Bill Lawrence Q-Filter, an analysis of...

[antigua]

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.





Pics:

[JohnH]

Interesting that these are within range of making a home-made version. When you analyse guitar circuits with added inductors, such as Varitone etc, it always seems like the most interesting tonal variations would come from having a few more Henries than that to dial in with a pot.

[stratotarts]

It would be interesting to see the curves with the treble bleed circuit in place.

[antigua]

May 2, 2018 8:47:22 GMT -5 stratotarts said:

It would be interesting to see the curves with the treble bleed circuit in place.

I used your integrator with the 200k ohms/470pF switch toggled on in order to show a "real life" end result, so the treble bleed would be across a volume pot voltage divider we're pretending is on "10", so the treble bleed would be out of circuit in this case. Do you mean assuming the volume were turned down to 5? We could model that with LTSpice, but I suspect you'd just sort of see a a more aggressive low end roll off due to the combination of high pass filtering. 

[blademaster2]

I used a 1H inductor in the bridge tone circuits of all of my guitars. It definitely is more interesting sounding to me than capacitor tone circuits.


On my guitars if you use the neck pickup with its tone turned down all the way, and then mix in the bridge pup using the volume control with this gadget in place on the tone circuit (also turned down to low) it makes a pretty decent wah-wah sound.

[JohnH]

I tried modelling your SSL1 Q filter test with GF. It came out pretty close (as it should, since my model was based on your pickup!)




Some variation is due to the pot, where yours was with a real 250k tone pot and the model assumes a theoretical 250k with a bilinear taper with 10% at mid turn.

Also, my plots assumed no tone pot/Q filter at max, eg like no-load, since that matched the test better. Is that what you had?

[antigua]

May 2, 2018 17:04:27 GMT -5 JohnH said:

I tried modelling your SSL1 Q filter test with GF. It came out pretty close (as it should, since my model was based on your pickup!)




Some variation is due to the pot, where yours was with a real 250k tone pot and the model assumes a theoretical 250k with a bilinear taper with 10% at mid turn.

Also, my plots assumed no tone pot/Q filter at max, eg like no-load, since that matched the test better. Is that what you had?

I was using a 250k audio taper pot, together with the 200k test load. What input impedance value do you use? I modeled it also with LTSpice and got a result similar to yours, but the practical plots looked nicer and were more realistic, but the good news it that it can be used as a means of selecting an ideal inductance value for a given selection of pots and pickups. 

The inductance changed pretty readily just by loosening or tightening the retaining screw that holds the two pot cores together, so if you modeled curved differ, another thing to try is adjusting the Q filter's inductance value. 2.2H was measured with with screwed tightly, but if it was a little looser it could drop down in to the 1H range. 

I ordered a few ferrite pot cores to make some of my own high inductance inductors. It's looks like a fun and easy undertaking. 

[JohnH]

I was trying to model the same as the test, with a 200k 470pF load, plus the Q filter through another 250k pot. I got the same peak frequencies and general shape, so I also agree that such models can help design with these gadgets.

[pj]

I suspect the little cr network is the network that bill used to suggest for putting in series with the coil to limit the severity of the cut by the Q filter, in much the same way as the 100k resistor in the Gibson Vari tone. I've got a copy of the paperwork somewhere I'll try and find it and upload. Bills preferred treble bleed values were 330pf and 80k if I remember correctly.

I also have a Kent Armstrong Tone Choke which I suspect is much the same thing though it is potted.

[pj]

[antigua]

May 5, 2018 20:13:33 GMT -5 pj said:

I suspect the little cr network is the network that bill used to suggest for putting in series with the coil to limit the severity of the cut by the Q filter, in much the same way as the 100k resistor in the Gibson Vari tone. I've got a copy of the paperwork somewhere I'll try and find it and upload. Bills preferred treble bleed values were 330pf and 80k if I remember correctly.



I also have a Kent Armstrong Tone Choke which I suspect is much the same thing though it is potted.

That's interesting. I'll model that up and see what pops out.

Note that you're missing a connection on the DP/DT from bottom left to top right.

I think the general idea of allowing the Q filter to swap with a typical cap by way of push pull is a good mod. I think it could even be pre-fabricated and sold to guitarists who want bass and treble tone control functionality, without having to source a choke or Q filter and follow wiring diagrams.  

[pj]

Yes that would be useful for people who don't want to get too involved.

That's as the diagram came from Bill/Becky.

There's also another one I'll load.

The way its connected on the diagram makes for a tone in one position with the coil disconnected and then brings the coil and resistor in the other position.

I'll see if I can find the other diagram he sent and try and get lucky with the image uploader again. Cheers

[pj]

[antigua]

[oops... brb]

[pj]

Perhaps it's as much to stop the cap popping. I found less resistance was better think I went with 10k and 22n

[antigua]

May 6, 2018 12:38:38 GMT -5 pj said:

Here is a tone control wired as depicted, and 250k pot with a 0.02uF C and 10k R in parallel across the center and right lugs, with the left lug connected to hot, and the right lug connected to the inductor and then to ground:



And here is a typical tone control with and inductor in place of a cap. The other side of the pot's resistance is left open.



So it does appear that the wiring that includes the cap and resistor pair help to retain some low end, compared to wiring it like a normal cap.

If the resistor is removed, leaving only the cap, there is an interesting band stop around 800Hz, similar to a Veritone circuit:




If the cap is removed, leaving only the resistor across the center and right lugs, then the circuit is similar to simply putting a resistor in series with the tone control itself. It appears the low end is not attenuated as dramatically if the real resistance of the tone control never reaches zero: 




If the resistor is set to a higher value of 50k, more low end is retained and the Q remains flatter. A trim pot might be helpful for getting a perfect balance of increasing treble without gutting the low end. 

[pj]

Thank you for going to those lengths. Seeing the bode plots and models makes me wonder whether I wouldn't have been better fiddling a bit longer and trying smaller cap values that would intersect the inductors slope a little further up, leaving the resistor out for a higher Q. Working on the half the cap twice the frequency principle, it would be quite easy to put a notch in the mids similar to that created by the Fender tone stack. A great option for those with champ-like simple valve combos to have a less in-your-face rhythm tone.

Thanks again for your exploration and effort.

[antigua]

Even though I have a Q Filter on hand, I was interested in just trying to make one, so I bought some pot cores and magnet wire and wound the bobbin up with a drill. Since the drill didn't have a turn counter, I don't know how many turns of wire it took, but I found that I pretty much needed to fill the bobbin up in order to get past 2 henries, and I guess that after about three to four hundred turns it was maxed out, and the final inductance was around 3 henries.

The Q Filter currently costs about $34, and all said and done, the parts needed to make my own cost about that much, but I have enough parts to make a total of five inductor / Q Filters, and plenty of wire to spare. 

pot core on the left, Bill Lawrence Q Filter on the right:




Put together, using a Popsicle stick with some drilled holes as a means to join the tiny magnet wire with the much thicker hookup wire:




The Q Filter wiring scheme suggests that the tone control also incorporate a cap and a resistor on the control pot, so that once the tone control reaches the minimum, the cap and resistor are in series with the inductor / "Q filter", which yields an outcome similar to a Gibson Veritone control.



As you can see the schematic recommends several different resistances, to taste, so I used a 20k trim pot instead:




What I found testing by ear is that the bass response increases as the resistance is increased.

I made an LTSpice model to get a more exact idea of what has happening:




The bright green line represents near zero resistance (bass drops off sharply), and the lines that rise above the green lines represent increasing degrees of resistance, up to 25k ohms (bass increases, mids decrease).

Where as a Veritone toggles between caps in order to move the mid scoop frequency, the capacitance value is fixed in this case, so the variable resistance serves to alter the amplitude of the bass and the mid scoop. If you were to replace the 0.02uF cap with a smaller value, the mid scoop increases in frequency, while a larger value cap decreases the frequency where the mid scoop occurs.

I settled at about 10k, to get some bass roll off, but not a complete bottom-out, as you can see happens with the bright green line. If you go for the full 25k, there is virtually no bass roll off at all, and you're left with a tone control that slightly scoops the mids while pushing the resonant frequency upwards by about 1 to 2kHz. If you're the type of person who hates a tone control that takes away bass, you might like the Q filter with a 25k resistor, but if you're buying/making a Q filter in order to get a counterpoint to the stock tone control, 10k seems like a good choice.

With a bright Fender neck pickup, the Q filter with 10k resistance almost produces sound like an acoustic guitar. It's easily more convincing than a piezo in a Strat, so if you're interested in piezo pickups for Strat, I'd definitely suggest this as an alternative. Here's a demo someone made with a Tele, you can see what I mean:

[MEDIA=youtube]YnSX1Fyy7Oc[/MEDIA]

[frets]

Hi Guys😇,

This is a thread I’ve been interested in of late given Antigua’s subsequent analyses of a Q-Filter’s enhanced Q factor over the Q factor of a regular Mouse Xicon Audio Signal Transformer of the approximate same Henries; AND, me building a Ritchie Blackmore Tone Control for a client.

I want to find a smaller “core” than a pot core, preferably cylinder type; and am wondering if Litz Wire would dramatically increase the Henries over a more standard magnetic wire. The goal would be to create the smallest highest Henrie (3H+) Q filter. Any thoughts on the subject?

[b4nj0]

I don't believe Litz wire would be suitable C. We don't concern ourselves with RF "skin effect" in guitar wiring, and in any event it is beyond tedious removing the individual insulation from all the fine constituent wires in order to take advantage of all of them. Maybe Litz wire can have the insulation burned off with the heat from the soldering iron like some enamelled copper wire? My experience with using Litz wire has been less than fruitful. It also ought to make things bulkier than they need to be. Unless you know something that I don't (and I've already learned that's entirely possible with you!)

e&oe ...

[gckelloch]

The Wilde Q-Filter is 1" square by 1/2". I used double sided tape under the pickguard to secure it sideways next to the V control in my rewired Parker NiteFly. The bass Q-filter should be the same size and is 3H if you can't find a smaller one.

Nice work here. The Q-FIlter should actually be called an L-filter, but it does come with parts to make it a Q-filter. I think “Q” may be in reference to how it affects the Q of the mid-dip as you turn it down, but I dunno.

The inductance measurement with a meter may not be accurate because it doesn't account for the Q-factor, but I guess you can determine the actual inductance value by using the pickup testing device, no? My N & M Microcoil also measures ~2H with my LCR meter, but the actual value is 1.8H. Bill used an equation involving the Q-factor to get the correct value for a pickup. He said most pickup makers don’t know how to determine the correct value. Here’s Scott Lawing’s method:
lawingmusicalproducts.com/dr-lawings-blog/electrical-parameters-and-pickup-performance-part-ii-inductance

I don’t think the 3H Q-filter would increase the high end in the middle range of the knob with an Fe core pickup because the pickup Z is very high. In fact, the 1.8H Q-Filter is highly recommended with Wilde’s highest inductance NF series ~5H L298 bridge models. Of course, the peak would be at a higher freq with any pickup using a lower capacitance cable.

Yes, tension on the coil increases the inductance. It is tightened specifically for the given value. It makes more sense to use thicker wire to keep the Z as low as possible with the given amount of turns, and so the coil is big enough to be compressed with the pot core. I don’t hear any added hum or buzz when I use mine.

[antigua]

Jun 6, 2021 15:10:08 GMT -5 frets said:

I want to find a smaller “core” than a pot core, preferably cylinder type; and am wondering if Litz Wire would dramatically increase the Henries over a more standard magnetic wire. The goal would be to create the smallest highest Henrie (3H+) Q filter. Any thoughts on the subject?

Not litz wire, but just finer gauge wire. You can get more loops on there if it's finer. 

I bought pot cores on eBay. They sell some with the pot and the coil former together, which makes assembly easy. 

This one looks pretty small www.ebay.com/itm/251273912244?hash=item3a81179fb4:g:agEAAOSwlkpb6rmK , I don't know if you can achieve 3 henries with it, though. It has a coil former and electrical connectors built in, which is very convenient. 

[frets]

Thank Guys!!😸

Antigua - thank you for finding me those pots. I would like to reduce my expense while improving on the Q Factor. I appreciate all your help.

[gckelloch]

Jun 9, 2021 0:52:40 GMT -5 antigua said:

Jun 6, 2021 15:10:08 GMT -5 frets said:

I want to find a smaller “core” than a pot core, preferably cylinder type; and am wondering if Litz Wire would dramatically increase the Henries over a more standard magnetic wire. The goal would be to create the smallest highest Henrie (3H+) Q filter. Any thoughts on the subject?

Not litz wire, but just finer gauge wire. You can get more loops on there if it's finer. 

I bought pot cores on eBay. They sell some with the pot and the coil former together, which makes assembly easy. 

This one looks pretty small www.ebay.com/itm/251273912244?hash=item3a81179fb4:g:agEAAOSwlkpb6rmK , I don't know if you can achieve 3 henries with it, though. It has a coil former and electrical connectors built in, which is very convenient. 

Not much thinner and may be taller. I don't see how you can compress the coil in it without breaking the former, so you'd have to add more thinner wire to bring the inductance up enough, and then the impedance would reduce the peak too much, no?