Loading a low-ish impedance pickup

[JayGunn]

I'm getting ready to wind a split-coil bladed pair with magnet steel (cut from transformer core CRNGO steel) as a 7-layered layered blade and neo magnets. Wire will be 36ga, somewhere between 2000 and 2500 turns. With two coils connected in series like a P-bass pickup, this is going to give me around 600 ohms DCR.

I noted in bbsailor's incredibly useful contributions that he suggests as a rule of thumb to have about a 1:40 ratio between the pickup DCR and the load it's driving. What I want to do is connect this and another pickup I'll make next to a 25K blend pot. BUT to give the pickup a steady load, I want to drive the blend pot in the reverse way from how it's usually wired.

Below you'll see an extract from Seymour Duncan's wiring diagrams showing the usual way to wire a blend, which most of us already know, and the way I think would be better for this application. Downstream from this would be an op-amp based EQ section, so the output of the blend pot will go to a very high impedance, and the pickup really will 'see' 25k...I believe.

Any problems with this?

John

[bbsailor]

I'm getting ready to wind a split-coil bladed pair with magnet steel (cut from transformer core CRNGO steel) as a 7-layered layered blade and neo magnets. Wire will be 36ga, somewhere between 2000 and 2500 turns. With two coils connected in series like a P-bass pickup, this is going to give me around 600 ohms DCR.

I noted in bbsailor's incredibly useful contributions that he suggests as a rule of thumb to have about a 1:40 ratio between the pickup DCR and the load it's driving. What I want to do is connect this and another pickup I'll make next to a 25K blend pot. BUT to give the pickup a steady load, I want to drive the blend pot in the reverse way from how it's usually wired.

Below you'll see an extract from Seymour Duncan's wiring diagrams showing the usual way to wire a blend, which most of us already know, and the way I think would be better for this application. Downstream from this would be an op-amp based EQ section, so the output of the blend pot will go to a very high impedance, and the pickup really will 'see' 25k...I believe.

Any problems with this?

John

[ATTACH=CONFIG]21117[/ATTACH]

JayGunn,

Your use of a highly magnetic laminated core could make the impedance of the pickup very high compared to using metal slugs or alnico magnets. Consider hooking up the series coils to your preamp with about 1M input impedance and then listen to the sound. Then, put a 25K resistor across the output of the pickup and listen for any sound changes. If the high frequency sound changes then that tells you that the loading of the 25K ohm resistor is affecting the sound because the inductance of the pickup is higher than a typical pickup and needs a higher resistance load or less turns on the coil.


The typical guitar pickup with 6000 to 8000 turns of fine wire has between 40 and 100 pf of self-capacitance in addition to 300 to 400 pf for the guitar cable causes a resonance in the 2 to 5KHz range. The normal pot load, 250K for single coils and 500K for humbuckers allows the resonance of the pickup to occur because of minimal loading at resonance or planned loading to tame the high frequency harmonics.

Assuming your pickup will have about one tenth the number of turns compared to a high Z pickup the resonant point should be well up into or beyond the audio spectrum however the core you are using will make a very high inductance coil with between 1000 to 1250 turns on each coil. Try to measure the inductance and Q using an Extech LCR meter. If your pickup has about 4H of inductance (simply speculation and what if) it will have slightly over 25K ohms of reactance (XL equals 2 X Pi X frequency X inductance in henries) at 1000 Hz and will start tapering off the higher harmonics. When using the Extech LCR meter, multiply the AC resistance at 1000 Hz by the Q value to obtain the XL reactance value which should be close to the result using the above formula.

I hope this helps?

Joseph Rogowski

[Mike Sulzer]

That core should not make much difference from a normal one for inductance (lower eddy currents, of course). It might seem counter intuitive, but a short open core with very high permeability does not give much high inductance than a lower permeability one. This is because the field lines have such a long path to traverse in order to close the loop. The inductance rises a few times over that of air core, and permeabilities greater than about 10 do not make much difference, although results vary somewhat with the actual core geometry. I have verified this with measurements, and somewhere I found a reference to this effect, but cannot remember where.

[JayGunn]

Assuming your pickup will have about one tenth the number of turns compared to a high Z pickup the resonant point should be well up into or beyond the audio spectrum however the core you are using will make a very high inductance coil with between 1000 to 1250 turns on each coil. Try to measure the inductance and Q using an Extech LCR meter. If your pickup has about 4H of inductance (simply speculation and what if) it will have slightly over 25K ohms of reactance (XL equals 2 X Pi X frequency X inductance in henries) at 1000 Hz and will start tapering off the higher harmonics. When using the Extech LCR meter, multiply the AC resistance at 1000 Hz by the Q value to obtain the XL reactance value which should be close to the result using the above formula.

I really appreciate hearing from the experts here! I won't have access to an LCR meter until the Spring. I was just planning to note the resonant peak frequency by wiring the pickup in series with a 1 meg ohm resistor, applying a signal from a generator, sweeping the frequencies with the generator and observing the voltage across the pickup itself.

My actual plan, starting within a week, is to wind two pairs of coils, one pair with 7 layers of silicon transformer core steel and one pair with a solid mild steel core. Both cores being around 5mm (3/16") thick. I'm in India this season and the mild steel is sold as HR-1 (HR=hot rolled) and it had like .008-.012 carbon--decently low.

I thought I'd see whether inviting eddy currents in the thickish mild steel, and rejecting eddy currents in the laminated core would make an interesting measurable? audible? difference. I'm going to laminate the transformer steel core with layers of mylar, bonded with epoxy.

I have also been wondering whether all the effort the high impedance pickup winding community puts into shaping the height and center frequency of the resonant peak simply goes away for lower impedance pickups. Who cares where the peak is, if it's above the range my amp can reproduce? SO I THINK all I care about is whether eddy currents start reducing highs in the audible range. That's the gist of the experiment.

Phase 2 of this messing around will be to wind a wide aperture pickup with the geometry of MM pickups--i.e. the poles or blades about 1" apart. I got some 430 stainless sheet 1mm thick in the 'big city' yesterday (Pune India), and I thought I'd make blades out of that. I'll give that pickup more turns than the split coil set since it'll be a bridge pickup and I want the output to be similar between the neck/split coil and the bridge/wide aperture humbucker.

If I learn anything useful, I promise I'll post results. Thanks for your interest.

John

[RedHouse]

...Any problems with this?

John

[ATTACH=CONFIG]21117[/ATTACH]

The lower diagram doesn't look right 2 me.

If the center (wiper) leads move toward either end of the pot travel, both coils, and the reamp input would be grounded, are you sure this is the correct interpretation of the SD diagram?

[JayGunn]

The lower diagram doesn't look right 2 me.

If the center (wiper) leads move toward either end of the pot travel, both coils, and the preamp input would be grounded, are you sure this is the correct interpretation of the SD diagram?

Good observation!

No it isn't a variation of the SD diagram, it's using the pot really in the opposite way. And as drawn, you're right. When the wiper is at one end or the other it shunts all the signal to ground. My bad. The problem is that I connected the 2 wipers in the picture.

I have looked into this bassackwards use of the blend pot and now I think I understand the issues better. If you do a search on "op amp mixer schematic," you'll see dozens of versions of the same thing in which multiple inputs (in our case pickups) join to go to a single op amp inverting input. And the thing I'm calling "blocking resistors" in the second image makes the isolation for each pickup's signal so it doesn't find a path through the ground of the other pickup's side of the blend control. The reason this resistor works is that the point on the schematic marked 'virtual ground' is the place the signal 'wants' to go, rather than to pass backwards out the resistors. The op amp keeps this point at ground potential, do or die.

Here's an example of the common mixer usage of an op amp using the inverting side for input:



Here is the improved drawing showing separation of the 2 wipers, the blocking resistors and the virtual ground point. In my case I am going to clone the original MM 2-band preamp for this low impedance experiment. That preamp is loaded by a 220K resistor. I am settling here for 100K, but even 25K or maybe 10K would work fine for this isolation.



Anyway that's how I think it will work and I'll know in a month or so...got my blades laminated with mylar and epoxy over night, so I'm a few days away from starting winding.

John