megnetic fields around toroidal transformers

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Replaced the power tranny in a music amp, the connection wiring insulation
was getting brittle. The old tranny is a standard E-I lamination type,
replaced it with a toroidal (35-0-35, 300VA), mounting it the same spot. The
result is less than satisfactory, with the new tranny there's way too much
buzz on the output with no signal, when I move the internal preamp module
away the level drops, so it's clearly related to a field around the
transformer.

I thought toroidals were claimed to be low magnetic field, is that a myth?



Re: megnetic fields around toroidal transformers

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**Depends. GOOD quality toroidals radiate less overall flux than a typical
EI tranny. Cheap, crappy ones (Jaycar, anyone?) may not. They may be
operating in saturation, or they may be responding to excessive DC on the
power lines. And of course, the manufacturer has probably oriented the
original EI tranny for the lowest radiation into sensitive circuits.

Examine the output waveform on a CRO. See if it is a clean sine wave. Bet it
isn't.


--
Trevor Wilson
www.rageaudio.com.au



Re: megnetic fields around toroidal transformers

"Trevor Wilson"
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** Fraid they all use the same kind of grain oriented silicon steel and
enamelled
 copper wire.

The only relevant variable here is in the symmetry of winding distribution
on the core.


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** Pure fiction.

Toroidal trannies never operate in saturation -   long as they are
being used with the intended supply frequency and voltage.

The Imag figures are extremely low, only 20mA for a 300VA type.


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** The OP said the hum was in induced his circuit -  not audible
   hum from the tranny itself.


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** And may well have include a flux shorting band  - ie an
    external copper strap.


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** ROTFL  !!

The mains voltage waveform is never a "clean sine wave" !!!!

The secondary voltage waves of transformers feeding rectifiers and filter
caps are also NEVER clean sine waves.



.....  Phil





Re: megnetic fields around toroidal transformers

"Bruce Varley"
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** It not a myth -  but there is still a significant external field at very
close range.

Makers only test their trannies with no load or resistive loads  -  so they
only get 50Hz hum fields.

However, when the load consists of rectifiers and filter capacitors, the
external hum field develops  high frequency components and they can create a
nasty buzz in nearby audio circuits.

Try rotating the tranny for least noise and route all signal and any ground
wiring away from it too.


....  Phil





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