Stupidly wonderful tone control

[Mark Hammer]

I was busy trying to finish a 4049-based pedal I'd like to try and give to David Lindley this weekend when he comes to town (assuming I can bump into him at the festival).  I've liked him for over 30 years and wanted to say thank you.  In any event, I wanted to insert a simple variable lowpass filter treble-cut control and was just about to wire up a toggle with two different passive 2-pole filter settings when something ridiculously simple occurred to me.  Simple enough that I'm sure it must be used somewhere (though I can't remember where).

Here's the deal.  I had wanted to use a two-stage RC lowpass filter just ahead of the output volume control.  Simple enough: two series resistors just ahead of the volume pot, and a small-value cap to ground after each series resistor.  Of course, if you ignore the caps for the moment, the series resistors make the volume pot behave as if it is a higher value pot "turned down", so I was going to need to figure out how to compensate for the volume drop/difference in setting A vs B.

Then it hit me.  I took a 10k linear pot and soldered a .018uf cap from the wiper to ground.  The "output" of this pot went to the input of the 50k volume pot.  The cap and whatever pot resistance there was between the wiper and the "input" leg of the pot formed a variable lowpass filter.  because the 10k remained a constant series resistance, regardless of where the filter was set, the interaction between volume and tone pot was made negligible.  To make sure there was an identified maximum treble point, I stuck a 1k fixed resistor in series with the input lug of the tone pot.  Fired it up and it works like a charm.  In my instance, the corner frequency ranges from about 8.8khz down to about 800hz, giving a pleasing range of "roundness" to the tone.

Those familiar with the Proco RAT "filter" control will recognize that it is a similar type of single-pole variable lowpass filter where the resistance of an RC pair is varied.  The thing is, the filter control and output level pot are isolated from each other by a FET buffer in the Rat.  IF you want to keep things simple, and IF the circuit has sufficient output that you can afford to "shave off" a couple of db by inserting an extra resistance in series with the volume pot, this does what the Rat filter does and works great.

I'm at language school right now so I can't post a drawing, but I'll try and do it later tonight.  In the meantime, I'm pleased with myself.

[cbriere]

Mark,
"Making the gift of a pedal to an apreciated guitarist"
I also had that tought. Last year coming back from a show here
in québec (mont-tremblant blues festival).
That was the band INDIGENOUS. It's a 3 member
blues guitar oriented band. So i went there, enjoyed the
show, had myself photographed with MAto (the guitarist leader).
I even asked the roadie,after the show, to take my camera and shoot the
floor right where the pedals where laying. Back home i could study
his pedals arragements.
On the way home, remenbering the show i just
experienced,  i told my wife that i wished i had one of my pedals
in hand for that show. I would have gave it to Mato with my email,
phone number and ask him some comments on it.
What a great reward it would be to hear from a guitarist you like,
good comments on a design. ANd ultimatly see that he use live.
cbriere

[Doug_H]

That's a good idea, Mark. I used the rat tone control in an old shaka derivative but it affected the overall volume. (I didn't use the buffer to isolate it from the volume control.) If I understand correctly, you are attaching one end lug as an input and the other as output, keeping the max pot resistance in series. Then the wiper goes through the cap to ground. That should work really well- and it's simple. I like it!

Doug

[MartyMart]

That's great Mark, very simple and it works !!  

Marty.

[Paul Marossy]

I think I follow that, but a picture would sure help.  :oops:

[Mark Hammer]

That's a good idea, Mark. I used the rat tone control in an old shaka derivative but it affected the overall volume. (I didn't use the buffer to isolate it from the volume control.) If I understand correctly, you are attaching one end lug as an input and the other as output, keeping the max pot resistance in series. Then the wiper goes through the cap to ground. That should work really well- and it's simple. I like it!

Doug

Thanks for the flattery, Doug, and yeah that's exactly how it works AND why I like it too.

But for one little bug that I hope to work out before the weekend, I like the way it sounds a lot.  I used my version of the Anderton Tube Sound Fuzz (i.e., op-amp gain stage w/gain of 50, and attenuator before CMOS invertor stages), but stuck Anderton's midrange frequency booster ( http://www.muzique.com/schem/ca-eq.gif ) between the input gain stage and the 4049 invertor stages to get some nice "gronk" out of it.  It is about as quick and dirty a version of a pedal with pre and post EQ as you can get but sounds nice and thick.

The nice thing about Anderton's frequency booster is that it can be made simple (on-off) or complex (multiple resonant frequencies, variable boost), and is defeatable with an SPST switch.  That means you can implement a second stompswitch in a 1590BB chassis (which I did) to kick in the pre-boost for a different tone and get two different sounds from the same distortion.  Simple and convenient and if you have an extra op-amp kicking around (e.g., you used 3 op-amps from a quad) well worth the few extra parts.  BTW, the unit is usable with a +9v/4.5Vref supply.  Those of you who have a taste for sticking fixed-position wahs ahead of an overdrive should take a peek at it.  It won't do cut, only resonant boost (i.e., flat above and below boost zone), and while the resonant boost is not huge, it is quite noticeable, and just enough to push a distortion stage into frequency-specific rudeness.  I used a 3-way toggle to select between .1uf and .027uf caps in series (boost around 940hz), .027uf only (boost around 740hz), and .1uf only (boost around 200hz).  It might be more to my liking with the same arrangement but .033uf and .068uf caps instead.  This would give boosts at around 900hz, 600hz, and just under 300hz, which makes for a better spread.  That being said, it yields three easily distinguishable characters, plus a fourth if you cancel the boost.  Big time ROI in my view.

[audioguy]

I think I follow that, but a picture would sure help.  :oops:

I second that emotion.

[Mark Hammer]

I *will* make a drawing later tonight, but for the moment imagine one capitol T standing up and another on its side just to the right slid right up against the first one.  The one on the right is the output volume pot.  The stem of the T standing up is a cap to ground.

[puretube]

what`s the little "bug" you mentioned?

[aron]

I had wanted to use a two-stage RC lowpass filter just ahead of the output volume control

I thought you wanted 2 of them?

Did you use a dual ganged pot with each element in series for a total of 20K or?

Makes sense and it is simple.

[Doug_H]

I had wanted to use a two-stage RC lowpass filter just ahead of the output volume control

I thought you wanted 2 of them?

Did you use a dual ganged pot with each element in series for a total of 20K or?

Makes sense and it is simple.

Yeah, if you wanted a 2 pole filter you would need 2 and a dual gang pot should work real well for that.

FWIW I just ran a sim of this and there's an obvious advantage to this over the rat style control where the pot is wired as a variable resistor.  Esp with lower value volume pots, the db level with this control stays consistent whereas with the rat it varies all over the place. Of course it makes sense when you think about it but it's nice to see it printed on a graph.

If you can afford to eat a few db (and with most of these fuzzblasto circuits you easily can...) this is an elegant way of implementing a variable LPF tone control that maintains a consistent volume level.

Doug

[Mark Hammer]

Okay, I just posted it at http://hammer.ampage.org  And just because it comes up now and then, a simple left click is confirmed to work fine.

Thanks for the diligent corroboration Doug.  I'm pleased with my homebrew Rat clone, so the FET buffer obviously works well, but if you want to keep the parts count low, this works like a charm.  I can easily see inserting it into a DOD250/YJM308/Dist+ clone, especially if one uses a 50k-100k volume pot.

Aron,
In a previous version of the same basic pedal (the green CMOS drive in the "family shot" at my site), I used a switchable 2-pole passive filter, and was going to do so here as well.  It sounds great but when I realized I could easily implement a tunable filter without having to fart around with level balancing, I went with that.  I still advocate use of 2-pole filters as a terrific sounding alternative, but went simple this time.  And yes, you could use a dual ganged pot the same way, though the trick is to have the tone pot's value be relatively small in comparison to the volume pot's value.

Ton,
The "bug" was unrelated to the tone control.  I realized on the way home  tonight that I had foolishly stuck a DC-blocking cap between the input gain stage and the frequency booster stage, without having provided any means to rebias the frequency booster stage.  As you can imagine, it tended to perform intermittently.

[Paul Marossy]

Oh, now I see. I totally misinterpreted that one.  :shock:  :oops:
Thanks for sharing that Mark.  

[puretube]

cc.: "bug": I had suspected s.th. similar...  

[RCZ53]

I built the Tube Sound Fuzz years ago when it appeared in Guitar Player magazine. I never really appreciated it until giving it another listen recently. One problem I have is that it has so much gain that the volume pot can only be open a tad or the output is just way too much. Any thought on taming it a bit? This is the early schematic BTW. Maybe a prime candidate for your Tone Control, Mark.
thanks

[puretube]

log pot ("audio-taper")... ?

[Mark Hammer]

I built the Tube Sound Fuzz years ago when it appeared in Guitar Player magazine. I never really appreciated it until giving it another listen recently. One problem I have is that it has so much gain that the volume pot can only be open a tad or the output is just way too much. Any thought on taming it a bit? This is the early schematic BTW. Maybe a prime candidate for your Tone Control, Mark.
thanks

I tinkered with it and settled on something very close to this: http://www.geofex.com/FX_images/mhtsf.gif  I like it a whole lot more than the original, though I wasn't aiming for any sort of serious fuzz.

Remove the 2-pole active lowpass on the right (from the 100k pot half all the way up to, but not including, the 10uf cap), and insert the SWTC between the 10uf cap and the output pot and you'll be home free.  Personally I like to keep the potential volume so that I can set the drive low for just a little grit and still have enough output to overdrive an amp.

[electrictabs]

it's great Mark
i believe all the "tone miracles" are probably created by something like a crazy  :idea:  or accident and that's the cool little thing about it.

[aron]

One problem I have is that it has so much gain that the volume pot can only be open a tad or the output is just way too much.

Simple mods page:

http://www.diystompboxes.com/cnews/mods.html

[Eric H]

That's slick as heck, Mark.

-Eric