How can I convert bass, mid, and treble knobs into a single tone control?

[rjda]

Like on the M800 overdrive or the kickstart distortion

 

[Alan W]

That’s a really general question, and I think in most cases would be very dependent on the entire circuitry of the audio path. There are two main varieties of single knob tone controls. The most common is just a treble bleed, so as you turn it, it sounds muddier. The other is a bit more advanced, usually moving a peak point for a midrange tone, so it’s neutral at noon, bassier before noon and brighter afternoon.

In many ways, it’s those little side circuits, the lay of frequency passing, that are what make each “pedal” sound distinct.

But it’s an interesting question, and I think if someone with greater overall knowledge of the project range here—which for me, means almost anyone else that is reading this thread—could suggest a two or three tone control schematic to look at, and discuss ways of removing and recombining the tone control into a single knob. Could any of them keep their “sound?”

But, but, I also gotta say that not only do I really like having that 3rd control (mid) there, I love it when there is a 4th knob that moves where the mid freq. is centered. Unit67 does this in a lovely way, as do the Tubesteader pedals. I think what attracted me to this style of control was playing bass amps that often had both a low mid and a high mid inflection point.

 

[giovanni]

I am not sure this is easy to do in general. There may be some pedals for which it’s easy to merge the controls (although I can’t really imagine what tone stack topology would let you do that, and how the merged controls would work) and others hard.
However I would recommend a different approach: move the less “useful” or interesting controls to a trim pot inside the pedal or replace them with a pair of resistors that you can socket and change later (depending on the topology, one resistor may be enough).

 

[BuddytheReow]

My first reaction was to put a BMP tone stack in there. I reread the question and now I have a question. Fully CCW it’s only bass and CW only treble and at noon it’s a mid hump? Sounds like you want a variable band pass filter

 

[rjda]

I'm trying to put two preamp pedals, a power amp pedal, a reverb pedal, and some other stuff into a chassis and hopefully make a two-channel combo amp. The chassis can't really fit a bunch of knobs from the pedals, so my main goal is to have less of them.

I am not sure this is easy to do in general. There may be some pedals for which it’s easy to merge the controls (although I can’t really imagine what tone stack topology would let you do that, and how the merged controls would work) and others hard.
However I would recommend a different approach: move the less “useful” or interesting controls to a trim pot inside the pedal or replace them with a pair of resistors that you can socket and change later (depending on the topology, one resistor may be enough).

Thanks for this idea. I see that the M800 already has 6 trim pots inside of it, and one is labeled "tone" so that seems like something to check out. I was also considering using concentric pots to combine 2 knobs into 1, but it's hard to find them with values matching the build guide. Do pot values affect the sound a good amount?

My first reaction was to put a BMP tone stack in there. I reread the question and now I have a question. Fully CCW it’s only bass and CW only treble and at noon it’s a mid hump? Sounds like you want a variable band pass filter

I'm not really sure honestly I read that a tone knob on a guitar was a low pass filter, but I don't know if it's the same on pedals/amps. I will look into the variable band pass

 

[Feral Feline]

Be lazy like me and install an internal trimmer for the mids, and then wire up the bass and the treble onto a dual-gang pot so that when turned all the way to the left the treble is reduced completely and the bass is saturated, and conversely turning the knob to the right removes bass and adds treble...

You could make treble and bass knobs a fixed value (dial it in on the breadboard or pedal, then remove the pots and measure each one and install the next closest fixed resistor value... and just leave the mids knob.

Internal trimmers for bass and treble and just the mids knob...

Seems to be a few ways to go about it.

 

[swyse]

I'm trying to put two preamp pedals, a power amp pedal, a reverb pedal, and some other stuff into a chassis and hopefully make a two-channel combo amp. The chassis can't really fit a bunch of knobs from the pedals, so my main goal is to have less of them.



Thanks for this idea. I see that the M800 already has 6 trim pots inside of it, and one is labeled "tone" so that seems like something to check out. I was also considering using concentric pots to combine 2 knobs into 1, but it's hard to find them with values matching the build guide. Do pot values affect the sound a good amount?



I'm not really sure honestly I read that a tone knob on a guitar was a low pass filter, but I don't know if it's the same on pedals/amps. I will look into the variable band pass

Most of the trimmers are for biasing the Jfets, The pot values will affect where the filtering happens usually pretty dramatically.
TSC in the web
You can use this to see the differences pots values would make. Could you use smaller pots? They make pots with the shaft already being a knob basically so you could maybe use those to condense the controls?

 

[Feral Feline]

You can get those trimmer pots with really long/tall black shafts, that way you can drill a hole and use them on the outside of the pedal.

 

[Passinwind]

I'm not really sure honestly I read that a tone knob on a guitar was a low pass filter, but I don't know if it's the same on pedals/amps. I will look into the variable band pass

Take a look at the Tilt EQ format. That does about as much on one knob as anything I've seen. Not necessarily the greatest for midrange tweaking though.

 

[mdc]

@rjda are you specifically using the kickstart and the M800 boards?

Are you looking to have a single, universal tone control for the ENTIRE combined thing or are you trying to have separate tone controls for each board (just one knob instead of 2 or 3)?

Can you provide a block diagram for what you're actually trying to do?

 

[mdc]

Anyway +1 to @giovanni 's suggestion of just using internal trimmers in lieu of the external knobs on each individual board. Set and forget, and if you feel like tweaking further you just have to pop the back off of the enclosure.

It would be simple enough to add a passive HPF at the input of the whole shbang and a passive LPF at the output if you wanted some global controls.

Or, depending on what circuit blocks are going where and what's switchable and how, you could maybe pick and choose one knob from each board to keep external based on where it falls in the chain. Maybe the bass control from the first circuit, the mids from another, the treble from whatever is last...?

Or make everything internal and add a dedicated active EQ board into the mix?

You can kinda do anything.

 

[HamishR]

Here's one idea - On a Fender Deluxe reverb there is no midrange pot. The tonestack is much the same as that in a Super Reverb but the mids knob is replaced by a 6k8 resistor to ground - which sounds much like a Super Reverb's tonestack with Mids on around 7-8. The mids pot is a 10K in a Super Reverb.

So you could replace Bass and Mids with fixed resistors. If necessary use pots to find the sweet spots, measure them then replace with the closest resistor value.