blend pot as treble/bass cut?

[mwnovak]

Hi all. I'm cross-posting a question that I had tacked onto another thread (TBX with P-90s?), since I realized that thread's title addresses a slightly different question. Hopefully this isn't a problem.

I'm interested in wiring a pot that does the following:

1. cuts treble when rolled counter-clockwise from center
2. cuts bass when rolled clockwise from center
3. is "neutral" (but not no-load) in the center position
4. loads the pups like a normal 500k tone pot

For the pot itself, I'm looking at either a Bourns 500k MN blend pot or the Alpha 500k blend pot sold by StewMac. Not sure if these differ in function (some threads indicate that the Alpha pots attenuate both signals in the center position, but I dunno if this is true), but have one of each on the way.

For wiring, I've found the following diagrams:

Phostenix's TBX mod

an old Gretsch tone control

and this "Craig's" mod.

Each purports to do the same thing . . . but each is also a bit different. So one question is whether these diagrams actually accomplish a bass/treble cut tone pot, and if one approach makes more sense than the others?

My other question is about the load presented to the pups: is it possible to wire this pot in a way that presents a "normal" 500k load to the rest of my guitar's circuit? I've read that when a 500k blend pot is wired as a pup blend, it mimics two 500k volume pots wired in parallel . . . which presents a 250k load. Wondering if this applies here and if there's a way to avoid or correct that?

For what it's worth--if I can sort this out--the treble/bass tone pot will go into a circuit with 2 P-90s, a 3-way switch, and a single volume pot with a push-pull "bypass" (i.e. bypass gives: pups > switch > output jack) .

Any help or guidance here would be appreciated.

--Matt

[Jef Bardsley]

A "pan" pot would attenuate -3dB in the center, a "blend" pot will (should) not. The Alpha pots I've used do not.

No worries on the load, the tone pots aren't in parallel with each other - only the treble pot will be in parallel with the volume pot. However, I do like a 1 Meg better than a 500K for the bass circuit (more cut).

At a glance, the circuits presented are all the same, which is good. Not sure what the 1M resistor on Phostenix's does, but it can't do much (and doesn't seem to do anything in a quick SPICE sim). Craig's wiring seems simplest, but just jump the cap between the two outside lugs, no need for the extra wire.

If the 'bypass' is too bright, add a 1 Meg resistor to ground to it, and work down from there until you like it (250K would be "stock").

[mwnovak]

Hey Jef, thanks for the help here.

So the loading isn't an issue because, in this application, the top and bottom "pots" of the blend pot aren't wired in parallel as they would be for a volume/pup blend?

As I mentioned in the other thread: lots of learning happening on my end right now, so I'm grateful for any explanations.

I'll try to draw this up in the next couple days and post back here for a sanity check before I sit down the with soldering iron.

Thanks again,

--Matt

[Sensible Musician]

great idea - good creativity!

couple thoughts: first understand how a blend pot works. one pot works from CCW to center detent; the other pot works from center detent to CW - does that make sense? you can imagine that from one extreme to center the knob moves the full range of a normal pot; then the knob magically jumps to a different pot to work it over its entire range.

both pots are always in the circuit. a blend pot on which each of the pots has a 500k trace presents the same "load" to the circuit as a single 250k pot. it has the same effect as two individual 500k pots

to get the same effect as a single 500k pot you would want to a)look for a 1M blend pot, which is going to hurt the interface or b) cut the tracks in each 500k pot so that they "hand off" to each other. never cracked open a blend pot - having a hard time picturing how they are constructed...

anyway if you get your blend pot working the way you want you can use any LPF and HPF you like.

consider learning to read schematics and some real basic electronics, focusing on passive filters. you don't need much for guitar, but you need more than you currently have to be able to get predictable results

happy hacking!

[mwnovak]

Originally Posted by Sensible Musician

both pots are always in the circuit. a blend pot on which each of the pots has a 500k trace presents the same "load" to the circuit as a single 250k pot. it has the same effect as two individual 500k pots

Yep, this was/is my main concern.

How the blend pot loads the circuit comes down to how the pot itself is designed, right? Because there's not actually a "hand off" between one side of the pot (CCW to center) and the other side (center to CW): it's that, past center, one remains full up while the other begins to taper. At least, that's how I read the Bourns MW literature. But what I actually need is a blend pot where only half the pot is in the circuit at any given time, correct? Hence your suggestion to cut traces.

Once again, thanks for all the good info.

--Matt

[Jef Bardsley]

With the Alpha (and from what I can see the Bornes is the same), there are two separate pots on one shaft. There is no electrical connection between the pots unless you make one.


BTW, I have both bass and treble controls on several guitars, and I do cut both at the same time. Your blend pot idea is interesting, but you might be better served by a simple concentric pot.

[Sensible Musician]

matt: my advice at this point is to make a quickie prototype and find exact values for the sounds you want. you may be happy with a regular 500k blend. you could find out in minutes by installing separate no-load pots for each control. if disconnecting one really makes the difference you need, you know which direction to investigate in order to make your single-pot interface work. if you find that the net parallel resistance of 250k at the center detent doesn't bother you IRL, you can cross off a step for free!

don't change your design because it's hard. change your resolve. maybe the solution will end up being cutting tracks. maybe you will spend an afternoon combing catalogs and calling manufacturers. maybe you'll do it with a microcontroler and vactrols...

i seem to write this in every thread but i just thought of another way to say it: the actual circuit is the way to marry your sound design with your interface design. when you get down to drawing a schemo, those other two things are already set. i.e. you find the sounds you want, which includes particular values. second step you design the interface you want to control your sounds with. finally you sit down with more blank paper than you think you'll need, a pot of coffee, and a fresh eraser to draw your schemo/brainstorm prototypes. you're done when you're satisfied with the real world performance of what you specifically want.

i.e. a guitar schematic is the answer to the question, "how to do i get these values from this interface?"

seems like you're on track. just keep track of where you are on the track, you know? if you have real clear stages of design and everything is a compartmentalized, little problem, things get real easy real fast

good luck,

michael

[Jef Bardsley]

Again, there's no "250K at the center detent". The pots are separate. He's putting one pot in parallel with the volume control, the other in series. With the knobs on '10', the pickup will see the same as load as with stock wiring.

[Sensible Musician]

Doh right one side is a HPF. I blame lack of schemo - certainly not my lack of attention, ageing, effects of musician lifestyle LOL

Okay I went back and re-scanned this thread and the OP from your other thread. Tell me if these are your design criteria:

• One knob tone control
• cut bass
• ...but keep a functional, typical "tone" control

?

If that's the case you might consider:

• go ahead with your blend pot idea
• concentric knobs
• HPF on a trimpot inside the guitar set to static value, which can be bypassed on any manner of switch, e.g. push/pull

In any case there really is no shortcut for the order I mentioned: find your sounds, design the interface, then figure out the circuit needed to make that real. Sounds like you are still looking for sounds.

Focus on your neck pickup. Put each filter on its own pot and see whether you like the effect of no overlap (which might lead you to use the blend pot) - or maybe you will like the cool, utile notched sound of both working together (maybe dual concentric if you really do want the interface space of one knob). One thing at a time.

Anyway sorry for adding noise to thread. I still stick by what I said about compartmentalizing. Trying to solve everything at once gets everyone jumbled.

[mwnovak]

Hi guys. Thanks for all the feedback and discussion here, I appreciate it.

In terms of looking for sounds, I agree that the open variable right now is whether a passive HPF will give me what I want (at the ears) . . . and if so, what component values will get me there . . . and whether I may be better off with an HPF+LPF on separate knobs.

Before doing anything else, I'll try converting my existing tone pot from LPF to HPF with a 0.001 cap and see what that gives me: if I'm happy with the result, I'll feel pretty good about going forward with this blend pot idea. If not? At least I know.

I promise, though, that I haven't arrived here willy-nilly: I've been having a blast learning about how different components and component values shape tone, and experimenting--one change at a time, usually with a/b recordings--to find what I like. What I've encountered with this treble/bass blend pot, however, are the margins of my understanding in terms of implementation . . . which is where the learning happens.

Thanks again, and I'll post back here as I continue to work through this.

--Matt

[Phostenix]

Originally Posted by Jef Bardsley

Not sure what the 1M resistor on Phostenix's does, but it can't do much (and doesn't seem to do anything in a quick SPICE sim).

The 1M resistor across the tone cap is there to discharge the cap when it is disconnected in the No Load center position. That helps to eliminate the audible pop that often happens with no load tone pots when you come out of the no load position.

[Phostenix]

My explanation of the modified TBX tone is here:

https://sites.google.com/site/phostenixwiringdiagrams/modified-tbx-tone-control

If you look at Craig's 2 pot bass & treble cut diagram, both controls will cut when turned clockwise. The problem with using the standard blend pot is that in order to get the low cut from center to CW, that pot needs to increase in resistance from the center detent to fully CW. A blend pot will do the opposite.

One of the pots will go from 0 ohms to 500K when rotating from CCW to center. That's the one that works for high cut (typical tone control). From center to CW, that pot stays 500K.

The other pot will do the opposite. It will go from 0 ohms to 500K when rotating CW to center and stay 500K to fully CCW. If you wire up Craig's setup with a dual pot blend pot, you'll have full bass cut in the center position.

I went with the TBX pot for this reason. I actually mod the pot itself so it's more ideal (I cover this on the page & in a video).

I have this modded TBX in several guitars now & it's really useful. I like the no load position with humbuckers to get extra bite and the low cut is great.

[Jef Bardsley]

Originally Posted by Phostenix

The 1M resistor across the tone cap is there to discharge the cap when it is disconnected in the No Load center position. That helps to eliminate the audible pop that often happens with no load tone pots when you come out of the no load position.

Hah! Clever. That, unfortunately, is one of the things SPICE simulations don't reveal.

Originally Posted by Phostenix

If you look at Craig's 2 pot bass & treble cut diagram, both controls will cut when turned clockwise. The problem with using the standard blend pot is that in order to get the low cut from center to CW, that pot needs to increase in resistance from the center detent to fully CW. A blend pot will do the opposite.

Duh! You are quite right. I should have caught that. Thanks for the benefit of your experience.

[Phostenix]

I just looked through the Bourns guitar pot data sheets and I can't find one with the taper that would work for this application. I'd love to be able to use a $3 pot instead of the TBX, but I can't find one that has the right functionality.

[mwnovak]

Thanks to Phostenix and Jef for that latest round of insight here.

So the key bit regarding implementation is that an LPF rolls off more treble as resistance decreases (allowing higher frequencies to ground), while an HPF rolls off more bass as resistance increases (forcing lower frequencies . . . to get blocked at the cap?). So--to go from full treble cut at full CCW, to neutral at center, to full bass cut at full CW--the desired pot should roll from 0 at full CCW to 500k (in my case) at center, back to 0 just CW of center, and then up to 500k again (or 1meg, or whatever desired value) at full CW.

Which is why Phostenix relies on the TBX pot, since it has this behavior more-or-less out of the box (albeit it with a 250k pot on the tone side).

::scratches head::

Okay. That helps me understand the problem better.

To the question of sound, I rewired my existing tone pot as a bass-cut to hear the effect and see if this was something I wanted to pursue. While I heard almost no discernible cut with a 0.001 cap on a 500k pot, I threw on a teeny 470pf (0.00047) cap and . . . yep. I don't know if the value is dead-on (will try this with slightly smaller and larger cap values), but that's good stuff right there. With a slight twist, it gave me a lot of what I was looking for: slightly muted, tighter bass that "backgrounds" nicely while fingerpicking arpeggios. More twist? Punchy twang that's maybe a bit thin, but not nearly unusable.

Good news is that I like it and it's an effect I'd love to have at my fingertips. Bad news is that I'm still sussing out ways to build it. There are worse predicaments, to be sure.

Frankenstein 500k/1meg TBX mod, anyone?

Thanks again, and I'll update this thread with any progress.

--Matt