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Torroid Transformer Hum
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peterb View Drop Down
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    Posted: 20 Mar 2017 at 8:39pm
I am trying to find a way to reduce the self hum from my Power Amp Mains transformer.
The transformer is this one: RS Components 223-8881
The circuit is an Unregulated one with snubbers on the rectifiers and 8 x 2200uF output capacitors on both the +ve and -ve outputs.

The only suggestion I have read is rotating the trafo can reduce induced hum, as it moves the field, but my problem is hum from the trafo itself, even when the current draw is minimum, about 100mA per rail.

I would be pleased to get some suggestions!
Peter
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Dual 505-1, Cyrus CD T, DIY 80W MosFet amp and PreAmp, 2xKEF 103.2
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Points: 16314 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 21 Mar 2017 at 5:42am
Try reading this:

http://sound.whsites.net/articles/xfmr-dc.htm
That none should be able to park up and enjoy the view without a smartphone and the knowledge in how to use apps
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Points: 16314 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 21 Mar 2017 at 6:06am
But then again, if mounted on sheet metal, it might just be a mounting issue.

The transformer is a large inductor and will influence its mounting surface causing it to resonate. Stiffening it up often solves the problem.

E.G., and although done with two frame transformers, mounted onto a metal sheet, and no wiring in-between, excitation of one primary produced an output (although very small) in the other's secondary!
That none should be able to park up and enjoy the view without a smartphone and the knowledge in how to use apps
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peterb View Drop Down
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Points: 354 		Post Options Post Options   Thanks (0) Thanks(0)   Quote peterb Quote  Post ReplyReply Direct Link To This Post Posted: 22 Mar 2017 at 10:19am
Thanks for the input Graham. Very interesting article, still reading it......
I forgot to mention that Mains input filters are also in the chain.
Mains in to PreAmp via RS 455-8929 > Relay > Cable > PowerAmp via RS 456-0950.
The arrangement is so that turning the PreAmp on switches on the Power Amp.

The trafo is mounted in the base of a HIFI2000 dissipante case and because its potting compound stops well short of its outer, I had to place a piece of plywood under it so as to space it off the case and allow its cables to exit without being 'cut' by its outer. The long mounting bolt passes through the case base, plywood and trafo and uses a nut captive in the recess of its outer. It is fairly tight.
Peter
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Dual 505-1, Cyrus CD T, DIY 80W MosFet amp and PreAmp, 2xKEF 103.2
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I am aware that some high power amps also suffer from from transformer hum which can be distracting if located near listening position - some models within a long standing Canadian company spring to mind..
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Points: 175 		Post Options Post Options   Thanks (0) Thanks(0)   Quote gwebster Quote  Post ReplyReply Direct Link To This Post Posted: 23 Mar 2017 at 12:13pm
Looking at the picture for the transformer on RS comp's web site and linking this with your comment that "I had to place a piece of plywood under it so as to space it off the case and allow its cables to exit without being 'cut' by its outer", I suggest you have the trafo mounted upside down - it shouldn't need the ply spacer.

Also, have you tried rotating the trafo? It really can make a difference.

Geoff
(Genera still sitting here)
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peterb View Drop Down
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Hi Geoff, thanks for your input.
I considered mounting as per picture to avoid a spacer but that would mean ignoring the hex recess in the other side which I assumed was for the fixing bolt to use. I even rang RS tech support and asked them what did the manufacturer recommend, they had no information! The manufacture's site shows them leads down, but also doesn't detail mounting. 

I haven't tried rotating yet as the trafo leads are very short and single core and so inflexible. I would have to cut them off and replace with heavy weight stranded wire to accomplish it. It is on my list of actions to try, but not yet at the top!
What I have tried and had some success with is getting the Mains from another socket instead of the PreAmp. I ran an extension lead from a socket close to the Consumer Unit (Kitchen) to the Amp (Sitting Room) and the hum level was notably reduced !! I want to repeat this with some variations but need the house to myself to do it safely.

Haven't forgotten your Genera, but I still don't have my 2M Blue. Ortofon cannot provide a replacement because they can't get the stylus rubber mounts to build it, or any other 2Ms !!

Peter
Peter
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Dual 505-1, Cyrus CD T, DIY 80W MosFet amp and PreAmp, 2xKEF 103.2
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