What is this tone control doing ?

[toddebner]

Newby to this hobby, looking for some help understanding a strange "active" tone control circuit from the DOD FX55b Supra Distortion.  It doesn't appear to do anything when using the pedal.

http://www.geocities.com/toddebner/tonectl.html

Can anyone help me understand what this tone control does at each end of the pot?

Upon first review, my calculations tell me that when the wiper is at the top of the pot (on the linked schematic).  That the Gain is 6, with the low pass at 2.2kHz, high pass at  74kHz (if the 120p/18k parallel is a high pass filter). Conversely, when the wiper is at the bottom,  The gain is 6 with the low pass at 5Hz and the high pass remains at 74kHz.

Therefore, when the wiper is at the top, it should rolloff bass/mid's up to 2,2kHz, right ?

However, then I think,  Is the gain affected by both 3.3k resistors at the same time ?  

And does the filter actually change or is it constant?

Todd :?:

[Mark Hammer]

Happily, I found the one machine at work/school which did not have the calculator applet rendered off-limits, so I can offer some real information.

The "tone control" is an interesting one.  Essentially it alters the gain settings of two parallel paths in a non-inverting amplifier.  If you look at the schematic of a Proco Rat and several other similar pedals that use a non-inverting op-amp for the clipping stage, it is not uncommon to see two parallel ground-leg resistor/cap combinations.

Note that the gain in such amps is set by selectively "bleeding off" feedback signal, thereby pushing the op-amp itself closer towards open loop gain.  (Think of it like having a drunk 19 year-old, with fewer adults and laws to constrain him/her).  The feedback resistance and the ground leg form a voltage-divider/attenuator.  The smaller the resistance in the ground leg, relatve to the feedback resistor, the feedback signal is "bled off" to ground, and the more gain the op-amp will now have.  It's like taking your foot off the brakes.

If several parallel ground legs are used, with different DC blocking cap values, you can shape different gains for different portions of the spectrum.  Keep in mind that the low-end rolloff of the op-amp will be given by the formula F = 1/(2*pi*R*C).   Choosing the cap and resistor values "just right" you can create more gain for the high end than the low end.  What's nice is that the second resistor/cap combo will also produce a certain gain which is flat from the boost point down to wherever.

Let's tinker with this unit a bit.

When the pot is set midway (we'll assume it is linear), we have two ground legs, each consisting of 50k+3k3 and a cap.  On one (.022) side the low end rolloff is set at 136hz.  On the other (10u) side, the rolloff is at less the 1hz.  In each case, the gain is set at 1.35.  So a tiny bit of boost but with the max bandwidth set by the 10uf cap, no rolloff to speak of.

Turn the tone pot hard to the .022 side.  The resistance on that leg is now 3k3.  With a .022uf cap, that gives a gain of 6.45, with a low end rolloff at just under 2.2khz.  The other leg, with a resistance of 103.3k and a 10uf cap, yields essentially a gain of 1 with a rolloff so low we'll say there isn't one.  So, essentially, at the one pot extreme, we have a slight boost above 2.2khz.

Twist the knob the other way.  Now we have a 10uf cap to ground with 3.3k of resistance in front of it.  The other leg has R=103.3k and .022.  The high resistance leg yields a gain of essentially one, with a low end rolloff of 70hz, and the low-resistance leg yields a gain of 6.45 with a low-end rolloff of just under 5 hz.  So, at the one extreme we get a bit of top boost, and at the other end we get full range boost, and in between we get varying mixtures.

Make sense?

[/b]

[toddebner]

So, at the one extreme we get a bit of top boost, and at the other end we get full range boost, and in between we get varying mixtures.
[/b]

Mark,  Thankyou very much.  Now I am able to model it.

This explains why I don't hear much difference on this control.

I have tried adjusting all the values to see what happens.  I'm assuming that I need to increase the gain to have an effect, so I replaced the 3.3k with a 1k on the top loop.  Resulting in a gain of 19 above 7.2kHz.  So I would need to replace the .022 cap with a .047 or .1 to keep the freq down around 3kHz.

If I replace the lower 3.3k with a 1k, I get the boost to 19 as well, but this will provide a full range boost.  

It appears this would have a lowering of the boost as you turn the pot to the middle.  What would this sound like >

Would this give the illusion of a traditional tone control ?

Any suggestions on modding this to make it work more like a traditional tone control?

Thanks.

Todd

[Mark Hammer]

Well, I'm not sure what you want the control to do "more" of.

For instance, let's say you wanted more of a "bite".  Okay, replace the 120pf cap in the feedback loop with 1200pf.  That will give you a rolloff around 7.4khz.  Sounds pretty low, huh?  Okay,  Let's say you replace the 3k3 resistor with 2k2 on the "high" side of the tone control.

Working out our calculations at max treble, that will provide a gain of 9.2 starting around 3.3khz.  Since some of the frequency content above that point will be hiss resulting from the distortion, the 7.4khz rolloff will tame that, while the extra gain in the upper midrange will yield more bite, in theory anyways.  The other extreme of the pot should be little different from before.

Your own calculations and suggestions are good, though. And yes, doubling the cap value may work out well.  If you are going to aim for that extra gain in the high end, though, I would recommend increasing that 120pf cap to some value that will tame the hiss to come from that extra high end gain.

There are lots of permutations and combinations.  Maybe this will give you some ideas.

If you were hoping for it to behave like a BMP tone control, though, it won't.

[toddebner]

Well, I'm not sure what you want the control to do "more" of.

Hi Mark,  Thanks for the input, I'm really starting to understand this.

What I'm looking for is how to use this control in context to the rest of the circuit.  Which is linked below.  

http://home.comcast.net/~eric.erickson/DODFX55b.jpg

This DOD FX55b really gets harsh at more than 9 o'clock gain with loss of Bass.  

I want to make it more of a smooth overdrive if possible  (I know, it would be faster, easier, and probably cheaper to use a different pedal, or build one myself, but I'm using this project to learn with).

I've found that if I mod the gain stage to reduce the upper gain to 400 / 500  with a low pass of 159hz and a high pass at 2.7khz(at top gain level).

Now, I'm trying to make the tone control usable as it doesn't appear to do anything.

This tone control comes after the Gain/Drive stage and a back-to-back diode to ground hard clipper.

Therefore, if I'm grasping your teaching, This tone control will give a slight treble boost.  

Implementing your suggestions, should give a little more treble boost and roll off the hissing.  I like that.

Given, that there is distortion coming from the gain stage and the back to back diodes, would you change your suggestions on the high cut or others.

Thanks,

Todd

[Mark Hammer]

I have a very different looking schem for the FX55B than you link to.  Though the one I have looks more like a factory schem than yours does, the values shown in yours make far more sense, so I'm going to use yours for the moment

Cap C2 shares the same problem that the identical cap does in the MXR Dist+ and the DOD250, namely that it cuts out bottom at max gain (rolloff around 338hz by my reckoning).  In conjunction with the 3.9M resistor, the 120pf cap provides a highend rolloff around 340hz.  As Alfred E Neuman would say, "Blecch!".

To "smooth out" this pedal, consider doing the following:

1) Change the 3.9M resistor to 1Meg, and the 120pf cap to 100pf.  
2) Change the 470R resistor to 2k2.

This will give a gain range of about 3 to 450, and a hi/lo rolloffs at max gain of 72hz and  1590hz, more in the zone of what you likely want.

The tone control shown here actually works quite differently than the one you showed earlier, so we are sort of back to square one.

In this configuration (assuing it is correct), at full counterclockwise, the 100k pot presents a 0 ohm path between C8 (.1uf) and pin 6.  That puts the .1uf cap in parallel with the 120pf one, giving a capacitance of .10012uf in parallel with an 18k feedback resistor, and a high-end rolloff (6db/oct) of  88hz...assuming it is correct.

Personally, I like the other "wrong one" better.  Maybe we better verify which circuit-schem is the correct one before getting too involved.

[toddebner]

Personally, I like the other "wrong one" better.  Maybe we better verify which circuit-schem is the correct one before getting too involved.

First of all, thankyou for all the in-depth help on understanding this.

Second,  I verified the circuit, the closest one to my pedal is the the full schematic that I linked to.  Just to be clear here is the link again.

http://home.comcast.net/~eric.erickson/DODFX55b.jpg

I had already succombed to replacing c8 (.1uf) with a jumper as it didn't make any sense to me.  Then we're back to the original tone control design.

(My pedal is different from the linked one, in that it has 3.3k resistors for R5 and R6.  These are the same values in the first tone control link I posted).

Regarding the gain stage, the pedal actually uses the same board as the DOD fx50 , so there is an empty place to place a jumper from the pot.  So  I'm able to connect the wiper to pin 2, the counterclockwise terminal to the lowpass filter (470ohm/1uf) and the clockwise terminal now connect to Pin 1.

I then change the R3/470ohm to 1k and voila, I get 1-500 gain, with 3-159 lowpass rolloff and a high pass going from 13khz(no gain) to 2.7khz(max gain).

This is getting fun, I'm really starting to get this stuff.

So now on to the tone.

Assuming that I put a jumper across c8, I can adjust the treble end of the tone control to get a treble boost, and then adjust the C4/120pf cap to reduce the rolloff as you suggested to eliminate the distortion hissing.

Can I do something with the counterclockwise rotation of the tone to reduce treble/boost midrange or bass?

Can I put an inductor in the circuit to create a high pass circuit that as the pot resistor increases toward the low side, the circuit passes higher and higher freqencies and boosts the mids and lows

I haven't seen this kind of application in any of the stompbox schematics, just trying to understand.

Thanks again, Todd

[Mark Hammer]

I then change the R3/470ohm to 1k and voila, I get 1-500 gain, with 3-159 lowpass rolloff and a high pass going from 13khz(no gain) to 2.7khz(max gain).

You had me a little confused there for a sec, because of your terminology, but I think I get it now.  Changing R3 from 470R to 1k will make the min resistance on the ground leg 1k, which, in conjunction with a 1uf cap, will give a low-end rolloff of 159hz at max gain.  This would more properly be referred to as "highpass", since it permits content above 159hz to be amplified more (it IS a rolloff, not a brick wall, after all).  NOTE, however, that whatever this RC combination does to low-end rolloff and gain, it does nothing to HIGH end rolloff.

Leaving R23 (3.9M) intact still permits a gain which is (on paper) ridiculously high.  In conjunction with 120pf in parallel capacitance, the high-end rolloff this produces starts around 340hz and attenuates more above that.  

My feeling is that you need to start with the bandwidth of the clipping stage before knowing what needs to be done to the tone stage to "idealize" it.  Again, if it were me, and if my goal were to smooth out the sound a bit, I'd reduce the gain of the clipping stage, and hoist the high-end rolloff.  In some respects, this circuit cries out to be turned into a Distortion+.  How could you do this?  Easy.  Change the 3.9M resistor to 1M.  Change the 120pf cap to 47pf.  That will set max gain at 1001 (min at around 3) and high-end rolloff around 3.3khz.  Lastly, eliminate D1 and D2.  I suppose the dual sets of diode pairs one often sees in circuits like this serve a purpose, but it seems odd to clamp the signal in a gain stage at an amplitude set by diodes, and then feed that output past another pair of diodes with the same clipping threshold.  Somebody needs to explain to me how this yields any appreciable change in degree of clipping.

Far better, I would think, to replace D1/D2 with a back to back pair of red LEDs.  These would have a higher clipping threshold, hence produce less (and some say smoother) clipping in the gain stage.  Most importantly, though, the output of this modified stage would be hot enough to yield additional clipping from D3/D4.  My guess is that this combo will start to sound a bit like a Boss DS-1, albeit with a different sort of tone control.

Re: C8  If you jumper it, then at one extreme of rotation, that path will be a zero ohm feedback resistance, making your tone stage a unity gain op-amp and having NO effect on tone (as well as no boost, which it normally has a bit of).  Better, then, to lift C8 if you want to monkey with the tone, to bring the circuit back to the topography we saw in the first circuit fragment you linked to.

Finally, the schematic you link to shows a rather interesting switching scheme.  Normally, the "flip" and the "flop" of a flip flop are used to alternately lift or complete the path of the dry and effect signal to the output buffer.  You will see here that the two FETs used for switching are tied to the same part of the flip-flop, so that the two FETs both go high and low together.  How does this switch?  When Q1 gets turned on, that provides a low resistance path from the input buffer (U1a) to the output buffer (U1b).  When Q2 gets turned on and goes lowe resistance, this forms a low resistance path to ground in conjunction with R9 (220k), that makes up the ground leg of a voltage divider formed by R4 and Q2/R9.  So, in bypass mode, it gets easier for the clean input to make its way to the output, and harder for the much of the distortion signal to get to the output.  My guess is that you may still hear some "hair" from the distortion even in bypass mode because that attenuation is not perfect.

There.  That's enough to chew on for a Sunday morning.

[toddebner]

this circuit cries out to be turned into a Distortion+.  How could you do this?  Easy.  Change the 3.9M resistor to 1M.  Change the 120pf cap to 47pf.  That will set max gain at 1001 (min at around 3) and high-end rolloff around 3.3khz.  

...replace D1/D2 with a back to back pair of red LEDs.  These would have a higher clipping threshold, hence produce less (and some say smoother) clipping in the gain stage.  Most importantly, though, the output of this modified stage would be hot enough to yield additional clipping from D3/D4.  

lift C8 if you want to monkey with the tone, to bring the circuit back to the topography we saw in the first circuit fragment you linked to.

Over the weekend, I did these mods and the pedal sounds soooooo much better.  Much smoother with the distortion and the upper ends of the gain.

In regards to the tone circuit, I took your advice and pulled the C8 cap, then changed c7 to .047, r5 to 2k2, and c9 to 1220pf.  This gives me a "High-end" rolloff of 7.2kHz and a low-end rolloff/high-pass that ranges from 5Hz @ 6.45 gain to 1.5kHz @ 9 gain.

Did I get the terminology straight?

The tone control now gives a treble boost at cw and a full range boost at ccw.

Just curious, what is the impact to tone at the middle of the tone control ?  I have gain of 1.34 with a low-end rolloff of 65hz.  Does this have any effect on the sound that we would hear ?

[Mark Hammer]

First, pleased to hear you like the changes.  I consider it an honour to improve the tone of any musician in "music central" (Austin).

Second, your terminology is spot on.

Third, the revised tone control really only adjusts how much treble boost there is.  It doesn't provide any cut.  The *old* one with C8 did, however.  Perhaps you may wish to experiment with other smaller values of C8 to get a wee bit of treble taming.

[toddebner]

First, pleased to hear you like the changes.  I consider it an honour to improve the tone of any musician in "music central" (Austin).

Thankyou, it's good to be considered a musician!!

Perhaps you may wish to experiment with other smaller values of C8 to get a wee bit of treble taming.

Ok, I'll give it a look, the high end of the tone, does give a bit too much treble.

However, I don't understand how c8 influences the tone circuit?  at one end, c8 ends up being in parallel with c9.  (Does it impact r6 & cll?

At the other end, c8 is in series with the 100k pot, and both of them are in parallel with c9.

How do I calculate high/low pass frequencies with c8 in the mix ?

[Mark Hammer]

Quite frankly, and maybe it would improve with a redraw, the tone control circuit is pretty idiosyncratic and does not make a helluva lot of sense to me.  That being said, if the extreme "bass" side involves placing C8 in parallel with C9, then it stands to reason that changing C8 for somewhat smaller or larger values would yield different effects on the tone.

[nbabmf]

I had an FX50-B Overdrive Plus come in for modding.  The owner said he didn't care for the way the tone control worked -- too woofy below 11 o'clock and too shrill above 2 o'clock.  He also didn't care for the excessive crunch on the boost end of the drive control.  Gotta love when they use a linear taper where a log taper belongs to fool the consumer into thinking there is a boost!  That's not a very large range to be confined too, so I did my research... which led me to this thread!

I did a lot of tinkering this afternoon, and here's what I ended up with:

D1 & D2 - pair of silicon diodes in series (instead of one)
R3 - 1k (bigger)
C8 - jumper (instead of 0.1uF)
C9 - 100pF (smaller)

Jumpering C8 killed the boost in the tone control stage, but it seemed to make the pedal a lot less obnoxious.  The owner was pleased with the result.  I would have liked more signal boost lower on the drive control, but that's just me. I think auditioning different values for C8 would likely yield a happy compromise.