Treble and Bass
In my tone capacitor video, I demonstrated the typical use of the capacitor as a low-pass filter in a guitar tone control. On a home stereo, this would be called a treble control, as it is used to roll off the high-end treble frequencies.
But what if you also want a bass control- a high-pass filter to roll off the low bass frequencies?
In the circuit diagram shown here, from the G&L S-500 guitar, you’ll find both a treble and bass control. They call it their PTB system (which presumably stands for passive treble bass, but I’m just guessing here).
In this circuit, you can see the standard tone control R/C network labeled “Treble”. This is a 500k audio taper pot in series with a .022uF cap, wired from the main guitar signal to ground. Since caps pass high frequencies but block low frequencies, this cap passes the high frequencies through to ground (making them silent, therefore rolling of the treble). You’ll notice there’s also a .001uF cap in parallel with that control. Since capacitors in parallel add together, the addition of this .001uF cap sets a minimum amount of treble roll-off.
You’ll also see a second R/C network in series with the guitar signal to the output labeled “Bass”. This is a .0022uF (2200pF) cap, in parallel with a 1Meg potentiometer. Remembering again that a cap passes high frequencies and blocks low frequencies, you can see that this cap, being in series with the guitar signal, will pass the highs and cut the lows. The potentiometer controls the amount of signal passed directly through.
There will of course be some interplay here between these two tone controls, as the the cutoff frequency of an R/C network is determined by the capacitance and resistance values.
This reminds me of the passive single-pot tone control found in the classic fuzz pedal- the Big Muff Pi. This single potentiometer controls bass cut on one side, and treble cut on the other, with a fairly pronounced mid-scoop. It’s a great, and distinctive sound. 
As a passive tone stack however, there is quite a bit of signal loss- I haven’t actually tried it (or any variation of it) directly in a guitar. This circuit really benefits from some additional gain/boost (like the insane amount of gain in the Muff!)
Much has been written about the Big Muff tone control, by the likes of Jack Orman, Beavis Audio, and etc.
March 22, 2012 @ 7:22 pm
Thanks for the schem, I’ve always wanted to do this with a regular guitar circuit.
April 1, 2012 @ 2:51 am
Check this out as well:
http://www.custom-sounds.com/epages/Gagar.sf/en_US/?ObjectPath=/Shops/CustomSounds/Categories/articles/moretone
This site has some other great articles, there is quite good info on r’n’r guitar strings history so to say. Shame they don’t update it more often though.
April 8, 2012 @ 8:38 pm
Cool, thanks for the link!
John
April 9, 2012 @ 1:32 am
Oooh, nice, thanks.
Skiffle Cat Jazz Guitar Upgrade : manchesterguitartech.co.uk
June 26, 2012 @ 12:56 pm
[…] the circuit right took some time, both to design and install. I based the tone stack design on G&L’s PTB system, explained here at Planet-Z but changed the values so that it worked well with the […]
November 3, 2012 @ 3:53 am
Hi!
I have a S500 and it didn’t come with the 0.001uF.
So I pre-installed one (using a couple of clamps) and A/B the guitar. You have some treble roll off with it, as suspected – its a 0.001uF treble bleed to ground, constatntly. But really, I prefer without it. It’s like the guitar looses some highs (not much, believe me), but if rather do it with a EQ than to lose it permanently.
I think G&L uses this PTB system also on other guitars, like the Legacy model. The main difference between S-500 and Legacy is the pickups and the (simple) mod of adding the bridge pickup to any combination on the 5way switch – this way you could have B+M+N or B+N (Tele style).
Anyway, believe me when I tell you this PTB system is WAY better than anything. I mean, the treble pot really works! The bass pot really works also. Its a USEABLE implemented EQ on the guitar. We have infinite tone possibilities with this wiring configuration.
Thank you John, to share all this information with us.
November 3, 2012 @ 11:08 am
Thanks for the report Zed! Yes, that .001uF cap (C3 in the schematic) seems totally optional- it just sets a minimum amount of treble roll off. If you like a brighter sound, go without. That’s what I would do!
John
December 20, 2012 @ 10:12 pm
John,
Just stumbled across this discussion tonight. I own both a Legacy and an S500 as well as some standard strats. I suspect that the addition of C3 is, to some extent, G&L’s accounting for the difference in electrical characteristics/response of MFD and “normal” alnico pickups. I’ve only ever seen that .001uF cap in guitars with MFDs. I can’t find the capacitance and inductance specs for MFDs anywhere or I’d shove it in my spice model to see what the deal is. By the way, for anyone with the PTB circuit w/alnico pickups, setting the volume to max, treble pot to around 7-8 (~250k on the log pot) with the base pot on 10 duplicates the frequency response of standard strat wiring with tone and volume pots wide open.
December 23, 2012 @ 4:22 pm
Cool – thanks for the info.
John
Satchurator Volume Mod - Planet Z
August 22, 2013 @ 6:53 pm
[…] the other, with the middle position being scooped. It’s a great tool, but as I mentioned in my Treble and Bass article, there’s quite a bit of signal loss in this type of tone […]
January 5, 2014 @ 7:20 pm
Hey John,
Thanks! I knew somewhere someone else had to be doing this with the bass roll off. I just happened to stumble upon it when I was trying to think of ways to thin the bass when running pickups in series. Definitely a useful addition to tone adjustment when running non-standard pickup configurations and other non-standard wiring.
February 9, 2014 @ 3:43 am
Am going to try, using a 750K with a 250K Pot, as i feel that from 750-1000K the bass comes in to effect. Can use a Push/Pull Switch to by pass it, or i can make a Dual Pot and the 2nd Pot can by pass it (with a bit of touching up)
December 25, 2014 @ 8:35 am
Very nice summary of the PTB circuit. I have been thinking of implementing it, but with a 50s/Fezz Parka type mod with the treble cut (low pass) filter on the output of the volume control, but I wonder if that will have other consequences for the bass (high pass) filter. Then of course, I wonder about moving both filters to be after the volume pot. Any thoughts on these two variants?
December 25, 2014 @ 9:53 pm
I’m not familiar with the fezz parka mod. My recommendation is to get some alligator leads and test out the circuit in the various configurations that interest you. Only start soldering when you’ve found something that works for you.
-John