Why 28 ga, not 32 on low Z pup?

[dcoyle]

Assuming the choice was based purely on audio engineering, any idea why Gibson would choose to use 28 ga on their low impedance les paul recording bass?

For a given number of turns, it seems that larger gauge would put some turns farther out from the stronger magnetic field close to the magnet, thereby lessening signal output.

On the other hand, there would be less noise build up on a coil with a lower DC resistance. But, if mag field intensity falls off as the square of the distance and noise current falls off by Ohms law as a first order I = V/R, a worse signal to noise ratio, ensues, it seems to me.

So why did Gibson use 28 ga?

Thanks for any thoughts.

Dan

[bbsailor]

Assuming the choice was based purely on audio engineering, any idea why Gibson would choose to use 28 ga on their low impedance les paul recording bass?

For a given number of turns, it seems that larger gauge would put some turns farther out from the stronger magnetic field close to the magnet, thereby lessening signal output.

On the other hand, there would be less noise build up on a coil with a lower DC resistance. But, if mag field intensity falls off as the square of the distance and noise current falls off by Ohms law as a first order I = V/R, a worse signal to noise ratio, ensues, it seems to me.

So why did Gibson use 28 ga?

Thanks for any thoughts.

Dan

The quick answer is: (1) to match input impedance of the transformer used to raise the low pickup impedance to a high impedance and (2) to pass some subjective listening tests reported by Jack Casady.

Joseph Rogowski

[J M Fahey]

Hi dcoyle

some turns farther out from the stronger magnetic field close to the magnet, thereby lessening signal output.

Don't worry about that, it won't happen.
The important fact is that if you make a larger diameter coil, the flux *density*, that's to say the amount of "lines of force per square mm" will be lower, because the surface will be larger, but the *total* amount of lines of force will be the same.
Remember that the magnet , or some iron yoke, goes within the coil.
That square law you mention would apply if you had an air-core coil and a simple magnet a few inches away from it.
As an acoustic example: if you have a loudspeaker in free air, your sound pressure will fall with the square of the distance, because you are creating a spherical wave in free air, but if you get, say, a 12 inch pipe glued to the rim of a 12 inch speaker, you will have a flat wave that will carry a long distance.
That pipe is an acoustic equivalent of the iron parts (pole screws, rivets, blades, whatever) in a pickup.

[Mike Sulzer]

For a given number of turns, it seems that larger gauge would put some turns farther out from the stronger magnetic field close to the magnet, thereby lessening signal output.

On the other hand, there would be less noise build up on a coil with a lower DC resistance. But, if mag field intensity falls off as the square of the distance and noise current falls off by Ohms law as a first order I = V/R, a worse signal to noise ratio, ensues, it seems to me.

What matters for the law of magnetic induction is the changing flux across the surface of the loop. The fact that the field is weaker farther away is important because a larger loop still contains the flux closer to the core. However, very large loops can contribute less because of the return field lines. A field line that passes through the core from the string must find its way back to the string, and it points in the opposite direction on the way back. If a loop is large enough it will contain that field line both down and up, and thus cancel. This is not a very large effect for most pickup coils. Most of the filed lines within the coil but outside of the core are nearly horizontal (They escaped from the core.), and have little effect.

Magnetic fields far from the source fall of with the cube of the distance. This is because they are dipole in nature. There are no magnetic monopoles, as far as anyone knows. Magnetic fields can vary in complicated ways close to the source, depending upon its details.

[David Schwab]

(2) to pass some subjective listening tests reported by Jack Casady.

I've heard this mentioned in passing, but never any detail. I assume this was for his signature bass, which is of course a copy of the LP Signature bass which had a slightly different pickup from the LP Triumph bass, though the original LP Signature had the same oval shaped pickup.

What I had heard was that the current Gibson people really didn't know how to make the pickup exactly, so they used him and his old LP Sig bass as a reference.

Oddly enough, they seem to be putting some kind of low Z pickup in the Epi EB-0 bass, but without the transformer. Or at least some pickup that reads about 2K and it wound with heavy wire.

I'm planning on making myself a LP Triumph bass with some NOS parts (neck, & pickup parts) so when I do I'll post it up on here.

I've never worked with wire that heavy. Should be interesting.

[bbsailor]

I've heard this mentioned in passing, but never any detail. I assume this was for his signature bass, which is of course a copy of the LP Signature bass which had a slightly different pickup from the LP Triumph bass, though the original LP Signature had the same oval shaped pickup.

What I had heard was that the current Gibson people really didn't know how to make the pickup exactly, so they used him and his old LP Sig bass as a reference.

Oddly enough, they seem to be putting some kind of low Z pickup in the Epi EB-0 bass, but without the transformer. Or at least some pickup that reads about 2K and it would with heavy wire.

I'm planning on making myself a LP Triumph bass with some NOS parts (neck, & pickup parts) so when I do I'll post it up on here.

I've never worked with wire that heavy. Should be interesting.

David,

The volume pot value on the LP low impedance pickup circuit is 2.5K. The impedance ratio of a transformer reflects a load back into the pickup based on the square of the turns ratio (TR). So, a 2.5K pot is 100 times lower than a 250K pot value assuming a 1 to10 turns ratio and if the turns ratio is 12.6 (Shure A95U) it would be equal to 158.7 times 2.5K or about 400K. You need to know the impedance ratio, hence the turns ratio of the transformer you choose to use to start your planning, unless you plan to go active.

The Jack Casady low impedance pickup is about 24 ohms (if I remember correctly). The A95U transformer is rated at 150 ohms on the low side to 41K on the high side with a 150 ohm source impedance. That impedance ratio provides +22db gain or a TR of 1 to 12.6. The rule of thumb is that the load on the source should be between 5 to 10 times the source impedance. That puts the transformer impedance of 150 ohms with between a source impedance of between 15 to 30 ohms to ensure an optimum voltage transfer.

Another rule of thumb is that the load on a pickup should be about 40 times more than the DC resistance of the pickup to minimize loading. Using this rule of thumb puts the pickup resistance between 30 and 60 ohms (60 X 40 = 2.4K).

Normally, guitar pickups with little metal in the core or nearby have an impedance about 1.2 times the DC resistance. Since this is a bass pickup, you want the better matching to be in the lower frequence range of the Extech LCR meter. The subjective qualities come in when evaluation the output of the low Z pickup, volume pot on board and only the transformer located at the amplifier end of the cable.

I just picked up an Audix "Professional Matching Transformer" on sale at the Guitar Center for $20.97. It has a rated impedance ratio of 600 ohms to 50K ohms impedance. The low side has a Rated DC resistance of 166 ohms and and high impedance resistance of 2250 ohms, measured: 153.4 ohms and 1.598K ohms DCR. I calculated the turns ratio to be about 7.5 using inductance ratios measured on the Extech LCR meter.

The point is that you can try different matching transformers that have a variety of turns ratios to listen for any sonic or tonal changes.

If you assume that the average turn is 0.5ft then each ohm of AWG 30 uses about 20 turns while each ohm of AWG 28 is about 31 turns. The balance is now between the available wire area available on your design, the impedance of you final wound coil, the input impedance of your chosen tarnsformer and the fine tuning of the pickup coil to transformer to select the desired sound. That is why some bass low impedance pickups have a few taps at different turns to allow for tonal selection by selecting various number of coil turns to feed the input transformer. You can establish some turns limits by calculating the maximum amount of both AWG 28 and AWG 30 that will fit on the coil. Example: AWG allows about 100 turns per inch while AWG 28 allows about 79 turns per inch. Assume a bobbin wire window of 0.75" tall by 0.25" wide will allow a maximum of about: AWG 30 - 75 times 25 of about 1875 turns; AWG 28 - 60 times 20 of about 1200 turns. 1875 turns divided by 2 equals 937 feet or about 96 ohms for AWG 30. 1200 turns divided by 2 equals 600 feet or 39 ohms for AWG 28.

I'm sure that other people with experience matching coils to transformers can jump in and add some of their own obsevations or matching tips.

Here is a web link that covers the theory of matching low impedance moving coil step up transformers that can be applied to this pickup analysis. Audio Note Kits - Moving Coil Step-up Design

I hope this stimualtes a good technical discussion.

Joseph Rogowski

Additional thought: Design for an optimal match then go 70% of optimal winds to about 150% of optimal to give yourself a range for tonal matching by selecting various winding turns in the coil.

[dcoyle]

JM Fahey

Thanks for your thoughts.

I didn't really think it was as simple as the field falls off as a 2nd order, because I think that the winds near a bar magnet, such as are in the LP recording PUP, (what with the largeness of the bar and closeness of the wire) makes it behave more like the Biot Savaart rule, where the drop off is closer to first order. I couldn't begin to write the integral that the actual magnetic field is, so I was just trying to indicate the slope of gain vrs resistance would be different with different gauges.

Thanks

Dan

[Mike Sulzer]

I think Jim was talking about the decrease of the field as the distance from the string increases, that is, along the pole piece. The pole piece does indeed act as a kind of pipe that stops the field from falling off so fast. In the absence of the pole piece, the field from the string would fall off as the cube of the distance since the string is like a small dipole. (Dipole - Wikipedia, the free encyclopedia)

I believe that Dan was discussing what happens for turns that are farther out from the pole piece as a result of using larger wire. This would not make any significant difference for the distances involved.

[dcoyle]

Joseph,

Your xfmr matching explanation really makes sense. Thanks for laying it out.

To anyone choosing transformers, IME, there is no advantage, from a hifi point of view, to using a transformer with any more gain than you need.

Not only could it require thicker wire in the pup bobbin, which may be hard to fit, but stray inductance and self capacitance of the transformer winds are worse at higher turns ratios. Both are the enemy of treble. Whether that high freq drop off is significant for bass pups is another matter. Certainly if it's a Jensen or Lowther it's less likely...

Mike,

I was indeed talking about the drop off of the mag field for the coil wire.

I'm a little surprised to see your point that it doesn't much matter at the distances involved. I have almost zero winding experience, but when I spread filings on paper over magnets, I see changes over the distance that the LP Bass pup extends.

Thank you everyone for the education.

Dan