Low Impedance Pickup Research

[bbsailor]

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.

And you do not.

Suppose you double the number of turns on the pickup. You get twice the voltage; you use half the turns ratio. The pickup coil has four times the inductance (if you hold the geometry the same). But the impedance transformation ratio of the transformer is the square of the turns ratio so you end up with the same inductance looking back into the secondary. Even the resistance looks the same because you use smaller wire when you increase the number of turns.

So where is the improvement? In fact, transformers are not perfect, so you actually lose something.

Mike and all

Sorry for my delay in responding to this thread but I just returned from a 10 day vacation in Europe and a cruise up the Rhine River from Amsterdam to Switzerland. I had no cell phone, access to the internet or desire to stay connected as I immersed myself in European culture, architecture and history. But for now, it's back to reality and comments about pickup technology.

Joe Gwinn is quite right about about keeping the the primary loop resistance low and maintaining the integrity of the joint when threading the loop through a toroid current transformer. To make the loop joint, I have been using thin copper tubing, tinning the wire making and silver soldering the joint. To minimize heat thansfer to the toroid you can just use two large alligator clips as heat sinks. The CSE187L is an E-I laminated core type with a single loop of AWG 12 that requires two solder connections.

Mike is correct about transformers not being perfect. So where is the advantage to current based transformers? This discussion should be going in the direction of improved signal to noise ratio. High impedance pickups make good induced noise antennas with shielding attempts to minimize the noise but with some eddy current effects impacting the sound. Current-based pickups have a lower noise pickup, espicially when you ground the E-I laminated core frame to the low Z loop and send the signal out of the guitar in a balanced microphone way. Even in the unbalance mode, the noise is still lower. The amount of wire on a current transformer (CT) is less thus the distributed capacitance is less. With lower noise the output of a CT can drive the higher gain of an XLR microphone input preamp and then into a guitar amp for a wider range sound. Add some frequency shifting resonant peak EQ circuits (EMG-BQC) and you can tune a CT pickup to sound like most any traditional high impedance pickup.

The Alumitone interleaved "U-shapes" cores wrap around the metal frame so as to not need any secure connections of the low Z loop. It is inductivly coupled and uses two coils that have very fine wire and that are connected in parallel from what I can decuce from the Lace published resistance specifications. The coils be connected in either series or parallel for a tonal shift change. To get the output comparable with high impedance pickups, I suspect the number of turns on each small coil on the "U-shaped" laminated core is about 10,000 turns. Maybe Mike can do some reverse engineering on the physical core sizes with Dave's coil size input and offer some possibe turns ratios if the coils are in parallel or series. I believe the wire gauge is AWG 44 but a close examination of one of the Lace under- shell coils should put that to rest pretty quickly. The under-string loop resistance and turns ratio affects the tonal coloring or voicing of the pickup to favor the lower or higher frequencies.

I have offered up my research to kick off this topic, and a few folks are starting to expand on my initial posts and are making nice current-based pickups. Rick Toone lives near me and saw some of my earlier posts on this forum and invited me to his shop. He needed a very thin pickup that did not need the traditional hole behind the fingerboard. With a CT-based pickup you can arrange the "U-shaped" laminated cores on each end of the pickup so the only recess that is needed is about 1 sq inch on each side (to accommodate the coils) about 3 inches apart to not interrupt the acoustic string energy transfer from the neck to the bridge.

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

Thanks for your interest and I hope to see more comments and refinements in these new types of pickup.

Look at Bajaman posts for his clever CT design.

Thanks

Joseph Rogowski

[Mike Sulzer]

And why not put one or more additional turns on the very low Z pickup? Frequency response and resonance will probably not drop into the audible zone, and you'd get more current to work with, right? That would mean a lower turns current transformer. Heck, make the primary coil out of silver to keep the DCR as low as possible.

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.

And you do not.

Suppose you double the number of turns on the pickup. You get twice the voltage; you use half the turns ratio. The pickup coil has four times the inductance (if you hold the geometry the same). But the impedance transformation ratio of the transformer is the square of the turns ratio so you end up with the same inductance looking back into the secondary. Even the resistance looks the same because you use smaller wire when you increase the number of turns.

So where is the improvement? In fact, transformers are not perfect, so you actually lose something.

[Rick Turner]

SMC Music Oh, well...

[Rick Turner]

Brilliant, Joe! I'll look into it.

[Joe Gwinn]

I have no way of measuring that. However, I checked the Lace site and they do list that info:

Lace Alumitone Bass Bars
Resistance: 5.0K
Peak Frequency: 3800
Inductance: 2.7 henries

Lace Music Products | Since 1979 | Electric Pickups

I would guess that the Peak Frequency (3800 Hz) is the resonant frequency; this is far lower than expected, and explains why it's sold for Bass.

The inductance is far larger than expected from ordinary current transformers, unless they have a whole lot of turns. The transformer will also cause the inductance of the one-turn primary to appear, suitably transformed.

[Joe Gwinn]

Don't forget to trademark "Silvertone". If it's still available.

[Rick Turner]

And why not put one or more additional turns on the very low Z pickup? Frequency response and resonance will probably not drop into the audible zone, and you'd get more current to work with, right? That would mean a lower turns current transformer. Heck, make the primary coil out of silver to keep the DCR as low as possible.

[David Schwab]

What's the inductance, as measured while connected to the pickup turn? That's the core of Mike's question.

I have no way of measuring that. However, I checked the Lace site and they do list that info:

Lace Alumitone Bass Bars
Sizes: 3.5", 4.0" and 4.5"
Position: Neck, Bridge
Resistance: 5.0K
Peak Frequency: 3800
Inductance: 2.7 henries

Lace Music Products | Since 1979 | Electric Pickups

[Joe Gwinn]

Yes, the DC resistance is 5K. The output level is like that of a regular pickup.

What's the inductance, as measured while connected to the pickup turn? That's the core of Mike's question.

[David Schwab]

Yes, the DC resistance is 5K. The output level is like that of a regular pickup.

[Mike Sulzer]

In context, the Alumitone's transformer reads 5K, not 200K. Not a very big coil. Not a lot of winds.

5K is the dc resistance? If so then what one needs to know is the impedance as a function of frequency with the pickup attached. It would probably fall within the range of what you can do with an ordinary pickup

[Chris Turner]

I am puzzled by this interest in single turn pickups.

This type of pickup could also find a nice home at the end of the fingerboard of an EUB. A single loop of wire could easily conform to the short radius.

[David Schwab]

A good example of this advantage can be seen on Rick Toone's Skele guitar.

Hey, it's our very own bbsailor!

[David Schwab]

In the neighborhood of the resonance with the cable, a pickup can easily be higher than 200K.

Yes, but in the context of his post, he was talking about winding a transformer that high.

that's a thousand times as big an impedance, and hence, the transformer would have to have around 32 times as many windings as the AS104

In context, the Alumitone's transformer reads 5K, not 200K. Not a very big coil. Not a lot of winds.

[Mike Sulzer]

There might be different reasons, but my own interest was triggered by the fact that with this kind of pickup, you don't have to route a large cavity in the structurally critical area where the neck joins the body. A good example of this advantage can be seen on Rick Toone's Skele guitar.

That is an interesting project!
Yes, it is a bit tough to make a very thin pickup by ordinary means; bobbins become extremely flimsy if you try to make them from thin material so that the flat work does not occupy too great a percentage of the available space. An approach I have considered is to make 12 small flat coils so that thin bobbin material is not so much of an issue. The pickup body would be a piece of plastic of the right thickness, drilled out to hold the coils and milled on the back for conductor paths, etc. The pole pieces would be short ferrite beads with thin (1/32") neo magnets on top, of course.