Shielding a power transformer

[cooldude666]

I've been helping a friend with his Sunn Alpha 6 PA/mixer. He's using its really lush reverb for recording purposes. I'm trying to quiet the circuit down a bit and the major offender is PT hum being picked up by the output transducer. How can I shield this puppy? Anyone have a good source on big metal boxes I can put around the PT? Unfortunately I'm pretty locked in to where the components can live. Even rotating the PT 90 degrees is out of the question given space constraints. Unless anyone has another solution I can try, I feel as though I'm left with shielding the PT. Is there a better means of shielding that a big metal box? Thank ya'll

[Chuck H]

IMHE you'll have limited success with shielding. If there is any room at all to relocate the pan you may find a quiet spot without moving it very far at all. I've installed reverb pans in head cabinets that were dead quiet in one spot and hummed like crazy moved one inch any direction. Might be worth a try if it's possible.

[Mick Bailey]

Make sure the pan is oriented correctly to position the output transducer at the 'correct' end - it may need rotating 180 degrees. Putting a box around the PT can sometimes have the opposite effect and produce more hum. The material, placement, construction and shape needs to be thought out and even then may not produce any meaningful result, especially if there's a fault on the circuit somewhere else.

So ask yourself if it would have been built with this level of hum from new. If the answer is no, then there could be a grounding problem (bad RCA ground connection) or the pan replaced with one that has the incorrect grounding configuration for that particular piece of equipment. Some pans are only grounded at one end.

[Gtr_tech]

This is a powered mixer right? Well if all it will ever be used for is an effect, I would unhook the output section and use an external pwr supply for the channel circuitry. Should'nt be that hard...

[R.G.]

Are you SURE it's radiated M-field from the PT? This devolves into a few other questions:
1. Have you looked at the hum on a scope and is it mains frequency or 2x mains? If it's mains, it MIGHT be radiated M-field. If it's 2x, it's rectifier ripple.
2. If it's 2x, temporarily bridge the filter caps with equal-size additional ones. If the hum changes, replace the filter caps. In fact, if it is more than 10 years old, replace all the electro caps in it. Odd are at least one of them needs it.
3. If it's 1x mains, do all the work needed to correct/update the ground wiring to eliminate hum before you go trying to destroy the world to shield the PT. It is rare for equipment like this to have decently-done grounds as they're understood today. Here are some common issues:
- Are the input connectors' grounds tight and properly returned to the circuit , not the chassis?
- Using the outer chassis as a ground return. There should be exactly one connection between the chassis and signal ground, and exactly one connection between AC mains safety ground and the chassis for lowest hum.
- Improper connection of the power ground to signal ground. The + and - filter caps have their centers connected together. Connecting signal ground to one side of the wire connecting them will introduce hum from the rectifier pulses. These should be connected by a substantial conductor, and the signal ground return should connect to this in the resistive center.
- Improper connection of AC safety ground to chassis. It needs its own bolt. The old technique of connecting it on one of the power transformer mounting bolts is not reliable. A loose safety ground can cause very audible hum.
- Improper wiring of the power transformer CT to the filter caps. This wire must go the the center conductor between the two filter caps and no where else. Not doing it this way makes for increased hum.
- Snubbed rectifier diodes/bridge. Ordinary power rectifiers have a "slam-off" when they cease conduction. The power and edge speed of this turn-off can make the wires ring at RF, at 2x power line frequency inducing audible hum.

There are many more that do not involve the PT.

On the PT, is this the original PT? And does it have a copper "belly band" and steel end bells? If so, chances are mods to the PT won't help much. But you can't tell unless all the rest of the hum adders have been neutralized.

[Chuck H]

Yupper. Not often recognized is that pan reverb recovery circuits require a ton of gain because the pan output is under 10mV!!! Usually closer to half that! Getting it up to a useful signal level is opportunity for all kinds of noise to creep in. This is especially true because reverb circuits are often regarded second to the dry signal and they are often located mid way or later in the circuit. A bad place to be when trying to generate more gain than is required for the main instrument signal overall. I would even go so far as to say that pan reverbs are USUALLY too noisy and that it's less common to find one that's only acceptably noisy.