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A little advice on transformer position???

75.143.239.3

Posted on October 6, 2009 at 12:37:09

amioutaline?
Audiophile

Posts: 309
Location: Great Smoky Mountains
Joined: August 29, 2008

Most commercial amps have the OPT-PT-OPT sandwich with 90 degree rotation of the PT the only accomodation to magnetic fields.

I think I recall Paul Joppa invoking a 4" (minimum) rule and so built my Simple SE thusly:

Of course I had lots of real estate (there's a big choke and 3 motor run caps under the hood)and it also put the PT close to the HT side of the board.

Was I fooling myself? Are the commercial guys right? Or should I repeat the Simple SE position for the 300B I'm about to build, aesthetics be damned, with a Hammond 372JX that dwarfs that little Allied?

RE: A little advice on transformer position???

Paul Joppa

22:47:31 10/09/09

RE: A little advice on transformer position???

Caucasian Blackplate

19:25:28 10/07/09

RE: A little advice on transformer position???

triode3

18:40:02 10/07/09

Well it works doesn't it :)

Bas Horneman

13:29:37 10/07/09

RE: A little advice on transformer position???, posted on October 9, 2009 at 22:47:31

Paul Joppa
Industry Professional

Posts: 5028
Location: Seattle, WA
Joined: April 23, 2001
Contributor
Since: July 1, 1999

I've had pretty good results with the OPT as close as 1/2 inch when the orientation and symmetry were just right. (This is with TFA-2004 and PGP8.1 transformers, as well as some custom units we've used.) And I've had bad problems at 3" distance when they were not just right.

Testing is easy enough if you have the patience. The power transformer can be wired to 120v - just tape off the secondary leads - the magnetic induction does not depend on the load current. You can put a voltmeter across a winding of the OPT, plate choke, or whatever iron you propose to put close to the power transformer. Move it around and find for yuurself how these two parts interact. You can load the winding you are measuring with an appropriate resistor and compare with the unloaded condition, so you can deduce the effective source impedance of any hum field you are picking up. This will give you a good idea of the actual resultant hum.

RE: A little advice on transformer position???, posted on October 7, 2009 at 19:25:28

Caucasian Blackplate
Industry Professional

Posts: 4444
Location: Seattke
Joined: June 18, 2004

Orientation will provide far better isolation than distance, unless the distance is quite large. I have never had a problem with the OPT-PT-OPT sandwich before (with the PT turned 90 degrees to the other), but I can say that if you having something like an unshielded interstage or input transformer, all of the sudden distances *and* orientation become super critical.

Proper transformer grounding will also keep things quiet.

RE: A little advice on transformer position???, posted on October 7, 2009 at 18:40:02

triode3
Manufacturer

Posts: 119
Location: South Louisiana
Joined: August 23, 2005

Hrmm, it has been my experience that you can put them closer
than you think. Mike can go into more detail on metals suppressing
magnetic field lines, but the bells on the transformers help.
I find that with iron at 90 degrees, you have to get pretty
close to get some induction. This 100WPC amp, for example, had
no interaction between the power and output, and the power was
pretty serious, with many windings and high secondaries. 900uf
in the power supply. The gap between iron is barely over one inch.

Just my $0.02. Perhaps SET's are more sensitive? I find that
most hum there is actually improperly designed power supplies,
at least in the ones I have seen.

MQ100WPC

Well it works doesn't it :), posted on October 7, 2009 at 13:29:37
Bas Horneman Distributor or Rep Posts: 3549 Joined: March 28, 2001	I usually stick to what works. However....why not try a mock build first....takes more time...but that way you'll know just how close everything can be. Kind regards, Bas To infinity and beyond!!!

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