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How to use 20K pots for hi-Z guitar pickup?
How to use 20K pots for hi-Z guitar pickup?
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Old 25th January 2019, 09:25 AM   #21
Kay Pirinha is offline Kay Pirinha  Germany
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Well, that's exacdtly what I've said in #15!
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Old 25th January 2019, 07:19 PM   #22
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Kay Pirinha View Post
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.


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Old 26th January 2019, 02:29 AM   #23
JohnDH is offline JohnDH  Australia
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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.
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Old 26th January 2019, 03:20 AM   #24
Kay Pirinha is offline Kay Pirinha  Germany
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Aah, you want a hand operated wah-wah?!? Voltwide has just mentioned it in #16! An apt circuitry would be that of a state variable filter - at least some part of it.



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Old 26th January 2019, 05:29 AM   #25
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by JohnDH View Post
...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.


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Old 26th January 2019, 07:22 AM   #26
JohnDH is offline JohnDH  Australia
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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.
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Old 26th January 2019, 07:56 AM   #27
Kay Pirinha is offline Kay Pirinha  Germany
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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.


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Old 26th January 2019, 11:01 PM   #28
JohnDH is offline JohnDH  Australia
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I also reckon opamps are easier, and purer, but here is my discreet transistor version, FWIW:


Click the image to open in full size.


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.
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Old 26th January 2019, 11:35 PM   #29
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Kay Pirinha View Post
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/en2...y-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
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Old 27th January 2019, 06:06 AM   #30
Kay Pirinha is offline Kay Pirinha  Germany
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As per datasheet, a TL06x opamp only draws 0.2 mA per amplifier, so only 0.4 mA for the TL062 .
Decades ago, I've successfully »activated« my '78 Fender Jazz Bass with a set of three TL061's as buffers.
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