Rack Mount Equipment and Magnetic Fields.

Keith Hilton · From: 248 Laurel Road Ozark, Missouri 65721

Most of the guys playing steel have a container of rack mount equipment. The container has various rack mount multi-effect units, tuners, and some even have big rack mount amplifiers. Each of these devices has "it's own" power supply. Many of these rack mount devices are grounded to case. All of them have a different ground potential. I have listened to guys on the FORUM talk about a product for racks called, "HUM FREES". I assume they are small plastic pieces that keep metal from touching metal. Correct me if I'm wrong. Keeping case metal from case metal probably does cut down on the hum problem. What really spooks me most is the magnetic field given off by anything electrical. To prove my point, take a Peavey ProFex II, and move it to about 3 inches from a single coil steel guitar pickup. Then move it away! We have all these multi-effect units, turners and big amps stacked together in racks---all giving off their own magnetic field. Worse yet, all powered by a different power supply, and all with a different ground potential. No wonder guys complain about HUM! With arrangments like this, how could you keep from humming? I would like to see someone think up shielding material that could be put over each unit. In this way, the magnetic field given off by a particular unit could be shielded, and would have no effect on other equipment.
Pre-amps sometimes get blamed for hum problems, and this confuses the issue. The truth of the matter is that Pre-amps only increase the signal strength. Any hum that is present in the signal only gets amplified by pre amps,on it's way to the power amp.
Lowering signal strength does lower hum, but it does not get rid of it. I firmly believe that a lot of hum originates from separate power supplies with different ground potentials, and from magnetic fields.

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[This message was edited by Keith Hilton on 06 March 2000 at 07:34 AM.]

Jack Stoner · From: Kansas City, MO

The "ground hum" problem is not unique to musical instrument equipment. It has been around in audio equipment for years.

As far as musical equipment, there are so many places for external influences to affect the sound that it's a wonder it works as well as it does.

The first place is the guitar. Single coil (non humbucking) pickups.
Next is the open wiring in a guitar. Nothing is shielded under the guitar, e.g. at the neck selector switch and at the output jack.

Then you can get to the cables, effects and the amp. Some equipment tends to induce more, such as the external power transformers which are not shielded with grounded (metal) covers. Some effect units, in an attempt by the manufacturer to package the device in as small a package as possible (e.g. a 1 rack space unit) sacrifice some internal shielding or use digital switching power supplies.

Then you get to the amplifier, which many times does not have the amplifier completely shielded - many amps mount in wood (not metallic and grounded)which leaves one side of the amp chassis unshielded. The externally mounted reverb pan (spring unit) is not completely shielded and can introduce hum.

Then after that, you have the many areas on a stage that have AC floating all over it.

Come to think of it, it's a wonder anything works as well as it does.

Rich Paton · From: Santa Maria, CA.,

Hi guys! Great thread.

There's two types, "electrostatic" shielding", and "magnetic shielding", and each is an attempt to eliminate/minimize the leakage of a field from a source, and/or eliminate/minimize the effects of external fields on components and circuits which are sensitive to such fields.

Most undesirable, stray sources of "electrical noise" contain energy in both the electrostatic and electromagnetic domains, which simultaneously exist and radiate at a 90 degree angle to each other.

The only truly effective means of electromagnetic shielding is through the use of " mu metal ", a special ferrous alloy which is expensive, heavy relative to common steels, and not particularly easy to tool or work with.

Thus, most shielding schemes you will see are of the electrostatic variety, such as conductive guitar cavity paint, metallic tapes, braids, sheet metal under a pickguard, and window screen stapled into a amp cabinet where the open open part of the chassis mounts against the inside of the cabinet.

Exceptions exist as the noise frequencies reach the "RF" or radio frequency region and above. Here shielding can be designed and built to reflect the noise back, rather than attempt to stop it from passing through a shield.

The wonder of all the hodge-podge gear we lash together actually "working" is in part due to the phenomenon of "psychoacoustical masking", where the ear/brain processes the louder (signal) portions of sound, and ignores ("masks") the lower (noise) ones.

The subject of undesirable noise, as it applies to electrical engineering and manufacturing, is a huge one. My first job out of the Navy in 1975 was as an RF/microwave "development/R&D" technician.

To implement effective equipment noise reduction techniques, I had to utilize a dozen or so engineering books, stacks of spec sheets, about $100K worth of test equipment, the knowledge of some fine engineers, and lots of good old trial-and-error, as well as luck!

[This message was edited by Rich Paton on 06 March 2000 at 04:30 AM.]

Mark Amundson · From: Cambridge, MN USA

Hum reduction in PSG systems is somewhat a self inflicted item. The old system of guitar->pedal->amp works really great if your happy with the tone and have decent cabling.

Once you start fooling around with "separate this" and "wall-wart powered that", you now are dealing with problems needing engineering assistance. Once in the rack, take advantage of all the balanced audio connections you can make.

Good manufacturers are somewhat responsible for keeping their magnetic fields contained with their units. What they can not ensure, is how customers will interconnect them. I regard "hum-frees" as a last-ditch effort to cure a ground loop. Good rack boxes with proper power distro usually show no problems.

Mark,

Keith Hilton · From: 248 Laurel Road Ozark, Missouri 65721

Great answers guys. You guys really hit the nail on the head. I personally think the problem is going to get a lot worse. People keep adding more and more electronic gear. Every day there are more and more electronic devices. Electronic technology has advanced so fast that a lot of musicians don't understand how to fully operate the gear they use. In 1950, people used hand operated can openers. Today they are electric. Try running a can opener 3 inches above single coil pickups and hear what a magnetic field sounds like.
I personally feel that noise reduction is a area wide open for new products, and inventors.

I am like Jack, I wonder sometimes how any of this stuff even works! It seems to be something people learn to live with, in degrees of moderation. I suppose there isn't a simple answer. As Jack said, there are so many sources, and so many possible entry points.

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