according to which modules are being used, which means it doesn't match the mains frequency
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> On 7 Jul 2013, at 19:20, "anoop.sahal" <draks@...> wrote:
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> >
> >
> > I think all coil based voltage transformers will emit noise. It is often due the coils attracting and repelling one another at each voltage cycle change.
> >
> > Winding them tightly helps initially but the coils lengthen as they age and they loosen up. Varnish helps delay the loosening by binding the coils.
> >
> > Is the hum mains frequency matched
> >
> > That would indicate that the above case is likely.
> >
> > I have only known solid state transformers not to hum.
> >
> > i would mount the PSU boards on Rubber washers to minimise making the case a sound box.
> >
> > Any concurrance
> >
> > Regards Anoop
> > --- In Doepfer_a100@yahoogroups.com , "isoldeandisobelle" <isoldeisobelle@> wrote:
> > >
> > > Hi - Thank you both for your replies.
> > >
> > > I am not sure what you mean by ''mechanical'' but, the sound is coming from the transformer itself. It is not too loud but I can hear it from a short distance away. The earth cables are all connected properly. My other 6U case is connected to the exact same power supply and makes no noise - this is what concerned me
> > >
> > > --- In Doepfer_a100@yahoogroups.com , Florian Anwander <fanwander@> wrote:
> > > >
> > > > Hello
> > > >
> > > > I assume you mean a mechanical hum. Right If so, then it should be
> > > > related to the method how the powersupply is fixed to the housing.
> > > >
> > > > Florian
> > > >
> > > >
> > > > On 07.07.13 00:31 , isoldeandisobelle wrote:
> > > > > Hi gang -
> > > > >
> > > > > I have a 6U case and a 9U case, the 9U PSU hums (which changes in frequency when different modules are in use.) - the 6U does not do this.... is this ok
> > > > >
> > > > >
> > > > >
> > > > > ------------------------------------
> > > > >
> > > > > Yahoo! Groups Links
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