Fender Prosonic transformer coupled noise

[Mick Bailey]

I'm looking at a Custom Shop Prosonic that's pretty noisy with a 50hz (uk frequency) buzz. This is prevalent in standby and also with the output tubes removed, in standby.

The OPT is extremely close to the PT and the voltage is 300mv RMS on the secondary with the tubes removed. A piece of sheet steel slid between the two takes the rasping edge of the buzz to turn it into a hum, but it's still at an annoying level. I wondered if Mu metal might have a better result, or whether this will create unwanted side-effects.

Anyone got an economical fix for this?

[Prairie Dawg]

Are the transformers situated in parallel or at right angles to each other? Because if they're parallel to each other you might try turning one 90 degrees. Just a thought. Also, is there any real estate available to move the output transformer away from the PT? Have you tried some copper foil shielding on one or the other?

[Mick Bailey]

They're at right angles on a really tight layout, but it may be possible to re-locate the choke, then move the output transformer along about three inches. Wanted to avoid drilling the chassis in case these ended up being future classics, but it may come to this to get the amp in shape. I'll give the copper shielding a try first to see if that works.

[Mick Bailey]

Tried the copper, but it didn't have any effect. It may need a ferro-magnetic screening can, as the lines of force from the PT probably cut straight through copper. Temporarily moving the OPT/choke to replicate a Twin Reverb layout causes internal feedback when the gain is turned up - possibly why the transformers are mounted the way they are - must be some reason for the odd proximity.

[Steve Conner]

Why is this even a problem? If the customer can't be persuaded to accept it, surely the easiest solution is to arrange the standby switch so it also shorts the speaker terminals?

I have an amp that does it too, but when in "play mode" the hum goes away. The feedback loop in the power amp rejects the hum, like any other external disturbance.

Copper and iron shield magnetic fields in different ways. Copper reflects the field by induced currents, iron attracts the field into itself. The proper way to apply a copper shield to a transformer is as a "belly band" aka "hum strap", whereas iron is used in the form of a box or can. I've seen transformers from hi-fi equipment that use both.

[Chuck H]

Good point. The OP doen't say if the hum is present out of standby. I might let it go if it isn't. But if the customer wan't it fixed then I think whatever improvement can be had with a shield is the best thing since Mick doesn't want to mod the amp in an obvious way.

[Mick Bailey]

The hum doesn't go away in play mode and is added to by the normal background hiss/hum. It's a particular problem when mic'd up.

I've cut it down a good deal by inserting a 3 1/2" square of 18 gauge steel wrapped in neoprene between the transformers - a somewhat larger piece than I used earlier. I would say that it's now just about acceptable, but not ideal.

I noticed while doing this is that the OPT has a strongly magnetised core that will snap a screwdriver to it at about 3/8", even after the amp has been turned off for a while. Something odd happening here.

[Steve Conner]

Transformer iron shouldn't be capable of retaining any significant magnetism. I think it's more likely your screwdriver is magnetised, than Fender accidentally made a batch of OTs out of high-carbon steel.

[Chuck H]

+1, magnetic screwdriver.

I built an attenuator that would whistle if you got within four feet of it using single coil pickups. The offending component was a 250mH air core inductor. My solution was to use two inductors in series flipped so that the EMF was reversed. To avoid inductive cancellation I made a shield sandwich with a coil, a layer of copper at a slightly larger diameter, a layer of insulation, another layer of copper and then the other coil. Seems to work fine. My point is that you be able to do better with more efficient shielding material than steel. A layer of copper, an insulator, a grounded layer of steel, an insulator and then another layer of copper, all at slightly larger dimension than the transformer profile should block virtually all the induced hum.

[Mick Bailey]

Definitely not the screwdriver. I checked this straight away.

There is however very little resistance when reading across the laminations - the core reads like a solid block. I've got an old Twin Reverb that looks like it came out of the sea with a really rusty OPT and it doesn't have any where near as low resistance as the Prosonic which reads 5 ohms max.

I'll temporarily swap the OPT to see if a known-good one makes any difference.

[Gtr_tech]

If the chassis is steel and the pwr and output xfmrs are in close proximity to each other, you'll get coupling between the two. So what is happening is the pwr xfmr is inducing a current into the output xfmr via magnetic coupling thru the chassis. Nothing you can do about this unless you can move either xfmr for a larger gap between the two.

[Mick Bailey]

Swapped the transformer and connected it up with much longer leads to experiment with positioning. Anywhere near the PT and it's the same story. I'll stick with my shielding arrangement as it seems to be the best compromise without re-designing the layout. I did find another reference (unresolved) about the very same problem, so not just this particular amp.

[Enzo]

Why not send a message to Fender customer service, and describe your situation. They may have something useful to add. If not, at least email costs nothing.