I want to make some pickups for a guitar I'm building, and I'm planning on going with active single-coils wound with some 36 AWG wire that I have around.
Having never made successful low-z pickups before, I'd like some advice. Right now the plan is to wind 300-500 turns of 36AWG on a handmade bobbin, with these 1/8" x 1/2" neodymium magnets as polepieces. I would use a Stratoblaster preamp circuit.
Would it be likely that this setup would yield a high output? And, would the neo magnets be too strong even though they are significantly smaller than normal AlNiCo's?
I've got some of those very same rod magnets I'm using for a bass pickup... but I don't normally use rod magnets. But they surely aren't any stronger than some of the neo bars I'm using. But then again rod magnets for poles exert more pull than a charged blade, and we are talking about guitar strings.
I also think 500 turns of 36 isn't going to produce enough of an output to be very useful without a step-up transformer or something, even before the preamp. I was doing about 1000 turns of 42 which worked well.
And of course 42 wound be the ultimate gauge to use for everything, no?
The original Alembic formula was about 1500 turns of 40 AWG.
I'm not actually set on the shape or size of the pickups, so I could put on 1000 or 1500 turns of #36. That would be at least in the same league output-wise as 1000 turns of #42, right?
Also, is there really any reason not to do something like cascade or cascode two JFETs to get a very low output pickup to a usable level?
By the way, David, your pickups are sounding great in my bass.
-Marvin (really though, Andy Gamble)
Yeah, the more turns you can get on there, the better.
My very first low z pickup had so little output that even a stratoblaster JFET circuit didn't help much.
I just made some Jazz bass sized humbuckers based on those skinny pickups I made for you.
If you want to get the best signal to noise ratio from a low impedance source, you do not use a FET in the first stage. FETs have a fairly high noise voltage and a very low noise current. Thus they are good for high source impedances where the noise current has the dominant effect.
One can use a junction transistor for a low impedance source. They have low noise voltage, but higher noise current. The noise current is not so important when the impedance is low.
An op amp with junction transistors at the input stage would be a possible choice. (like the LM837, but it is a quad, more than you need.)
The rule of thumb for scaling down a high impedance pickup is to use 1/10 then number or turns or about 700 to 750 with AWG 36. Then the impedance ratio scales by the square of the turns ratio between the coils, 10 squared or 100. The volume pot of 500K at high impedance scales down to 5K at the lower impedance. The tone capacitor will be larger by a factor of 100 also.
Try using a 500 ohm to 50K ohm microphone matching transformer using an XLR mic cable from the guitar to the matching transformer mounted right at the guitar amp input. As an alternative you can feed this directly into an XLR input of a microphone mixer and get a low noise, high fidelity sound. The microphone input has a higher gain than a guitar amp and typically uses lower level signals in the 5 to 20 millivolt range where the guitar has a pickup output in the hundreds of millivolts.
AWG 36 winds at about 200 turns per inch so you will need to make a Strat type single coil pickup to fit enough wire (about 750 turns) on it. As an alternative you could make a low impedance humbucker with about 300 turns on each bobbin or as much as will fit.
With 700 turns, you should get about 120 ohms resistance. With 750 turns, you shoud get about 130 ohms.
The low impedance pickup will not have the traditional resonant hump in the 2KHz to 4KHz range that defines high impedance pickup sound. The resonant peak will be well above this range and provide more high frequency harmonics.
Low impedance guitar pickups are an interesting variation on 70 year old high impedance pickup designs and patents.
The Stratoblaster FET preamp is mainly used for high impedance circuits to prevent loading by the volume control and cable capacitance. By offering the pickup coil a high input impedance (1 to 10 meg ohms), the frequency response is extended somewhat. Since you are in the impedance range of 500 ohms, you don't gain much benefit by using a high input impedance FET circuit.
If you are into experimentation, you can use a transistor arrangement such as common base transistor amplifier configuration that is useful for matching low input impedances to higher impedances. With a gain of 20X you could get away with about 300 turns (about 150 ohms impedance) and obtain an amplified output near a traditional high impedance pickup or directly feed a microphone mixer but maybe a little too low for feeding a microphone matching transformer.
For a more detailed explaination, see the following web site: http://www.langcaster.com/Pickup-Anthology.html. If you use his formulas to calculate impedance, note that the value of pi is missing from the formula. It should be: Z= 2pi F L (where pi is 3.14159, F is 4000 Hz and L is 10H in his example).
Let us know how it works out.
Last edited by bbsailor; 04-10-2009 at 05:43 PM.
Thanks for your replies, everyone.
I have enough wire to mess around a bit, so I think what I may do is wind a pickup with about 500 turns, then mess with various configurations of bipolars and JFETs on a breadboard. If I can't get that to work right, then I'll rewind with more like 1000 turns and experiment on the breadboard some more.
I am planning on using 1/16" x 1/2" neodymium rods as polepieces, and #36 wire.
Try using two 1/16" magnets spaced about 1/8" apart so the string sits between them. Then, there is less chance of the string be damped by being too close to the magnet directly below. You can also try this with 1/8" X .5" long magnets.
I have used the K&J 1/8" X .5" neo magnets. I use two small round bobbins that fit side by side, using 1/8" X .5" neo magnets from K&J that fit below the string spacing of the E and A strings just forward of the bridge. Then, I wind about 10 ohms DC resistance of AWG 32 wire on the bobbins until filled. I wire one lead of each bobbin together as a common ground. Then the other two leads are the hot leads that go to a 1/8" stereo female jack. I use a 10 ft long 1/8" male plug to a 1/4: stereo make plug cable going to a footswitch box that contains a miniature audio output transformer (8 ohms to 20 K ohms with about a 1 to 50 turns ratio). I use a SPDT momentary push button switch that when up passes the low E string to the transformer and when pressed down sends the A string to the transformer. The output of the transformer goes to an octave divider and only receives one note at a time thus minimizing the glitch sounds from dual notes confusing the octave divider. This allows the two lower guitar strings to go down an extra octave with no permanent guitar modifications.
You can also make it with two momentary push buttons, one for each string, but you need to press the buttons to activate the octave divider sound rather than make the E string the default in the up position.
This makes a fun project.
The other day, I realized that my friend had a humbucker that he tried to wind, but it didn't work. I had a bunch of #30 wire around, so we decided to rewind the bucker with that and see what would happen. We wound it up, ending up with something like 17 ohms DCR, then connected it to two cascaded Stratoblasters.
Interestingly, it turned out to be probably the best-sounding pickup either of us had ever heard. Very responsive, very complex tone. The output was a bit less than a passive pickup when plugged into a tube amp. Still enough to get a nice overdrive in a dimed 5E3. Confusingly, when we plugged it into my solid state bass amp, that pickup was several times louder than a Les Paul Jr's P90.
I'll have to mess with a few more preamp setups, but it looks to me like that pickup will be permanently in a guitar very soon.
Check out this web link below for a common base amplifier configuration. This is used to convert speaker low impedance into a higher impedance but will also work with low impedance pickups in place of a microphone matching transformer. Try the NPN confiruration.
I couldn't get that common base circuit to sound right for some reason. The only BJT's I had around were 2N3904's, maybe they don't do too well in that configuration.
However, the JFET equivalent of that circuit, common gate, worked fairly well. It sounded very good, but very low output. So, I sent the output of that into a Stratoblaster. The tone was awesome, and it had enough output to send a 5E3's bright channel into hard distortion with the volume on 4! I had to remove the Stratoblaster's bypass cap to get the gain down to reasonable levels. It works great.
This has me rethinking my plan for the guitar I'm building. I'll probably go with no more than a couple hundred turns of 36, in a short and small pickup, with this preamp.
Here's the schematic for the preamp.
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