How to use 20K pots for hi-Z guitar pickup?

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if any active electronics is put between the PU and the guitar cable, the missing cable capacitance should be substituted by a discrete capacitor.

Agree, and to add to the discussion: cable capacitance is typically in the region of 100 pF per metre of guitar cable, so use around 300 pF to compensate for the buffer if you normally use a 10-foot / 3 metre guitar cable, and around 500 pF if you use a 15-foot / 5 metre cable.


-Gnobuddy
 
I was just thinking about how to use the second 20k pot, for some tone adjustment. Rather than just load down the output with a basic treble-dulling circuit based on 20k, a more interesting effect might be to get the effect of sweeping that parallel cap value discussed above. Ie, the typical peak and fall of a passive pickup driving a cable gets swept from say 6khz down to say 1khz, or switched off. Ive got an idea for how to do that, needs a couple more transistors.
 
...sweeping that parallel cap value discussed above.
The easy way is to bootstrap the other end of the capacitor with a voltage that's in phase with the input, but adjustable in amplitude, up to nearly equal to the input voltage. Varying the amount of bootstrap varies the apparent capacitance. It works just like Miller capacitance, but the in-phase voltage reduces the effective input capacitance rather than increasing it.

I threw together a quick LTSpice simulation to show how it works. V1 / L1 / R1 represents the guitar pickup. The first op-amp buffers the pickup, R2/R3 is a linear pot that controls input capacitance, and the second op-amp bootstraps the input cap (C1) with the voltage from the pot.

I used a dual op-amp, but it can also be done with a JFET in place of the input op-amp, and a BJT emitter-follower replacing the second (bootstrap) op-amp.


-Gnobuddy
 

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Yep, that's what I was thinking too. An opamp version is probably the way to go, with a pure performance and very few parts. How much current would a dual opamp need?

But I have an all discreet version spiced up, just because I like them. It reminds me of tinkering with transistors in the '70's, when I was supposed to be doing high-school homework. Ill post it later.
 
I also reckon opamps are easier, and purer, but here is my discreet transistor version, FWIW:


buffertone2020.gif



It's based around the jfet buffer, with a couple of bjt's to lower output resistance for feeding the 20k pots and making sure the capacitance feeding back to the pickup is done via a low impedance.


But it looks like it would work, and draws 1.1mA in theory, and can produce a clean output swing on +/-3V.


btw, the pickup is modelled with 6 components. This is a whole different story but it is tuned to very closely match the output and impedance of specific tested pickups, in this case an Seymour Duncan '59 humbucker. These models have been developed at Guitarnuts2, for several dozen types, based on very detailed measurement by others there.
 
If I'd use opamps, I'd opt for the TL062, which is a low current dual opamp that drains the battery by only a few mA.
I was looking at the Texas Instruments datasheet for their TL072 just a few days ago. Supply current is specified as typically 1.4 mA per op-amp. Twice that for a TL072.

A typical 9V alkaline flat battery has a capacity of around 500 mAh ( https://www.newark.com/energizer/en22/alkaline-zn-mno2-battery-9v/dp/81F157 ). A TL072 drawing 2.8 mA would therefore take approximately 180 hours of use to drain the battery. Assuming a fairly keen hobby guitarist who plays for 5 hours a week, that battery would last for 35 weeks or nearly nine months.

I don't think most guitarists would find it unreasonable to have to replace a battery a couple of times a year. I wouldn't want to leave a battery inside a musical instrument for much longer than that anyway, just to make sure it doesn't die, leak, and corrode the electronics inside the guitar.

So, to me, an ordinary TL072 seems like a perfectly reasonable choice. And even the through-hole version is very small, which is a good thing when you're trying to fit your electronics inside a solid-body guitar!

Like John, I too have built discrete JFET / BJT buffers into guitars. Like John, I prefer their softer overload characteristics. But by the time you add all the resistors and capacitors necessary to bias and decouple them, the circuit ends up bulkier and more complex than a plain-jane op-amp or two.

You can use a stereo 1/4" jack in the guitar and a stereo cable with TRS plugs on both ends to send DC power up the cable into the guitar, with a special power supply (or battery box!) plugged in between guitar and amp. But if the guitar is going to be used anywhere outside the owner's house, it's a good idea to make sure it works even without the special cable and power supply...so an onboard 9V flat battery is definitely a good idea.

(I made the mistake of building the power supply into my home-made guitar preamp, nor did my guitar have a 9V battery inside it. Which, of course, meant my guitar couldn't be used with any other amp...stupid mistake!)

-Gnobuddy
 
My understanding is that when specs say that alkaline 9V batteries are good for 500-600mA.H, that's to drain them down to half voltage? If thats right, then somewhat less capacity is really available if we want to maintain adequats volts. A new one can give about 9.5V, I tend to ditch them at around 8 to 8.5V.
 
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My understanding is that when specs say that alkaline 9V batteries are good for 500-600mA.H, that's to drain them down to half voltage? If thats right, then somewhat less capacity is really available if we want to maintain adequats volts. A new one can give about 9.5V, I tend to ditch them at around 8 to 8.5V.
With a current drain of <0.5mA, the 9V Alkaline lasts for about 1yrs inside my guitar. And I often forget to remove the plug.
 
...the 9V Alkaline lasts for about 1yrs inside my guitar.
That's kinda where I was going - most of the casual, hobby guitarists I've met replace the battery in their active bass or electroacoustic guitar about once a year, just to be on the safe side. Nobody wants to experience having the battery die, leak, and corrode the precious electronics, so better not to leave it in for years on end, even if it hasn't died.

I know a couple of semi-pro players who do regular gigs in front of real audiences. Most of them replace the battery much more often as a precaution, simply because nobody wants their instrument to go silent in mid-song in front of an audience.

It would have been nice if guitar manufacturers and guitar amp / preamp manufacturers had got together and agreed on a standard to supply DC power up the guitar cable, in the same way that most microphones accept "phantom power". After all, most acoustic guitars these days are not pure acoustic, but have an onboard pickup and preamp. The majority of bass guitars are active too, except for the cheapest ones. So we shouldn't be having to send millions of 9V flat batteries into the world's landfills every year, just so we can keep playing our basses and acoustic-electric guitars!


-Gnobuddy
 
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That's kinda where I was going - most of the casual, hobby guitarists I've met replace the battery in their active bass or electroacoustic guitar about once a year, just to be on the safe side. Nobody wants to experience having the battery die, leak, and corrode the precious electronics, so better not to leave it in for years on end, even if it hasn't died.

I know a couple of semi-pro players who do regular gigs in front of real audiences. Most of them replace the battery much more often as a precaution, simply because nobody wants their instrument to go silent in mid-song in front of an audience.

It would have been nice if guitar manufacturers and guitar amp / preamp manufacturers had got together and agreed on a standard to supply DC power up the guitar cable, in the same way that most microphones accept "phantom power". After all, most acoustic guitars these days are not pure acoustic, but have an onboard pickup and preamp. The majority of bass guitars are active too, except for the cheapest ones. So we shouldn't be having to send millions of 9V flat batteries into the world's landfills every year, just so we can keep playing our basses and acoustic-electric guitars!


-Gnobuddy
I did this many years ago and drilled a fat hole into my strat to mount an XLR socket. It did not take very long until I switched back to 1/4" Jacks. Imagine a plug that is fixed in its position - the twisting cable drives you mad.
Another aspect is the possibility to play wireless.
But there should be alternatives to 9V supply.

For guitar, one (or two) 3.6V/1200mA Lithium batteries should do the trick as well and last much longer than 9V Alkaline.
 
It did not take very long until I switched back to 1/4" Jacks.
I've used stereo 1/4" jacks, sending +9 V DC up the "ring" connection. It works, but there are problems. One bad thing about 1/4" jacks is that the ring gets shorted to ground and tip as you insert the plug. It also gets grounded permanently if you plug into a mono jack (instead of a stereo one.) So the +9V line has to be designed to tolerate a continuous short-circuit to ground (by using a series resistor, etc.)

For guitar, one (or two) 3.6V/1200mA Lithium batteries should do the trick as well and last much longer than 9V Alkaline.
A few years ago, Fishman released some guitar pickups (trademark name "Fluence") that did not use a traditional pickup coil wound with copper wire. Instead, they used a stack of printed-circuit boards, each with a few turns of copper track printed on it. The idea was to have perfectly consistent "coils" - and so perfectly consistent sound - in every pickup manufactured.

Output and self-inductance of the printed coil was very low, so Fishman also included a preamp built into the pickup to bring up the signal level and EQ it to sound like a traditional electric guitar pickup.

As a final touch, Fishman powered the whole thing with a small lithium-polymer battery, which they claimed would last much longer than alkaline (as you said.) Also, of course, you wouldn't have to toss it in the recycling when discharged, instead you just recharge it and use it again.

Well, that recharging process turned out to be a fatal flaw. When a guitarists finds out during sound-check just before she goes on stage that the battery in her guitar is dead - and that it needs an hour or two to recharge - that is completely unacceptable.

Even worse, you cannot just pop in another fully charged battery, because lipo packs like this cannot be found in every corner store, and the pack wasn't designed to be easily replaceable anyway. It was designed to stay in the guitar for many years, after all.

What actually happened in this situation is that the guitarist would either be unable to perform, or have to borrow somebody else's guitar and struggle through the gig with it. Either way, once the angry and frustrated guitarist got home, those Fishman Fluence pickups were going to get ripped out of the guitar permanently, along with the troublesome lipo pack. Then it would either go in the trash, or on Craigslist, for some other unfortunate sucker to buy.

For this reason, I believe this is one case where an old-tech 9V alkaline battery is a far superior solution to a lithium pack. The 9V battery can last six months to a year, so replacement cost is low. It doesn't self-discharge, it doesn't burst into flames as lipo packs can, every savvy guitarist carries a couple of spare 9V batteries in the gig bag. Worst case, you can quickly buy a replacement 9V battery just about anywhere you are likely to have a gig.

None of this applies to a hobby guitarist who only plays at home or with friends, and is under no pressure to perform on time. So for hobby use, a lipo pack might be fine.

(The fire hazard would still deter me from using one. Literally tens or hundreds of millions of electronic devices have been recalled during the last ten years by Apple, Samsung, Sony, Lenovo, and numerous other manufacturers due to problems with flaming lithium batteries.)


-Gnobuddy
 
Oh, I know, but in this case, seven times less current draw doesn't translate to seven times more battery life. Batteries have a finite shelf-life, after all, so even at 0.4 mA draw, a battery won't last for 63 months instead of 9. It will still only last for maybe a year.
Remember that in a guitar with two PU's you'll need five opamps, two per PU in the variable resonant peak tone control that you designed, and another one as a mixer/buffer right before the cable. So the calculation is 1 mA for TL06x's vs. 7 mA for TL07x's. Both these values will drain a LR-6 by far more than it's self discharge, I suppose ;).
Best regards!
 
We haven't heard from Bassamp, the OP, in a while, and we may have wandered from the design brief with this interesting discussion. I'm picturing this with just two 20k pots total, after any pickup switching. So just a single stage of buffer/tone is needed.

Stripping right back, the simplest buffer I ever build had one jfet, one resistor and one cap. The jfet was one with a fairly high Vgs, an MPF102, which can be biased with gate at ground (so hardwired to the vol pot, no other biasing parts) and still get a few volts at the source. Current draw was <0.2mA from 2x3V Lithium buttons.
 
If your preamp has 1MOhm impedance you are fine. Take it and later on you may switch some resistor of your taste (100~500k) parallel to the pickup.

As You will definitely need a parallel resistor resembling all impedances of that passive guitar circuit (vol and tone pots) plus the input impedance of the amp. Otherwise the sound will be shrill and harsh, moreoverly different from the original sound.

And one single op amp may be enough if You set up a virtual earth addition circuit.
 
Stripping right back, the simplest buffer I ever build had one jfet, one resistor and one cap. The jfet was one with a fairly high Vgs, an MPF102
I built a buffer pedal exactly like that in 2018, hand-selecting a J113 JFET that had a Vgs of about 1.8 volts.

After I built it I found a few J111 FETS in my parts-box. Those have a higher Vgs and might have been a better choice. Then again, even with humbuckers, I've never heard my J113 buffer pedal produce any audible signs of clipping.

My buffer pedal does have a few more components: two LEDs and their series resistors, a footswitch, and a 50k pot (I wanted a lower resistance one, but 50k is what I had in the junk-box.)

The pot is used to drop the guitar signal by a chosen amount for rhythm playing. The footswitch selects either full guitar output, or the signal reduced by the pot. A red LED lights up for full signal strength, a green one when you select the reduced output for rhythm playing or low-volume second guitar parts.

That pedal turned out to be very useful, and it has had a permanent home on my tiny pedalboard since I built it. I sing as well as play guitar, so I really don't have enough brain left over to precisely turn a volume knob up and down. Now I don't have to, I just stomp on the footswitch to go from rhythm to lead or vice versa!

The old (August 2018) thread on that buffer pedal is here: An Unboost pedal for electric guitar

-Gnobuddy
 
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