One-Knob Pickup Variable Resonant Frequency using a Buffer

[Charlie Honkmeister]

EDIT: moved to Tone Control Discussions by sumgai, Dec 5 '16. Explanation below.


Everyone,

Here's a link to my post on successfully demo'ing a one-knob variable resonant frequency control for electric guitar.  It is extremely simple and the version I demo'ed used completely off the shelf parts including a commercial buffer.   This provides an onboard tone control which is IMHO vastly better than a conventional passive tone control.  

The version I demo'ed in a Strat is able to vary a single pickup resonance from about 1.4 KHz to above 5 KHz.   This provides an extreme amount of versatility while retaining clarity even at low resonant frequency settings.   

link

Please follow the link including the two other links to the "theory" posts,  and I would be happy to clarify or share more information on this technique.

Take care,

Charlie

[newey]

CharlieH-

Hello and Welcome to G-Nutz2!

We've had a good deal of interest in active tone controls, as well as in using buffers onboard one's ax for various other purposes, such as combining piezo and magnetic pickups or to eliminate cable capacitance losses. Your concept is an interesting one. Active tone controls can also allow for separate bass and treble attenuation.

[Charlie Honkmeister]

Newey,

Thanks for your welcome to the forum.   I really like the simplicity of this idea,  and also that the buffer is fully functional as a buffer, as well.

I am prototyping two onboard buffers,  one with an opamp and one with discrete JFETs,   to offer a lower cost option than the CAL Redeemer buffer I used in the demo guitar.  If there is some interest,  I will update the forum with progress, and availability of boards.

Take care,

Charlie

[sumgai]

Charlie,

Yes, let me also welcome you to The NutzHouse!

I've moved the entire thread because it is a well-reasoned and complete circuit, ready to go. That, and the fact that it is not a question looking for answers/help, make it an ideal posting for this sub-Forum, where most folks interested in Tone Controls will be headed anyway, or so I should think. Makes it easier to locate for future reference, too.

sumgai

[newey]

sg- I second the moving of the thread, it does belong here.

CH- I didn't know if you had spotted JohnH's post on adding a homemade piezo to a Strat. This includes his buffer circuit, which is useful for other purposes as well.

guitarnuts2.proboards.com/thread/3718/adding-piezo-pickup

John has also explored the buffer-in-a-jack idea, which was mentioned in the link you provided.

[Charlie Honkmeister]

Thanks for the link to the piezo.   I really like the preamp/mixer design on that one.  

In turn on the piezo front ,  I have to plug something that I don't make but definitely recommend.

brennerusa.com/

Mark Brenner has come up with a merged saddle assembly for Strats and Tele's  (and other guitars too,  I have one on my PRS SE)  which uses a dirt cheap undersaddle piezo transducer.   It's called the piezo-one and is available on Ebay as a kit including a stereo Switchcraft jack for 65.00. 

www.ebay.com/itm/Brenner-piezo-one-Strat-bridge-hybrid-guitar-conversion-saddle-kit-/262423716808?hash=item3d19ac23c8:g:6TMAAOSwu4BVq8dX

Here's the version for the Tele vintage three-saddle bridge.

www.ebay.com/itm/Brenner-piezo-one-Tele-bridge-hybrid-guitar-conversion-saddle-kit-/262609927649?hash=item3d24c57de1:g:eXEAAOSwLs5XLNXg

This thing sounds great as a piezo and just listening to it as a bridge, it works very well too. 

I am using it on my tone control demo Strat,  with a Fishman Powerchip to provide the piezo buffer.

Being able to mix the piezo and variable resonance tone control magnetics onboard,  results in tonalities I have never heard from a Strat before.


-Charlie

[Charlie Honkmeister]

Just a quick note on buffers:  

The buffer which drives both the cable and the variable rez tone control has to be able to drive about a 2.5K load to ground and have fairly low output impedance.   So simple single-JFET buffers can work, but the source resistor has to be so low that the buffer is drawing quite a bit of current.   

There's three buffer approaches which seem to work and give acceptably low (well under 1 mA,  and ideally close to 500 uA) current drain:

1.   JFET Borbely follower.  This is just a JFET version of a White cathode follower.

2.  Hybrid MOSFET/JFET  modified Borbely follower  (JFET on the bottom as current source, MOSFET on the top.)

3.   Low current low noise op amp.   My first proto will be with the OPA188 which has under 500 uA current draw typical.  

I have PCB designs and SMT boards for (1) and (3) above and will be doing those proto boards in the next couple of months.

The PCB's can be used for piezo buffers as well.   Please let me know if you are interested in my posting the schematics for these.

Just as a teaser, here's the PCB top side for the opamp version.


Take care,

C.H.

[ashcatlt]

We had a thread not too long ago about variable capacitance and ways to achieve it.

I took it over to DIYSB too because i thought it might be of interest to them and they are a little more oriented toward active electronics.

I originall posted this message in the Spice thread because I missed your request to bring this part of the discussion over here. From the schematic I tracked down on the other end, it kind of looks like you are using a variation of the "driven capacitor" concept, but I'm not really sure and haven't read all of the threads yet.

[Charlie Honkmeister]

Thanks!

Yes,  the variable capacitor with a buffer idea is exactly what I implemented for the variable resonant frequency control.  It looks like several people picked up on this in roughly the same time frame.  

The nice thing is that the buffer can both provide the benefits of a buffer in eliminating tone dependency on the length and quality of the cable,  and also drive the variable capacitance tone control.

What I'm pursuing is high quality buffers which have very small current drain.  The scenario is that there may be 3 buffers in an instrument,   one for each pickup as in a Les Paul 2H configuration,   and one buffer for a piezo bridge. 

So the target is about 500 uA  drain per buffer.  

[JohnH]

Dec 26, 2016 15:28:13 GMT -5 Charlie Honkmeister said:

Thanks!

Yes,  the variable capacitor with a buffer idea is exactly what I implemented for the variable resonant frequency control.  It looks like several people picked up on this at the same time.  

The nice thing is that the buffer can both provide the benefits of a buffer in eliminating tone dependency on the length and quality of the cable,  and also drive the variable capacitance tone control.

What I'm pursuing is high quality buffers which have very small current drain.  The scenario is that there may be 3 buffers in an instrument,   one for each pickup as in a Les Paul 2H configuration,   and one buffer for a piezo bridge. 

So the target is about 500 uA  drain per buffer.  

For a good quality x1 buffer with low current, you can do a lot worse than using a jfet in source-follower mode. Run it with about a 10k to 15k source resistor, with bias to about 5V at the source, and there is the target 0.5mA current drain. The output impedance is a lot less than that 10k value, more determined by the characteristics of the jfet. 2N5457 is a good choice, but jfets are getting harder to find.

Here's a thread about various jfet buffer configurations:
guitarnuts2.proboards.com/thread/3150/jfet-buffer-cable
I had buffers working with even less current than that.

And here is one about using jfet buffers to mix magnetic and piezo signals:
guitarnuts2.proboards.com/thread/3718/adding-piezo-pickup

[Charlie Honkmeister]

JohnH,

Thanks and I do appreciate the links to the buffer design and mixer threads.

I do like and have used simple single-JFET buffers before, but what I have been chasing is low-inductance pickups which without any tone control,  have a self-resonance of about 8-10 KHz.  This is to be able to get really acoustic-like tonality and/or flat response to over 8 KHz when desired.

Using pickups with inductances in the 1 to 1.2 Henry range and then attempting to do the variable capacitor thing with a buffer,  requires the buffer to have a relatively low output impedance, for consistent resonant peak amplitude as you sweep the tone (resonant frequency) control.   If you decrease the drain resistor to lower the output Z of the buffer,  the single JFET buffer starts to become a bit of a current hog for the signal swing you want to get out of the buffer to drive both the tone control pot and the volume pot,  on out through the cable.  

There's two main ways around this that I'm pursuing.   One is to use an opamp buffer with a low current opamp  (example,  TI OPA188, and others),  and the other is to use a 2-JFET configuration where the bottom JFET is a voltage-controlled current source in push-pull.  This is called a Borbely follower,  after Erno Borbely,  who came up with it as a JFET version of the tube amp world's White cathode follower.   It is only a bit more complex than the simple JFET buffer.  Here's my version of the complete system, with resonant/lowpass switching on a push-pull pot switch.   (BTW through hole TO-92 J201's will work with this,  SMT version has different specs and doesn't work for current draw):



I have the SMD board design for this version,  but am actually going with a board which uses a Linear Systems LSK389A ultra-low-noise JFET pair.

The opamp version is extremely straightforward as well; just substitute a non-inverting opamp buffer for the JFET follower.

I have built the opamp version and have just gotten boards from OSHpark to build the LSK389A version.  

The version I have in the demo guitar uses a commercial very high quality buffer module,  the Creation Audio Labs Redeemer.   However the current consumption of the Redeemer is about 1.8 mA  and I need to have a lower cost lower current buffer overall for general use as a commercial product.

The current plan after testing is to open source the board designs on OSHPark,  and maybe do a through-hole version of the board if there's some interest in such a thing.

-Charlie

[Charlie Honkmeister]

Here's some typical  response plots of the general circuit above in resonant (movable peak)  and lowpass (tone control is just lowpass or treble control) modes,  controlled by the switch S1.  I've improved the circuit since then to be able to get more even height peaks in the middle pot positions in resonant mode, but this should convey the idea of what the circuit above can do.  The curves are at 10% points in the pot value.   Please note that at about the 90% point of the tone pot in lowpass mode , the pickup resonance is  nulled out and the response is flat within a dB or so to 8 KHz.    

[christopher]

Hey, I know it's been a while since this thread was created, but I've got a few questions about it:

How would I have to adjust the electrical values of certain components based off of the pickups?Wilde L45s (2.0H inductance) in my case.

Another question is how would series/parallel and out of phase options be influenced by this on my control? I assume it wouldn't cause the control to operate in an undesirable manner.

Oh, lastly, does anyone has any sound clips or videos of this control in use?

[Charlie Honkmeister]

Christopher,

I apologize that I haven't seen the board in a bit.   I will try to answer your questions.

1.  Yes,  the component values have to be recalculated based on the measurements of the pickups you are using.   I measure the pickups with the techniques found on this board,  and plug in their values into circuit modeling.   I use CircuitLab  (www.circuitlab.com.) 

If you can measure the pickups (inductance measured at 100/120 Hz, DCR, capacitance)   I can plug those values in to get a good resonant frequency and Q range for the tone control,  or, alternately,  I should be able to share the Circuitlab link for the circuit so you can do this yourself (I think that you can run values for a shared link;  you just can't save the circuit unless you are a member.)

2.  Generally,  different switching arrangements work the same way you would expect with a normal passive circuit (parallel, series, coil splitting, etc.)  The overall inductance (primarily) changes with the switching arrangement so the "range"  of resonant frequency adjustment changes.  (This is oversimplified but mainly accurate.)    In general,  I have found that the variable cap tone control is so versatile that I don't find the need for extremely complicated switching arrangements.   But they will work just fine with this circuit.   

3.  I don't have any sound samples yet since I am pursuing the "right" tone settings for several configurations (2 humbucker,  3 single coil,  etc.)   and still tweaking to get the best tones.  But I am very close and will put up some sound clips fairly soon.  I'm also going to do a longer post on this technique since the implications of this are pretty interesting -  basically with this platform you can "tone-engineer"  an instrument's electric voice within limits to suit the instrument construction, pickups, artist, genre, amp/signal chain, etc.   

Please feel free to drop me a PM if you need some more information.  

I agree with your taste in pickups -  formerly Bill and Becky,  and now Becky and Shannon are making some great ones.

-Charlie

[christopher]

Thanks, I'm only on here occasionally too so it might be easier if you just gave me the calculations that I need and I can figure out the numbers my self.

More about the switching scheme of my guitar. So you say with this fancy tone knob I won't need all that? In case your interested this is how it's wired, I adapted it from Briguy on the SD forum:


S1 volume that controls phase (it automatically selects the proper pickup based off the rest of the circuit)

3-way rotary switch. This switch works in combination with the 3-way pickup selector to get all the neck/middle/bridge in position 1, neck+middle/neck+bridge/bridge+middle (all in parallel) in position 2, and neck+middle/neck+bridge/bridge+middle (all in series) in position 3.

And then the 3rd pot would be variable resonant frequency.


With the variable resonant frequency control, what parts of this wiring do you think would be made "obsolete"?

[Charlie Honkmeister]

Nov 3, 2018 13:07:53 GMT -5 christopher said:

Thanks, I'm only on here occasionally too so it might be easier if you just gave me the calculations that I need and I can figure out the numbers my self.

More about the switching scheme of my guitar. So you say with this fancy tone knob I won't need all that? In case your interested this is how it's wired, I adapted it from Briguy on the SD forum:


S1 volume that controls phase (it automatically selects the proper pickup based off the rest of the circuit)

3-way rotary switch. This switch works in combination with the 3-way pickup selector to get all the neck/middle/bridge in position 1, neck+middle/neck+bridge/bridge+middle (all in parallel) in position 2, and neck+middle/neck+bridge/bridge+middle (all in series) in position 3.

And then the 3rd pot would be variable resonant frequency.


With the variable resonant frequency control, what parts of this wiring do you think would be made "obsolete"?

I tend to believe the 2.0 Henry inductance for the Wilde L-45's,  but if you can measure it exactly with an LCR meter like an Extech or DER EE DE-5000, and also get me the DC resistance and either the pickup capacitance or the self-resonant frequency of the pickup(s),  I can work the circuit simulation to give you a good range.   It would be easier to share a Circuitlab link using a good set of starting values.  But you need a good set of measurements on your pickups to really tweak the resonant frequency range of the tone control. 

If we ignore output changes as you change your switching configuration,  the electrical main effect of doing a pair of pickups in series rather than parallel is to double the inductance of the combination when they are in series and halve the inductance when they are in parallel,  relative to a single pickup.  This affects the resonant frequency given a fixed load capacitance or a certain fixed point on the variable frequency tone control.   That being said,  having a quick way to change parallel to series combinations can give a volume boost and a drop in resonant frequency before cutoff,  which might be useful for, let's say,  a solo break in a song.

The variable resonant frequency tone control  typically will be dialed in for a bit under a 1 1/2 octave range,  let's say for an example  from about 1.7 KHz  to about 4.3 Khz,  which covers humbucker tonality all the way through bright Strat or Tele type tonality with cable loading,  all by itself.  This works the best with the widest range of amps and pedals.   So I would say that  sticking with either all series combinations or all parallel combinations of two pickups would be OK without having to switch to the other mode, and having both modes as alternatives to the normal single pickup selections is redundant.  

I would vote for keeping just "normal"  and series modes available because the lower resonant frequency at cutoff in series versus normal single selections or parallel is  "better"  for hitting good distortion tones from a pedal or amp, and it also gives you an output boost for the two-pickup combinations. But it's really all about what you prefer, your style/genre of music,  and whether you are going to play the instrument live and don't want to deal with a lot of switching in mid-song.    

The phase switch wouldn't be redundant in any case.

-Charlie 

[davidofchatham]

I find this idea very interesting! After some digging, I found the current link for the Music Electronics Forum discussion referenced in the first post: music-electronics-forum.com/forum/instrumentation/guitar-tech/44187-variable-resonance-pickups-fun-with-buffers/. Unfortunately the links there to other discussions don't work. 

Charlie Honkmeister, do you have any updates on this project? Will there be a ready-built version at some point? (I'm just getting started with guitar wiring — circuit boards are way beyond my skills at the moment!)

[reTrEaD]

Mar 30, 2022 10:53:09 GMT -5 davidofchatham said:

I found the current link for the Music Electronics Forum discussion referenced in the first post: music-electronics-forum.com/forum/instrumentation/guitar-tech/44187-variable-resonance-pickups-fun-with-buffers/. Unfortunately the links there to other discussions don't work.

Hey David, you'll have to wait for Charlie for answers to your other questions. Meanwhile, these might be valid links from that thread to the other threads on that forum:

Variable Resonance Pickups - Some Prior Art and Links
music-electronics-forum.com/forum/instrumentation/pickup-makers/38763-variable-resonance-pickups-some-prior-art-and-links

medium Z pickups with gain
music-electronics-forum.com/forum/instrumentation/pickup-makers/42154-another-run-at-medium-z-around-1-henry-pickups-and-preamps
or perhaps
music-electronics-forum.com/forum/instrumentation/pickup-makers/36320-

[davidofchatham]

Mar 30, 2022 14:32:57 GMT -5 reTrEaD said:

Hey David, you'll have to wait for Charlie for answers to your other questions. Meanwhile, these might be valid links from that thread to the other threads on that forum:

Variable Resonance Pickups - Some Prior Art and Links
music-electronics-forum.com/forum/instrumentation/pickup-makers/38763-variable-resonance-pickups-some-prior-art-and-links

medium Z pickups with gain
music-electronics-forum.com/forum/instrumentation/pickup-makers/42154-another-run-at-medium-z-around-1-henry-pickups-and-preamps
or perhaps
music-electronics-forum.com/forum/instrumentation/pickup-makers/36320-

Thanks, reTrEaD! Looks like I didn't dig long enough. At one point I did try to sign up on that forum so I could search better on there, but they're evidently not accepting new sign-ups at the moment.

I hope this project hasn't been abandoned!

[reTrEaD]

davidofchatham, I'm not a member of that forum. There is a dropdown marked with ▼, associated with the search function there. If you click that, you can tick a checkbox to limit your keyword(s) to search only titles. Or you can click on Advanced Search which will take you to this page:
music-electronics-forum.com/search?q=&searchJSON=%7B%7D&AdvSearch=1

Advanced search is handy. It returns fewer results you'll need to sift through.