Metropoulos Forum

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.

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

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.

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

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.