Neve EZ1290 toroid field being picked up by microphone input xformer

[777funk]

Is there a way around this besides using a separate enclosure?

I originally built this pre using an 18VAC door bell transformer and it worked great (very quiet) without this proximity effect on the input. This is not at all the case with the large toroid I'm not using. I may have to go back to the door bell but I wanted to see if there's a way to block the field inside the enclosure to where it won't be picked up by the input tranny.

 

[777funk]

I wonder why Carnhill doesn't wrap their input transformer in Mu Metal can like most others do (Jensen for instance)?

 

[gary o]

Costs money I guess ...

 

[prh]

I wonder why Carnhill doesn't wrap their input transformer in Mu Metal can like most others do (Jensen for instance)?

They do.

http://www.audiomaintenance.com/acatalog/CA-18-VTB9045M_extended_info.html

The pin footprint of the non-shielded transformer is slightly different that of the shielded one.

 

[777funk]

It works now! Got rid of the toroid and everything's much better. I still wonder why they don't mu metal enclose all input transformers. On a low signal to noise ratio area like the input of a mic pre, it seems like this would be important. I guess Neve didn't have trouble with this (these look a lot like the Marinairs). But were the Marinairs metal outside? These have a plastic shell I believe.

 

[emrr]

Many mic inputs have mu-metal shielding, but it's still only good for -30dB hum reduction.  This is often not enough by itself when close to a power transformer.  Very few modern transformers are offered with multiple shields for -60 or even -90 reduction.  In the old old days pro equipment power transformers were never allowed in the same chassis as audio iron, even with multiple shield audio transformers.  It's important to not make assumptions in layouts.  Glad you cured it. 

 

[mitsos]

What is the VA rating of your toroid and how many channels is it feeding?