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Humcancelling a single coil

Discussion in 'Pickups & Electronics [BG]' started by wyliee, May 6, 2004.


  1. wyliee

    wyliee

    Jul 6, 2003
    South Hill, WA
    Hi everyone,

    I have a second hand F Bass BF6. It is a fantastic sounding bass and one I seriously think I could keep a lifetime. Unfortunately, it does have one problem.

    Some kind of Lindy Fralin humcancelling PU was dropped in the bridge position. It's an excellent pickup and I want to keep it.

    However, the neck PU is a single coil and I get the standard buzz when I turn it up regardless if the bridge PU is up or not. It's a great sound and matches well with the Fralin. I just want to get rid of the buzz. I had a tech look at the wiring and it looks fine.

    Any recommendations on getting rid of the hum? I know some Alembics do/did come with dummy pickups in them to cancel out any noise. Is there some way I could drop something in the electronics cavity? Suggestions please.

    Thanks,
    Eric.
     
  2. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Hi Eric, several thoughts.

    One is, you might check into the guitar wiring menu on guitarnuts.com, especially the part about "star grounding". That may or may not solve your problem, but on general principles it's beneficial to follow good wiring practices.

    The second is, the F bass is an active instrument, isn't it? I'm not sure what a BF6 is, is that the fretless version, and if so is it a passive-only instrument? If it's passive, you might want to check with George Furlanetto at fbass.com, he's used some very interesting hum-cancelling methods over the years. For instance I have an early model fretless 6 that has two hum-cancelling dummy coils mounted at right angles inside the cavity, they look exactly like pickups (and they may in fact be pickups) but they're wired strangely and George can probably tell you what that's all about.

    Lastly, there's only one surefire way I know of to get rid of hum from a single coil pickup, and that's to use an onboard differential active preamp. In other words, you have to lift your pickup coils from ground. Most manufacturers (including F bass) ground one side of the pickup, which is a bozo no-no when it comes to hum. I did this mod to an early model active BN6 (Studio version) that I bought around 1991 or 2, when I got it it had a horrible hum problem, but now I can stick it right up next to a fluorescent light fixture and it's quiet as a church mouse. Drop me an e-mail if you'd like to see the schematic for this mod, it's very easy and if you're handy with a soldering iron it can be done with about 20 dollars in parts and a couple of hours' worth of construction. It does require a battery though, so if your instrument is passive you'll need to put a battery somewhere in the cavity, but if you have an active instrument you can use the battery that's already there.
     
  3. geshel

    geshel

    Oct 2, 2001
    Seattle
    You're confusing the matter, because you're talking about two different kinds of interference as if they were the same thing.

    You cannot make a purely single-coil pickup humcancelling by lifting the ground. You must use another coil of wire to cancel out any induced hum (60Hz type stuff).

    Lifting the ground like you mention might affect the high-frequency interference picked up (like from fluourescent lights). Proper shielding will go a long way towards this, too.
     
  4. wyliee

    wyliee

    Jul 6, 2003
    South Hill, WA
    Thanks for the response. BF6 is the fretless model. It has the old fashioned V/V/T configuration. Like other F Basses, the output is pretty low. A friend of mine has designed a nice, high quality boost only preamp and I had that installed. Otherwise, it is passive. I'll have to check with George. Don't know why I didn't think of that earlier!

    I am curious about your mods too.

    Thanks.
     
  5. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    "You cannot make a purely single-coil pickup humcancelling by lifting the ground. You must use another coil of wire to cancel out any induced hum (60Hz type stuff)."

    Well, I understood the issue to be that there is unwanted hum, and the object to be getting rid of it. The statement above is correct as far as it goes, but my method is not to "cancel" the hum, it is to prevent it from being picked up in the first place. The rationale is somewhat complex from a physics standpoint, but my method definitely works, I've used it on at least fifty basses without any problems. The issue is, that "induced hum" doesn't necessarily come from the pickup itself. In a perfect world, the ground on your passive guitar would be the same as the ground on your amp, but in real life that's never the case, 'cause instrument cables have resistance, capacitance, and inductance, and merely having a long cable can lift the instrument ground and cause your whole cable-pickup-electronics system to become a gigantic antenna. The DIAP guarantees that no part of the coil is grounded, and secondly that the wires from the pickup to the electronics are as short as possible (as distinct from having the ground wire travel all the way through the instrument cable and back into the amp). That way, the likelihood of stray EMI crossing the surface area of the ground system is almost nil, and in most cases if the pickup casing is correctly grounded this will be more than sufficient to alleviate the problem.
     
  6. geshel

    geshel

    Oct 2, 2001
    Seattle
    I'm sorry, but you don't understand the physics involved. The pickup coil will have a current induced in it by whatever changing magnetic field goes through it. This is a law.

    You cannot do anything by "not grounding" the pickup to prevent this. It is very simple: if the pickup detects the 60/120/180Hz etc from the string/magnet interaction when playing a C note, it will pick up the same frequencies of electromagnetic interference from any leaky devices nearby. It can't selectively detect some magnetic field changes and not others at the same frequency.

    A hum-cancelling pair (whether the other coil is active or passive) is the only way to cancel the effect (you cannot prevent it from happening in the first place without shielding the pickups against magnetic fields, which would mean the whole bass (and player) would have to be wrapped in a thick layer of metal). This is, actually, a balanced system at that point - it has to be magnetically balanced, not electrically balanced.

    Most of the details and explanation in your post is concerned with higher-frequency EMI such as radio interference -- where the capacitance and inductance of wires and ground connections after the pickups comes into play.
     
  7. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Geshel, all due respect man, but I don't need a physics education. I have a degree in it, and I've been desigining and building bass amps for the better part of 40 years. Also I'm a studio tech who happens to specialize in grounding problems, that's how I make a big part of my living. Grounding is NEVER as simple as the theory would suggest.

    Again, my point is, it is incorrect to consider the pickup in isolation. Your statement that you "cannot" do anything by not grounding the pickup and etc. might be true "in theory", but it doesn't apply in the real world. Hum problems "almost never" result from pickups in isolation.

    It's okay though, I don't want to argue the point with you. All I can say is, try my method, it works.
     
  8. MAJOR METAL

    MAJOR METAL HARVESTER OF SORROW Staff Member Supporting Member


    If you have time could you explain to me the diffrence between a true single coil and a humcancelling pickup and what stops the hum. Thanks
     
  9. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
  10. MAJOR METAL

    MAJOR METAL HARVESTER OF SORROW Staff Member Supporting Member

  11. geshel

    geshel

    Oct 2, 2001
    Seattle
    Explain to me, then, why this system has never been sold on any production guitars? There is a HUGE market for pickups that sound like single-coils but are hum-cancelling. There is no question that, if what you say was true, it would be all over the place. Yet, I've never read anywhere else about such a possibility. The page you link to seems to contradict what you say.

    And, you know, as far as "in theory" goes, that is sort of a one-way thing: "in theory, you can do that, but in practice it is not that simple". You don't very often read, "in theory, you can't do that, but in practice all you have to do is. . . ." -- because then you would have to change the theory.

    Please, just explain to me this (my degree isn't in physics): how can you remove some of the signals that the pickup senses, but not others at the same frequency and the same time? Thank you.
     
  12. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Well, my method is a version of a "balanced connection" from the pickup to the nearest electronics. Crusty old audio engineers have been advocating this method for nearly 60 years (and Les Paul himself was a big proponent of this, he used several guitars that he had customized with balanced connections). For some reason, the manufacturers haven't found it cost effective to do this, although some will do it as a custom job or a one-off. Balanced connections are commonly used in small-signal applications, especially microphones, for the exact reasons I'm referring to. Ever try grounding one side of a mic? Try it sometime, you'll see (and hear) exactly what I'm talking about.

    Again, the point of my method is not to "remove" hum. The point is to minimize its influence in the first place. The only part of the pickup that should ever be grounded is the casing. That way, the ground will do what it's supposed to do, which is to shield the wires carrying the signal. Otherwise, you have one leg of your signal connected to the very part that is most likely to pick up hum (the ground plane, which as I mentioned is far from a perfect ground). "Ground" is mainly a theoretical concept, in practice it's extremely difficult to achieve. My claim is that induced hum in guitar pickups usually has more to do with "ground loops" that it does with pickup coils. As with many ground loops, the symptom may appear in one place while the problem comes from somewhere entirely different. One of the things that an active preamp does is that it forces the guitar's grounding system closer to "actual ground" (as defined by the amp), that's part of why it's so effective at reducing hum.

    This, by the way, is what first made Alembic famous. Ron was able to solve Phil Lesh's grounding problems when the Dead started playing larger venues and the backline was often thirty feet or more behind the band. He was able to modify Phil's bass to allow him to use fifty foot instrument cables with little or no hum pickup, by installing an active preamp with a proper grounding system.

    This stuff is all over the internet, just search on Google and you'll find plenty of literature. Also the Audio Cyclopedia is an excellent reference on the theory and practice of ground loops. It's a little esoteric 'cause not many people understand the physics behind it, it does in fact obey "theory", but the issue is that most people consider the guitar to be the black box, and my claim is that that's inadequate. One can't understand hum in pickups without considering the instrument cable and the amplifier too. Heck, in my forty years as a tech I've solved plenty of peoples' hum problems without ever touching their instruments.

    Grounding is not a "simple" science. My dad made an excellent living for over fifty years solving grounding problems that no one else could tackle. And even in today's high tech world, you'd be amazed at the number of issues that simply interconnecting two pieces of equipment can cause.
     
  13. geshel

    geshel

    Oct 2, 2001
    Seattle
    You're still completely missing my point. I know what balanced connections are. They nullify any interference picked up through the length of the connection. They cannot nullify any interference picked up before the balance point begins, because as far as the balancing system is concerned, that interference is part of the intentional signal.

    A single-coil pickup is an antenna. It will pick up 50/60 cycle hum (and whatever else in the audio frequency range) from any nearby radiating devices. The only way to cancel that hum is by using another coil of equal proportion.

    You cannot make a single-coil pickup hum-cancelling by using downstream electronics.

    This is a fact, plain and simple, and if you continue to try to convince people here otherwise you are perpetuating a fraud. This is my last post in this thread. Instead of trying to convince me further, I suggest you spend your time with a patent lawyer because you've just done the impossible.

    P.S. The "early-model F-Bass BN6" you mention in your first post had humbucking pickups! (split-coil)
     
  14. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Well, it's obvious we're not talking about the same thing. I don't have the time to explain it any more than I already have. This isn't anything novel, nor is it rocket science. I don't dispute that pickups "pick up" hum and other EMI. All I'm saying is that in 99.99% of the cases, this is exacerbated by factors that have nothing to do with the pickup, and if those other factors are addressed, the pickup behavior will become tolerable.

    By the way, you're wrong about my F bass, and how could you possibly know what kind of pickups are in my bass? Try to be civil, and avoid making assumptions and wild accusations. Please. :)
     
  15. geshel

    geshel

    Oct 2, 2001
    Seattle
    Sorry about the assumption. I've never seen a BN-6 with single-coil pickups.
     
  16. :confused:

    my understanding (correct this if I'm wrong) was that Les Paul's balanced output guitars and basses (eg. Les Paul Recording, and the one the Jack Cassady sig. is based on) used low impedance pickups and internal impedance-matching transformers.

    -microphone coils don't pick up hum as they're low impedance,
    same with low-impedance guitar and bass pickups, which use either a transformer or an internal preamp to enable them to be connected to conventional amplifier inputs.

    also my understanding was that the only way to stop a high impedance single coil pickup from picking up hum was by combining its signal with another coil with the hum out of phase- be it another pickup or a phantom coil.

    -although Vigier use some circuit that cancels hum in their Excess bass- how does this work?
     
  17. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Well, impedance probably has more to do with the ability of a coil to transmit signals, as distinct from picking up stray EM fields (especially in circuit, and relative to other factors). Consider Alembic's Series 1 electronics, for instance, which use low impedance pickups and still require a dummy coil.

    This is probably one of those cart-before-the-horse deals. There's two issues, or maybe three, and it's important to separate them. The first and most important issue is, the actual "'picking up" of the unwanted signal by the pickup. As geshel correctly pointed out, once the pickup has the signal, you're screwed, and there's nothing you can do but try to "cancel it out" by whatever clever means. However, you can sheild the pickup, and use other physical methods to minimize the actual pickup of the unwanted signal. If you can prevent the induction in the first place, you won't have to worry about "cancelling" anything. That's why, if you ground one side of the pickup coil, you're really defeating the purpose of the shield. In that case, all you're doing is adding surface area to what is probably already an ungrounded antenna. Which leads to the second issue.

    The second issue is, the "grounding" of the instrument. If your ground plane isn't at ground potential, then anything that's "grounded" in the instrument is actually contributing to the signal that you're hearing in the amp. Even if your instrument is perfectly wired according to star grounding and all the other wondeful methods, it might matter not a whit if your ground plane gets a decent size AC signal impressed on it. It doesn't even have to be especially strong. The magnitudes in question can be calculated, and here's where the impedance comes into play. It does have an influence, and it's relative to the other parts of the system that connect the pickups to the amp. It's not so important in the pickup by itself, but once the pickup's connected to something, and especially if that something already has an unwanted signal on it, then the usual AC math can be used to predict (estimate) the interactions.

    And the third issue is, the instrument cable and the "rest of the system". Even humbuckers aren't perfect, and in a poorly wired instrument, they can be just as bad as single coil pickups (well, okay, "almost" as bad). If you're getting hum when you're walking around the room, how much of that is due to the pickup, and how much is due to the "rest of the system"? Consider that a 60Hz signal has an incredibly long wavelength (5 million meters or something like that), and it's only the part that crosses the coil that creates a potential difference and induces hum. A pickup is pretty tiny compared to a 20 or 30 foot instrument cable. And, the transformers in your amp probably have many more turns of wire than your pickups do.

    There are many ways to deal with unwanted hum. For instance, those with tube amps may be familiar with the "hum balance" control, it's usually a 100 ohm pot across the filament supply. In a way, it's a clever electronic approach to "humbucking". Maybe Vigier does something simlar, who knows. My guess is they've discovered that their hum isn't actually coming from the pickups, it's coming from somewhere else (or at least the root cause, the "enabler" if you will, is somewhere else), and they've addressed the problem through clever electronic means. Just a guess though, I'm not familiar with their basses.

    Electronic instrument systems are pretty complex. There are many parts, and you can pick up unwanted hum at any and all steps along the way. If you start by addressing the pickup and the wiring in the instrument, you'll be way ahead of the game. Why do most manufacturers ground one side of the pickup coil? Probably because it's convenient to do so, after all most instrument amps are single ended. And really, that should be okay. A well designed and built passive instrument will do just fine with an amp that's working correctly. The thing is, people hear hum and right away they say, "oh, it's the pickup", when in reality it may only be a symptom of a deeper underlying problem.

    Consider this very common experience: take a single coil instrument (a Strat or something), and use it with an amp of your choice. Chances are good that you'll get some hum as you're walking around the room. Now take that same guitar and same instrument cable, and connect it to a different amp, and you won't hear any hum at all, it'll be quiet as a church mouse. Why is that? It's an example of what I'm trying to point out, which is that in any given system, induced pickup hum may only be a symptom of an entirely different problem. I've worked on amps where I can make the single-coil hum appear and disappear just by moving a wire in the power amp section. A ground loop in the amp can definitely cause induced hum symptoms in the instrument. I've seen this hundreds of times in my tech career.

    Really, us musicians don't have it so bad. If you want to hear some horror stories, talk to the radio engineers, especially the ones that work with relatively high frequencies. There was a guy at RCA that actually committed suicide 'cause he couldn't figure out why two identical circuits exhibited different behavior with EMI. It can be very tricky stuff, it really gets down to very fundamental physics and in no way is it as simple as saying "the pickup causes the hum".

    This thread is a little frustrating, it's difficult to encapsulate the relevant concepts into a few paragraphs. The Audio Cyclopedia has a good discussion on grounding, and there are some excellent resources in the radio world too (even the ham guys are pretty good with this stuff, it's been a while since I've pickup a Radio Amateur's Handbook but as I recall there was some good info on EMI and grounding in some of those manuals). Anyway, I'm not really an expert on this stuff, I"m just "good enough" to find work every now and then, and I picked up a lot of practical intuition from my dad, who spent a good part of his life de-hummifying studio and broadcast equipment.

    All I can ask is, dig a little deeper. Don't believe everything you read on the Internet. Don't even believe me! Check it out for yourself. There's lots of really cool hum-related experiments you can do with a guitar and an amp and a few wires.
     
  18. ah, I'm beginning to get it now.

    although I often use an XLR-to-1/4in mono jack plug lead to connect a mic to my 4track input (ie. grounding one side of the mic coil), with no hum problems.
     
  19. wyliee

    wyliee

    Jul 6, 2003
    South Hill, WA
    <Raising hand and coming out from under the table.>

    If F Basses don't have single coil pickups, then what do I have?

    You can see them at: http://home.earthlink.net/~wyliee The green fretless is the one I want to fix.

    Now I'm more confused than I was reading the above responses.

    -Eric.
     
  20. geshel

    geshel

    Oct 2, 2001
    Seattle
    I thought that all F Basses w/ those pickups were dual-coil pickups. The recent ones can be split into single coil by pulling up one of the knobs. Before the straight pickups were used, the BN-6es had the split-coil (humbucking) pickups like your black fretted 6.

    A good way to know is: does the hum you are having a problem with go away when both pickups are on (eg, it's only a problem with just one or the other pickup)? If so, then yes they are single coil. If it doesn't, it might be that they are humbuckers. Or, it might be the type of interference that humbucking pickups wouldn't do anything to fix (so the jury is still out if they are single coils or humbuckers), and a better shielding/grounding setup like nonsqtr mentioned would do the trick.

    And, of course, you can always ask George. :)

    P.S. that is a beautiful trio!