Les Paul's Low Impedance "Staple Back" Pickups

[Josh]

Yesterday I had the privilege to work on some of Les Paul's personal pickups and one of his guitars. The guitar is a (most likely) 1954 Les Paul Custom previously owned by Les Paul himself. One of his crazy Frankenstein LPs. The pickups were also his, but I don't know if they were previously installed in this guitar or not. I had to design and make a mounting plate to work with the pickups as to not alter the guitar.

I tested the pickups with my Integrator, and made some comparison plots. The Integrator I had set to 0db assuming that would be better for the low as hell impedance / low output pickups. The pickups are nearly flat across the audible spectrum, and pretty closely matched. Both are done with heavier gauge wire using scatter winds. I had to repair/replace the leads coming from the pickups.

The following tests were done with my new leads connected:
Ls @ 1kHz, Rd @ 1kHz, Cp @ 100kHz, Q @ 3kHz, 10kHz

LP SB 1:
Ls 4.06mH
Rd 4.0926Ω
Cp Nope! (tried up to 200kHz, couldn't do it)
Q @ 3kHz: 12.94
Q @ 10kHz: 12.13

LP SB 2:
Ls 3.8mH
Rd 4.056Ω
Cp Nope! (tried up to 200kHz, couldn't do it)
Q @ 3kHz: 11.987
Q @ 10kHz: 12.26

Here are the bode plots:





Here's the guitar with the pickups on the plate I made, and a photo of the guitar complete with Les Paul's wacky pickguard:




I borrowed the (probably) period correct P90s that were in the guitar so I can test those as well at some point.

Thanks,
Josh

[Josh]

In a week or two I will be rewiring the electronics to be what Les Paul intended. Currently it's dumb, but I'll be making the electronics essentially match the LP Recording model setup, which will work great with those insanely low impedance pickups.

Now I need to find a transformer that would work well with it. The internet tells me it was a Shure A95U or A95UF, but I can't find those as standalone transformers. I may have to scavenge one out of one of those adapters.

I'm just guessing, but from what's in the electronics compartment, it could be an early prototype for what eventually became the LP electronics setup on the Recording and similar models. That's just me speculating though.

[mikecg]

Hello Josh,

You have a real museum piece there, a real 'war-horse', and there can't be much wire on those pickups?
I would be interested to know the approximate gauge and number of turns.
It looks like some kind of 'test bed' instrument, and it has clearly been subject to a series of 'modifications' - over the years.
Have you plugged it in yet?
How does it sound?

One of a kind!

[Josh]

Sept 12, 2023 13:14:38 GMT -5 mikecg said:

Hello Josh,

You have a real museum piece there, a real 'war-horse', and there can't be much wire on those pickups?
I would be interested to know the approximate gauge and number of turns.
It looks like some kind of 'test bed' instrument, and it has clearly been subject to a series of 'modifications' - over the years.
Have you plugged it in yet?
How does it sound?

One of a kind!

Yes, this was one of Les Paul's prototypes, we think it might have been the prototype for the Recording series LPs. My friend is convinced this might be the guitar known as the "G5" - but I have no idea. Les Paul had a few guitars like this that he would swap electronics and pickups and do all sorts of crazy things. LP or Mary Ford would play this guitar, with a goose neck microphone plugged directly into the body on the XLR jack near the pickup selector switch. It's pretty crazy.

For the record, it's not my guitar, it was brought to me to have Les Paul's pickups restored and installed, and soon the electronics too. I'm currently trying to source decent pots in the correct values, which is not so easy. 1k and 2k isn't too bad, but of course he had to have 2.5k pots. lol

Here's a recording with my friend testing the LP plugged into the line in of an MP573 preamp using a low to high impedance converter. However, I haven't setup the guitar yet (will do after electronics are done), so the intonation is currently meh, and the strings were brand new (still stretching), etc. I also need to work some more magic on the nut the next time I have the guitar here.

[antigua]

Sept 12, 2023 10:46:20 GMT -5 Josh said:

Yesterday I had the privilege to work on some of Les Paul's personal pickups and one of his guitars. The guitar is a (most likely) 1954 Les Paul Custom previously owned by Les Paul himself. One of his crazy Frankenstein LPs. The pickups were also his, but I don't know if they were previously installed in this guitar or not. I had to design and make a mounting plate to work with the pickups as to not alter the guitar.

I tested the pickups with my Integrator, and made some comparison plots. The Integrator I had set to 0db assuming that would be better for the low as hell impedance / low output pickups. The pickups are nearly flat across the audible spectrum, and pretty closely matched. Both are done with heavier gauge wire using scatter winds. I had to repair/replace the leads coming from the pickups.

The following tests were done with my new leads connected:
Ls @ 1kHz, Rd @ 1kHz, Cp @ 100kHz, Q @ 3kHz, 10kHz

LP SB 1:
Ls 4.06mH
Rd 4.0926Ω
Cp Nope! (tried up to 200kHz, couldn't do it)
Q @ 3kHz: 12.94
Q @ 10kHz: 12.13

LP SB 2:
Ls 3.8mH
Rd 4.056Ω
Cp Nope! (tried up to 200kHz, couldn't do it)
Q @ 3kHz: 11.987
Q @ 10kHz: 12.26

Thanks for sharing this info. If someone wants to create replicas of these pickups, they can use this info. 

I see you have Ls at 1kHz, if your meter can do 100Hz, that's usually necessary for pickups with steel cores, because the eddy currents present a resistance, but since these are AlNiCo staples, the higher frequency probably didn't hurt too much in this case. I usually do Q at 1kHz just because there are times when 3kHz can approach the resonant peak of some high inductance pickups, but in this case that's a non issue of course. I'm surprised the capacitance didn't measure, but I'd guess it's because the pickup is of such a low impedance that it doesn't look capacitive to the meter.

It's kind of funny that all that can be said about the resonant peak is that it's somewhere north of 100kHz. The loaded plot in this context is probably not too useful, since with low impedance pickups you'd use smaller pot values, but for all intents and purposes the response is flat, and only are not flat if you do like EMG or Fishman and attach a filter circuit to the pickup. 

[Josh]

Sept 15, 2023 9:43:22 GMT -5 antigua said:

Thanks for sharing this info. If someone wants to create replicas of these pickups, they can use this info. 

I see you have Ls at 1kHz, if your meter can do 100Hz, that's usually necessary for pickups with steel cores, because the eddy currents present a resistance, but since these are AlNiCo staples, the higher frequency probably didn't hurt too much in this case. I usually do Q at 1kHz just because there are times when 3kHz can approach the resonant peak of some high inductance pickups, but in this case that's a non issue of course. I'm surprised the capacitance didn't measure, but I'd guess it's because the pickup is of such a low impedance that it doesn't look capacitive to the meter.

It's kind of funny that all that can be said about the resonant peak is that it's somewhere north of 100kHz. The loaded plot in this context is probably not too useful, since with low impedance pickups you'd use smaller pot values, but for all intents and purposes the response is flat, and only are not flat if you do like EMG or Fishman and attach a filter circuit to the pickup. 

I'll be replacing the electronics in the near future with the correct low resistance components, essentially along the lines of the LP Recording model. Sourcing the pots/decade switch is a little tricky, but I have a few things picked out if his friend at Gibson doesn't find anything.

Let me know if you have any requests for specific measurements. I plan to also measure the impedance when I have it back in my shop. Last time I only measured DCR. I can measure Q at 1kHz, I'd be surprised if it's much different. There was almost no change between 3k and 10k, which surprised me. 

My meter range is 20Hz to 200kHz. I think to see capacitance on these pickups, you'd need a crazier LCR meter that goes into the MHz range. I'm just guessing of course, but I was getting stupid negative readings which is usually a good sign that the freq range is way off to get a valid measurement.

I can do a real loaded measurement of the pickups installed in the guitar. I'd like to see a comparison with that. I'm also curious to see how the decade switch affects the resonant frequency. In theory, the idea is that adjustments on the decade switch move the RP and make it possible to sound like other pickups. 

I don't know how much time I'll have to do with testing after the guitar is done, but I'll try and do loaded(real)/unloaded measurements, and also try a couple positions on the decade switch. If time permits, I'll also try with and without the imp conversion transformer engaged. 

Thanks,
Josh

[stratotarts]

The integrator, if it's one of mine, can not measure accurately above 30kHz. Specifications are here: github.com/KenWillmott/integrator/wiki#specifications

It would be better to use some other arrangement like an impedance bridge, first focus on finding the actual self-resonant frequency, then try loading with a few low values between 600 ohms and 10k ohms. I agree that the resonant frequency will be very high, for this reason the load for a normal kind of resonant peak is hard to calculate, some iterations of testing would probably be necessary to extract really revealing data.

If you have the "original" impedance transformer, it would be nice to have measurements on that too.

A local jazz player has one, he let me hold it but not play it. I asked, really just to have a closer look at it. He was really nervous about it, I would be too.

[Josh]

Sept 17, 2023 21:14:06 GMT -5 stratotarts said:

The integrator, if it's one of mine, can not measure accurately above 30kHz. Specifications are here: github.com/KenWillmott/integrator/wiki#specifications

It would be better to use some other arrangement like an impedance bridge, first focus on finding the actual self-resonant frequency, then try loading with a few low values between 600 ohms and 10k ohms. I agree that the resonant frequency will be very high, for this reason the load for a normal kind of resonant peak is hard to calculate, some iterations of testing would probably be necessary to extract really revealing data.

If you have the "original" impedance transformer, it would be nice to have measurements on that too.

A local jazz player has one, he let me hold it but not play it. I asked, really just to have a closer look at it. He was really nervous about it, I would be too.

Yes, I built the integrator based on your kicad project. While it would be interesting to get a higher accuracy reading over 30kHz, we can't hear any of that, so I don't think I'd invest much time. I might test the pickup directly up to 200kHz or so just to see if something pops up.

The circuit uses a 2.5k 20% audio taper put, and the rest of the controls can be bypassed (a 2.5k linear bass pot, and a 1k linear treble pot).

The internet claims the imp transformers were the Shure A95U. Accurate or not, they weren't highly loved, but I ordered a vintage one anyway. We also have somebody at Gibson looking for parts for this.

I will get more measurements in general the next time I have the guitar.

[stratotarts]

The point of looking at the high frequencies would not be to know what it sounds like, but to determine and verify the effects of an appropriate load. I suppose if the stock pot values are 2.5k as you say, you could test using that (or half that, since the tone pot is in parallel), and examine the first 20kHz like usual. My expectation would be almost a flat line across the entire audio spectrum, however the magnets characteristics will probably bend it a little. With low impedance pickups, the plot is always going to be very predictable because the resonant frequency is so high. If high fidelity was really what people wanted to hear, modern pickups would probably be mostly low impedance because they would satisfy that requirement, and would also be easier to wind. Output isn't nearly as important as it was before high gain transistor input stages became commonplace. There's also the fact that high impedance (normal) pickups are in an ecosystem of amps, pedals and effects that inter-operates mostly seamlessly. Hence the use of impedance matching transformers or preamps.

You may be aware, there is an entire forum dedicated to low impedance pickups, small wonder due to their special properties. If an audio engineer naively designed pickups, they would be low impedance because they are flat. But, everyone has gotten used to the low pass filter response, it produces pleasing tone, so there isn't much demand for flatness. You can see examples of marketing aimed in that direction - "wide range" humbuckers from Fender, and the Betts "high fidelity" Filtertron design for example. Yet neither of those actually exhibit flatness, so it's a kind of doublespeak or "playing to the crowd".

[Josh]

I think it would be simpler to test the pickups loaded in the guitar with the actual low impedance circuit for the "loaded" plots. Then there's no need for speculation, as the guitar is getting the Recording model circuit either way. I can do that up to some high frequency number, at least 200kHz, maybe higher if the peak doesn't show up. It might be fun to also test with the high imp transformer active.

I'm not aware of the low imp forum. Is that on this site, or another? Low impedance doesn't excite me either. It's cool for this historical guitar, and LP's wackiness, but I like modern pickups.

Lace Alumitones play the HiFi marketing game too, but at least they generally sound good. Hmm, I should plot one of them I have loaded in a guitar on my wall.

Thanks,
Josh

[ms]

If you load a pickup with the actual circuit, there is no need to study it much past a few KHz.  Guitar speakers fall really fast above 5 KHz (some even lower), and simulation software should too to reproduce the actual situation.  On the other hand, if you want to understand the complete pickup circuit, measure it unloaded to as high a frequency as necessary to include all detail.

[ms]

By the way, are those  pole pieces made of steel or permanent magnetic material?  My best guess is that they are steel, and there is are alnico magnets underneath as in a P-90 with the steel pole pieces replacing the P-90 screws.

[Josh]

Sept 19, 2023 6:24:47 GMT -5 ms said:

If you load a pickup with the actual circuit, there is no need to study it much past a few KHz.  Guitar speakers fall really fast above 5 KHz (some even lower), and simulation software should too to reproduce the actual situation.  On the other hand, if you want to understand the complete pickup circuit, measure it unloaded to as high a frequency as necessary to include all detail.

I already have the unloaded measurement plots. Though I suppose going further to see if the peak shows up unloaded might be interesting. After that I'll get the real loaded plots. I'm doing this for the sake of measuring and having the data available more than anything else. 

You know, I don't recall seeing separate magnets like a P90, but that doesn't mean they weren't there. If I remember to look, I'll check next time I have the guitar.

[ms]

Sept 19, 2023 15:30:16 GMT -5 Josh said:

Sept 19, 2023 6:24:47 GMT -5 ms said:

If you load a pickup with the actual circuit, there is no need to study it much past a few KHz.  Guitar speakers fall really fast above 5 KHz (some even lower), and simulation software should too to reproduce the actual situation.  On the other hand, if you want to understand the complete pickup circuit, measure it unloaded to as high a frequency as necessary to include all detail.

I already have the unloaded measurement plots. Though I suppose going further to see if the peak shows up unloaded might be interesting. After that I'll get the real loaded plots. I'm doing this for the sake of measuring and having the data available more than anything else. 

You know, I don't recall seeing separate magnets like a P90, but that doesn't mean they weren't there. If I remember to look, I'll check next time I have the guitar.

There might be no peak. If the poles pieces are sufficiently conductive, the eddy currents induced in them might be sufficient to damp out a peak, if this possible peak is high enough in frequency.

[Josh]

Anybody have a link to that low impedance forum?

Thanks,
Josh