Low Impedance Pickup Research

[David Schwab]

Actually, the fact that you have a one turn pickup with an inductance similar to that of a normal pickup (see specs David quoted above) suggests that you do not gain much of anything by using a transformer and fiddling with the ratios.

I think much of that inductance comes from the transformer coils (I believe they use two for hum canceling). For the "hotter" voiced Alumitones, they wind the transformer hotter. They don't change the primary coil at all.

One complaint when they first came out was they were to thin sounding, so then they increased the winds on the transformer.

Similarly the old Les Paul Signature guitar used low Z pickups with a transformer with different taps to get different tones.

[David Schwab]

This 60 Henries seems too high but can 60 Henries be possibly the correct value. It seems more realistic that it is more like 6 Henries. What do you think?

I think they meant 6.0 H.

[Mike Sulzer]

Summary: Low noise, less wire, wider frequency response and thinner physical size all offering guitar makers some new alternatives.

Joseph,

That sounds like a wonderful vacation. I hope you have returned refreshed and ready to go!

Yes, those are the potential advantages. But how well do they apply to a single turn pickup, and do you have to go to a single turn to realize them?

1. Comparing a one turn pickup to one with 10,000 turns in terms of hum pickup from electric fields, the single turn pickup is better by 80 db (signal to hum ratio improvement). You do not need so much reduction, and so a pickup with maybe a few hundred turns would be good enough.

2. Realizing the full benefit of wide bandwidth and high output requires a transformer at the amp (like the original Gibson recording pro). A single turn pickup requires a transformer in the guitar since the impedance is too low to send over a cable with connectors, etc. Therefore, you would need two transformers to realize the full frequency range when using a single turn pickup, one to go to an intermediate impedance, such as a couple hundred ohms, and a second at the amp to produce the normal voltage level.

3. A low impedance pickup still needs to reduce hum from magnetic fields with a canceling scheme, like a humbucker.

4. A single turn pickup is certainly a natural for a thin design. Other designs are possible, but harder.

[Mike Sulzer]

I think much of that inductance comes from the transformer coils (I believe they use two for hum canceling).

It should not. Audio transformers are designed to have very good coupling between the windings. If the coupling is perfect, there is no series inductance in the output from the transformer no matter how high the inductance of the coils is.

Imperfect coupling (leakage flux) results in such inductance, known as a leakage inductance. Remember a discussion of pickup impedance, where the effect of eddy currents in the cores was treated as imperfect coupling to a transformer secondary.

[Mike Sulzer]

Joe Gwinn,


On the same web page Lace published the following information. Lace Aluma J-Bass

• Position: Neck, Bridge
• Resistance: 5.0K
• Peak Frequency: 3800
• Inductance: 60 henries

This 60 Henries seems too high but can 60 Henries be possibly the correct value. It seems more realistic that it is more like 6 Henries. What do you think?

Thanks

Joseph Rogowski

I wonder if that resistance is just the resistance of the secondary coils or if it includes the resistance of the pickup loops, as tremendously increased by the transformer turns ratio?

[Joe Gwinn]

Joe Gwinn,


On the same web page Lace published the following information. Lace Aluma J-Bass

• Position: Neck, Bridge
• Resistance: 5.0K
• Peak Frequency: 3800
• Inductance: 60 henries

This 60 Henries seems too high but can 60 Henries be possibly the correct value? It seems more realistic that it is more like 6 Henries. What do you think?

I don't believe 60 Henrys for a minute. Even 6.0 seems pretty high, although I suppose it isn't impossible. While transformers will change the inductance or capacitance presented to one side to a different value seen from the other side, conservation of energy still holds.

I bet they used a LCR meter to measure it, and got bit -- most LCR meters simply don't work on low Q inductors such as pickups, yielding wild answers. The Extech being one of the few exceptions.

I got 40 H and 60 H results when I used a B+K LCR meter on a pickup, which flummoxed me for a while. This is what led me to build the Maxwell-Wein bridge, which in turn allowed me to show that the Extech got the right answer. Unlike the B+K.

[MR COFFEE]

Magnet structure question for David S.

David,

Thanks for the great pics of the Alumitone earlier in the thread. The pic of the top of the pickup look like there is a junction in the magnets part way across the pickup. Is this the case? If so, what do you think that's about?

And would you check the poling of the magnets and let us know where the north and south poles lie? I.E., both are poled top N, bottom S, both are poled side-to-side and the N poles go toward the center aluminum rib, and the S poles go toward the edge aluminum ribs, etc.

The interest and research into very low impedance pickups by you, Joe, and other luminaries around here is fascinating. Keep it up!

Thanks,
mr coffee

[Helmuth Lemme]

Nobody seems to know how guitar pickups work. Read this and you will be
informed:

BuildYourGuitar.com :: The Secrets of Electric Guitar Pickups

With my Pickup Analyzer I can measure the frequency response of any pickup
under real operating conditions.

The DC resistance of a pickup or a transformer is an irrelevant magnitude.
Most manufacturers tell a lot of nonsense. The inductance is essential.
Because of the winding capacitance in parallel and eddy currents in an iron
core the reading depends on the measuring frequency. It must be measured
at lowest frequencies as possible, e. g. in the order of 100 Hz. Measuring at 1
kHz or more will lead to false results. If a pickup manufacturer publishes
inductance figures (only a few do) you cannot rely on these.

I experimented a lot with low impedance pickups, especially with the Gibsons
used in the Les Paul Recording (1971). The transformer is of very poor
quality. It has high losses by eddy currents and loose coupling. In the "Les
Paul Signature" of 1973 (a golden semiacoustic, looks like an upper half of an
ES335 combined with a lower half of a Les Paul) Gibson used a high-quality
studio transformer with different taps on the primary side, selected by a rotary
switch. So one can simulate three different pickup inductances. I measured all
the frequency response curves.

The very first invention of a low-impedance pickup probably was made by the
British guitarbuilder Jim Burns. See US patent no. 3,249,677. He never had
success with it.

[Possum]

Helmuth, welcome, some of us own your book. I emailed you once about your pickup analyzer and how much it costs and if it has English instructions, never got a reply. Would like to know more about how it functions in detail.

Alot of us us the Extech LCR meter, which has test frequencies of 120hz and 1khz. It also read AC resistance. I find that it is useful to have the 2 frequencies so one can see what is happening in the bass frequencies a little bit. While inductance is useful, personally I find AC resistance MORE useful and usually look at that first. If you add more iron into a pickup design usually you see the inductance go DOWN so its hard to compare one pickup against another by how high the inductance reading is, but AC resistance always goes up the more wire you add or more iron, or putting covers on etc. I don't quite see how only using a 100HZ test signal would be useful, its seem a very limited look at things.

I have to say I don't see the appeal of these one turn pickups either, they don't sound particularly good to me and to me they are just too bright. A good Filtertron sounds way more musical and pleasing than those Alumatones, but I guess they sell them, I just wonder if they remain on player's guitars or after a year get replaced by something more traditional. They DO look cool, I will give them that!

[Mike Sulzer]

Helmuth,

I was interested to see that your measurements predict a dip in the frequency response (below the resonance) of a pickup with significant eddy current losses. This would be due to the effects of the cores, etc. I suspect that a lot of work that Possum has done on his PAF sound-a-likes is in getting the eddy currents right by choosing the proper metal in order to reproduce this dip accurately.

Mike

[Helmuth Lemme]

Pickup Analyzer

LIPR2.10 18. 8. 10

Possum,

Sometimes e-mails get lost in spam filters, sorry.

The price of the new PC-coupled Pickup Analyzer is 850 Euros or
1100 US-$ (with today's exchange rate) plus air mail freight costs
(order of 60 $). An English instruction manual will be included. The delivery
time is about four weeks. It is totally hand-made. I do this in the evenings
beside my regular job.

The price does not include the software which was not developed by
me. It must be purchased separately at:
audioTester
(English version available). The price is 39 Euros (51 $). You can
test it free for one month, after that you have to pay.

The measured frequency response curves are much more interesting than all
measurements with the RLC meter. My one also works with 120 Hz and 1000
Hz. Only the 120 Hz inductance figures are useful. The 1000 Hz figures are
good for nothing, as well as the AC resistance figures.

I also tested a Lace Alumitone. The characteristic is not far away from a
standard humbucker except a slight roll-off the bass end, caused by the
transformer. Unfortunately it is rather microphonic. When you touch it with the
pick you hear a loud click in the loudspeaker.

There is no typical sound of low-impedance pickups. The transfer
characteristic can be trimmed with external load capacitors and resistors just
like with normal high-impedance pickups. I developed a new 11 position
"Decade" switch for the Les Paul Recording with exponential capacitance
steps. It works much better than the original one.

----

Mike,

Yes, the dip is caused by the eddy currents. When you wind a pickup with
lower impedance for test then you can separate this effect from the LC
resonance effect.

[David Schwab]

David,

Thanks for the great pics of the Alumitone earlier in the thread. The pic of the top of the pickup look like there is a junction in the magnets part way across the pickup. Is this the case? If so, what do you think that's about?

They used two shorter magnets instead of one longer magnet. I don't think there is any reason other than not having to order longer magnets. It's probably a size they already had. They act as one long magnet.

And would you check the poling of the magnets and let us know where the north and south poles lie? I.E., both are poled top N, bottom S, both are poled side-to-side and the N poles go toward the center aluminum rib, and the S poles go toward the edge aluminum ribs, etc.

One magnet is North up, and the other is South. Assuming you mount the pickup with the wires exiting to the right (transformer on the left), viewed from the top, the magnet closest to the neck is South.

[David Schwab]

Nobody seems to know how guitar pickups work. Read this and you will be
informed:

BuildYourGuitar.com :: The Secrets of Electric Guitar Pickups

Helmuth welcome to the forum. Your web page and your pickup analyzer has been discussed here a few times in the past. Nice to have you here.

[MR COFFEE]

Hi David,
Thanks VERY much for the pics of the alumitone design.

A couple of questions, if you'd be so kind.

1) the pic of the top of the alumitone looks like there is a line across the magnets as if two lengths are butted together. Is that the case?

2) what are the poling of the magnets used? I.E., is the South pole of both magnets toward the center bar, North toward the outside aluminum bars of the frame; North up toward the strings, South toward the pickup cavity\guitar body, or what?

3) are the magnets those rubberized ceramic strip magnets, hard ceramic or ??

I've never seen a real Alumitone in person.

Thanks,

mr coffee

[David Schwab]

A couple of questions, if you'd be so kind.

This is the first one I ever saw in person as well.

They are hard ceramic magnets. They are butted together to make one longer magnet, so think of it as two magnets. One is south up and one is north up.

I'll post some more photos when I get a chance.