Hum, Polarity, and Phase in Guitar Pickups

[PTChristopher2]

There was an earlier thread somewhat on the subject, but I do not want to mix the basics in with emotional attachment to opinion, so a new thread on the basics.

But to clarify:

- A single coil PU picks up ambient noise, mostly in the form of indoor AC power hum. No news there.

- This hum has a phase. It goes positive at particular time, and it goes negative at a particular time.

- A substantially similar signal that is 180 degrees out of phase with this hum would "cancel" the hum if combined in the same signal.

- The hum comes from the coil, and is not associated with the magnet in the PU.

- You can make a 180 phase change in the STRING VIBRATION signal in a PU by changing the magnet polarity, OR changing the coil direction, OR reversing the coil leads.

- You can make a NET ZERO change in a STRING VIBRATION signal by changing the magnet polarity AND changing either the coil direction or coil leads. You have thus changed the phase of the hum by 180 degrees, but made no change at all in phase of the string vibration signal.

- This is how an HB PU works. One coil has north magnetic poles and a given coil configuration. This is run in series with a second coil with south magnetic poles and an opposite coil configuration. The result is that the guitar string vibration signals are in-phase, while the AC hum signals are out of phase. The hum is thus bucked.

- If we run two single coil PUs on a guitar, they are typically run in parallel with the switch in the middle position.

- These two PUs are traditionally of the same magnetic and coil orientation. They hum. The hum of one PU is in-phase with the hum of the other.

- To many, there seems to be an opportunity here. If one PU is reversed in both magnetic polarity AND coil configuration, we could enjoy a humbucking feature when the PUs are used together.

- Two PU's in parallel are similar (often slightly lower) in net volume vs. a single PU.

- The two coils in an HB PU are run in series. Running these two coils (OR two separate PU's) in series results in a much stronger signal (but not quite 2X as strong) vs. one of the coils.

- The hum canceling effect of the having reversed coils works both in series and in parallel - both within a single HB PU, or in a combination of two PUs.

- Sooooooooo,...

- If you had two P-90's and they had both opposite magnetic orientations and opposite coil configurations, the hum would cancel when both PUs are used.

(I am glossing over an aspect of the coil reversal to keep it straightforward.)

And the prickly part:

It is not clear in electromagnetism, or mechanics, that there is any perceivable difference in the phase of a signal.

If one genuinely measured a difference between the sound of a pair of P-90's when one was "reversed" vs. the other in both magnetic and coil polarity, this difference could be associated with many factors supported by actual principle.

- The coil of one test PU can be different from the other even if both are exactly the same length and DC resistance. (At the moment, my excuse of a brain can come up with three ways by which this can happen.)

- The magnetic fields of one test PU can be different from the other.

- The strings on the test guitar can be partially magnetized by the "north" test PU, then placed over the "south" test PU, yielding a potential difference in sound for those who feel they are remarkably attuned to such things.

- The PU position vs. the strings is different.

So there are many ways in which one could mis-attribute a sound difference to PU polarity without it being "all in your head".

Of course there is still the significant possibility of one "perceiving" what is not a genuine sound difference at all. But this suggestion can be upsetting.

All in my opinion - albeit with some associated objective principle.

Chris

[339 in june]

Great post, Chris !
Very clear and understandable explanation, as usual coming from you.

Much appreciated

Thanks

[PTChristopher2]

Why in-phase can sound "out of phase":

If we play in the middle position on a traditionally wired Telecaster, or one of the in-between positions on a Strat, the resulting sound has a slight "quack", as some say, that we associate with running two pickups with the string signal "out of phase".

This is because even when two PU's are in-phase with each other, parts of the string vibrations are out of phase.

Staying with the Telecaster:

The primary string vibration from the fret to the bridge will have the string moving "up" vs. one PU at the same time it is moving "up" vs. the other PU. The signal from this vibration through each PU is in-phase.

But there are MANY harmonic vibrations going on at the same time on the string. These mostly involve wavelengths that are subsets of the primary vibration. Some of these harmonics will result in the harmonic vibration moving "up" vs. one PU and "down" vs. the other. This harmonic will exhibit some degree of "phase cancellation" even though the PUs are in phase.

Sometimes this is barely noticeable, sometimes is lends a pleasing sparkle, sometimes it sounds too thin and nasal.

(Can you tell that I am putting off some paperwork that I should be doing?)

Chris

[Woody Sound]

My PRS C24 has a 5-way blade. Postions 2 & 4 (the "in between" positions) are bridge humbucker with neck single, and both pickups in single. I wish I could find out which coils are used (inside/outside), and whether they're series, parallel, in or out of phase, etc. I really like especially the #4 setting (2 singles), and I don't know how to duplicate it in another guitar.

[PTChristopher2]

In position #4, there is arguable reason to favor the "hot" end coil. In any case, it is easy to check.

Plug in to an amp and switch to position #4.

Take a very small screwdriver, or other small metal thing, and tap the pole pieces and screws lightly. (This guitar typically has no PU covers.)

It will likely be very obvious which coil is active on each PU.

It is very likely that the PU's are in-phase. An out-of-phase signal from the single coils of two humbuckers would be extremely weak and thin.

But surely a call to PRS would be interesting on the details of these switch positions?

Chris

[Woody Sound]

In position #4, there is arguable reason to favor the "hot" end coil. In any case, it is easy to check.

Plug in to an amp and switch to position #4.

Take a very small screwdriver, or other small metal thing, and tap the pole pieces and screws lightly. (This guitar typically has no PU covers.)

It will likely be very obvious which coil is active on each PU.

It is very likely that the PU's are in-phase. An out-of-phase signal from the single coils of two humbuckers would be extremely weak and thin.

But surely a call to PRS would be interesting on the details of these switch positions?

Chris

Aha! Found this. Obviously the slug side is inside coils. But they still don't address parallel/series/phase, which is where I believe the quackiness comes from. At least for the non-tech people.

Attachment 14795

[PTChristopher2]

OK on the inside vs. outside. But can you check as mentioned to see which coils are active in position #4?

If they are parallel and out of phase, the volume drop would be extreme.

A sound sample will make it far easier to speculate - but a call to PRS may help.

Also, a pic of the control cavity can reveal much as well.

Also, you can plug a cable into the guitar, then check to output DC resistance with an ohm meter. If you put all knobs at 10 and then let us know the DC resistance in all 5 switch position this will give useful info.

[Woody Sound]

OK on the inside vs. outside. But can you check as mentioned to see which coils are active in position #4?

If they are parallel and out of phase, the volume drop would be extreme.

A sound sample will make it far easier to speculate - but a call to PRS may help.

Also, a pic of the control cavity can reveal much as well.

Also, you can plug a cable into the guitar, then check to output DC resistance with an ohm meter. If you put all knobs at 10 and then let us know the DC resistance in all 5 switch position this will give useful info.

Good idea, thanks, will do that as soon as I have the chance.

[oldane]

Why in-phase can sound "out of phase": ....

But there are MANY harmonic vibrations going on at the same time on the string. These mostly involve wavelengths that are subsets of the primary vibration. Some of these harmonics will result in the harmonic vibration moving "up" vs. one PU and "down" vs. the other. This harmonic will exhibit some degree of "phase cancellation" even though the PUs are in phase.

Yep. One can even get a "quacky" strattish sound with just one neck PU and a carefully selected picking position. I can get that sound when picking in a position which on a Strat would be midways between the neck and the mid PU. It works best with a pointed pick and a single coil PU. A blues guitar playing nephew of mine didn't believe this, so for the fun of it I recorded the sound with my partscaster (which is an ideosyncratic home assembled instrument with only a HCC in the neck position, set up with heavy flatwounds) and e-mailed him the MP3 file. He heard the "quack" clearly and and told me not to BS him and that it was made with my Strat with the neck and mid PU in tandem. (I can get the same quack with my Painter, but he might have heard the reflection of the hollow body which would have given him a clue he shouldn't have). I have yet to prove it next time we meet with a guitar and an amp on hand.

[xuoham]

Great lesson, thank you !

[PTChristopher2]

Hiya xuoham,

I was posting more as a clarification and completion of my view - not as a lesson that is to be taken as the word of "ultimate wisdom".

Glad if it helps, even better when it kicks off other things like Oldane's comments about playing in a way that also gives the sort of notches in frequency response that we associate with out-of-phase quack.

Chris

[GNAPPI]

Great thread Chris! Another quirky attempt at noise cancelling was in the Gibson P100's.

From what I read they reverse wound a second coil around the bobbin (whether the second coil was grounded or wired with the output coil I'm unsure) and used that as a choke of sorts. I have an ES-135 with these and they work well at cancelling the noise, but the downside seems to be a loss of signal (volume and sound envelope) not seen in a true single coil. Have you had experience with these.. anyone?

[PTChristopher2]

Yeah, P-100s are stacked HBs. The coils are one on top of the other, as opposed to side by side in a "normal" HB.

I never thought they sounded bad at all. Sticking to Gibson PUs only, I suppose they do lack the sort of characteristics that made P-90s or PAFs beloved.

PAFs sample a relatively long segment of string yet do not get muddy, and P-90s have single coil spark yet do not get shrill. In my opinion.

Then there are fat HBs that some love for output level and Benedetto A6/B6 style midrange.

But in my opinion, the P-100 just sits there with moderate output, decent but not sparkling highs, and seemingly much less of the P-90s dynamics. But I have never really measured anything on P-100s, so this is just vague subjective response on my part.

If I bought a guitar with P-100s in there, I'd certainly work with EQ and a variety of picks with the full expectation of finding many fine sounds.

Chris

[mad dog]

I have three 2 x P90 guitars now. Two have one p/u reverse wound, for hum cancelling in the middle position.
And one without that option. This last one (a Les Paul Special), happens to have the noisiest P-90s, at least in my home environment. I'm going to see if I can get it to be hum cancelling, as that is too useful a feature not to use. Haven't yet heard the same set of p/us hum cancelling vs. non hum cancelling in the same guitar, so no idea what sonic trade off there is, if any. I have very much liked the tone of the reverse wound sets (Lollar, Wolfetone, SD Antiquites and now Reverend) that I've heard, so it's not an issue with me.
MD

[PTChristopher2]

>>> This last one (a Les Paul Special), happens to have the noisiest P-90s, at least in my home environment. I'm going to see if I can get it to be hum cancelling, as that is too useful a feature not to use.

Are you planning a new PU for one position, or will you change the magnets' polarity orientation and internal leads in one of the existing PUs?

[mad dog]

Chris:

I'm going to ask a tech friend about the magnet orientation and internal leads swap, as you mention. Not sure enough of my own electronic skills to do it meself. I'd rather do it this way, as the pickups sound rather nice. (Gibson P-90s, in place of the P-100s that came stock in this 1990 guitar.)
MD

[PTChristopher2]

It would be a rather easy thing for a tech in my opinion.

[entresz]

I'm partial to the Bill Lawrence L609 pickup-
sonically it's a cross between a jazzmaster pickup and p90. a fat sounding single coil with an extended top end. reminds me a bit of the dearmond rhythm cheif 1100s in a way.
it's noiseless, but it has two coils wound sideways, kind of like the gibson EB bass pickups.
sounds distinctly single coil to my ears, but no hum.
I have mine fitted in my 1977 Gibson L6-S (a bill lawrence designed guitar).

Would love to fit one to an archtop at some stage. Would be good in something like an ES350 I imagine, or even an L-5 to get more of a 50's sound, but without the hum.

[mad dog]

It would be a rather easy thing for a tech in my opinion.

Chris:

You're right. It should be easy. I'm actually sort of torn about what to do. I like that Gibson neck P-90 a lot. The bridge not as much. At the same time, have a used set of Wolfetone soapbars (Mean/Meaner) sitting in a box here. I know from previous experience how good those Wolfetone P-90s are. And they are reverse wound for hum cancelling in the middle. Trying to decide if I should just have the Wolfetones swapped in, then figure out how to mod the bridge p/u on the Gibson set on my own.

MD

[pingu]

Yeah, P-100s are stacked HBs. The coils are one on top of the other, as opposed to side by side in a "normal" HB.

I never thought they sounded bad at all. Sticking to Gibson PUs only, I suppose they do lack the sort of characteristics that made P-90s or PAFs beloved.

PAFs sample a relatively long segment of string yet do not get muddy, and P-90s have single coil spark yet do not get shrill. In my opinion.

Then there are fat HBs that some love for output level and Benedetto A6/B6 style midrange.

But in my opinion, the P-100 just sits there with moderate output, decent but not sparkling highs, and seemingly much less of the P-90s dynamics. But I have never really measured anything on P-100s, so this is just vague subjective response on my part.

If I bought a guitar with P-100s in there, I'd certainly work with EQ and a variety of picks with the full expectation of finding many fine sounds.

Chris

are P100s humbucking Chris ?
if so , do you know how they manage to
reverse the magnet polarity in that design
(or any other 'stacked' humbucker for that matter)

I can't work it out !

[PTChristopher2]

>>> You're right. It should be easy [to RWRP my existing P90]

You need to reverse the magnets. The inboard edges of the magnets, touching the pole piece screws are one polarity, and the outside edges are the opposite polarity. So you swap the inside and outside edges.

Then you need to reverse the wiring of the coil INSIDE THE PU on a Gibson P-90. So what was connected to the shield of the PU lead is now connected to the center wire, and vice versa. This is the part requiring some minor care.

[PTChristopher2]

EDIT:

I believe that I am way-wrong in part of this post, so I have deleted it and will try again below:

[PTChristopher2]

OK attempt #2.

I still do not have a P100 here, but I made a quick test with a magnet and long slug.

So,...

******************************

Hiya Ping',

>>> are P100s humbucking Chris ?

Yes. The coils are oriented to the ambient room electromagnetic noise in the same direction, so via reversed winding direction (or in principle, reversed wiring of the coil ends), the lower coil will hum much the same as the top, but this hum in the lower coil will be 180 deg. out of phase with the hum in the top coil.

>>> if so , do you know how they manage to
reverse the magnet polarity in that design
(or any other 'stacked' humbucker for that matter)

All stacked HBs are not the same. But yes indeed, you have found the "Say what?" part of the design.
If the magnetic polarity through the two coils is the same, but the coils are reversed, then the hum will be cancelled, but so will the parts of the string vibration signal that are the same in each coil.

Ideally, one might decide to use a Faraday Cage, or other method to magnetically isolate the two coils yet somehow get the two coils to sense string vibration signals in a way that has both coils contributing to your target sound results.

I forget the guy's name, but he had patented some PU design in this direction.

(I am being loose with use of "Faraday Cage" as a term here.)

I do not have a P100 here, and I have no recollection of ever measuring the coils on one. I really do not like wild speculation on a forum, so please note that while I am not pulling this stuff out my backside, I am not writing with an actual P100 or data in-hand:

The P100 is a stacked configuration. It relies on reversed coils to buck the hum.

The P100 has no magnetic isolation between the coils. It relies on the mid (between the coils) position of the magnets and the much greater distance from the strings to the lower coil to do two things:

There is a much weaker string vibration signal in the lower coil. This significant difference in the two string vibration signals means that net signal maintains a remarkably large amount of the top coil's signal.

I have no way to measure the magnetic fields in a P100 configuration, but with the mid position of the magnets, there should be two separate "lobes".

If the edge of the magnets touching the pole pieces is north, then both the top of the poles at the top of the PU, and the bottom of the poles at the bottom of the PU would be north.

These would then form two separate lobes with the south outside edges of the magnets. These lobes would have opposite flux vs. the coils.

I HAVE NO DATA TO SHOW THIS - but it should result in a very weak signal from the lower coil, but one that does not cancel the similar content in the upper coil.

This would be relatively easy to test with a P100 in hand.

Again, this post has some opinion that is not confirmed.

Chris

[PTChristopher2]

If this mess is unclear, I can make a drawing of a P90 and a P100 this evening showing the difference in the magnetic configuration and how the mi-position of the magnet works.

It would be great to combine this with a measurement of the individual coil output and the combined output.

[PTChristopher2]

I just checked this with a small compass and a quick mockup of the P100 configuration. Indeed, we end up with opposite magnetic fields through the two coils.

Sorry for the now-deleted initial ass-hat post.

Chris