Modeling A Guitar For Circuit Simulation

Guitar effects have come a long way from the jangly, unaltered sounds of the 1950s when rock and roll started picking up steam. Starting in large part with [Jimi Hendrix] in the 60s, the number of available effects available to guitarists snowballed in the following decades step-by-step with the burgeoning electronics industry. Now, there are tons of effects, from simple analog devices that would have been familiar to [Hendrix] to complex, far-reaching, digital effects available to anyone with a computer. Another thing available to modern guitarists is the ability to model these effects and guitars in circuit simulators, as [Iain] does.

[Ian] plays a Fender Stratocaster, but in order to build effects pedals and amplifiers for it with the exact desired sound, he needed a way to model its equivalent circuit. For a simple DC circuit, this isn’t too difficult since it just requires measuring the resistance, capacitance, and inductance of the overall circuit and can be done with something as simple as a multimeter. But for something with the wide frequency range of a guitar, a little bit more effort needs to go into creating an accurate model. [Iain] is using an Analog Discovery as a vector network analyzer to get all of the raw data he needs for the model before moving on to some in-depth calculations.

[Iain] takes us through all of the methods of figuring out the equivalent impedance of his guitar and its cabling using simple methods capable of being done largely by hand and more advanced techniques like finding numerical solutions. By analyzing the impedance of the pickup, tone and volume controls, and cable, this deep dive into the complexities of building an accurate equivalent circuit model for his guitar could be replicated by anyone else looking to build effects for their specific guitars. If you’re looking for a more digital solution, though, we’ve seen some impressive effects built using other tools unavailable to guitarists in days of yore, such as MIDI and the Raspberry Pi.

20 thoughts on “Modeling A Guitar For Circuit Simulation

  1. The most impressive guitar effect is undoubtedly distortion and feedback and can be easily achieved by turning your Marshall stack up to 11 and standing right in front of it with guitar in hand. (Ear protection recommended).

    1. im partial to crunch myself. amplify, clip, and amplify again. mine is using a pair of general purpose opamps, and a pair of red leds to chop the signal. very useful for playing thrash riffs.

      its a design i got off the intertubes and modified to use components i had on hand at the time. it called for a higher quality audio opamp, but i used a generic. i also didnt have the correct potentiometers, so i just used what i had on hand. some where linear when they should have been logarithmic, so im effectively getting half the range, but it still sounds good.

      1. You may want to experiment with asymmetric clipping, such as what is produced by a classic fuzz box, or your opamp based one provided you use different diodes (and/or a different number of them) for each half wave.
        This article might turn quite useful to understand why that pedal sounded so good.

        And yes, it can be built with more modern parts, but I would try to source germanium transistors over silicon ones any day because of their better sound. I’ve rebuilt the original schematic with a few modifications using different Ge transistors ant it always sounded great; my best one used some surplus unmarked parts in TO5 case with grounded base, probably RF parts of military origin, but also generic AC/ACY/SFT transistors sounded great, as will many ex soviet parts still sold online from eastern Europe.
        AC128s are overpriced but sound exactly the same as other much cheaper Ge parts; you don’t need them at all.

      1. that goes without saying. my personal guitar usage is that the thing stays in a closet for months, nay even years, at a time until i read about some new effect pedal or other guitar upgrade. i make a project out of it, build the box, do the upgrade, whatever and noodle around for a week or two and then return it to its closet. i know it doesn’t make up for the lack of practice and general neglect of what little musical ability i have. but i have no delusions of rock’n’roll grandeur, i know full well that im not a real musician. im too lazy for that.

    2. You might be on to something.

      Some of the best tones have come from that setup, but the players get hearing problems because of it, so people who want an enjoyable life tend not to do that anymore.

      But we are HaD readers. We know that ear-killing volume isn’t the only way to excite sympathetic resonance in strings.

      Maybe a small speaker embedded in the guitar bridge that plays the pickup signal, amplified, right into the strings?

      Hendrix feedback and blooming at bedroom volumes. Cake and eat it.

      1. i never used anything other than a crappy chinese made no-name practice amp. having a number of diy pedals and effect boxes certainly makes it sound almost decent. things you wouldn’t need if you had a few more knobs to play with.

  2. How does the frequency response affect the circuit? Shirley it’s the other way around. My nonsense detector needle is pegging.

    Jimi Hendrix?

    Les Paul.

    Link Wray, Rumble.

    Buddy Guy.

    1. I found myself wondering why also. I can see wanting to design an amplifier system that has at least the bandwidth as the pickup but how challenging or costly is that for audio range signals to target a wide band? The cable from the audio jack is going to have a characteristic impedance. So, I think the concern is to match the characteristic impedance of the cable with a load across a high impedance amplifier would be input criteria.

      1. Too little bandwidth is hardly ever an issue. This is relevant when dealing with arcane effects like the Rangemaster and Fuzz Face, which have low-ish input Z and specific desired interactions with the guitar. Everything else in guitar world uses bridging (~1M input Z).

  3. I’ve often felt that the pickup, two pots, a cap, and a jack is so little tech for an electric guitar. Lithium battery, pickup, 10 band EQ, Digitech processor, e-bow circuits, amp, and speaker is more to my liking.
    Portable Pink Floyd.

  4. I don’t think you’re supposed to connect shield gnd to analog gnd, you don’t want an antenna to be connected to anything like small signal inputs or anything at all really. It’s supposed to be a faraday cage around your electronics.

    1. With a two-conductor cable (which is the standard for electric guitars), I don’t think you have any choice but to ground the connector to the same place as the rest of the electronics.

      One could argue that tip-ring-sleeve connectors would be better because then you could use the sleeve and shield ground (and connect it to a faraday cage, as you propose), and then use the tip and ring for signal and signal-ground. And I don’t think anyone would argue.

      But last I checked (admittedly, more than a decade ago) every product made to accept a guitar signal just uses 1/4-inch 2-conductor inputs.

      3-conductor cables and XLR plugs are common for microphones, so equipment does exist to accept such signals. But I’ve never seen a guitar that uses them.

      1. I saw a Kawai guitar on the cover of the Musical dealer zine ten or more years ago with a XLR jack. All pickups are floating 2 wire and are one side grounded in install.

  5. During the late 70s and 80s, I constantly saved to buy the latest and greatest effect. Now, I use nothing but the Guitar, the Amp and an old off brand Chorus pedal, as well as an old DeArmond volume pedal that weighs a ton. I don’t perform anymore (I retired in 2009), just in the band that accompanies the choir at mass. My acoustic runs through the church’s system and the AC 15 for my ES 335 is just behind me, the line out also running through the board. Almost exactly the set up I used the last 10 years we performed.

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