Low Impedance Pickup Research

[Rick Turner]

What about conductive epoxy? Or conductive grease in addition to the mechanical connection?

[Joe Gwinn]

What about conductive epoxy? Or conductive grease in addition to the mechanical connection?

I have not tried either, but I'm skeptical. Conductive epoxy (grease) is typically silver dust in epoxy (grease), and depends on contact resistance from dust particle to dust particle. Silver will corrode to form a black film. The film is soft and not that good an insulator, but still switches and relays using silver contacts are invariably designed to wipe on make and break, so the rubbing will keep the contacts clean.

[David Schwab]

One fix would be to bend that side of the top plate over and slot it the way Lace does it.

Here's one of their "bass bar" Alumitones. They assemble the transformer cores around the frame, so I don't know how that would work with the current transformer.

[Chris Turner]

I'd also like to build one of these pickups. Very cool stuff!!

However, is there not a way to get around having to use the Shure A95U (or similar)? Could you not use a transformer with a higher turn ratio? And if that's the case, does anyone know of a good, easy-to-buy-from (preferrably off-the-shelf...) source?

Anyone?

[Joe Gwinn]

Lace is building their own current transformers. There is no magic here.

[Rick Turner]

If you can wind pickups, you can wind tiny transformers.

Heck, if Gibson could make inductors out of bolts, any of us should be able to do this stuff!

[David Schwab]

If you can wind pickups, you can wind tiny transformers.

Anyone have an idea how to wind a transformer like that?

Heck, if Gibson could make inductors out of bolts, any of us should be able to do this stuff!

That cracked me up the first time I saw it! A bolt, a few washers and a nut! Do you know if that was Bill Lawrence's doing? I know they used those in the L6-S and Ripper bass. It was a humbucker inductor too.

[David Schwab]

Lace is building their own current transformers. There is no magic here.

Exactly. I pointed out the bent side in response to melting the current transformer in the above design.

[Joe Gwinn]

Anyone have an idea how to wind a transformer like that?

Little transformer laminations and bobbins are catalog items. You will have no problem using a pickup winder to fill the little bobbin.

Another alternative is to note that the fancy bit of punched copper sheet going through the current transformer need not have that shape. A piece of heavy copper wire threaded through the current transformer, formed into shape, and soldered into a ring well away from the current transformer will also work. You may need to make an aluminum (not copper) clamp to hold the wire being soldered and to keep soldering heat from reaching the transformer. Insulate the wire as needed using teflon tubing, which will not melt from soldering.

Alternatively to the aluminum clamps, if the current transformer has been fully wax impregnated, simply hold the transformer end under water while soldering.

In all cases, use a BIG iron or a torch to do the soldering. Quicker is better. Soldering should take literally one second, maybe two. It helps if the wire ends have been pre-tinned.

[alexoest]

I'd also like to build one of these pickups. Very cool stuff!!

However, is there not a way to get around having to use the Shure A95U (or similar)? Could you not use a transformer with a higher turn ratio? And if that's the case, does anyone know of a good, easy-to-buy-from (preferrably off-the-shelf...) source?

I'd like to know too, but I guess they'd have to have a lot of windings. I've tried to figure out how many:

Instead of increasing the impedance to something that similar to a microphone (I believe around 200 ohms) as we do with the AS104, the transformer would have to increase the impedance to something similar to a traditional pickup (around 200.000 ohms). 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 (since the impedance increases by the square of the number of windings, and the square root of 1000 is around 32). So, in order to get an output impedance similar to a traditional pickup, we're looking for a transformer with 32 times as many windings as the AS104. The AS104 has 500 windings so that would mean that we need a transformer with 16.000 windings (32 times 500).

I might have made one or more mistakes in the above, but unless I'm far off, that's a lot of windings. I don't think such a transformer exists. If I'm wrong, I'd like to know.

[edit: In this thread, bbsailor mentions current transformers with 5000 windings]

/Alex

[alexoest]

Little transformer laminations and bobbins are catalog items. You will have no problem using a pickup winder to fill the little bobbin.

---<snip>---

Alternatively, if you don't have a pickup winder, you might use a sewing machine thread bobbin - and a sewing machine for winding. I made a between-the-strings pickup that way.

/Alex

[David Schwab]

I'd like to know too, but I guess they'd have to have a lot of windings. I've tried to figure out how many:

Instead of increasing the impedance to something that similar to a microphone (I believe around 200 ohms) as we do with the AS104, the transformer would have to increase the impedance to something similar to a traditional pickup (around 200.000 ohms). 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 (since the impedance increases by the square of the number of windings, and the square root of 1000 is around 32). So, in order to get an output impedance similar to a traditional pickup, we're looking for a transformer with 32 times as many windings as the AS104. The AS104 has 500 windings so that would mean that we need a transformer with 16.000 windings (32 times 500).

I might have made one or more mistakes in the above, but unless I'm far off, that's a lot of windings. I don't think such a transformer exists. If I'm wrong, I'd like to know.

What pickups are 200K?

The transformer on the Alumitone pickup I posted in the photo reads 5K. I have no idea what gauge wire it is.

It's not unheard of to wind 10,000 turns on a small bobbin. Look at P bass pickups. It's not a lot of winding on a small bobbin.

[Mike Sulzer]

What pickups are 200K?

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

A 3 Henry pickup would be about 40K at 2KHz.

The impedance approaches the dc resistance only at very low frequencies.

I am puzzled by this interest in single turn pickups. Why not just wind a regular pickup with fewer turns? The inductive impedance drops roughly with the square of the number of turns as turns are removed, while the voltage openly drops linearly. The "advantages" of low impedance (flat frequency response, low electrostatic hum pickup) are realized without reducing the number of turns very many times. If you need to boost level, you can do it at the amp with a transformer or a preamp with bipolar transistor input or some really low noise FETs.

The transformer should go at the amp because then it is not loaded by the cable capacitance.

[alexoest]

I am puzzled by this interest in single turn pickups. Why not just wind a regular pickup with fewer turns?

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.

[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.