Tx hum cure?

Been living with John for 2 week now (John Shearne amp that is!)
Before i pass my thoughts on the amp i want to cure one problem, transfromer hum
it is quite audible from around 2-3 foot away.
I thought of cork tile under the Tx (toroidal) but unsure about the use of cork with the heat of a Tx.
Any other ideas on how i can cure the hum?

 

Penance, do you think it is a vibrational problem or a electrical one ?, have you had the top off the amp?, is the mounting bolt tight?, does the hum change pitch if you swop power leads say? WM

 

I occasionally had hum from my Rotel RA-971mkII transformer at my old address in southampton. I suspect that dodgy electrics was the cause, as a DC offset will cause a transformer to hum. The hum was intermittent, and has not been noticed since moving to gloucestershire, and also the trichord powerblock should prevent DC offsets making it to the kit...

 

its not a class A amp is it?

 

I agree, I think the Transformer is reacting to noise in the mains. Have you tried any of the isolation techniques around ?

If you go for mains cables make certain you use the sort that act as filters, Eupen, TTS and others. These will limit the ammount of noise reaching the transformer, and you should be able to hear the difference.

Is the trans. a torroidal or square frame type?

 

Morning all

No its not class A (i think)

WM, i think its electrical. ill check the bolt tonight and try a different power lead, hadnt thought of that!

Pete, it's toiroidal. I have a eupen on it but ill try my other one tonight. Ill also try it on my diy filter and see if that helps.
What isolation techniques should i try? (its early and my brain hasnt woken up)

 

are your speaker cables and interconnects shielded?


dont dismiss trying this one, unshielded interconnects increased the hum in my system till I recently got rid.

alternatively, maybe an earth loop? - Ive had these too. A dangerous/easy way to check is to remove the earth wires on the plugs for a quick demo and see if it goes, if it does theres your culprit!!

Wellies and rubber gloves at 6 paces!


cheers
Chris

just a couple of easy things to try there to narrow it down

 

Your working on the right lines......

Try taking the isolation off and see if it makes it worse, if not then the problems likely to be internal. You could check your ground lines to the mains.
Make certain your not creating ground loops. Take an earth line from the amp's binding post and take it straight to the earth pin of the
main supply plug of the system....RA has an expensive wooden thing that does the same, but he has more terminals and so can sar earth a whole system.
Have you run the amp with now external connection other than the speakers? Is the hum more or less?


How's the tuner doing ?

 

Hi Pete

The hum seems to stay the same no matter what is connected, ill check the earthing tonight.

The tuner is getting there, was hopeing to sort it completely last week as i had a week off work. Turned out to be a bad week, working on the house and getting nothing achieved

 

Did you manage to eradicate the Hum ?

 

I tried most things but to no avail
only thing i didnt do was to remove the case and check the mounting of the TX, ill try that tonight if i get time.

to much study at the mo

 

If your going inside the amp, have a look at the decoupling caps to ground [true ground]. I suspect the problem is more to do with internal resonaces in the toroid. Changing the bolt tension may help and simple damping under and around the donut should provide some help. The final solution, the penultimate solution would be to swap the toroid for another of the samw value or even two smaller ones for each channel.

 

I tightened the bolt last night and it is a bit quieter, ill try some damping material over the weekend.
If i ever finish this assignment that is

 

Hi,

A lot of Txfmr's are mechanically noisey, windings buzzing..... Audio Grade??? Txfmr's are available...but at a price. I went through a LOT of manufactures of Traffo's and only a few of them offered to make the things quiet...at a price!!! Lintron being the one that I chose...

Regards

 

sounds like maybe i should live with it, just play it louder

 

I think you have the beginings of a solution. just don't over tighten that bolt or you'll hear a plinck andthe rattle of small change [actually large change] as you buy a replacement.

The extra damping should help more, damping the base of the amp's case as will also reduce the transmitted vibrations.

best materials.....
for the case automotive damping pads. For the trans, extra layers of rubber or similar, closed cell foam if possible you get an air suspension then.

 

i didnt try anymore lst night, but over the weekend ill try that.
I think i have a small piece of an old camping mat, doubled over it would be about 1/2" so i guess that would do

 

Just a single piece top and bottom of the trans. above and bellow the rubber [rubber insulating the trans.]