Designing a Guitar Stompbox to Emulate Cable Capacitance

[Marvcus]

Hello there.

First of all, I am not an EE but I have some Computer Engineering experience.

I noticed that the guitar sounded different when I used different cables. Had a look around and it looks like the capacitance of the cable is the culprit.

The complete circuit of the guitar->cable->amplifier creates a low pass filter because of the interaction of the pickups(EM Inductors)/tone control circuit(Capacitors and resistors)/capacitance of cable/amplifier circuits. These three components create an RLC circuit which will have a cutoff frequency and a resonant peak in the circuit's frequency response.

You can avoid the effect of the "low pass filter" by using a cable with an extremely low farads/foot rating.

Here is a site I found that talks about cable capacitance.

https://www.shootoutguitarcables.com/guitar-cables-explained/capacitance-chart.html

With a cable with a low enough capacitance the cutoff frequency of the low pass filter is too high to be noticeable.

However, sometimes the frequency response resulting from the use of a high capacitance cable is desired.

The main electrical engineer for Gibson guitars in the 70s wrote the analysis in the following link.

http://www.billlawrence.com/Pages/All_About_Tone.htm/CableandSound.htm

In the linked article he states that the effect of a cable with a high capacitance could be duplicated by using a capacitor with an equivalent capacitance.

I can probably get the desired effect by purchasing a parametric EQ and placing it between the cable and amp in the signal chain but that stompbox costs about $180 Canadian(BOSS GE-7) and I would have to play around with the sliders to get the same effect as a cable with a different capacitance.

I would like to build a stompbox placed between the cable and amplifier with a rotary switch that selects between several capacitors and a direct line. I assume that the capacitor has to be placed in parallel with the cable in some way to alter the capacitance of the circuit but I can't figure out how to design the circuit.

I dropped out of Electrical Engineering in first year and changed to Computer engineering so that I wouldn't have to design circuits myself.

Is there anyone on this forum who knows how to design the circuit?

Any help is appreciated.

[GibsonGM]

Hi, welcome to the forum!   

Why not make a box with a range of caps from 50 to 150pF, and a switch to select them. Each would just go to ground across the output jack, really...nothing else needed.  You are really placing the cap from signal to ground, but the shield of your cable IS ground.

[FiveseveN]

I have a similar setup in most of my guitars. Gibson caught onto the idea in the late '50s: the Varitone is slightly more involved but same principle: https://farm4.staticflickr.com/3155/2862193610_b82184f436_o.jpg
Here's a deeper discussion on the effect of load capacitance and more, with measurements and graphs: http://www.buildyourguitar.com/resources/lemme/
And if you want to go all the way down the rabbit hole... http://www.sugardas.lt/~igoramps/article57.htm

[idy]

Yep, rotary switch, as many positions as you want, they have "stoppers" that allow you to select that. Up to 12, but you will find you only like a couple. Then either leave one position empty (no cap) or add a bypass stomp. One end of each cap to ground, one to the switch. The other ("common") end of the switch goes to the signal wire of the cable, or the stomp switch if you want to add bypass.

This thing you are talking about building is a handy utility for those who go down the rabbit hole. I have a couple of "tins" that I have set up with caps on rotary. When someone wants to try a different tone cap on a guitar, or I want to try them out in a tone control, I can do that before I solder.

[andy-h-h]

Back in the day when wireless was first becoming a thing, some guitarists didn't like the change in tone (no more 40ft cables).   The solution they used was to roll up the 40ft cable at the receiver end...   

Interesting to look at the table and see that some with the highest capacitance cables are made by Canare, Belden and Mogami, which are considered the go to cable for mic leads and installation cable.  I guess low noise / insulation was prioritised over capacitance? 

And yes, a rotary with or even a simple three switch would do the job.  Some pedals incorporate an impedance pot to further mess with the signal - could go all out and include both. 

[Rob Strand]

100pF to 1nF at whatever steps you like is more like reality.

Interesting to look at the table and see that some with the highest capacitance cables are made by Canare, Belden and Mogami, which are considered the go to cable for mic leads and installation cable.  I guess low noise / insulation was prioritised over capacitance? 

Probably going overboard.   Some mic cables have ridiculously high capacitance per meter (or foot).
With a low impedance mic it's not a big deal but for guitar pickups the high-end loss is excessive.

[Mark Hammer]

All audio cable is rated in terms of capacitance-per-linear-foot (or something similar).  This implies that differences in cable capacitance matter more the longer the cable.  So, an 8-incher, used to patch pedals on a pedalboard, won't really differ in any measurable (and certainly audible) way if made of dollar store shielded cable or AAA-grade unobtanum (quality of shielding aside).  HOWEVER, use of cheap higher-capacitance cable WILL be quite audibly different from lower-capacitance grade is we're talking 12-20ft cables from guitar to pedalboard.  When trying to "convert" people to understand about the effects of loading and cable capacitance, I generally recommend the following (very persuasive) experiment.  Plug your guitar directly into your am with the longest cable you own.  Heck, if you have a true-bypass pedal of any kind, run your guitar into the amp through TWO such connected cables.  Okay, now plug your guitar directly into the amp via the shortest patch cable you own that can physically manage to connect guitar and amp input.  Where the heck did all that treble come from?!? 

Assuming capacitance ratings are within reason (i.e., NOT comparing 22pf vs 1000pf/ft), most audible differences can be eliminated if the signal source is low output impedance.  Remember that the capacitance of the cable behaves like a cap to ground with a series resistance - in other words, a single-pole lowpass filter.  The rule of thumb for such filter topologies is that the more current one can pass through, the faster that capacitor charges up.  So, a 1k resistance in series, with 10nf to ground, has a high-end rolloff beginning just under 16khz.  Increase that 1k to 10k, and the rolloff moves down to just under 1600hz.  If we make that capacitance to ground 1nf (1000pf), the rolloff with 1k series resistance is just under 160khz, and the rolloff with 10k is just under 16khz.

The underlying principle is that "if the output impedance of the source is low, then the cable capacitance matters less, unless the cable is very long, or the cable capacitance on the high side".  Conversely, if the source impedance is higher (e.g., a guitar), and "loads" down the signal, then cable capacitance starts to matter more, even in modest lengths.

Craig Anderton had a simple project in Guitar Player magazine,thirty-five some years ago, that simply consisted of a rotary switch and succession of fixed resistors that mimicked a guitar output that was more "loaded down".  Incidentally. this is why I have to regularly remind folks that all the received wisdom about tailoring the top-end of your humbucker-equipped OR single-coil-equipped guitar with choice of volume-pot value is true, PROVIDED YOU LEAVE YOUR VOLUME UP FULL ALL THE TIME. The moment you roll the guitar volume down a bit, part of the pot's resistance is placed in series with the pickup/s, and the resistive leg of the pot from wiper to ground is reduced; both of which result in loading down the guitar signal and increasing the importance of low capacitance in that first cable.

[idy]

How different are the action of the tone control and the effect of longer/shorter cables?
Don't you already have a curly cable simulator built into your guitar?

[andy-h-h]

100pF to 1nF at whatever steps you like is more like reality.

Interesting to look at the table and see that some with the highest capacitance cables are made by Canare, Belden and Mogami, which are considered the go to cable for mic leads and installation cable.  I guess low noise / insulation was prioritised over capacitance? 

Probably going overboard.   Some mic cables have ridiculously high capacitance per meter (or foot).
With a low impedance mic it's not a big deal but for guitar pickups the high-end loss is excessive.

That's a fair point re mic impedance.  I guess mentally I was looking at what I thought to be a high-quality product at the bottom of the list, without fully considering the application.  thank you

[mozz]

10pf to 550pf. Should be enough. 4 sections will be on switches if needed. Up to 2200 pf. Taken from a piece of test equipment a few years back , got the idea and never finished the project. Wanted to see how the guitar would sound, how it interacted with pedals/amps/buffers. 10 or more projects/pedalboard ahead of this, so maybe finished by 2025-30.

[Rob Strand]

That's a fair point re mic impedance.  I guess mentally I was looking at what I thought to be a high-quality product at the bottom of the list, without fully considering the application.  thank you

When I was young I made some beautiful guitar cables with Neutrik connectors.  I chose the best looking coax from a place that sold really good stuff off the reel.  Costed a bundle.  After a while I realized they sounded dull ... only to find it was mic cable with a significant capacitance.   Sometimes we learn the hard way 

10pf to 550pf. Should be enough. 4 sections will be on switches if needed. Up to 2200 pf. Taken from a piece of test equipment a few years back , got the idea and never finished the project. Wanted to see how the guitar would sound, how it interacted with pedals/amps/buffers. 10 or more projects/pedalboard ahead of this, so maybe finished by 2025-30

I feel sad when I see those.  I had sooo many and at some point I threw them all out when I did an "old technology" clean-up, along will boxes of tubes,  working tube amp chassis, germanium transistors and diodes.   All before I started playing guitar.

[andy-h-h]

Here's an article on the topic, or more specifically vintage curly cable care of Premier Guitar:  https://www.premierguitar.com/gear/the-vintage-coiled-cable-simulator-mod

But vintage cables (which I measured after purchasing a large cache of '60s NOS spiral cords on eBay) could have a capacitance of up to 400pF each meter!


 

[CheapPedalCollector]

Guitar cable is more like a lot of very small resistors and very small capacitors in a chain.

As for how tone controls and volume controls react with a pickup, it depends on how they are wired. I've noticed a lot inconsistencies even with my own guitars which generally use the same control layouts and part values and pickups. They can still sound radically different, and I have no explanation why. People say wood doesn't matter, I'm not so sure about that anymore.

[Mark Hammer]

How different are the action of the tone control and the effect of longer/shorter cables?
Don't you already have a curly cable simulator built into your guitar?

Depends on how and where the Tone control is wired in.  Most guitars will have it tied to the input lug on the volume pot, while some will tie it to the wiper instead.  The first kind is largely isolated from the output, while the second kind interacts more with the output and especially the loading presented by the volume setting.

[Marvcus]

Awesome.

Thanks for the help. The switches, box, capacitors, and jacks are in the mail.

[amptramp]

There is another way to tune a cable.  Some high impedance sensors use a guard ring cable which is triaxial with the centre wire for the signal, the outer core for shielding and the inner core between them is driven by a buffer that is driven by the signal.  Since the core wire and the inner shield braid are driven by the same voltage, there is no voltage difference between them and effectively, the capacitance disappears entirely.  Ideal for high frequencies at high impedance.  If the buffer were to be replaced by a non-inverting amplifier with variable gain, you could get the effect of a cable with variable capacitance set by the gain control.

[Quackzed]

different capacitors will lower the resonant peak of a pickup but will also roll off above this peak more quickly, the treble rolloff is steeper. this can cause a sharper more 'cawked wah' sound than the natural q and roll off a pickup usually has, you can fix this by lowering the output impedance with a smaller value output  load resistor. this reduces the peak a bit and also reduces the steepness of the high end roll off to more natural and less intense amount. here is a link where it is discussed...

https://guitarnuts2.proboards.com/thread/7914/impact-resonant-peak-tone-pickup

[brett]

I recently bought a cheap ($15, I think) 6 m cable from Aldi. It was crazy high in capacitance.  650p, from memory, and sounded awful (my amps typically have 12AX7 preamp and 470k input impedance).
Cutting a 2 m and a 4 m cable from it produced a nice short lead 200p and a slightly dull ("Jimi Hendrix tone") lead.
There's a reason Jimi used those curly leads.