Overdrive question...

[vanessa]

In my world wide web travels I came across this photo of the guts of an overdrive from a boutique builder. I don't want to name the builder or the model as my intent is not to reverse engineer their pedal. What I did notice that's really cool is how few parts there are and how it looks very similar to the ROG tube reamer with a few exceptions, one of them being this unit also has a bass and a treble control.
I've looked into having separate bass/treble controls for builds in the past and I found that it really takes up a lot of space on the circuit board. Looking at this one it looks like it can be done with very few parts. Can someone tell me how this can be done with such few parts (like this one) or point me to a low part schematic for separate bass & treble controls?

[brett]

Hi Vanessa
in something really simple like this, I'd try simple low pass and high pass filters in the feedback loop of the op-amp.  2 caps 2 resistors and 2 pots and you've got it.  A 22k resistor and 0.01uF cap roll off at about 720Hz (around E above middle A ?), so those values would work quite well.

Interesting post.  I've never thought about this much.  Always used the BMP 1 knob tone control....
cheers

[MartyMart]

Like Brett said, my guess would be simple low pass/ high pass and using the
other half of that 4558 to bring the gain back up from the tone's volume drop.
That would leave first half of the opamp for a similar "reamer" setup.
I've built a LOT of OD's and in my limited experience, the more "simple"
ones sound the best !  ( reamer  Easy drive etc )
I've made a version of my "Mr Drive" with a BMP and that sounds great !

MM.

[spudulike]

Get the Seymour Duncan Tonestack Calculator - there are several simple tone control schems and you can futz around with the component values and see the results.

[stm]

I would bet  that Bass and Treble controls in such a simple TS workalike are implemented in the following way:

1) Bass control is implemented varying the resistor to ground on the (-) input of the first opamp, just like in ROG's Tubereamer.  This resistor affects both gain and bass content of the signal.

2) Treble control is nothing less that the standard TS tonecontol, or even a simpler RC filter with a variable resistor.

In case I'm wrong with the above, the other possibility is that the second opamp is configured for Treble and bass controls (which is a quite popular tone control topology). For an example take a look at Sans Amp GT-2 tonecontrol.  If you want  to add a midrange control as well, you can see what I posted at the Layouts Gallery (this would require another opamp however).

http://aronnelson.com/gallery/STMs-Circuit-Ideas/Modified_Sans_Amp_GT_2_equalizer_with_midband?full=1

Regards.

[vanessa]

Bret, I did not know you could make a separate bass and treble control with such few parts! So what you're saying it's just 1 cap with 1 resistor for the bass cut and 1 cap with 1 resistor for the treble cut? (Plus the pots).

Marty, I think I understand you. You're saying that there would be a volume drop after the tone circuit so you would want to make use of that second half of the opamp after the tone circuit (or before?) to make up for a volume drop?

[PaulC]

This is my timmy pedal - it is not a simple TS work alike.  Also the design was made in 1997 - a bit before the tubereamer if I'm not mistaken.  It does not use a variable bass/gain resistor like stm talked about, nor does it use the second opamp stage for a tone control ciruit.   This is something that helps pay the bills, so I'd rather not see the circuit dissected just yet.  No circuit ideas came from the DIY area, so you don't have to worry about it having any ganked ideas from here. 

PaulC

[MartyMart]

Marty, I think I understand you. You're saying that there would be a volume drop after the tone circuit so you would want to make use of that second half of the opamp after the tone circuit (or before?) to make up for a volume drop?

Yes, I would use it after ... exactly
MM

[b_rogers]

so this is the timmy.. it gets good reviews over at HC>

[Mark Hammer]

Regardless of what does or doesn't happen in the pedal pictured, what gets called treble and bass in pedals can vary substantially in terms of what it does and how it does it, much the same way that what gets called "stereo" can happen in a multitude of ways.  That's not to say that anyone is cheating in their naming of controls.  Rather, you spot a way to change tone, and you ask yourself "What would I call it so the control made sense to the minimally informed user?", and the answer comes back treble and bass.  Some pedals have Baxandall-type boost/cut controls, where others have simple cut like a Fender/Marshall tonestack.

One fairly simple way to get adjustment of the bottom and top end in an overdrive pedal is to use dual paths.  For instance, in many of Joe Gagan's designs, you'll often see two parallel input caps, one with a variable resistor in front of it.  The "pot-assisted" cap is always much bigger than the other one.    As input caps, these limit the low end of the device up front.  As the pot resistance in front of the larger cap is reduced, the smaller-value cap plays less and less of a role in determining that low end limit, and the low end is set by the combined parallel capacitance of the two caps.  So, in a sense, that pot ahead of the larger cap acts like a bass trim/cut control.  Other than the pot itself, only a single extra part is used.  If the pedal has other sorts of tone shaping downstream that trim the treble to achieve some sort of tonal balance, such a control may feel like a bass boost control, rather than bass restoration.

Another type of dual-paths approach is one you'll often see in pedals that employ pre-emphasis/de-emphasis for noise reduction, or simply to alter the tonal balance in some manner.  If you look at the standard non-inverting op-amp, the gain is set by the joint action of the feedback resistance and the ground resistance (from inverting pin to ground).  The op-amp doesn't particularly care if each of those two resistances comes from a single resistor or a hundred.  However, the bandwidth of the op-amp is affected by the joint action of each of those resistances and their associated caps; being the feedback cap in one case and the cap to ground in the other.  If two paths to ground are provided, each through a different value cap, it is possible to provide two separate bandwidth products, one of which can be varied to achieve treble boost..

So, say I have a 470k feedback resistance, and a 10k resistance to ground, through a 0.1uf (100nf) cap.  This will give a gain of 48 and a low end rolloff starting around 160hz.  Suppose I provide a second path to ground, though, using a 2k2 resistor and a .047uf (47nf) cap.  The low-end rolloff of these two parts will begin around roughly 1540hz.  But here's the kicker: the gain for content above that point will be roughly 214!!  That's some kinda treble boost.  The high end will, of course take the path of least resistance...literally, such that you'll have a relatively flat response from 160hz up to just over 1500hz, and a serious boost above that.

You can probably see where I'm going with this.  Make that second resistance variable, and you can futz with the degree and range of high-end boost.  Once again, apart from the pot itself, we've only added a cap and resistor (you'll need a minimum fixed resistor value to prevent instability).

Now, I'm not saying that this is what is in the timmy (if it's Paul's bread and butter, I certainly hope not), but it IS a way to achieve what could be *labelled* as treble and bass functions within the context of an overdrive pedal, without a lot of fuss, space, or fancy design, which was Vanessa's original request.  Is it "treble" or "bass" of the same type found on a car stereo or an amp?  Not at all.  Does it have that same degree of flexibility?  Hah, you wish!  Will it change how your top and bottom end behave in the context of overdrive?  Absolutely.

[Brian Marshall]

here is a really simple tone control... ive used it in a number of circuits including the flying tomato fuzz.


its pretty easy to implement but for best results needs a buffer after it... if you go straight to a volume pot after it, bass response will get loaded down, and the treble control will alter the volume.

it wont add bass or highs, just cut them... but ive had good luck with it for stomp boxes.

[vanessa]

Wow thank you guys for the help. I really did not think you could make such a simple bass/treble setup. The schematics I've seen were more along the Sans Amp type like stm said. Those look cool but take up a lot of space on the board. So like Brett I would just go with a single tone control for a lot of my "Frankenstein" builds.

I'm going to check these out, if anyone can think of other "stupidly simple, bass, treble controls" please chime in.

Paul, I don't want to dissect your pedal. I don't think that has happened here. I also do not believe the question of "who's" pedal that was would have ever have been brought up. If it was I would not have answered that question. I've heard that you do great work and that you're a great guy, in fact you've helped me out once. I built your "Fat Boostered" over at GGG, and really like it, thank you! This is a 'Do it yourself' forum, and I just wanted to know how you can build a bass, treble control like that with so few parts (i.e., do it myself)? You're a member of the forum, maybe you can chime in?

[David Barber]

Another type of dual-paths approach is one you'll often see in pedals that employ pre-emphasis/de-emphasis for noise reduction, or simply to alter the tonal balance in some manner.  If you look at the standard non-inverting op-amp, the gain is set by the joint action of the feedback resistance and the ground resistance (from inverting pin to ground).  The op-amp doesn't particularly care if each of those two resistances comes from a single resistor or a hundred.  However, the bandwidth of the op-amp is affected by the joint action of each of those resistances and their associated caps; being the feedback cap in one case and the cap to ground in the other.  If two paths to ground are provided, each through a different value cap, it is possible to provide two separate bandwidth products, one of which can be varied to achieve treble boost..

So, say I have a 470k feedback resistance, and a 10k resistance to ground, through a 0.1uf (100nf) cap.  This will give a gain of 48 and a low end rolloff starting around 160hz.  Suppose I provide a second path to ground, though, using a 2k2 resistor and a .047uf (47nf) cap.  The low-end rolloff of these two parts will begin around roughly 1540hz.  But here's the kicker: the gain for content above that point will be roughly 214!!  That's some kinda treble boost.  The high end will, of course take the path of least resistance...literally, such that you'll have a relatively flat response from 160hz up to just over 1500hz, and a serious boost above that.

This approach works very nicely at providing texture and eq change when there are clippers in the FB loop, I use that on a few pedals and call it "presence" make the lower freq resistance adjustable and you get more texture control and shape/size of the low end.

I think Mark covered that quite well, so I am pretty much babbling saying "Me Too!"

[PaulC]

Hey Vanessa,

I didn't mean to come across like I did.  I know you weren't going to copy it, and to be honest I'm not that worried about it because it is getting to be an old design.  I was just more concerned that you guys didn't think it was some type of knock off on the reamer circuit that was talked about. 

Hey Mark and David,

    There's another way to look at the pre/post emp circuits beyond what's been talked about.  If you look at how they're done in noise reduction type circuits the pre and post eq'ing are mirror images of each other.  The highs get boosted (or you could say the lows get cut ), and then the opposite happens leaving you with a flat responce again.  In noise reduction this is in the upper khz, but you could bring it down into distortion pedal values.  what's nice about this way is when you're gaining out you have a nice bit of pre/post eq'ing, but when you roll back the gain, or the volume on your guitar, you're back to being pretty much flat - no mid bumps for the clean stuff.  You'd still need some cool eq control after everything, but it does work pretty well as a nice starting point.  You could even be a tricky bastard and make the pre/post eq'ing points selectable.  You could have thicker and thinner sounding distortion, but with it still flat when rolling back the volume.

    Back to work...  PaulC

   

[gaussmarkov]

i would appreciate it if you all would just keep chatting.   

[Mark Hammer]

This is going to be considerably more complicated than what Vanessa started off on, but in response to Paul's last note, imagine you had a resonant boost before clipping.  I like to use this for that gronky Billy Gibbons sound where specific notes get all wolf-ey on me. Now, imagine you had a complementary resonant cut after the clipping stage, and the boost and cut were ganged.  The harmonic content would be note-sensitive, but the output level would be relatively consistent (let me clarify: the harmonic content is always relative to picking strength - it is the degree of *advantage* that notes have which the boost/cut varies). 

Now imagine you could vary the stagger.  Keep in mind that a resonant pre-boost will produce more of certain harmonics for certain notes, and that a resonant cut, post-clip, could be used to adjust the harmonic balance and which harmonics were emphasized for that note range.  With the boost and cut ganged, you could shift which note range you wanted the "extra touch" for.  With a "stagger" control, you could roughly attenuate specific ranges of harmonics for specific ranges of notes.

Something like this is easily implemented with a standard circuit like this one:  http://www.geofex.com/Article_Folders/EQs/parmet.gif
...except that one EQ section gos before the clipping stage and one goes after.

The amount of pre-emphasis/deemphasis (boost/cut) is set by reverse wiring a dual-ganged 10k linear pot (using the example shown).  The global up/down shift is simply a dual-ganged 1meg pot, and the stagger is simply another pot (variable resistor) in series with the 1meg section in the post-clip resonant filter.

[PaulC]

There's another way you can do the mid emp/deemp thing that works pretty well with boosters that use the bypass cap gain control.  Most of those blend in a cap that gives a full freq responce, but you can make it alot smaller forless bass content, and then series an inductor with it to  give it a big mid bump to the gain.  Then follow it with another stage that uses a dual pot (the first side was used for the  emitter/source gain control on the first tranny), and have it parallel the collector/drain with another cap/coil to de emp the mid bump bringing you about flat in the eq curve again.  then follow it with some cool eq controls to mate it up with your amp. 

The goal isn't to have a flat eq responce for the overdrive because that's not the best sounding thing.  What it does is help the design to never sound real bad because there is some pre/post eq'ing going on while still giving you the ability to have a flat clean sound if you want the pedal to be able to be used as a booster - that's what makes it different from what you'd find in something like a screamer - even clean you always have that mid bump.

Later, PaulC

[brett]

Hi.
Very interesting thread.
I like the absolute simplicity of things like Brian Marshall's schematic above.
It reminded me of the minimalist tone control in the 18 watt amp.  You can see it in the schematics at www.18watt.com.
2 knobs, and 3 capacitors give both tone and volume controls.  It intrigues me.
cheers