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Thread: Variable Resonance Pickups - Fun with Buffers

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    Variable Resonance Pickups - Fun with Buffers

    Everyone,

    I just got back from the Arlington guitar show where I demo'ed my Strat (1997 NC Affinity) with the variable resonance tone control I have been working on. The reaction was overwhelmingly positive with multiple players. Sounded fabulous through high-end tube amps.

    The goal of this demo was to demonstrate the concept using only off-the-shelf parts any guitar tech could install. The Tone control gives one-knob control of the pickup resonant frequency from about 1.4 KHz to over 5 KHz. This covers Gibson humbucker tonality through Tele through bright Strat. In positions 2 and 4 the resonance shifts up a bit because of the parallel pickups and covers about 2 KHz to over 6 Khz, which gives incredible "quack." The resonance can be switched out to provide a flat pickup response past 8 KHz.

    The 3 pickups were generic Chinese dual rail single coil-sized humbucking pickups which were about $8.50 each shipped. The key reason for using these was because in parallel coil mode these had low enough inductance and stray capacitance to make the technique work. The surprise was that these pickups sounded fabulous when in this circuit.

    The key electronic component was a Creation Audio Labs Redeemer buffer. This both buffers the pickup signal and provides a low impedance output to work the variable capacitor feature.

    Besides the buffer, the tone pot, and volume pot, there are two capacitors and two resistors needed for this to work. The schematic shows three caps but the Redeemer has an input cap so I deleted the 100 nF cap. I put a switch on the volume pot for lowpass/resonant mode with an extra resistor to allow hi-fi flat response to over 8 Khz, for recording/plugins.

    Here's the schematic.

    Redeemer Buffer with Resonance- White Rail.pdf

    If you want some ideas on how this works, please refer to my two posts. One is on variable resonance pickups Variable Resonance Pickups - Some Prior Art and Links,
    and one is on medium Z pickups with gain Another Run At Medium Z (around 1 Henry) Pickups and Preamps. For this build I used conventional pickups and no gain.

    Not all of you read the Pickup Makers forum, so I thought this would be of general guitar tech interest.

    To make this technique work, you need to know the inductance, DC resistance, and stray capacitance to ground of the pickup windings. Using Circuitlab, you can simulate the response and pick the correct resistors and caps to get the resonance range you want. But you can generally make this work with any pickup with a reasonably low inductance, below 1.5 Henries. Many "vintage" or low-wind humbuckers will fit the bill when the coils are paralleled.

    Take care and questions/comments are welcome,

    Charlie

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    Very cool, thanks for the links. As it happens, I've recently been working on a DIY onboard bass preamp with a variable frequency/variable resonance format for the treble tone control section. Nothing new or earth shattering about that, of course, and no bass pickups I typically work with have inductance anywhere near under 1.5H. But I do also happen to have a CAL Redeemer kicking around in a rarely used active DI box, and a possible candidate guitar...hmm. Name:  Yel_wink.gif
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    Passinwind,

    You can still use the technique with higher inductance pickups if they have very low wiring capacitance to ground. For example I can do this with Bill Lawrence L-90's which you can get at 2.0 and 2.8 Henries, because they have very low capacitance of about 90 pF. Using separate wires instead of shielded multi-conductor cable also helps; a lot of capacitance is in the cable.

    The real target is to have the self-resonance of the pickup very high -- 8 to 10 KHz or more. You can always bring the resonant frequency down with capacitance but you can't bring the resonant frequency above what the pickup itself limits you to.

    If you wind your own pickups or can have them wound to spec, you can easily get in the right range. The tradeoff is fewer winds = lower output. That's why one of my posts included a 12 DB FET preamp to boost the signal along with a FET buffer to provide the variable resonance control.

    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.

    If you (or anyone else) needs to get correct component values, until I can figure out if a public CircuitLab URL will work for anyone without paying for the service, I can give you the correct values if you give me the inductance (ideally measured with an Extech or DER EE LCR meter at 100 Hz) , DC resistance, and capacitance to ground of the pickup wiring. Or, maybe it's a pickup I already have or have access to, to measure. Most published pickup specs are either incomplete, are total BS, or done with unknown test conditions.


    -Charlie
    Last edited by charrich56; 10-16-2016 at 04:18 AM.

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    Thanks Charlie, appreciate those insights. It'll be a while before I get to even contemplating working on my guitar, and what I'm doing right now for bass is working quite well and is pretty simple to work out in LTspice. Here are some sims of my current config's response curves, which I have confirmed by measurement:

    pw3blpf.jpg

    The whole board build including prototype PCBs was cheaper to build than buying a Redeemer, and does bass and mid EQ as well. My goals are a lot different than yours in this case though; I'm not trying to make my Travis Bean sound like an Alembic or Wal bass, even though I'm loosely copping their general sweepable resonant filter formats. But to make that work optimally, it's actually helpful to kill the pickups' native LRC resonant peaks, which makes me all the more interested in your work.
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    That looks like a very versatile preamp/filter you have there. I've always admired bass players and builders for being out front in using active onboard electronics . Guitar players are a traditional finicky lot for the most part in going active, but things are coming around.

    You can kill the LRC peak with just the right value of load resistance as you probably know, but it doesn't help you with losing the highs above the peak very much. My "sneaky trick" is to put the native peak at about 8-10 Khz so when the peak is there and I'm not bringing it down with capacitance to use for voicing, it's just basically a moderate treble boost which can be dialed back to "flat", or a moderate cut. I don't see much use in having response past 8-10 KHz for guitar.
    Last edited by charrich56; 10-16-2016 at 01:31 PM.

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    Quote Originally Posted by charrich56 View Post
    That looks like a very versatile preamp/filter you have there. I've always admired bass players and builders for being out front in using active onboard electronics . Guitar players are a traditional finicky lot for the most part in going active, but things are coming around.

    You can kill the LRC peak with just the right value of load resistance as you probably know, but it doesn't help you with losing the highs above the peak very much. My "sneaky trick" is to put the native peak at about 8-10 Khz so when the peak is there and I'm not bringing it down with capacitance to use for voicing, it's just basically a moderate treble boost which can be dialed back to "flat", or a moderate cut. I don't see much use in having response past 8-10 KHz for guitar.
    Buffering is a very good and easy way to isolate the pickup from the additional capacitance loading of the guitar cable and the amp input load. A very easy way to hear the difference is to make the "Tillman buffer". Just web search the words in quotes to get the construction details. Then you just plug the buffer, located in the guitar end of the plug cable, and you can hear what effect the coax cable and amp loading have on your sound. It was very educational for me when I tried this about 10 years ago.

    Joseph J. Rogowski
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    Joseph,

    Thanks and good to hear from you.

    The Tillman buffer will work for the resonant tone control, but you need to make sure that the source resistor is set fairly low, under 5K and maybe under about 2.5K. One of my circuits actually uses the 5K tone pot as the source resistor for the FET buffer. Generally you don't want to put DC through the wiper of a pot but if you are using the two fixed terminals it works OK, especially since you are just connecting a capacitor to the wiper.

    Here's a link for some buffer circuits. I don't agree with the article author's comments on buffers, but the links are OK:
    Electric Guitar Preamp Buffer*Circuits

    In certain situations with trying to set low frequency range resonance, too low pickup inductance, etc. you will need very low output impedance for the buffer, say less than 200 ohms or so, and will have to use a fancier buffer, or an opamp, to get the resonant frequency control to work right. Op amps make great buffers BTW as well (as long as they're unity gain stable), and you can use those.

    Unity gain is critical to getting the resonant control to work right in bootstrapping the capacitor. The closer you are to unity gain, the better the circuit will work and the more tweaking range you will have on the resonance of the pickup.
    Last edited by charrich56; 10-16-2016 at 08:04 PM.

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    I just made a URL for the Circuitlab simulation of the variable resonance control/buffer. Here it is:

    https://www.circuitlab.com/circuit/4...ffer-v2-opamp/

    This will allow you to look at and modify the circuit for a limited time, in demo mode without save or export capability. I don't know how long the time limit is for now.

    I am simulating the Redeemer with a generic opamp buffer.
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    A great idea

    Sounds great! I hadn't heard of the CAL Redeemer buffer so I had to look it up...

    Creation Audio Labs

    Kinda pricey especially when you figure in the $16.87 shipping charge for UPS ground...



    With all of the various buffering circuits in the public domain perhaps someone can come up with a single board solution incorporating the circuitry you came up with.

    OOPS! I see that you already did exactly that... My bad!

    https://www.circuitlab.com/circuit/4...ffer-v2-opamp/

    Thanks!

    screenshot_2016-10-17-12-31-01_20161017125405376.jpg

    Steve Ahola

    P.S. I really like the idea of using a buffer not just for "long cable runs" but for doing something creative with the sound of a pickup. Can you post a link to the particular $8.50 pickup you used?
    Last edited by Steve A.; 10-17-2016 at 08:40 PM.
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    Steve,

    Here's the same one on Ebay, it looks like exactly the same thing. A bit cheaper than the ones I bought. I preferred the type with a single plastic cover rather then the "open rails" look, and I preferred the single copper tape strip grounding the rails on the bottom. There's lots of listings for these and I think they are all about the same. Amazon has some too.

    New Dual Rail Humbucker Neck Pickup White 4 Wires for Electric Strat St Guitar | eBay

    The other type has open rails and uses a grounding copper tape ring around the windings. These work OK IF you unwrap the covering tape, vertically cut the copper tape so it doesn't make a complete loop around the pickup, make sure the copper tape break is insulated from re-touching, and tape it back. They have slightly more capacitance but still work for the purpose.

    New 1pcs Black Dual Rail Humbucker Strat Guiter Neck Pickup 4 Wire | eBay

    The main spec is that the pickup is rated at about 9K - 10K resistance; a lot of them say that they are 4.98 K per coil.

    I have a buffer circuit which I haven't built yet, which uses a hybrid of MOSFET and JFET in a mangled White cathode follower configuration. It looks great on headroom in CircuitLab but the MOSFET might be a bit noisy. I'll post it if there's some interest. Simple JFET buffers would work OK for this in theory and I will try one out very soon.

    There are so few parts to this depending on the buffer, that it would be really simple to do a single sided PCB for it. That's probably the preferred direction, but the Redeemer is very nice albeit expensive as you pointed out. I'm moving on to humbucker territory to see how that will go.


    -Charlie
    Last edited by charrich56; 10-18-2016 at 06:52 AM.

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    Steve,

    Here's a Circuitlab link for the discrete buffer I designed. Again, hasn't been built yet but in simulation has incredible headroom, doesn't look like it distorts with +-3.5 Volt swings which is surprising for 9V. Current draw is good too, 550 - 565 microamps. I have high hopes for this on sound quality. Noise may be an issue though on the MOSFET.

    https://www.circuitlab.com/circuit/k...r-v2-discrete/

    The LSK170 is a high-end super low noise JFET. Could use a J201 probably with some tweaks. 1 uF coupling caps are polypropylene, or Elna Silmic II, which are dirt cheap from Mouser.

    Part of the reason for posting all this stuff is to generate some interest and experimentation. If someone could come up with a better DIY buffer that is about $5.00 in parts, with a PCB which can be stuck to the back of a pot, has good battery life, and sounds great to the most discerning ears, that's great.

    I'm going to be working with some local builders to whom a $50.00 buffer (cheaper for dealer/OEM) isn't going to be a huge expense on a $2.5K guitar. Also, might be able to save quite a bit of money on pickups.

    But I'm still all in for the DIY and open source thing.
    Last edited by charrich56; 10-18-2016 at 05:59 AM.

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    Quote Originally Posted by charrich56 View Post

    Part of the reason for posting all this stuff is to generate some interest and experimentation. If someone could come up with a better DIY buffer that is about $5.00 in parts, with a PCB which can be stuck to the back of a pot, has good battery life, and sounds great to the most discerning ears, that's great.
    I have a few different opamps on hand that I've been planning to try out in that role, and I already have a pretty nice 2SK170 based buffer/preamp board and a Redeemer to use as benchmarks. If I manage to come up with anything good and cheap I'll be happy to share, but a lot of my candidate parts are well over $5 just for the chip. Many are SO-8 SMDs, so fitting something on a pot would be no problem at all.
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    Passinwind,

    Thanks! $5.00 was a stretch goal anyway. There are so many parts out there, that it would be a real service for someone to wade through them and suggest some good choices. I do like the old school methods of discrete FETs, etc. but modern opamps are very good. This is a somewhat stringent requirement because we want both low power consumption and low noise in the same part on a single 9V supply. It's no problem to get absolutely fantastic specs in an opamp which draws 5-7 mA per amp.

    On second thought, maybe the whole thing could run on 5V, so we can build in one of those USB phone charger/battery modules that I see on Ebay a lot. I do worry about headroom but those modules will regulate the 5V until the battery is almost dead. Then just put a micro USB jack on the instrument control cavity cover or something, and plug in a USB charger to charge it up. Using an internal phone charger/battery would work fine for battery life because very small LiPo batteries can have a large maH capacity and we really want to get at least 200-500 hours battery life ideally.

    Fishman has a 9V solution like this in both a Les Paul control cavity cover, and in a Strat trem cavity cover, but they want 100 bucks for them. Very nice and well engineered though.

    I used a Microchip op amp some time back (MCP6021) for a low impedance preamp, which was a single DIP-8 and also available in SO-8 and it was for 5.5 V max supply. This one has good noise specs , but 1 to 1.35 mA draw per amp. This part also has an internal VDD/2 reference voltage pin which saves 2 resistors.

    http://www.microchip.com/wwwproducts/en/MCP6021

    It just might turn out though, that simple JFET buffers will work just fine for this application. I am going to try J201 and LSK170A.

    I'm really interested in a good compact solution that would be simple for a small volume luthier/builder to install in their instrument.
    Last edited by charrich56; 10-19-2016 at 03:03 PM.

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    Passinwind,

    I'd love to get a peek at your bass preamp design and/or the 2SK170 buffer/preamp sometime. I already showed you mine, will you show me yours? 8-) 8-)

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    Quote Originally Posted by charrich56 View Post
    Passinwind,

    I'd love to get a peek at your bass preamp design and/or the 2SK170 buffer/preamp sometime. I already showed you mine, will you show me yours? 8-) 8-)
    I'll PM you. Some of the IP for the buffer is not mine, so I don't feel comfortable posting the schematic for that. Once I get the bass preamp fully dialed in I may do a board share for it on OSHpark, and will probably also make the design open source, including the LTspice model. It's really just cookbook stuff though, simple Sallen-Key low pass filter with the only trick being getting the alignment tweaked for good intuitive control feel. The board is 2.2" X 1.66" in through-hole config, I could definitely cut it down a good bit further with even just using SMD opamps and resistors and the same polypro and polyester film caps. Here's board #2:

    pw3b_lpf_2.jpg

    The 5 hole control format for the bass it's going in will be V-V-B-M-Treb Resonance-Treb Freq, with Bass and Mids on a stacked pot. Quiescent current draw is < 800uA. Here's my stompbox test fixture:

    lpfstomp.jpg
    Last edited by Passinwind; 10-20-2016 at 09:01 PM. Reason: Added picture
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    Here's some plots of the response curves with the values given in the schematic. The Redeemer buffer is simulated by an opamp and a 150 ohm resistor.

    First plot is in resonant mode, sweeping the Tone pot from 0 to 100% with a plot line every 10%. System Q is set for 7-9 dB peaks to emulate an unbuffered pickup loaded with tone(wide open) and volume controls and some cable capacitance.

    Second plot is in lowpass mode, sweeping the Tone pot the same way. With the Tone pot slightly dialed down, the response is darn near flat to slishtly past 8 KHz. It can be flatter than this but I selected the resistor value to give slightly more high frequency cut range around 5 KHz at the expense of slight peaking around 1 Khz.

    redeemer-buffer-emu-res-tone.png

    redeemer-buffer-emu-res-tone-lowpass.png
    Last edited by charrich56; 10-21-2016 at 02:40 PM.

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    There's a fairly long running thread on variable resonance pickups on Talkbass, here's the latest schematic from one of the main posters in that thread: https://www.talkbass.com/threads/a-d...#post-19323345
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    Charlie,

    That's a really good thread and I really enjoy looking at the designs there (including yours.) I followed the thread for a while; please correct me if this is wrong, but the thread is only talking about active post filtering, and not about making the pickup itself part of a sweepable or variable LRC circuit.

    I really haven't come up with a name for the buffered thing, and I knew that what I really wanted to convey was "variable resonant frequency pickup" rather than "variable resonance pickup" because the latter sort of implies that we are able to control both center frequency and Q of the whole thing as in a state-variable filter.

    So the goals are slightly different. We aren't going for as much sheer tweakability as much as we are going for very simple control of the resonant frequency of the pickup itself, which by itself gives enormous tonal variation, essentially re-voicing the instrument, while staying with the "traditional" general idea of what various magnetic pickups sound like, loaded by wide open tone and volume controls, and with some cable capacitance, without doing anything else. That's a relatively constant Q approach although we could revisit that with a fancier design.

    The secondary goal is to be able to provide a low impedance, flat (to 8-10Khz+) response signal out of the instrument to provide a good baseline for subsequent analog and digital processing downstream.
    Last edited by charrich56; 10-24-2016 at 02:40 AM.

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    Quote Originally Posted by charrich56 View Post
    Charlie,

    That's a really good thread and I really enjoy looking at the designs there (including yours.) I followed the thread for a while; please correct me if this is wrong, but the thread is only talking about active post filtering, and not about making the pickup itself part of a sweepable or variable LRC circuit.

    I really haven't come up with a name for the buffered thing, and I knew that what I really wanted to convey was "variable resonant frequency pickup" rather than "variable resonance pickup" because the latter sort of implies that we are able to control both center frequency and Q of the whole thing as in a state-variable filter.

    So the goals are slightly different. We aren't going for as much sheer tweakability as much as we are going for very simple control of the resonant frequency of the pickup itself, which by itself gives enormous tonal variation, essentially re-voicing the instrument, while staying with the "traditional" general idea of what various magnetic pickups sound like, loaded by wide open tone and volume controls, and with some cable capacitance, without doing anything else. That's a relatively constant Q approach although we could revisit that with a fancier design.

    The secondary goal is to be able to provide a low impedance, flat (to 8-10Khz+) response signal out of the instrument to provide a good baseline for subsequent analog and digital processing downstream.
    Yep, these are really pretty different forks, just hoping for a bit of cross pollination. I don't even own a passive bass, FWIW. Name:  Yel_wink.gif
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    Everyone,

    Here's an updated discrete buffer design which uses 2 JFETS (J201) and has some good characteristics, and should be easy to build. I believe that for a working pro musician level instrument, that the CAL Redeemer is a pro-quality, reliable solution, but for the DIY and experimenter crowd, this buffer should get the job done.

    At least one person has expressed an interest in doing an SMT PCB for this design and I will update on this thread on how that progresses.
    discrete-buffer-j201-v1x.png

    The Circuitlab version of the circuit is at:

    https://www.circuitlab.com/circuit/7...ffer-j201-v1x/
    Attached Thumbnails Attached Thumbnails discrete-buffer-j201-v1.png  
    Last edited by charrich56; 11-04-2016 at 07:05 PM.
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    Charlie,

    Thanks for the like. We are trying to do this so that the same raw PCB will work for both a 9V version, and an 18V version for the bass crowd. Going semi-audiophile with only low-ESR film caps in the signal path (no tantalum or ceramics). R1, R2, C1, C2 are on a plug-in DIP header so they can be customized for a particular application. It's looking like we can get close to a 1" x 1" board which would stick to the back of a pot. This will be useful just as a buffer, even if someone chooses not to implement the variable rez tone feature.

    -Charlie
    Last edited by charrich56; 11-07-2016 at 02:45 AM.
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    Quote Originally Posted by charrich56 View Post
    We are trying to do this so that the same raw PCB will work for both a 9V version, and an 18V version for the bass crowd. Going semi-audiophile with only low-ESR film caps in the signal path (no tantalum or ceramics). R1, R2, C1, C2 are on a plug-in DIP header so they can be customized for a particular application. It's looking like we can get close to a 1" x 1" board which would stick to the back of a pot. This will be useful just as a buffer, even if someone chooses not to implement the variable rez tone feature.
    Outstanding. Yesterday I finished up my last round of mods to my prototype board, confirmed that things work as expected, and finished up the necessary board revisions for v1.1. I'll try porting your design to LTspice later this week so I can do some "what-if" sims. Using a plug-in header for modding efficacy is a nice idea.
    --Charlie Escher

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    Charlie,

    Thanks!

    If you are firing up LTSpice, if you were interested in testing THD, I'd be interested, since CircuitLab doesn't have that facility. The Cadillac JFET to use here would be an LSK170A and I will be doing some boards with that. I had to ask Bob Cordell through Linear Systems for the SPICE model for that one. Let me know if you need it. I assume that you would already have the J201/ MMBFJ201 model or could get it easily.

    The target current drain I shot for is 600-700 uA. It's pretty easy to set with R8. That was the best compromise between battery drain and drive capability since we are driving about a 2.5K load if the tone control is used.


    -Charlie

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    Quote Originally Posted by charrich56 View Post
    Charlie,


    If you are firing up LTSpice, if you were interested in testing THD, I'd be interested, since CircuitLab doesn't have that facility. The Cadillac JFET to use here would be an LSK170A and I will be doing some boards with that. I had to ask Bob Cordell through Linear Systems for the SPICE model for that one. Let me know if you need it. I assume that you would already have the J201/ MMBFJ201 model or could get it easily.

    The target current drain I shot for is 600-700 uA. It's pretty easy to set with R8. That was the best compromise between battery drain and drive capability since we are driving about a 2.5K load if the tone control is used.
    I think I probably already have the LSK170A model, but I'll ping you if not. I'll be happy to run THD sims once I get things going. I can do S/N too, but of course modeling specific physical builds (ie shielding and layout) really accurately is still out of my wheelhouse at this point.

    That's a sufficiently modest power draw to make most people happy, I think.
    --Charlie Escher

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    I've been listening in the background as by sheer co-incidence I bought a single coil Far East p/u for $1.86 just to see if I could turn lead into gold. Been too busy to get further than that. No surprise it uses a rather weak ceramic magnet.

    Here are a few Linear Systems FET model cards that you might find helpful.

    .MODEL LSK389A NJF BETA=0.0378643 VTO=-0.4025156 LAMBDA=4.783719m IS=3.55773E-14 N=1 RD=10.6565 RS=6.8790487 CGD=3.99E-11 CGS=4.06518E-11 PB=0.981382 MJ=0.794653 FC=0.5 KF=5e-18 AF=1
    .MODEL LSK389B NJF BETA=0.0350699 VTO=-0.5375582 LAMBDA=4.951m IS=2.76212E-14 N=1 RD=7.82168 RS=6.9525888 CGD=4.36E-11 CGS=4.36E-11 PB=1.2 MJ=0.8892205 FC=0.5 KF=5e-18 AF=1
    .MODEL LSK389C NJF BETA=0.0278541 VTO=-0.800434 LAMBDA=0.0122435 IS=2.45217E-14 N=1 RD=12 RS=5.8 CGD=4.22E-11 CGS=4.23E-11 PB=0.9265487 MJ=0.6098477 FC=0.5 KF=4e-18 AF=1
    .MODEL LSK170A NJF BETA=0.0378643 VTO=-0.4025156 LAMBDA=4.783719m IS=3.55773E-14 N=1 RD=10.6565 RS=6.8790487 CGD=3.99E-11 CGS=4.06518E-11 PB=0.981382 MJ=0.794653 FC=0.5 KF=5e-18 AF=1
    .MODEL LSK170B NJF BETA=0.0350699 VTO=-0.5375582 LAMBDA=4.951m IS=2.76212E-14 N=1 RD=7.82168 RS=6.9525888 CGD=4.36E-11 CGS=4.36E-11 PB=1.2 MJ=0.8892205 FC=0.5 KF=5e-18 AF=1
    .MODEL LSK170C NJF BETA=0.0278541 VTO=-0.800434 LAMBDA=0.0122435 IS=2.45217E-14 N=1 RD=12 RS=5.8 CGD=4.22E-11 CGS=4.23E-11 PB=0.9265487 MJ=0.6098477 FC=0.5 KF=4e-18 AF=1

    Something in simulating pickups, a bit of a detail really, is that the voltage source is ideally modelled to rise at 6dB/octave as the coil output is proportional to the velocity of the strings. For the cheapo pickup I got, this is the model I used:

    LtSpice ASC file
    Cheapo_corrected.zip

    cheapo_sch.jpg

    My implementation plan is to include a graphic equalizer as part of the preamp so I've a very different direction. The VR idea is very attractive due to the combination of flexibility and simplicity. Anyway I'll bow out now and let you get on with it. Good luck and much success to you
    Last edited by nickb; 11-07-2016 at 09:05 PM.
    Experience is something you get, just after you really needed it.

  26. #26
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    Nick,

    Thanks for the modeling data and I appreciate your good wishes. Best of luck to someone who seems to share my hobby/passion/addiction.

    Some of the Chinese pickups are indeed made for a price, but they have gotten their act together in magnet fabrication and can make some good PU's. Sourcing from there has its perils but you can get good quality. I just needed some PU's quickly since I didn't have the spare time to wind my own, 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.

    I really want to support small, local luthiers, builders, and hobbyists and not just outsource and mass-market the whole thing. But I also can see that the VR tone control could find a home in some high-end instruments.

    On the pickup modelling part, a simple AC voltage source works fine through an inductor as a first-order model because the inductor has, as physics and luck would have it, an exactly opposite slope with frequency, and the modelled AC voltage source will supply whatever current is needed to maintain the same voltage. This matches what you would see with a real magnetic pickup which would otherwise be intolerably bright through a flat signal chain. It looks like you are doing a bit more sophisticated pickup model, which is great.

    I appreciate your comments on the flexibility and simplicity of the VR tone control. It somewhat amazes me that this approach was not picked up on, long ago.

    -Charlie

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    Hey Charlie,

    Joel de Guzman (Cycfi Research | Infinity in a Day, and his FB page) turned me on to a couple of op amps which are a combination of low current draw and low noise. I've got my eyeballs on the TI OPA188 and have designed and ordered some boards for a buffer using that part. It turns out that the MMBFJ201 SMD part isn't just a SMD version of the J201; it's got very different specs and is just about unusable for this application. The Linear LSK170A is good but you need two of them at $6.00 a pop. The LSK389A is also 6.00 but it is a dual, and would sound quite nice in this circuit.
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  28. #28
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    Quote Originally Posted by charrich56 View Post
    Hey Charlie,

    Joel de Guzman (Cycfi Research | Infinity in a Day, and his FB page) turned me on to a couple of op amps which are a combination of low current draw and low noise. I've got my eyeballs on the TI OPA188 and have designed and ordered some boards for a buffer using that part. It turns out that the MMBFJ201 SMD part isn't just a SMD version of the J201; it's got very different specs and is just about unusable for this application. The Linear LSK170A is good but you need two of them at $6.00 a pop. The LSK389A is also 6.00 but it is a dual, and would sound quite nice in this circuit.
    Have you found a reasonable source for the LSK170A and/or LSK389A? Trendsetter wants $24 for shipping a single piece and DIYaudio never seems to have the A spec pieces in stock.

    I haven't tried any CMOS based opamps in many years, but the slew rate of the OPA188 is a bit below my usual lower limit. Might be fine in this application though.

    Hope to get back to some circuit modeling soon. Things took an unexpected swerve when the first luthier I demo'ed the new preamp for asked me to provide a few samples for his NAMM booth next month. I built three different ones last week, but I fully expect that my friend will end wanting something a little different than any of these.
    --Charlie Escher

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    Charlie,

    I have gotten samples for LSK170A and LSK389A direct from the manufacturer, Linear Systems. I just got pricing from those from Kevin Hicks at Linear and basically any qty. up to past 200 both the 170A and 389A are about $6.00 each. Obviously the 389A is a better deal if you need more than one JFET in the circuit, as I do.

    They are good to deal with for small purchases, generous with samples, and they are aware of the audiophile DIY crowd, and also the high-end audio/recording crowd (because their JFET's are used in some mondo expensive ribbon mic preamps.)

    They also have other JFETS and they are a second source for some of the Fairchild, Intersil, Toshiba, etc. parts.

    My opamp SMD boards are just about to show up from OSHPark, and I'm going to build them first most likely.

    op-amp-buffer-board-v1.6-top.jpg

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    4042 Clipper Court, CA 94538
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    Quote Originally Posted by charrich56 View Post
    Charlie,

    I have gotten samples for LSK170A and LSK389A direct from the manufacturer, Linear Systems. I just got pricing from those from Kevin Hicks at Linear and basically any qty. up to past 200 both the 170A and 389A are about $6.00 each. Obviously the 389A is a better deal if you need more than one JFET in the circuit, as I do.

    They are good to deal with for small purchases, generous with samples, and they are aware of the audiophile DIY crowd, and also the high-end audio/recording crowd (because their JFET's are used in some mondo expensive ribbon mic preamps.)
    Cool, thanks. I have something else in the works that would definitely constitute a legitimate reason to request a few samples. Opamp based designs are one of my main comfort zones though, so I'll be especially interested to hear how your v1.0 board works out.
    --Charlie Escher

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    Charlie,

    No worries, you're welcome, I'm curious to see how the OPA188 works out as well. No big deal if it doesn't since there are so many choices for a single in a SO-8 package.

    The slew rate might be a concern but it should work out just fine for guitar where we are really staying below 1V peak to peak a lot of the time. We're a long way from the 741 design and all the nasties it did on overvoltage, latchup, and phase reversal. For an 18 volt bass design you are running a lot hotter signal I think so I would have more concerns there.

    The low power low noise , 9VDC+ choices Joel gave me were:

    http://www.ti.com/lit/ds/symlink/opa188.pdf 8.8 nV/√Hz 510 μA (max)
    http://www.ti.com/lit/ds/symlink/opa180.pdf 10 nV/√Hz 525 μA (max)
    http://www.ti.com/lit/ds/symlink/opa171-q1.pdf 14 nV/√Hz 475 ľA
    http://www.ti.com/lit/ds/symlink/lmv641.pdf 14 nV/√Hz 138 ľA
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    Quote Originally Posted by charrich56 View Post

    The low power low noise , 9VDC+ choices Joel gave me were:

    http://www.ti.com/lit/ds/symlink/opa188.pdf 8.8 nV/√Hz 510 μA (max)
    http://www.ti.com/lit/ds/symlink/opa180.pdf 10 nV/√Hz 525 μA (max)
    http://www.ti.com/lit/ds/symlink/opa171-q1.pdf 14 nV/√Hz 475 ľA
    http://www.ti.com/lit/ds/symlink/lmv641.pdf 14 nV/√Hz 138 ľA
    Effectively spec'ing noise often requires going a lot deeper than voltage noise density, especially for relatively high impedance inputs. I assume you're familiar with this doc: http://www.analog.com/media/en/train...als/MT-047.pdf ?
    --Charlie Escher

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    Thanks Charlie, that's a very good resource paper and I'm glad you tipped me off to it. I think I had run across it a while back but it's still one of the best concise articles on op-amp noise. I'll make sure I look at the current noise data as well on making a final pick. But for now the voltage noise density is a good quick selection yardstick. Discrete JFET may still possibly win on this one, but it's going to be fun to find out

  34. #34
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    The MT-047 design note looked familiar, reminded me of Walt Jung's old Op Amp Cookbook. It references two design books at the end, both costly as hardcopy, but available as free PDFs if you know where to look. Note: "costly" means ~$90 new. AYFKM?! Seems Elsevier bought the hardcopy distribution rights, applied academic/institutional price models, and they went from $30 to $90 overnight.

    download FREE at:

    The Linear Circuit Design Handbook PDF is in the Analog Devices educational material section.

    Walt Jung's last iteration of Op Amp Applications Handbook, 2005 is there, too.

    (edit) General noise considerations are in chapter 1.
    An instructive worked example of total noise starts in Chapter 6 page 150.

    Not so free:

    E-book Analog Circuit Design is still reasonably priced at $20/download on the SysCompDesign site.

    -drh
    Last edited by salvarsan; 12-14-2016 at 03:11 PM. Reason: addendum
    The lofted fewmet doth soon hew close to the whirling blades.

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    Just a quick note that I am following through on releasing an open source design for my variable resonance swept frequency bass preamp: https://www.talkbass.com/threads/the...reamp.1259692/

    That thread has generated well upwards of 500 views already, so I think there will be some real interest in the other Charlie's approach as well.
    --Charlie Escher

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