Variable Resonance Pickups - Fun with Buffers

[charrich56]

Hans,

I've simplified the schematic and here it is:



A couple of notes.

1. I'm using just a 1 uF filtering cap on the 9V supply but the one I'm using is a very high quality low ESR film cap. If you want to save cost, use a 4.7 uF electrolytic bypassed by a .1 uF decent film cap instead.

2. This buffer may be overkill for some applications. There are buffers out there which can do a good job for guitar but can use less power. For me, 650-700 microamps is acceptable for an onboard buffer. The big deal here is that my buffer has a very large signal dynamic range of over 6 volts peak-peak with a fresh battery. This makes it a very good choice for bass guitar where there are some big transients depending on the playing style.

3. If you are going to do an SMT version of this board, DON't use the MMBTJ201 which is "supposed" to be a direct SMD replacement for the through-hole J201. The specs are different and it won't work. Use a Linear LSK170A instead.

4. I'm very pleased with the sonic transparency and low noise of this circuit using LSK389A's and am probably going to do a board run and offer these for sale.

[charrich56]

And... yet another note on my just-posted buffer. I didn't invent this topology. This circuit is called a Borbely follower, after Erno Borbely the original designer, whom you should look up if you are interested in JFETs for audio and hi-fi amplifier designs.

The Borbely follower is a JFET implementation of the White cathode follower used in the tube (valve) amplifier world.

-Charlie

[rjb]

...and I needed 1 Henry. The ones I picked turned out sounding very good. I will be working with a local winder to home in on a good PAF-style humbucker wind to work with the VR control, and a couple of other types.

Using separate wires instead of shielded multi-conductor cable also helps; a lot of capacitance is in the cable....

In general, "hot" pickups won't work for this even in parallel coil mode. So an 8K DCR vintage humbucker might work where a 15K "hot" bridge pickup might not work for getting a Fendery sound since the resonance can't go high enough.

With an exhaustive/exhausting web search, I've found exactly one off-the-shelf full-sized dual rail humbucker that might work. (All the rest were "HOT rails".) Any reason not to try two DH11 "neck" pickups in a 2-HB guitar? Given specs are 7.8K and 3.8H. At $16.60 a piece, they're not as great a bargain as the Strat rails- but still about half what GFS would charge (if they had such a product).
https://www.aliexpress.com/store/pro...608.0.0.vJBpSO

Thanks,
-rb

[charrich56]

With an exhaustive/exhausting web search, I've found exactly one off-the-shelf full-sized dual rail humbucker that might work. (All the rest were "HOT rails".) Any reason not to try two DH11 "neck" pickups in a 2-HB guitar? Given specs are 7.8K and 3.8H. At $16.60 a piece, they're not as great a bargain as the Strat rails- but still about half what GFS would charge (if they had such a product).
https://www.aliexpress.com/store/pro...608.0.0.vJBpSO

Thanks,
-rb

I did the same search as you, and I did see those Donlis pickups. They should work fine. They have another model with a nickel silver base plate but it's a bit more expensive and I don't know if it's worth the additional money. I actually want to try some myself. You don't absolutely have to have the final inductance at very close to 1 Henry, or the neck pickup exactly matching the bridge pickup, for this to work. The general idea though is that the self-resonance of the pickup (interaction of the inductance and the capacitance) should be over 10KHz for best results in getting the high resonant frequencies for the Gretsch/Rick/Fendery sound.

Many bridge PU's are a bit too "hot" in a neck/bridge set to work well with the VR tone control and this drove me to using "neck" model rails in the first place so I wouldn't have to worry about pole piece spacing; sounds like you are there as well.

Sonny Walton has made me several sets of superb-sounding rail humbuckers to my specs with Mojotone parts. These are right on the money at 2.0 Henry/coil, CNC scatter wound with #41 AWG, and have 4 ceramic mags per pickup, Forbon base plate. If you want to go a little more high end, you can contact him at sonnywalton.com.

I have the Calig H61 Neo pickups to try out and will let you know what's up with those. They have monster neo magnets in them , over 1000 Gauss at the rails, so don't set them too close to the strings. The Calig's are a bit more expensive than the Donlis but look to be good quality.

I have also tested the California Guitar (CALIG) H67 neck rail pickup. Works fine , a bit high on resistance/inductance, but a good pickup.

The Bill Lawrence (made by Becky and Shannon) L-90 is 4-wire and can be ordered at 4.0 Henry. I think that the L-90's would be a "can't go wrong" choice for the system. At least one of the L-500 models is low enough wind I think but I haven't verified that.

My other suggestion, and if you want the "traditional" PAF look instead of the rail look, would be for a four-wire "vintage PAF" set with about 7.4 to 7.8 K DCR neck, and about 8.2 to 8.4K bridge. There's several imported "vintage PAF clone" sets out there, including Donlis, that seem like they are ending up close to the Seymour Duncan Pearly Gates set (which also tentatively looks like it would work as well.) Even though the pickups won't be completely matched on inductance, they still are close enough for a single-buffer, single variable resonance tone control, to give a good range of sounds. If you tune the system to the bridge pickup, the neck pickup will be a bit higher in resonance by itself, and that still sounds fine.

If you can't get the resonant frequency on the bridge as high as you would like , worst case, it's possible to get rid of the 4-conductor shielded cable, hard-wire for parallel coil mode, and come out with a twisted pair (like what's present in an Ethernet Cat 5 cable) and a separate ground wire. This will eliminate about 90-150 pF of capacitance. Most likely you will not need to do that to get good tone variation on the bridge, but it's an option. After Summer NAMM I'm going to continue researching a reasonable cost humbucker pickup set that will work really well with the system.

I will have single buffer boards available hopefully by the end of the summer, and will be doing a dual buffer board so that in a Les Paul/SG 2V 2 T type instrument, the bridge and neck pickup can have completely separate tone settings. In that case each buffer can be optimized for its own pickup so the issue of different bridge and neck inductances/DCR's will be non-existent anyway. In that case you would probably want slightly higher inductance/turns on the bridge anyway, for better volume matching, as long as you can get the resonant frequency where you want it for the bright cleaner bridge tones.


-Charllie