Bill Lawrence Q-Filter, an analysis of... May 2, 2018 2:32:13 GMT -5
Post by antigua on May 2, 2018 2:32:13 GMT -5
I bought a Bill Lawrence Q-Filter www.wildepickups.com/Q-filter.html in order to take it apart and see what's going on, and possibly to use it.
Altogether, the inductor shows an inductance value of 2.18H @ 1kHz. The Wilde Pickups website rates it at 1.8H, but it seems the value drifts rather easy if the retaining screw is loosened or tightened. The DC resistance is 49.6 ohms. I can't measure the wire gauge without undoing some of the coil, but it's certainly larger than 42AWG, and appears to be about a couple hundred turns.
The inductor is a ferrite pot core type, which allows it to achieve very high inductance relative to the turns of wire on the coil. 2.18H is in the ballpark of a Strat pickup, having nearly 8,000 turns of wire, but without much to show for a core aside from a few ALNiCo pole pieces.
With both pots in place, but without the steal screw through the center, the inductance reads 630.0mH SER @1khz, and about the same in PAL mode. Without the upper upper pot, the inductance drops to 8.94mH SER 16.03mH PAR @ 1kHz, which really goes to show the value of having a completed magnetic circuit.
Wilde Pickups also threw in a parallel resistor and cap treble bleed for free. The cap is 10nF and the resistor is 10k.
The name "Q-Filter" is not an especially fitting name, as it's little more than a high inductance inductor with a rather low series resistance. The tone pot that you connect the Q-Filter to would more rightfully be called a "Q filter", and it has this effect whether or not you use an inductor, or a typcal capacitor. Supposedly the "Q-Filter" name is a reference to the fact that the low series resistance allows the guitar pickup to maintain a high Q factor when used in conjunction with this rather high value inductor.
Here is a practical frequnecy response plot of an SSL-1 with a Q-Filter, wired in place of a tone cap, using a 250k pot.
To a large extent, this behaves like the reverse of a capacitor. There is a strong resonance at one frequency when the tone pot is all the way up, and another strong resonance when the pot is turned all the way down. When a typoical cap is used, then at zero the resonant peak is at a lower frequency, but with an inductor, it ends up at a higher frequency. However, throughout most of the pot's sweep in between, the resonances flattens out, and it's in that respect the tone pot is acting as a legitimate "Q" control, and this is the case regardless of whether a cap or an inductor is used.
The major difference though, between using a cap and in inductor, is that there is an overall drop in amplitude output, -6.3dB with the SSL-1, which is very a noticeable drop in volume.
Here is the same test contducted with a Seymour Duncan SH-1N, "'59 neck":
The curves are similar overall, although interestingly the full open resonance with the Q-Filter / inductor is much, much higher than the normal Q factor of the pickup as-is, which is almost completely flat. In this case the Q-Filter might cause the SH-1 to sound "muddy" throughout the sweep, becoming a lot more clear as you reach the bottom, but by the time you get to the bottom, there is also substantial drop in output.
The problem in this case is that the Q-Filter at 2 henries has only half the inductance of the SH-1 neck. If an inductor closer to 4 henries is used, the effects wouldn't be so dramatic, and it would likely be more usable. I notice that Wilde Pickups offers a 3 henry Q filter "for bass", so a person could just buy the bass version, and use that instead.
In general, if you were trying to use a parallel inductance to virtually "unwind" a pickup, you'd get the best result with an inductor that has an inductance which is close to that of the pickup. Since most "hot" pickups on the market, such as a JB, or an SSL-5, which you might want to "unwind" have inductance of well over 5 henries on average, a 2 henry Q Filter falls well short of that. You could make your own 5 henry inductor using the same basic parts, or just combine two or three Q Filters in parallel, though that multiplies the costs.
Here is an LC plot of the Q-Filter itself, which has a resonant peak of 12.3kHz, and a very high Q factor, which puts the intrinsic capacitance at about 26pF.
Wile Pickups sells the Q Filter for $24, though they can be made at home for much less. Similar ferrite pot cores with coil formers cost about $1-$2 a piece on eBay, and supposing you use 38AWG magnet wire, the spool might cost $10, but you'd have enough wire to create dozens of inductors.
If you just want one Q filter, it's easier to pay the $24, but if you wanted to create several of them, or if you wanted to match the inductor value to the pickups, making them at home would be a good way to go. You can see from the picture below what a pot core inductor looks like. It appears that it only has two to three hundred turns of wire, so they would be quite a bit easier to produce than a guitar pickup. You could simply use a drill as your winder, or wind it by hand if your wrist can take it.
In lieu of a wind counter, you could check the DC resistance and use that a way to gauge how much wire you've put on the coil former, though you'd probably want an LCR meter, such as the DER EE DE-5000, so that you could determine the exact inductance values you've achieved.