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megnetic fields around toroidal transformers

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?

Reply to
Bruce Varley
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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
Reply to
Trevor Wilson

"Trevor Wilson"

** 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.

** 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.

** The OP said the hum was in induced his circuit - not audible hum from the tranny itself.
** And may well have include a flux shorting band - ie an external copper strap.
** 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

Reply to
Phil Allison

"Bruce Varley"

** 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

Reply to
Phil Allison

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