My output transformers capture hum !! (passive Pultec)

[Cardinen]

Hi all,

I'm trying to solve a problem that is driving me crazy!
I've successfully built a fully passive dual pultec (no make-up gain, no PSU) inspired by this thread:

http://groupdiy.com/index.php?topic=49672.0

I'm using Edcord XSM 10K:10K as input trafo and XSM 10K:150 out feeding my mic pres as make-up.
Everything work as expected, except i have a strange hum picked by the output trafos.
At first i haven't notice this, when i calibrated the unit on my bench, hum level was at an incredible -103 using my Hardy M1 preamp, and -96 with G9.
When I put the case into the rack along with other gear the hum level goes up to -80 using the same preamps !! 
(see photo below)
After a lot of checking my ground connections,  preamps ground-lifts i discover the hum level is captured by out transformers, they are extremely sensitive to any metal close to it, including my rack!!
if approached my hand to the trafos i can see his extreme sensitivity even to 40-50cm!!
Moving the trafos inside the case don't solve the problems as they capture hum from air, i've built a metal box for screening the trafos but is even worst, hum level goes up + 3dB (tried to ground the box too without any change).

i guess the problem could be the open core style of the Edcor, and i'm thinking of swapping them with some incapsulated trafos like Lundahl (1530), do you think this could solve the problem ?

Ok, i could leave it on the rack and give a damn, but I like challenges !!

 

[Cardinen]

add an image.

ground is connected to case in just 1 point from xlr pin1 to star ground (can't see the black wire from photo)

 

[radardoug]

You are using the wrong transformers. Use Jensens or Sowters. Read up on transformer shielding.

 

[0dbfs]

Something like this may help:
http://www.ebay.com/itm/MU-METAL-MAGNETIC-STATIC-GUITAR-MIC-PHONO-LP-MRI-SHIELD-PARAMAGNETIC-HUM-REMOVER-/151239268601?pt=Vintage_Electronics_R2&hash=item23369018f9

Cheers,
jb

 

[CJ]

could be a ground loop from the other rack gear,

try measuring the hum voltage without the transformer connected to the preamp,

short the input terminals of the input transformer,

put a scope on the output sec and look for 60 cycle hum,

is the mic input balanced? if so try grounding the CT of the sec.

yo can also try reversing both pri and sec leads, this will put the windings at a different location in relation to any e fields,

 

[gyraf]

..make sure that the whole chassis/case/frontplate/lid is actually electrically connected to ground when assembled - reaction to hand-near-unit points towards electrostatic rather than magnetic.

Jakob E.

 

[Cardinen]

thanks everyone for the help, i finally solved !! 

I tried all the things you said to me, the whole chassis was correctly grounded with frontplate/lid/case, the CT of sec. has no effect.

The solution is a bit weird but it work:
I put the case into the rack without the lid and started to move the transformers whit a pencil looking at the analyzer... i notice the difference of e-fields was completely different than outside the rack; so i flipped them and... hum from -80 to -100!! After a bit of moving around i've found the best position for transformers and raised from the bottom of case with a wood block give me another 5db, now i have hum at -105 (photo below)

the weird thing is this don't happen when the unit is outside! if i tip the trafos out of rack hum level is worst while inside is completely different, now i have the reverse situation, inside rack hum at -105 outside -80... this make me smile  ;D

I wondering if a better screened trafos like Jensen or Sowter do the same, anyway at the moment i'm happy with my Edcor.

Thanks a lot, love this place! 

 

[CJ]

Jensen has e shields on their JT 10K-61, Sowter probably has a similar model,

 

[Cardinen]

yes, even Lundahl 1530 have the shield, maybe one day i'll try one of those...

Anyone planning to build a fully passive Pultec is warned: use good shielded transformers! 

 

[pucho812]

or make your own shielding with copper end caps and epoxy.