megnetic fields around toroidal transformers

[Bruce Varley]

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?

 

[Trevor Wilson]

"Bruce Varley" <bxvarley@weastnet.com.au> wrote in message
news:2OOdnQyxFqlEkYbWnZ2dnUVZ8vydnZ2d@westnet.com.au...

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?

**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

 

[Phil Allison]

"Trevor Wilson"

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

**Depends. GOOD quality toroidals radiate less overall flux than a typical
EI tranny. Cheap, crappy ones (Jaycar, anyone?) may not.

** 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.

They may be operating in saturation,

** 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.

or they may be responding to excessive DC on the power lines.

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

And of course, the manufacturer has probably oriented the original EI
tranny for the lowest radiation into sensitive circuits.

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

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

** 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

 

[Phil Allison]

"Bruce Varley"

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?

** 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

 

[Jerry G.]

From what I can remember the tranny's made for music amplifiers and
sensitive equipment are specially designed to have very low external
magnetic radiation. It does not take very much magnetic field emission
to have an affect on an audio amplifier or a television device.

Contact the manufacture to have an original tranny, and you should be
okay.


Jerry G.



On Dec 5, 8:45 pm, "Bruce Varley" <bxvar...@weastnet.com.au> wrote:

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?