Calculating R-C Filters without Resistors?

[GibsonGM]

Watch out for the sneaky foot sweep from the brown belt they have you spar with in the test. 

You know, at my gym, I'm actually that guy with the foot-sweeps beginners never see coming. You'd be surprised how much Judo you can learn from the internet... or maybe not.

But here I am on a hot April afternoon, still pondering how to calculate a freaking guitar tone-control.

I get it... the Pickup creates a highly Q dependent, complex Z, and the layman's formula may not work.

If Spice is needed, could someone give me some idea of how to set up a simulation?

1) get LT Spice IV:  http://www.linear.com/designtools/software/#LTspice

2) learn how to run a basic simulation (100's of tutorials online, just search).  Learn to simulate normal HPF, LPF.....then you'll need to make a LPF and/or HPF, and have a source to approximate the output impedance of whatever you're considering.   In practice, I simply 'make' the circuit I'm thinking of using (CE, CC, opamp gain stage, etc), which will get very close to real-world.  Close enough for us, anyway. 

3) Once you get going with it a bit, many here (including myself) would be glad to help you out with further questions!

LT Spice has a small learning curve, but don't let it throw you - you learn 2 or 3 sim commands, and you've got it made. 

[merlinb]

If Spice is needed, could someone give me some idea of how to set up a simulation?

1) get LT Spice IV:  http://www.linear.com/designtools/software/#LTspice

I would recommend Tina TI:
http://www.ti.com/tool/tina-ti
It has a much friendlier user interface than PSpice or LTSpice, making it a lot easier to learn.

[ashcat_lt]

The big problem with accurately simulating a specific guitar's T control is that it is highly dependent on the inductance of the pickup.  There aren't many manufacturers who will tell you that particular spec, and most of our meters wont measure it directly.  So you can build the model, but you have to guess at that most important of values. 

Anyway, go over to GuitarNuts and grab JohnH's GuitarFreak.  It's a spreadsheet (I trust his macros) that simulates a few common wiring schemes with sliders for the important values and some presets which include typical values for common types of pickups.  If nothing else, it illustrates how to model it in Spice and gives you some realistic values to work with.

[GibsonGM]

GuitarFreak looks REALLY interesting, but I couldn't get any of the links to get me to an actual file...everything is "file not found".  Maybe he's just doing maintenance (?).

Yeah, Ti is easier to learn than LT. I like old school code-like stuff, YMMV. 

[thehallofshields]

Wow. Guitar Freak is a pretty impressive spreadsheet.

Okay, so I've read a couple of threads on other forums (can't quite remember which) of debates on whether or not Multiple Caps on a Rotary Switch can achieve something different than a standard Tone Control; the single Cap with variable resistance to ground.

The proponents of the Tone Rotary configuration argue that higher cap values shift the Resonant Peak of the Circuit to a lower Q, while the standard Tone Pot simply lowers the amplitude of that Resonant Peak. Playing around with the C value (with the Red Slider set to minimum) in Guitar Freak for 30 seconds seems to confirm this.

This was part of the reason I started this thread, because I wanted to see for myself HOW that works. Well I still don't understand how a Cap with 250k-1M Resistance to Ground can determine the Resonance of a Circuit, but that's probably because I have a long way to go in understanding Inductance.

[thehallofshields]

Anyway, go over to GuitarNuts and grab JohnH's GuitarFreak.  It's a spreadsheet (I trust his macros) that simulates a few common wiring schemes with sliders for the important values and some presets which include typical values for common types of pickups.  If nothing else, it illustrates how to model it in Spice and gives you some realistic values to work with.

It's pretty interesting how they emulate a pickup:



The Inductance is in Series with the Resistance, which are loading the Voltage Source, and then a tiny bit of Capacitance in Parallel->Ground. There is no way on this earth I would have figured that out.

[PRR]

> interesting how they emulate a pickup

It is in half of Gus' posts. Worth reading.

[alfafalfa]

It's pretty interesting how they emulate a pickup:

Fantastic piece of work !  Thanks a lot.

[GibsonGM]

Works now!  Dude spent quite a bit of time putting that together....nice link, good project!

[ashcat_lt]

> interesting how they emulate a pickup

It is in half of Gus' posts. Worth reading.

Yes, in fact it's pretty standard, but John did put quite a bit of work into the spreadsheet and its presentation.

I also strongly recommend The Secrets of Electric Guitar Pickups, wherein we find out that even the "standard" RLC thing only comes close, and that there are other things going on which are even harder to measure in real world pickups.

While it's probably obvious - and maybe outside the realm of this conversation - I always feel compelled to mention that these graphs only show the frequency response of the filter made by the pickup and its attached components.  That is, this is what it would look like if you wired it up like a stompbox and fed white noise into it from an ideal voltage source.  Actually, I think it's probably a step function, but same-same.  You almost never get anything approaching white noise out of a guitar string.  The very first part of a sharp pick attack is sort of a half-assed step function, but it very quickly settles down into a harmonic series with proportions between the harmonics dependent mostly on the position of the pickup along the vibrating length of the string.  Again, beyond our scope here, but what I'm trying to get across is that any given strum on your guitar - viewed on a spectrum analyzer - is not particularly likely to correspond to the graphs you're seeing out of these models.  I've found that if you integrate a number of strums all over the fretboard you can sort of get close, but...

[Digital Larry]

This was part of the reason I started this thread, because I wanted to see for myself HOW that works. Well I still don't understand how a Cap with 250k-1M Resistance to Ground can determine the Resonance of a Circuit, but that's probably because I have a long way to go in understanding Inductance.

The resonant peak caused by your tone cap occurs when the tone pot is turned all the way down.  The resonant peak that you get when your tone pot is turned way UP is caused by other things, like the pickup's own distributed capacitance, the cable capacitance and whatever input capacitance the amp itself has.  These capacitances don't have much series resistance to ground.

I've done some analyses over at my own blog which you may find interesting (or not):

http://holycityaudio.com/ltspice-analysis-of-a-simple-guitar-tone-control/

[post edited to correct link]

[ashcat_lt]

I can't believe it didn't occur to me til just now that it would be relevant to this thread, but there's actually a whole section over at GuitarNuts where JohnH, the author of GuitarFreak, goes into some detailed discussion complete with graphs of simulations all about the effects of different values of different components in different types of passive guitar tone controls.

John's Tones

[Gus]

A post from 2011. 
http://www.diystompboxes.com/smfforum/index.php?topic=93834.0

The first link in the above thread can be viewed using the wayback machine the graphs do not seem to be saved.  That was the first site I found on the web for sims of guitars to amps.

I think of the simple guitar sims as an EQ that adds to the guitar electromechanical system.  The body and neck and other parts move differently at different frequencies.

A big thing that the sims show is the change with different cables from the guitar to amp the length and capacitance per foot is a cause of the tone change.  Yes cable can cause a change in sound.

Also check what happens when you turn down the volume control as it is turned down you start to lose the peak sometimes at 50K series(a 250K volume example is when the volume control has 50K from top to wiper and 200K from wiper to ground)