Low Impedance Pickup Research

[babysnake]

Using the volume pot creates the static, which is very loud. My cable is not balanced, as I use a single conductor wire with shield. Is it possible this is coming from the 48v phantom power the amp puts out on the xlr? I don't have a schematic and the manufacturer, Henriksen, did not reply to my request for help. I use a 10k pot, and this seems to work ok through the xformer side.

You can decouple it with a DC blocking capacitor in the guitar.

[Singer15]

To update - thanks for the feedback - I got a reply from Henricksen which stated that capacitors will work, or switching the 48v. I do get 48v between pin1 and each of 2 and 3. I may open the amp and install a switch, since I have no use for phantom power. Now I have another issue, which is tone control. I should state that I'm using 10k audio pots, which work ok for the volume. No cap that I try does anything to affect tone however. Is this because my tone pot is 10k? Should I swap the pots for 2.5k pots? I've tried several diferent 47uf caps just to see if one was defective, but neither work, and I'm sure they are not defective, since I have used them on other hi z systems.

[bbsailor]

To update - thanks for the feedback - I got a reply from Henricksen which stated that capacitors will work, or switching the 48v. I do get 48v between pin1 and each of 2 and 3. I may open the amp and install a switch, since I have no use for phantom power. Now I have another issue, which is tone control. I should state that I'm using 10k audio pots, which work ok for the volume. No cap that I try does anything to affect tone however. Is this because my tone pot is 10k? Should I swap the pots for 2.5k pots? I've tried several diferent 47uf caps just to see if one was defective, but neither work, and I'm sure they are not defective, since I have used them on other hi z systems.

Singer15,

The rule of thumb to minimize loading on a pickup is to use a pot value about 40 to 50 times higher resistance than the pickup resistance. If you look at most high impedance single coil pickups this is 250K pot values and 500K for most passive humbucker pickups. Pot values are in parallel with the typical high impedance amp input impedance which is about 1Meg ohm which is in parallel with the pot value. At high impedance the coax cable adds about 300pf capacitance for a 10 ft cable which tends to add extra capacitance across the pickup to affect the resonant frequency.

This capacitance effect on high impedance pickups does not occur with low impedance pickups as its effects are typically much higher in the frequency range beyond shaping the sound of the pickup.

It it is always a good idea to keep DC voltages off pots on low output pickups to minimize audible noise.

Joseph J. Rogowski

[Singer15]

BBSailor,
Am I to understand that tone controls do not work for low impedance? Or do I have the wrong capacitors? I note that the circuit diagrams for the Gibson Les Paul Lo Z models include tone control and standard caps.

[bbsailor]

BBSailor,
Am I to understand that tone controls do not work for low impedance? Or do I have the wrong capacitors? I note that the circuit diagrams for the Gibson Les Paul Lo Z models include tone control and standard caps.

Singer15,
As the low impedance pickup impedance is about 10 times lower than the high impedance pickup values, the capacitor values on low impedance circuits, such as yours, should be about 100 times higher than what you see on high impedance circuits.

Joseph J. Rogowski

[Singer15]

Singer15,
As the low impedance pickup impedance is about 10 times lower than the high impedance pickup values, the capacitor values on low impedance circuits, such as yours, should be about 100 times higher than what you see on high impedance circuits.

Joseph J. Rogowski

bbsailor,
Thanks - so what would be the code you suggest for a cap? I.e. 473 and 223 is what I have. Do I use 471 or 221? Cheers!

[bbsailor]

bbsailor,
Thanks - so what would be the code you suggest for a cap? I.e. 473 and 223 is what I have. Do I use 471 or 221? Cheers!

Singer15,

High impedance pickup tone capacitors range from about .02 to .1 uF (micro-farad). This would put a low impedance pickup tone capacitor in the 2 to 10 uF range.

Just alligator clip the capacitors you have across the pickup connection to the volume control and listen to the tone. The tone change should be the same as if you turned down the tone control fully counterclockwise.

Joseph J. Rogowski

[Helmholtz]

As the low impedance pickup impedance is about 10 times lower than the high impedance pickup values, the capacitor values on low impedance circuits, such as yours, should be about 100 times higher than what you see on high impedance circuits.

What is the reason for the factor 100?

[Helmholtz]

473 and 223 is what I have. Do I use 471 or 221?

Cap code 473 means 47nF or .047µF and 223 means .022µF. Increasing the third digit by 1 means multiplying by 10, e.g. 474 = .47µF

[bbsailor]

What is the reason for the factor 100?

Hemholtz,

If you look at the schematics of the low impedance Les Paul guitars, you will see volume pots 100 times lower in value, 2.5 K Ohms versus 250 K Ohms for high impedance. The tone capacitor values are about 100 times higher in value for low impedance pickups typically about 4.7 uF for the Les Paul low impedance pickups versus about .05 uF for high impedance pickups. It’s all about scaling the values to match the impedance environment. See link below
http://archive.gibson.com/Files/sche...PRecording.PDF

Joseph J. Rogowski

[Helmholtz]

It’s all about scaling the values to match the impedance environment.

Yes, but above you wrote "As the low impedance pickup impedance is about 10 times lower than the high impedance pickup values...".
So I wondered why the values of the electronic components should change by a factor 100 when PU impedance differs only by a factor 10.

Meanwhile I found actual low-Z PU data in Helmuth Lemme's book: A LP Signature PU has a DCR of 23 Ohm and an inductance of only 15mH. From this its impedance calculates to 44 Ohm @400Hz and 97 Ohm @1kHz.

This means that the low-Z PU impedance is at least 200 times lower than typical high-Z PUs and a factor 100 or more for pot values and caps now makes sense to me.

[bbsailor]

Yes, but above you wrote "As the low impedance pickup impedance is about 10 times lower than the high impedance pickup values...".
So I wondered why the values of the electronic components should change by a factor 100 when PU impedance differs only by a factor 10.

Meanwhile I found actual low-Z PU data in Helmuth Lemme's book: A LP Signature PU has a DCR of 23 Ohm and an inductance of only 15mH. From this its impedance calculates to 44 Ohm @400Hz and 97 Ohm @1kHz.

This means that the low-Z PU impedance is at least 200 times lower than typical high-Z PUs and a factor 100 or more for pot values and caps now makes sense to me.

Helmholtz

Coil inductance varies by the square of the turns differences. If you double the number of turns, the inductance is 4 times higher. If you have a coil with 10 percent more turns, the inductance will be 1.1 times 1.1 or 1.21 times higher inductance than the initial coil. Actual measurements may vary a little but these concepts are very good to keep in your mind to do quick mental engineering.

Joseph J. Rogowski

[Helmholtz]

Coil inductance varies by the square of the turns differences. If you double the number of turns, the inductance is 4 times higher. If you have a coil with 10 percent more turns, the inductance will be 1.1 times 1.1 or 1.21 times higher inductance than the initial coil. Actual measurements may vary a little but these concepts are very good to keep in your mind to do quick mental engineering.

Sure, I am familiar with the basics.

My question wasn't about the effect of different turns' numbers. I asked why a PU impedance ratio of 10 should necessitate an increase of tone cap value by a factor 100.
Turns out that the impedance of the low-Z Gibson PUs is at least a factor 20 lower than you assumed. This satisfactorily explains the high tone cap value.

[Singer15]

I got some 2uF caps (6 of them) and tried various combinations with the tone control. What seems to work is 4 of them in parallel, which I presume gives a simple sum of 8uF. The DC R of my PU is 130 ohms. The problem remains that with the 10 pot the tone control only comes into effect at the full limit of cutting, i.e. it has to be turned ccw almost fully before any treble bleed is noticeable. Do I require more capacitance or lower R pots or both??

[Helmholtz]

I got some 2uF caps (6 of them) and tried various combinations with the tone control. What seems to work is 4 of them in parallel, which I presume gives a simple sum of 8uF. The DC R of my PU is 130 ohms. The problem remains that with the 10 pot the tone control only comes into effect at the full limit of cutting, i.e. it has to be turned ccw almost fully before any treble bleed is noticeable. Do I require more capacitance or lower R pots or both??

I would try a 2.2k audio taper pot and whatever capacitance that suit your needs.