Transformer noise questions

All about iron and copper.

Moderator: VelvetGeorge

Post Reply
MGW
New Member
Posts: 4
Joined: Tue Dec 21, 2010 8:37 am
Just the numbers in order: 7

Transformer noise questions

Post by MGW » Mon Apr 29, 2013 4:46 am

I recently built a rack preamp and while trying to eliminate this buzzing noise, I discovered something. The trafo is a hammond 270CX. I removed the endbells so it would fit in the chassis. The preamp circuit is ground to the chassis at the input jack and nowhere else. The switching circuit is ground to the chassis by the ftsw jack on the back panel. The buzz I'm getting is a combination of 180hz and 300hz and other harmonics of this. The interesting part is that it goes away with the trafo isolated from the chassis. However, when I do this, the 60hz grows much louder. If I touch the iron (I know...not the best practice) the 60hz dies down to a tolerable level. I've probed pin 6 on the input stage and the buzz (with trafo bolted to chassis) is there.

There is also something interesting happening with the gain pots, if they are turned all the way down the buzz gets louder. What would make the circuit buzz by turning the gain pots down?

Any thoughts on where to go next.

User avatar
neikeel
Senior Member
Posts: 7231
Joined: Tue Dec 06, 2005 8:31 am
Location: Suffolk, England

Re: Transformer noise questions

Post by neikeel » Mon Apr 29, 2013 12:37 pm

Does the tranny have a screen that needs to be grounded to a dedicated clean grounding point??
Neil

User avatar
HTH
Senior Member
Posts: 600
Joined: Thu Nov 27, 2008 8:53 am
Just the numbers in order: 7
Location: Newcastle, UK
Contact:

Re: Transformer noise questions

Post by HTH » Mon Apr 29, 2013 3:01 pm

neikeel wrote:Does the tranny have a screen that needs to be grounded to a dedicated clean grounding point??
Not according to the wiring diagram... http://www.hammondmfg.com/pdf/EDB270DX.pdf" onclick="window.open(this.href);return false;

I know where you're coming from though, I've used a 370HX before which has an internal shield.
"If you can possibly manage the time, please play both sides at one meeting"

http://www.youtube.com/TeleCustom72" onclick="window.open(this.href);return false;

MGW
New Member
Posts: 4
Joined: Tue Dec 21, 2010 8:37 am
Just the numbers in order: 7

Re: Transformer noise questions

Post by MGW » Tue Apr 30, 2013 8:14 am

Nope...no screen. Would the removal of the endbells have any impact on this?

EFK
Senior Member
Posts: 86
Joined: Tue Mar 23, 2010 9:58 pm
Just the numbers in order: 7

Re: Transformer noise questions

Post by EFK » Wed May 15, 2013 6:57 am

I've found Hammond transformers to be very noisy comparative to other transformers. It's a mechanical hum that is transferred to the chassis where it can be easily picked up by various wiring, tube grids etc. This is why it goes away when you physically isolate the transformer from the chassis.

Post Reply