P-bass pickup hiss

[Mick Bailey]

I have a parts-built P-bass in for investigation where the owner tells me that during a gig it suddenly got noisy. He's screened it with copper, then removed that and used aluminium, then replaced the pots, and screened the inside of the covers.

It struck me that there was an excessive level of hum with the instrument and it was clear that none of the foil sections had any continuity whatsoever. The whole lot was effectively ungrounded and this was making things far worse. I've since removed all of this and eliminated hum completely, but I'm still left with a white-noise hiss.

To isolate the problem I've removed the pickup from the guitar and wrapped the coils in grounded foil with just the pole pieces exposed and it still has the noise, no better or worse than when fitted to the guitar. The pickup is from an unknown custom-builder and looks to be built to a reasonable standard. The total DC resistance is 13K and I have a grounding strip for the polepieces (which was the last step in eliminating mains-borne hum). There does seem to be more noise than I would expect - I can't recall another situation where I've noticed so much hiss, though generally there's perhaps been enough hum to mask any other noise.

What I'm struggling with is that the owner is adamant that this suddenly started and previously the bass was fine. All the rest of the wiring is good. Does anyone have any thoughts on this?

[big_teee]

If the pickups are made with Rod magnets?
Check that there is no short or continuity read from ground to the polls.
You will have to take the ground loose on the pole ground foil to check that.
I also like to tight twist the white black wire, or use shielded 2 conductor cable.
Also make sure you have them wired in RW, or RWRP mode.
If you are checking the pickups out of the bass plugged directly into the amp?
They will always be quieter mounted in the bass, than out loose.
T

[Mick Bailey]

They're separate pole pieces and there's no continuity from pole pieces to ground with the ground foil removed. When testing them out of the bass I had them fully wrapped in foil with just the tops exposed and plugged straight into the amp in order to eliminate the instrument from the equation. It didn't make a difference to noise level with these particular pickups. They're RWRP.

I have a torture-test for pickup noise; a particularly noisy 12v halogen light where one of the lamp connections is taken off to the metal underside of my guitar bench. I use it to track down and (hopefully) eliminate hum as it gives me a consistent and high-level of interference. This bass is now immune to that 50hz noise, but the white-noise is present even when nothing other than the amp is switched on. I've tried a different amp in a different location and still the same. It's the sort of hiss you get with a high-gain pedal.

My thought is that if screening was the issue then the instrument would pick up mains hum, too.

[big_teee]

Sounds like you have it a lot better.
I've had some of that white noise before.
Did you try resoldering the hookup wire at the eyelets?
Also you might try testing the pickups in reverse.
Hot to the black and ground the white.
I try to make things change, sometimes that helps isolate issues.
I usually settle with a grounded pickguard, and ground foil on the rear of the pickups.
You might try putting it back together and see where you are now.
He may be thrilled to death with the way you have it now?
13k is pretty hot, I like underwound, and usually settle for 9-10k.
Less than 10,000 turns per coil.
GL,
T

[rjb]

It's the sort of hiss you get with a high-gain pedal.

No answers; just questions:
Hunh- sounds like Johnson-Nyquist (thermal) noise?
No controls attached; plugged straight into amp?
Just out of curiosity, what happens if you insert a small capacitor across the leads?

-rb

EDIT:
PS - Yup, 13K is pretty hot....

[Mick Bailey]

Turning down the tone control to 8.5 eliminates the noise completely. A small cap would perhaps be the answer and I'll experiment with some values today to see if I can strike a balance between noise and tonality. I wondered about thermal noise. Since my last posting I soldered a screened lead onto the pickup leads and wrapped the bundle in grounded foil so that the only lead out was screened, and the noise was still there with no reduction.

The guy who owns the bass was warned he'd lose some highs with such a hot wind. So to make up he has to apply boost to give him the top-end he wants (he likes a mid-rangey, toppy sound with additional drive). So this makes things even worse. This is the first time I've encountered this being a problem. I get plenty of setups where noise comes in later down the line from pedals, but usually with instruments the issue is hum, hence my permanently-connected hum station.

I reversed the connection and get a little more hum with the same noise, so put it back. I did notice one lead was marginally soldered so redid the eyelets.

Edit: Just experimented with caps across the output and a 560pf pretty much kills it. To the extent that I tried it with his Mooer noise gate and it's quieter in (true) bypass than with the gate engaged. He has a lot of gain on top of that hot pickup - a Sansamp 'Leeds' driver and EBS Sheehan drive deluxe. The cap doesn't make a vast difference in taking off the top-end, just some of the clicky fret noise gone. I'll see how it goes.

[Chuck H]

FWIW I'm with rjb. I've never experienced any mechanism in passive guitar circuitry that causes hiss and I only know hiss as a consequence of thermal noise due to gain and resistance. That IS a hot pickup and that may contribute a little hiss? I wouldn't expect as much hiss production from a hot pickup though as, say, an amplifier stage with high gain and a large series resistance. I don't mean to insult at all since you already have chops, but have you tried plugging in another guitar to the same test circuit. If there is an inherent amount of hiss in the test circuit it may not have seemed important, until now when you're trying to eliminate noise from this bass. That is, I don't see how it could be the bass causing any significant hiss and the pickup may not be responsible for it.?. I'd be interested in what mechanism about passive pickup construction might cause undue hiss if anyone knows.

[rjb]

The guy who owns the bass was warned he'd lose some highs with such a hot wind. So to make up he has to apply boost to give him the top-end he wants...

:facepalm:

Edit: Just experimented with caps across the output and a 560pf pretty much kills it....
The cap doesn't make a vast difference in taking off the top-end, just some of the clicky fret noise gone. I'll see how it goes.

I'd say button it up, ship it.

-rb

[Mick Bailey]

The owner called by yesterday afternoon to collect and I gave him a demo. Firstly through a clean amp and there was zero hum and just barely perceptible difference in noise with the bass volume at zero or at full - you had to listen closely to detect it. In fact, it's probably now one of the quietest P-basses I've ever worked on. I also demo'd it with plenty of gain - much more than you'd use with a 6-string guitar - and it held up fine.

Chuck, I did say this was the first time I've encountered such a problem, and like yourself only know hiss in relation to high-gain circuits. The thermal noise works out to about 2uV, almost insignificant. I use my bench amp to test everything at a repair/test stage and whilst working on this bass, but I also checked it through a different amp when the customer bought it in, so he could demonstrate the fault. During investigating this I also worked on a few other guitars and pedals using the same bench amp, with no problems. Some of the shred guitars I get have very hot pickups 22k-24k and I've never had a problem with hiss. Plenty of other noise - picking up clicks, mains-borne interference etc, but not hiss.

There has to be some other mechanism at work. I think I've eliminated the possibility of external noise and the instrument itself. I know that some alloys can generate high levels of electrical noise near to their yield-point. Could a pickup do this if it was so tightly wound the wire was stretched to almost breaking? It seems like an extremely remote possibility and would require some contaminant as I don't think pure copper exhibits this tendency. By the same token, could a tightly wound pickup have microscopic movement in the winding that interacts with the pole piece magnetic field as the windings relaxed? Again, seems unlikely. Theorising is probably the best I can do with this; how does a pickup removed from an instrument and wrapped completely in grounded foil make a significant noise?

Nice subject for a PhD, but I'm just glad to this one out of the way. I didn't even make a pizza delivery hourly rate on that job.

[Chuck H]

Well since this thread is still going we may as well speculate on the hiss. Which seems now to me now to have been critically isolated to the pickup by Mick. And I think Mick is right that we need an expert in pickups and physics (Mike Sulzer of course) to evaluate speculations. I'll throw my $.02 in for the cause being a very high resonant peak as a result of the unusual pickup wind. If so then the problem might also have been reduced somewhat by changing the volume pot and shifting the peak to a different, and hopefully less audibly hissy frequency. Perhaps a higher value pot, which would also be brighter, could have shifted a resonant peak into a less offensive range and then a cap to shunt any remaining hiss could have preserved the most top end and still mitigated the hiss.?. Or, conversely, a lower value pot and a lower value capacitor.?. The basic idea being to shift the pickups frequency response in the hope of making the hiss less audible and then do less overall shunting.

[Mick Bailey]

I had the momentary though of changing the pot. I didn't rationalize it - just a fleeting thought that I neither dismissed nor acted upon and then it was gone. I should have tried it to see if it made any difference. Upon reflection a 500k or 1M pot with the cap to ground (or whatever it would need with a changed pot) would have been a much better solution. Reminds me of the recording pre-emphasis/de-emphasis technique. Boost the treble at the front end and cut it at the back, taking the hiss with it.

[big_teee]

Did you pot the pickups, after everything you added to them.
Potting always seems to help quieten things.
I like to pot the pickup after the fact.
T

[Mick Bailey]

The pickups were already potted, so I quit while I was ahead and decided to leave it alone.

[Leo_Gnardo]

FWIW I've had a rash of noise complaints lately, or distortion, etc. that seem to result from the use of 1) digital equipment near the afflicted sound gear and B: class D amps used near or in conjunction with formerly working perfectly sound gear. Makes me wonder if somebody with a cell phone, smartypants phone or maybe a bluetooth device or even one of those digital "wristwatch" devices got close to your customer's bass. Or if he or his cable got close to a class D amp or even close to cable connected to such an amp. Hiss isn't the sort of thing you expect from a guitar/bass pickup. Hum, buzz, even ticking from a nearby digital wristwatch, we've encountered these. Now there's a wide choice of noise-broadcasting gadgets to cause problems. Add to the list, LED lighting systems. You can get a terrific lighting system, low cost, low power draw & light weight but they make noise too, that can be picked up by instruments & audio gear. Helluva future we've arrived in.

[Mick Bailey]

I found a few references to screening that suggested an incomplete screen around a circuit can act as a resonant cavity. By incomplete this means anything that has an entry or exit through the screen. The screen acts as a high-pass filter, so regular mains-borne interference is eliminated, but GHz RF can enter and cause noise propagation. It would seem to be a remote possibility and I have no way of eliminating this. We live in a soup of EMI so it wouldn't surprise me if the noise source was an artifact of wireless equipment, phone masts, etc.