From what I've read about single coil hum, it can come from almost all sources of electromagnetic noise, such as fluorescent lights and all sorts of transformers. The only kind I can discern, however, seems to be coming from the transformer(s?) of my amplifier. When I am facing the amplifier or turned directly away from it, the hum is minimal, but if I turn perpendicular to it the hum reaches maximum volume. The physics major in me saw that angular distribution, and guessed that there is some sort of dipole radiation coming from the amplifier and detected by the pickups. In addition, my observations indicate that even when I have set my guitar to the bridge position, the middle pickup is contributing to the hum. I noticed this because, when the tone knob of the bridge pickup is at 1, adjustments in the tone for the middle pickup alter the level of hum. I'm confused as to how that works, but more importantly wondering about ways around the hum problem short of changing pickups. There is a baseline level of hum from the amplifier regardless of which pickup combination is engaged, a fix to which has been posted at
http://home.comcast.net/~machrone/bjr/bjrhum.htm
but this seems to be targeted at reducing electromagnetic interaction between the two transformers in the amp, and doesn't seem like it would affect the amount of noise radiated towards the guitar. Presumably the amplitude of the radiation falls off with the inverse of the distance from the source, so moving farther away should have some effect, as, of course, will playing while pointing the guitar right at the amp, but I'm wondering about some sort of internal fix along the lines of billm's mod, perhaps adding some sort of metal inside the amp that shields the area outside from the transformers' radiation. Any thoughts on this, or other suggestions about where to start in the quest to reduce hum? It's much more of a problem now that I have an overdrive pedal to play with, and certainly not a huge problem, but certainly an annoyance.
~Sam
This is a good place to start:
Playing guitar and never playing for others is like studying medicine and never working in a clinic.
Shielding magnetic fields isn't easily done. Better to move away from your amp and orient the guitar to reduce the hum pickup. Or if it bothers you that much, use humbuckers.
"A cheerful heart is good medicine."
Shielding magnetic fields isn't easily done. Better to move away from your amp and orient the guitar to reduce the hum pickup. Or if it bothers you that much, use humbuckers.
If the hum increases and decreases depending on which way you're facing it's RFI (Radio Frequency Interference). The link I posted will let you explore ways to shield the pickup cavities to reduce RFI created hum.
Playing guitar and never playing for others is like studying medicine and never working in a clinic.
If the hum increases and decreases depending on which way you're facing it's RFI (Radio Frequency Interference).
No. Plain 60 Hz. EMI as emitted from transformers is also directional. So is the EMI from CRT magnetic deflection coils, fluorescent lights, and from any other source. Remember those "lines of force" demonstrated by iron filings on paper with a bar magnet behind it? Similar curving lines of force connect the magnetic poles whenever a magnetic field is radiated, continuously or from an AC source. Where the lines of the local field at the pickup are parallel to the axis of the coil, the induced hum will be at maximum. If they're at right angles to the coil's axis, hum will be minimized. It's not always easy to predict what direction this will be at a given point some distance and direction from the source of the magnetic field. You just have to turn the guitar till you find the quietest orientation.
Amps don't emit RFI unless they're oscillating. If they're oscillating, that needs to be fixed.
"A cheerful heart is good medicine."
I get a nasty hum into my Strat when the light in the second floor hallway of our house is on. The light is controlled by three-way switches located upstairs and downstairs. The downstairs one is in the room where I practice, and is located behind a TV set.
The hum started happening after I swapped all the incandescent lightbulbs in my house out for compact flourescent lamps. I usually play my Strat with the pup selector in position 2, and it was a few weeks after the lamp changeout when I first switched to a single pup and noticed the hum so I didn't immediately associate the hum with the lamp change. At first I thought something weird was happening with the TV, as I'd get the hum when the guitar was oriented to have the pups pointing straight at the set. I was fooling around trying to figure it out when somebody upstairs hit the lightswitch and the hum went away and I realized what was going on.
So.... You sure it's your amp and not something else? :wink:
I'm actually pretty sure it's the transformer(s?) on my amp and nothing else. I removed the chassis from the cabinet to have a look around, and the hum depends on orientation with respect to the transformers even if I move the chassis around. The only other electrical device in the neighborhood of the amp is a power strip, which I determined was not the cause of the hum by moving it around while it was humming. I think I will put the amp back together pretty soon. For now I'll probably just face the amplifier when I have to play with lots of crunch in positions 1 or 3. Otherwise the hum isn't all that bad (although I'll need to take more care when recording).
~Sam
Most of the supposed EMI at 50, 60, 100 and 120 Hz is really either E (electric field) or M (magnetic field) not E-M (electromagnetic, of which RF or radio frequency waves or interference are a subcategory). One has to get several wavelengths away from a dynamic electric or magnetic field source before a true E-M wave has formed and self propagates in the fashion of a radio wave. The wavelength of 60 Hz is about 5000 km -- kinda far! Transformers, as well as pup coils are magnetic dipole antennas as mentioned in the original post. And Ric is correct, magnetic shielding is a difficult and usually minimally effective ... unless one uses somewhat exotic bimetalic shields that need annealing after they are shaped. Cancellation is the other technique. That's the principle behind humbucking pups and dummy coil pup systems. If simple shielding worked on magnetic fields, the humbucking pickup would not have been such a big innovation (not talking tone here). So the shielding in a guitar cavity does almost nothing for magnetic interference -- if it did, metallic pup covers would totally screw up magnetic pups. Instead, shielding addresses electric field interference produced by high voltage electric circuits and devices (flourescent tubes, CRTs, welders, spark gaps ...) and also higher frequency RFI.
Various techniques for post production magnetic shielding of amplifier transformers have been tried with limited success and sometimes even better success with seemingly bizarre, yet working configurations. One can effect a cancellation of interference -- say from one transformer to another (fixed situation) by allowing magnetic coupling to take place, but also setting up a capacitive coupling between those devices that manages to produce out-of-phase currents in the "receiving" transformer that cancel the magnetically induced currents. Again, this works for a fixed geometry, but is a useless solution for the guitar-amp scenario. Given a mixed combo of amplifier and guitar, distance and positioning are the best isolators. The energy transfer for magnetic coupling is inverse cube law. That means doubling the distance => 1/8 ratio reduction in coupled interference power relative to the original location. Double the distance again, and the interference is down to 1/64 that received at the original distance. Positioning works because most simple magnetic coils (transformers, pups coils ...) are magnetic dipoles antennas whose radiating and receiving patterns have end nulls (roughly toroidal coupling pattern ... a donut). A better quality power amp power supply transformer can improve the sitiuation, as the transformer core design and coil geometry determine the amount of magnetic flux leakage. Less leakage = worse magnetic antenna => less interfering magnetic emissions.
-=tension & release=-
And fluorescent lights, even of the compact variety, contain ballasts that radiate magnetic interference.
"A cheerful heart is good medicine."
Ummmm.....hmmmm..... Gnease beat me to it.
:P
-=- Steve
"If the moon were made of ribs, would you eat it?"