Could someone please explain this tone control to me?

[the_floyd]

It's the BJF Dyna Red tone control. I'm trying to incorporate it into a Rat build, and the way I expected it to work was that it sets a fixed cutoff frequency, and the intensity of the cutoff is increased as the pot's resistance is lowered. However, I set up the circuit, (with a 100k log pot, mind you) and the cutoff is barely present until the pot is rolled up all the way, at which point it is still a very subtle rolloff. It is highly likely I set up something incorrectly - my Rat board is a nightmare of tack-soldering at the moment - but it is also very likely that I simply don't understand how this circuit works.

As such, could somebody enlighten me on the workings of this circuit? Help would be much appreciated.

[Govmnt_Lacky]

Have you tried a 100K Linear or Rev Log pot?

If it is "all bunched up at the end of the pot" with the Log version, you may get it to spread out more with a Linear version. 

Good Luck 

[the_floyd]

Well, yes, I'm aware of the difference a linear pot will make - what puzzles me, though, is the seemingly minimal treble rolloff that it yields, and the fact that it seems to work in the opposite direction than I would expect it to.

[Govmnt_Lacky]

...and the fact that it seems to work in the opposite direction than I would expect it to.

Did you wire it up correctly? (At least according to the picture.)

From the looks of it, it appears that Lug 1 (far right when looking at back of pot) should go to Ground.

Lugs 2 & 3 should be tied together and also going to the 473 cap.

Is this how you have it wired up?

[MikeH]

This style tone control is pretty common, although different circuits will require different values for the cap and pot.  You'll have to experiment to see what works best in the Rat.

it seems to work in the opposite direction than I would expect it to.

You can fix this by wiring the pot in the opposite way that it is now.

[ayayay!]

I'd be willing to bet there's not much wrong with the way you've done it.  It's hard to say without seeing more of the circuit, but I think that .022 after the tone control is already cutting a lot of treble out.  You're only going to cut about 1Khz more out by using that cap/pot control setup.  Granted, that's a crucial area where that 1Khz is going to be quite noticeable, but I think you should try a 50k Audio pot, and make that .022 to ground a .01, and see if you like it.  

[Mark Hammer]

I concur with Jono. A 3k/22n combination gets you a shallow rolloff starting around 2.4khz.  When the tone pot is at min resistance, you bring in a 47n cap in parallel, which gets you a 69n effective value and a rolloff starting just under 770hz.  Yes, it's lower, but the rolloff is fairly shallow and that sort of rolloff will not be especially audible.  Really it's more of a fine-tuning-to-suit-the-normal-treble-response-from-my-amp control than any sort of character-shifting control.

[ayayay!]

Really it's more of a fine-tuning-to-suit-the-normal-treble-response-from-my-amp control than any sort of character-shifting control.

Yeah, Mark is a better conveyor of thoughts than I!  What we're trying to say is that what you're trying to do is mostly there, it just needs a little tweaking to your taste.   

[petemoore]

  Making either of the caps shown vertically [from signal path toward ground], a larger value will attenuate more treble.
  50kb is probably still a good choice and allow adequate treble pass when set to max R.
   I would add another cap to the cap between signal path and the pot, or make the 223 larger until..with the treble max, there's still more than enough treble for whatever application might come along. 

[Mark Hammer]

I don't think that control will really add anything to a Rat, quite frankly.  There is already a quite effective variable lowpass filter in there, which this won't really improve upon.

If the intent is to provide more tonal flexibility for a Rat, then I would recommend something involving the two ground legs on the op-amp instead.

The stock unit has a 560R/4u7 network to ground, and a 47R/2u2 network to ground.  These two networks set thegain differentially for the upper and lower portions of the signal.

Let's say the gain-setting feedback resistance is set for 20k (medium gain).  Via the 560R/4u7 path, the gain will be set to [20k+560R]/560R = 36.7 for the entire spectrum down to 60hz.  Via the 47R/2u2 path, however, the gain will now be set to [20k+47R]/47R = 426 for content around 1540hz and above.    In other words, the Rat sends the signal through a serious treble boost prior to clip.  With the Gain pot maxed, that second treble-boost path imposes a gain in the thousands (2128x to be exact).

What this suggests is that a more interesting way to modify the treble content is via a small - and I do mean small - adjustment to that 47R resistance.  Merely adding 220R to that 47R is sufficient to seriously drop the treble boost.  Making that path a resistance of 267R instead of 47R now makes the maximum gain 375x (still pretty serious when you consider that a Tube Screamer maxes out at around 118x), compared to the 180x max gain via the other path.  More importantly, the rolloff where that max gain of 375 is applied gets bumped down to 271hz, which is essentially a full range boost.

A simple 3-position toggle that selected 3 different resistance values (47R and two others) would get you a more pronounced character shift, which could then be augmented by the stock Filter control.  Start with a 270R fixed resistor, and use the toggle to add in either a 56R resistor in parallel for 46.4R (close enough, given 5% resistors), or a 150k in parallel for 96.4k, which makes the point where the differential gain kicks in 750hz, and sets max gain for that portion at 1038x.

If your goal is to change the character of the top end.  That'll take you places.

[the_floyd]

Thanks for all the responses. I did indeed wire the pot backwards, which I intend to remedy.

I've already tweaked the ground-legs, actually - but thanks for the recommendation. My Rat circuit in question has been a work in progress for quite some time, and it's a rather different beast at this point, if I do say so myself. The reason I sought out this particular tone control design was because I didn't like what the variable low-pass filter did with the 'presence' range - once I had attenuated the extreme, 'ice-pick' treble to my liking, I found that the pedal ended up sounding dull and muffled. As such, I assumed this design (or a variation upon it) would allow me to isolate my undesired frequency range, and attenuate it to my liking... now that I'm starting to understand how it actually functions, I'm becoming a little less certain of that, but I'll keep working on it. Thanks again!

[PRR]

What's to the left of the 3K?

You expect the full-up pot to be slightly larger than the source resistance, so as pot is turned-down it loads the source. Then the "473" cap makes it load only the highs, letting the bass pass at full strength.

So is there another 10K-50K to the left of the 3K shown?

[the_floyd]

What's to the left of the 3K?

You expect the full-up pot to be slightly larger than the source resistance, so as pot is turned-down it loads the source. Then the "473" cap makes it load only the highs, letting the bass pass at full strength.

So is there another 10K-50K to the left of the 3K shown?

Ah, that was stupid of me. I probably should've just posted the whole schematic; didn't want to step on anyone's toes, though. Its the opamp output, followed by a 1k resistor, a 1uf cap, and a pair of diodes to ground.

[the_floyd]

Just to put a bit of a post-script on this, I've decided to go with a 2-pole passive RC filter, as per Mark Hammer's post here: http://www.diystompboxes.com/smfforum/index.php?topic=75563.msg615660#msg615660. It seems to do what I'm seeking to accomplish, and appears to be a fair sight simpler as well. Thanks again.