Question about tone control frequencies...

[Projectile]

Okay, I've read the GeoFX "Technology of the Tubescreamer" article as well as the "AMZ tone control mods" article and I'm a little confused about the interaction of the various components.

Here is a link to the article for reference: http://www.geofex.com/Article_Folders/TStech/tsxtech.htm

 The calculation for the rolloff of the 1K resistor and .22uf cap to ground immediately after is  gain stage is stated as being 723Hz. That makes sense. Now, the boost stage is where it gets hairy. The GeoFX article claims that the R-C rolloff calculation for the cap in the feedback network is 3.2KHz when the filter is all the way open. He gets this number by calculating the combination of 220ohm resistor to ground and the .22 cap attached to the wiper, and I've also seen this same calculation repeated before here on these forums with the same numbers. That's what I don't get. Don't you have to include the 1K resistor that is also in the path of the feedback network to get the rolloff value? So wouldn't the rolloff actually be 593Hz? Could someone please explain to me why the 1K resistor is not included in the calculation for the R-C filter? I thought I was starting to understand this stuff, but now I am really confused.

It seems to me that it would make sense to if the rolloff was actaully (1K+220ohms)/.22uf which equals 593Hz, not 3.2KHz like the article explains, because that would much better explain the midrange hump in the Tubescreamer that everyone is always complaining about. If the first cap is rolling off everything above 723Hz, and then the feedback network is boosting everything above 593Hz, then that means you would get a big midrange hump between 593 to 723Hz.  That makes a lot more sense to me than a boost above 3.2KHz, which would leave a midrange dip between 723 and 3.2k when the tone control is maxed, which I don't hear at all in the tubescreamer.

Going on this assumption, then to get an even frequency response you just get rid of the 220ohm resistor to ground on the wiper. Now you have a 1K/.22uf=720Hz rolloff, followed by a 1k/.22=720hz boost, and they should theoretically cancel each other out!

So, I lost the 220 resistor and put the whole tone section on a bypass switch so I could flip between them. Guess what... with the tone control maxed and the 220 ohm resistor gone it sounded almost identical to the sound of the whole tone section being completely bypassed.  Hmmmm...  Not satisfied yet, I used a synth to run some fast frequency sweeps through the pedal and hooked it up to a spectrum analyzer. Yup, with the 220 resistor gone the response is nearly flat as a rail.

So, the for final answer to the question "How do you get rid of the mid-range hump in the Tubescreamer": jumper the 220ohm resistor in the feedback network, and of course, also do the typical fat mod to the gain stage. It seems that the whole tone stack on the tubescreamer is designed specifically to be midrange booster. The initial high frequency rolloff when you start to turn the tone knob down is just a consequence of boosting even lower midrange which sounds like a high cut when the gain of the whole stage is 1. That's why there is never a real treble boost or scooped sound with the tubescreamer. It just goes from more to less midrange honk.  I guess that's part of it's charm.


So is the GeoFX article wrong or am I just crazy? Ugghhh this is frustrating! Why do I have such a hard time understanding all of this stuff? Please help me understand this better.

...

Ohh, and one more question: How do you calculate the rolloff frequency for the small 51pf cap in the feedback loop of the clipping stage. I just don't understand how that works at all. Don't you need a resistor to ground somewhere after the cap in order to get a high pass filter?

[MohiZ]

Don't you have to include the 1K resistor that is also in the path of the feedback network to get the rolloff value? So wouldn't the rolloff actually be 593Hz? Could someone please explain to me why the 1K resistor is not included in the calculation for the R-C filter?

The 1K resistor is between the output and the negative input of the op-amp, and that's why I'm inclined to think it only sets the gain of the stage and does not effect the frequency response. The tone control is in between the negative and positive inputs. I'm sure you noticed it, but I think the current flows towards the output of the opamp and that's why the 1K resistor would be "after" the tone control in the circuit, and thus wouldn't affect the roll-off frequency of the filter.

Ohh, and one more question: How do you calculate the rolloff frequency for the small 51pf cap in the feedback loop of the clipping stage. I just don't understand how that works at all. Don't you need a resistor to ground somewhere after the cap in order to get a high pass filter?

The 51p resistor lets more highs through. That means the negative feedback for highs is bigger, and thus highs are attenuated. It is explained in the article. You can in your mind think of the Drive pot, the 51k resistor, the diodes and the 51pf cap as a single resistor with a fixed impedance at any given frequency, and the 0.047uF cap and the 4K7 resistor as another. The impedance of the first resistor is the parallel combination of the cap, the diodes and (drive pot setting + 51k in series). The impedance of the second imaginary resistor would be the series combination of the 0.047uF cap and the 4K7 resistor. These two imaginary resistors form a voltage divider between the output and ground. Obviously, the impedance changes with frequency.

I'm hoping someone else can phrase this in a more understandable way 

[Projectile]

Don't you have to include the 1K resistor that is also in the path of the feedback network to get the rolloff value? So wouldn't the rolloff actually be 593Hz? Could someone please explain to me why the 1K resistor is not included in the calculation for the R-C filter?

The 1K resistor is between the output and the negative input of the op-amp, and that's why I'm inclined to think it only sets the gain of the stage and does not effect the frequency response. The tone control is in between the negative and positive inputs. I'm sure you noticed it, but I think the current flows towards the output of the opamp and that's why the 1K resistor would be "after" the tone control in the circuit, and thus wouldn't affect the roll-off frequency of the filter.

I was under the impression that in a feedback network the current flows from the output of the opamp into the negative input (isn't that why they call it feed-BACK), which means it would have to pass through the 1K resistor before the cap to ground which I would think would have to be included in the calculation.  The current definitely isn't coming from the lug connected to the positive input of the opamp because I can disconnect that lug entirely and the tone control functions relatively the same for the first 1/4 turn of the knob.  Current wouldn't be flowing OUT of the negative input would it? That's not how I understand opamps work. Also none of this explains why the tone control magically becomes "ruler flat" when I remove the 220ohm resistor, which only makes sense if both networks are rolling off at the same point, 723Hz, one being a cut and the other a boost.

With AC, does current really "flow" in a particular direction anyway?  Doesn't it just oscillate back and forth, the output of the opamp being what "forces" that oscillation at the output based on what it "sees" at the positive and negative inputs? The only sense of "direction of flow" would be if you placed component "A" in front of that output, everything after that particular component will see the result of that component's effect on the current that is being oscillated by the output of the opamp.  In this case, the 1K resistor would be between the "force" of the output and the cap to ground, so it would have to be included in the calculation, right?

This is my first pedal build and I feel very strange having to argue with people that are obviously much more knowledgeable than I am, but something is telling me that this is not correct.   

Ohh, and one more question: How do you calculate the rolloff frequency for the small 51pf cap in the feedback loop of the clipping stage. I just don't understand how that works at all. Don't you need a resistor to ground somewhere after the cap in order to get a high pass filter?

The 51p resistor lets more highs through. That means the negative feedback for highs is bigger, and thus highs are attenuated. It is explained in the article. You can in your mind think of the Drive pot, the 51k resistor, the diodes and the 51pf cap as a single resistor with a fixed impedance at any given frequency, and the 0.047uF cap and the 4K7 resistor as another. The impedance of the first resistor is the parallel combination of the cap, the diodes and (drive pot setting + 51k in series). The impedance of the second imaginary resistor would be the series combination of the 0.047uF cap and the 4K7 resistor. These two imaginary resistors form a voltage divider between the output and ground. Obviously, the impedance changes with frequency.

I'm hoping someone else can phrase this in a more understandable way 

That actually helped a lot. It was very understandable. Thank you Mohiz.

I guess my questionis then: How do you calculate the rolloff frequency off a RC network with a resistor and capacitor in parallel? I tried doing a google search but i didn't find much that was helpful.  I see these parallel resistor capacitor filter networks in feedback loops all the time, but I've never seen any information on how they are calculated. I know this particular example becomes exceedingly complex, but let's just say we look at it in one particular state. With the diodes at full clip you can eliminate them, and let's say you have the pot at full gain, so that would be 551K in parallel with 51p in combination with 4.7K to ground. Now if I could figure out how to calculate that particular state I could start shifting numbers around to estimate different states, which would help me greatly to wrap my head around it. Anyone care to point me in the right direction? I appreciate the help. 

[SISKO]

You calculate de Frec cutoff in all the filters the same ways. The only thing that changes is the type of filter being them: Parallel, series, PIs, etc. Then, in a subcategory you have: Hig pass, low pass, band pass and stop band

[Projectile]

You calculate de Frec cutoff in all the filters the same ways. The only thing that changes is the type of filter being them: Parallel, series, PIs, etc. Then, in a subcategory you have: Hig pass, low pass, band pass and stop band

Okay, but how do I apply that formula to find the cutoff frequency of a filter that consists of a resistor and capacitor in parallel? I already did a google search and didn't come up with anything useful. If I already knew how to apply the R-C formula to this type of filter I wouldn't be asking. At least a link to some specific information would be useful.


The formula I am familiar with is   Fc = 1/ (2 . pi . R . C)

Do I just plug the parallel resistor into R with the parallel cap in C?


SO essentially this:   in-----l C l------------out
                                                |
                                                z 
                                                R                                           
                                                z
                                                |
                                                V


is calculated the same way as this?

                             in-------------------l C l-------------
                                              |                            |
                                              |                            |
                                              --------W R W--------------------out

[aziltz]

I was under the impression that in a feedback network the current flows from the output of the opamp into the negative input (isn't that why they call it feed-BACK), which means it would have to pass through the 1K resistor before the cap to ground which I would think would have to be included in the calculation. 

i learned early on to stop thinking about which way the electrons run...  because in all truth, the little buggers run the other way!

just thinking outloud here, this isn't meant to be a full answer or a lesson,
the opamp works by changing the output so the inputs match, so the 1k resistor is just showing the minus input a signal that's "so much" less than the output...  its the upper leg of a voltage divider, except that the bottom leg is frequency dependent (depends on the tone pot setting essentially.)  So i think the best way to thinking about that circuit is that it provides frequency dependent gain (or attenuation!). 

Set to one side, it filters out the highs going into the minus/feedback input, this translates to a boost of those frequencies at the output, and because of the filter cutoff, it effectively undoes the low pass filter that comes between the opamps.  At the opposite setting, it lets the minus see the full frequency response, so the output already has the highs cut from the low pass filter between the opamps.

Jack Orman (AMZ) does a great discussion of this tone control, as well as RG.

[Projectile]

I was under the impression that in a feedback network the current flows from the output of the opamp into the negative input (isn't that why they call it feed-BACK), which means it would have to pass through the 1K resistor before the cap to ground which I would think would have to be included in the calculation. 

i learned early on to stop thinking about which way the electrons run...  because in all truth, the little buggers run the other way!

just thinking outloud here, this isn't meant to be a full answer or a lesson,
the opamp works by changing the output so the inputs match, so the 1k resistor is just showing the minus input a signal that's "so much" less than the output...  its the upper leg of a voltage divider, except that the bottom leg is frequency dependent (depends on the tone pot setting essentially.)  So i think the best way to thinking about that circuit is that it provides frequency dependent gain (or attenuation!). 

Set to one side, it filters out the highs going into the minus/feedback input, this translates to a boost of those frequencies at the output, and because of the filter cutoff, it effectively undoes the low pass filter that comes between the opamps.  At the opposite setting, it lets the minus see the full frequency response, so the output already has the highs cut from the low pass filter between the opamps.

Jack Orman (AMZ) does a great discussion of this tone control, as well as RG.

Yes, this is all true, but it completely misses the point. What is in question here is not a general idea of how the filter works, but where the rolloff points specifically fall on the frequency spectrum when the filter is all the way open. GeoFX claims that the feedback network boosts frequencies above 3.2Khz, bringing back up most of the treble that the first 720Hz filter rolled off. I'm saying that this appears to be wrong and that the feedback network actually boosts above 593Hz, which falls BELOW the frequency of the first filter. This means that you get a broad midrange hump around 650 that rolls off gradually in either direction and then steeply in the highs. This is the characteristic midrangey sound of the tubescreamer. it never goes away because you never really boost the highs, just remove the hump by first boosting even lower frequencies and then rolling off more treble as you roll the knob back. The most important part of the midrange honk of the tubescreamer tone stack IS that 220 ohm resistor. If you jumper that resistor the tonestack appears to go flat. you get -720Hz rolloff and then +720Hz boost and the midrangy honk is gone.

The AMZ discussion is great if you are looking for alternatives to the original design but he also seems to entirely miss what is going on with that resistor. The article doesn't state anything wrong like the GeoFX article, but it kind of ignores that resistor's function, which is that it turns the whole tonestack into a midrange honk machine. The tonestack isn't really useful for anything else, so if you don't like midrange, then you are probably better off using one of his alternatives.

I've read a lot of articles an posts discussing the inner workings of the tubescreamer and the issue of it's midrangeyness comes up a lot, but I kind of shocked that no one has ever pointed out that you can make the tubescreamer flatten out almost entirely by just jumpering that resistor. I've done frequency sweeps through the pedal into a spectrum analyser and this appears to be exactly what is going on.

At least it appears that way...

But Really I'm a total newb at all this and it bothers me that my understanding of this circuit would contradict the published information that so many people are familiar with. What I really need is just someone to answer either, "Yes, that is correct the GeoFX article is wrong," or  "No that's not how it works, here is where your confusion is" and give me a clear and specific correction.

I am very interested in audio electronics, but I don't have anyone else to help me with this stuff, so I have to rely on forums like this one when there is a contradiction in my understanding. It is very important that I clear it up, so I can move on. I guess I really don't understand why that is so difficult. This isn't rocket science. They're just simple little audio circuits, right? But for some reason whenever I ask a specific, precise question on this forum just to help clarify my rudimetary understanding,  all I ever get are really vague and dismissive answers and it's is very frustrating. I just don't get it. I've never had this experience on any other internet forum. I come on here and I see all this really knowledgeable people designing and building really amazing things, so I know my questions must be simple and elementary, but it's so difficult to get any help or direct answers here. I've honestly had better results posting my electronics related questions on other, non-electronics related music forums than I have here, which I find very odd. The whole thing leaves me rather frustrated and makes me feel like I'm the butt of some kind of joke here. WTF? Am I just a complete idiot or something?

[aziltz]

this could easily be put to rest if someone would/could simulate that thing in spice.  I unfortunately don't have the skills yet.

sorry i missed your point the first time.  In the back of my mind i was thinking that the mid hump is set by the fixed low pass at 720Hz and the Cap to ground off the feedback of the CLIPPING amp, only because that's where the BYOC Overdrive II has its Mid Hump/Flat/Full Mod.


although, if the fixed roll off is >720Hz, and the Tone Control Boosts from 593Hz and up, that would "raise" the Mid Hump with the Tone Control wouldn't it?

I'm just as intrigued/confused as you are.  I grade for a undergraduate electronics class currently, and in 2 weeks we will be learning Spice in-depth.  I plan on adding this to my basic dissection of the Tube Screamer and other Standard Circuits.

Whether this is right or wrong or just one big grey area, thanks for bringing it up and pointing it out.

[Projectile]

this could easily be put to rest if someone would/could simulate that thing in spice.  I unfortunately don't have the skills yet.

sorry i missed your point the first time.  In the back of my mind i was thinking that the mid hump is set by the fixed low pass at 720Hz and the Cap to ground off the feedback of the CLIPPING amp, only because that's where the BYOC Overdrive II has its Mid Hump/Flat/Full Mod.


although, if the fixed roll off is >720Hz, and the Tone Control Boosts from 593Hz and up, that would "raise" the Mid Hump with the Tone Control wouldn't it?

I'm just as intrigued/confused as you are.  I grade for a undergraduate electronics class currently, and in 2 weeks we will be learning Spice in-depth.  I plan on adding this to my basic dissection of the Tube Screamer and other Standard Circuits.

Whether this is right or wrong or just one big grey area, thanks for bringing it up and pointing it out.

Sorry aziltz, that little rant wasn't directed at you. I appreciate your help. This forum just confuses me. I think I just need to go back to school. I'm probably too busy looking for things on the internet that would probably be better learned with the help of a teacher in a classroom.


Just to boil this down to two questions for anyone following this thread:

-Is the 1K resistor in the feedback network of the tubescreamer tone control calculated in combination with the 220 ohm resistor to ground, which sets the rolloff frequency of the .22 cap on the wiper below 720kHz when the tone control is turned all the way up? (This would appear to make the well circulated GeoFX article incorrect)

-If not, then why isn't the 1K resistor included in RC filter calculation?

 ...Note that according to my rudimentary testing, simply jumpering the 220ohm resistor to ground with the tone knob turned all the way up appears to flatten out the signal entirely, making it sound like the tone stack has been bypassed. This would seem to confirm that the 1K resistor is in fact part of the RC filter network and the 220 ohm resistor just offsets the rolloff frequency down compared to the previous 720hz rolloff, where without the 220 ohm resistor the opposite 1K/.22ohm cut and 1K/.22 boost would just cancel each other out.

Thanks.

[MohiZ]

Okay, but how do I apply that formula to find the cutoff frequency of a filter that consists of a resistor and capacitor in parallel? I already did a google search and didn't come up with anything useful. If I already knew how to apply the R-C formula to this type of filter I wouldn't be asking. At least a link to some specific information would be useful.


The formula I am familiar with is   Fc = 1/ (2 . pi . R . C)

It becomes very complicated to calculate it precisely, and that's why it's most often approximated, like so many other things in electronics  The low roll-off freq is approximately Fc = 1/ (2 . pi . R1 . C1) = 720 Hz and the high roll-off is Fc = 1/ (2 . pi . R2 . C2) = 5660 Hz, where in this circuit R1=4K7 C1=0.047uF R2=551K C2=51pF. In truth, the roll-off frequency is where the impedance of the imaginary resistors I was talking about in my earlier post is equal. That is, the parallel combination of R2 and C2 is equal to the series value of R1 + C1. This becomes an equation which I couldn't calculate  But in the same way, the roll-off of a single pole RC filter is the point where the impedance of the cap and resistor are the same, R = 1/(2 . pi . f . C), which can be re-arranged (or whatever, English is not my native tongue) to the familiar f = 1/(2 . pi . R . C)

Please read this, this is elaborated more at the end of the article: http://gaussmarkov.net/wordpress/parts/op-amps/op-amps-4-divided-negative-feedback/

With AC, does current really "flow" in a particular direction anyway?

There's really no current flowing in the input of the op-amp. It just reacts to voltage changes, as you said. Actually, I was talking about the DC current, in audio circuits you can think of the DC conditions and AC conditions as two separate things. But maybe you are right about that.
Nevertheless, if you take into account the 1K resistor, then on the same basis you should also take into account everything that comes after the op-amp as well. BUT, the whole point of an op-amp is to have infinite input impedance and zero output impedance. What comes after the op-amp shouldn't affect what is before it. Maybe I just trust R.G. Keen's knowledge too much but I think there is no mistake in his article.

But for some reason whenever I ask a specific, precise question on this forum just to help clarify my rudimetary understanding,  all I ever get are really vague and dismissive answers and it's is very frustrating. I just don't get it. I've never had this experience on any other internet forum. I come on here and I see all this really knowledgeable people designing and building really amazing things, so I know my questions must be simple and elementary, but it's so difficult to get any help or direct answers here. I've honestly had better results posting my electronics related questions on other, non-electronics related music forums than I have here, which I find very odd.

I think most people here are not really that much electronics gurus than they are effects gurus. Many people just learn this stuff by trial and error and by substituting different parts and getting insight at what everything does. It's all so imprecise anyway so your ears are your best guides. But as far as understanding the electronics go, nothing beats proper education and I've learned lots more from this basic electronics course that I'm attending than I've learned reading various pages over the Internet.

Anyway, I think this is a very interesting point you've made and I'm sure someone comes up with a definitive answer. I totally believe you when you say the freq response becomes flat by jumpering the 220 ohm resistor, but I'm just not sure I agree as to why it does so. Thanks for this thread! 

[aziltz]

after sleeping on this, i might have something worthwhile to suggest.

I'm not convinced that the .22uF and 220Ohm act as a low pass entirely, and RG's article argues this as well.  The gain of the non-inverting op amp is 1 + R2/R1 where R1 = 1K, and R2 = .22uF + 220Ohm, which means the gain is frequency dependent (we knew this).  What I'm trying to suggest though, is that this cannot be explained as a low pass filter exactly.  YES, the cut off frequency of R2 (Z2 really) is calculated the same way as any other filter, but its slightly more complicated than a simple roll off.  You could calculate the frequency response using complex impedance i suppose.  Let me rephrase, I plan on doing that calculation eventually when I'm trying to document some stuff for this circuit, but not today, sorry.

The other thought that just occurred to me, i misstated earlier that this action "undoes" the .22uF/1k roll-off, and that is incorrect.  It acts to level it off above 3.2kHz, yes, but frequencies between 720Hz and 3.2kHz are still only affected by the .22uF/1k.  THAT is our mid-hump, I believe.  It never goes away, since the active tone control only affects the content above 3.2kHz.

I do however, agree with RG's article when it describes the other end of the tone control.  The 20K Pot serves to isolate the .22uF and 220o from the feedback, and it provides additional roll off, pre-op-amp, above 3.2kHz.  He does not go into detail about the gain of the op-amp in this situation, which I think would be helpful to the reader, but i have a feeling it gets reduced to 1.  Again, I'm not 100% on this.


Fun discussion.  Again, I'm not trying to answer you entirely, I'm just learning this stuff currently myself.

One thing to keep in mine i think is, the the Clipping stage adds gain above 720Hz, and a lot of upper frequency content, so while a tone control or stack might seem like its limiting things a lot, its actually just reigning in the harmonics so that things aren't harsh.  I struggled with that for a while.

[Projectile]

after sleeping on this, i might have something worthwhile to suggest.

I'm not convinced that the .22uF and 220Ohm act as a low pass entirely, and RG's article argues this as well.  The gain of the non-inverting op amp is 1 + R2/R1 where R1 = 1K, and R2 = .22uF + 220Ohm, which means the gain is frequency dependent (we knew this).  What I'm trying to suggest though, is that this cannot be explained as a low pass filter exactly.  YES, the cut off frequency of R2 (Z2 really) is calculated the same way as any other filter, but its slightly more complicated than a simple roll off.  You could calculate the frequency response using complex impedance i suppose.  Let me rephrase, I plan on doing that calculation eventually when I'm trying to document some stuff for this circuit, but not today, sorry.

The other thought that just occurred to me, i misstated earlier that this action "undoes" the .22uF/1k roll-off, and that is incorrect.  It acts to level it off above 3.2kHz, yes, but frequencies between 720Hz and 3.2kHz are still only affected by the .22uF/1k.  THAT is our mid-hump, I believe.  It never goes away, since the active tone control only affects the content above 3.2kHz.

I do however, agree with RG's article when it describes the other end of the tone control.  The 20K Pot serves to isolate the .22uF and 220o from the feedback, and it provides additional roll off, pre-op-amp, above 3.2kHz.  He does not go into detail about the gain of the op-amp in this situation, which I think would be helpful to the reader, but i have a feeling it gets reduced to 1.  Again, I'm not 100% on this.


Fun discussion.  Again, I'm not trying to answer you entirely, I'm just learning this stuff currently myself.

One thing to keep in mine i think is, the the Clipping stage adds gain above 720Hz, and a lot of upper frequency content, so while a tone control or stack might seem like its limiting things a lot, its actually just reigning in the harmonics so that things aren't harsh.  I struggled with that for a while.

Interesting...

I think you are right that the interaction of the feedback network is probably more complicated than I have explained it, but it still doesn't explain what happens when you remove the 220 ohm resistor. I would suggest anyone to just try it. It takes five minutes to breadboard the tone control circuit. You don't even need the pot. Just use a resistor in it's place since we are only discussing the tone knob at full position. Then, put the whole thing on a dpdt bypass switch. If you jumper the 220 ohm resistor, and flip the switch back and forth, the two signals are remarkably similar. I can't say that they are exact, because there is always going to be some interactions when you are filtering and boosting signals like that, but it sounds nearly the same.

The only way I can figure out how to describe this is to say that the 1K resistor and .22uf cap act as a 720Hz rolloff in the feedback network, which essentially boosts the same frequencies that were just cut earlier by the same amount, giving a flat response to the whole filter network.  When I run sine sweeps through the circuit and look at it on a freqency spectrum analyzer I see the same thing: it looks ruler flat. When I put the 220 ohm resistor back in the circuit it is a little bit harder to see exactly what frequency is the center of the hump because the slope is so gradual, but I am definitely seeing a broad hump and it's peak is definitely much lower than 3.2Hz. What I am clearly NOT seeing is what is described in the GeoFX article. If frequencies were being rolled off below 720Hz and brought back up above 3.2Hz, then I would expect to see a dip in the frequency content between 720Hz and 3.2Hz. I am not seeing any kind of dip like that on the plot, and I don't hear anything like that either. The content between 720Hz and 3.2Hz would be the bulk of what most people consider "mid-range" content (I usually consider mids to be 300Hz-5KHz, but I couldn't find a standard definition), so it should clearly sound like a mid-range scoop if the filter works as described in the GeoFX article. I've never heard anyone describe the Tubescreamer as having a mid range scoop, period. It's always been known for it's mid-range presence.

The only thing I can figure is that the GeoFX article is wrong, which baffles me because that article has been so widely circulated that I would think that by now someone would have caught the error. I really wish I could just get someone to confirm this so I can move on. Where are all the "expert" engineers here that can calculate R-C filters in their sleep?  I get the distinct feeling that I am being ignored by the folks who could answer this easily. I can't blame them. They are probably sick of reading threads about tubescreamers, but thank you to those who have tried to help. It has been much appreciated.

[aziltz]

What I am clearly NOT seeing is what is described in the GeoFX article. If frequencies were being rolled off below 720Hz and brought back up above 3.2Hz, then I would expect to see a dip in the frequency content between 720Hz and 3.2Hz. I am not seeing any kind of dip like that on the plot, and I don't hear anything like that either. The content between 720Hz and 3.2Hz would be the bulk of what most people consider "mid-range" content (I usually consider mids to be 300Hz-5KHz, but I couldn't find a standard definition), so it should clearly sound like a mid-range scoop if the filter works as described in the GeoFX article. I've never heard anyone describe the Tubescreamer as having a mid range scoop, period. It's always been known for it's mid-range presence.

The 720Hz cut off is a low pass, meaning it cuts frequencies above 720Hz.  Our confusing tone control only acts to control things above 3.2kHz.  Considering the gain of the clipping stage is 1 below 720Hz and higher above, everything running into the tone section has a lot more harmonic content above 720Hz.  The Tone section attemps to reign that back in to mostly flat, and in the process, adds a mid hump somewhere bertween 702 and 3.2 like you said.  I don't think the Geo Article is incorrect here.

For reference, here's the section on Tone:
"Tone and volume control stages
Cutting out harsh high frequency harmonics seems to be one of the underlying principles of the TS series. Following the clipping stage there is a 1K resistor leading to a 0.22uF capacitor to ground. This acts like a simple RC lowpass filter, with the rolloff point being 723Hz. This means that the output of this stage is down 20db (10:1) at 7230 Hz, and another 6db (20:1) at 14KHz, close to the top of the audio range. From this simple lowpass filter, the signal goes to the active tone control stage. The control is a 20K potentiometer strung from the (-) to the (+) input of the second opamp section. The wiper of the control is tied to a series RC combination to ground. This RC is a 220 ohm resistor and a 0.22uF capacitor. As a series network, at frequencies above the point at where the capacitive impedance is less than 220 ohms (which happens at about 3.2KHz), the network just looks like the 220 ohm resistor. At frequencies below that point, the capacitor impedance gets larger as the frequency goes down until at some point the capacitor impedance is large even compared with the full resistace of the tone control (20K); this happens at about 36Hz, below guitar frequencies.

The tone control operations are easiest to see if you assume that the tone control is at one end or the other of its range. When fully toward the (+) end, the capacitor shunts the frequencies above 3.2KHz to ground; when fully toward the (-) end, the capacitor shunts feedback frequencies above 3.2KHz to ground. This means that at the (+) side, the signal gets another -6db/octave high frequency rolloff, while when it's at the (-) side the signal finally gets some treble boost, +6db/octave above 3.2KHz. Note that the "boost" actually just levels off the -6db/octave induced by the 1K/0.22uF network ahead of the active control stage, so the treble is just not being cut any more above the turnover frequency for the tone control stage when fully at "treble".

The opamp is set up as a noninverting buffer, which just means that there is no net signal loss through the tone control stage, a gain of 1 - if you can find a frequency where there isn't otherwise a boost or cut from something else. "

[Projectile]

What I am clearly NOT seeing is what is described in the GeoFX article. If frequencies were being rolled off below 720Hz and brought back up above 3.2Hz, then I would expect to see a dip in the frequency content between 720Hz and 3.2Hz. I am not seeing any kind of dip like that on the plot, and I don't hear anything like that either. The content between 720Hz and 3.2Hz would be the bulk of what most people consider "mid-range" content (I usually consider mids to be 300Hz-5KHz, but I couldn't find a standard definition), so it should clearly sound like a mid-range scoop if the filter works as described in the GeoFX article. I've never heard anyone describe the Tubescreamer as having a mid range scoop, period. It's always been known for it's mid-range presence.

The 720Hz cut off is a low pass, meaning it cuts frequencies above 720Hz.  Our confusing tone control only acts to control things above 3.2kHz.  Considering the gain of the clipping stage is 1 below 720Hz and higher above, everything running into the tone section has a lot more harmonic content above 720Hz.  The Tone section attemps to reign that back in to mostly flat, and in the process, adds a mid hump somewhere bertween 702 and 3.2 like you said.  I don't think the Geo Article is incorrect here.

Sorry, but I can't make a bit of sense out of that argument. If the first part of the filter is rolling off at 720Hz and the second part of the tone control is only controlling frequencies above 3.2KHz, then there is no way it could be adding a hump between 720 and 3.2K. It would be an audible cut in that range, regardless of how much hamonic content the clipping stage is adding there. If you think the gain stage is adding THAT much midrange content then just take it out of the equation all together. Let's just talk about the tone control by itself. I'll bypass the clipping stage and do my tests again.

This is getting ridiculous. Why is it so difficult for someone to confirm whether or not the 1K resistor in the feedback loop is supposed to be taken in account when calculating the rolloff frequency of the .22 cap and 220 ohm resistor to ground? It is a very simple question, and that is all I'm basically asking.  If you are trying to find a way to fit the theory in the GeoFX article to what I am finding, then start with the fact that the frequency response of the whole filter section goes basically flat when I jumper the 220 ohm resistor. That is something I can measure with near certainty.

In the meantime, does anyone knows of any good measurement software that can actually send out a sine sweep and generate a frequency plot? I would be very grateful. At the moment I am just using a digital synth with an LFO routed to a self-oscillating filter as a signal generator, and a audio visualizer tool as a specrum analyser. It's a pretty rough way of doing it, but it works for getting a general picture. I would prefer something that could give me a bit more accurate reading though. Thanks.

[Projectile]

duplicate post deleted.

[aziltz]

Forgive me for trying to help, perhaps I miss-communicated.  I was just pointing out that its a Fixed Low Pass at 720Hz, not High Pass as you said in the post previous to mine.

I think what I said makes a lot of sense, and the Geo article is in line with that as well, I'm just stating that I agree with RG's explanation thus far.

The clipping stage increases everything over 720Hz. 
Between Op-Amps, everything over 720 is reduced (6dB/Octave).
The tone control has a gain of 1, and can either cut above 3.2kHz at the input, or cut in the Feedback loop (adding more highs to the output).

I never claimed that the tone control added the mid-hump on it's own.  In fact, I believe its the combination of the Clipping Amp's Filtering and the Fixed Low-Pass between stages, but I haven't had time to calculate that.  That's just my hunch.  I don't believe you'll find a mid-hump without the Clipping amp.

On to your actual question:
The easiest way to see what the 1k Resistor does its to put a pot in there.  My bet all along has been that its just setting the gain to 1, but I cannot be sure, because I don't recognize the 20K Pot between (+) and (-) inputs as any typical op-amp circuit.  You seem capable of doing this.  I can suggest Spice or 5Spice or LTSpice, a program where simulating would be very simple.  I'd do it myself, but I haven't learned the ins and outs yet.  It's coming in a week or two for the lab I'm teaching.

EDIT:  Thinking a little more, I'd like to suggest that the 220 and .22uF is not a typical low pass filter in that, its not a simple voltage divider with upper leg R, lower leg C.  I see it more as, a side path, or a dead end side street.  (Getting figurative here i suppose)  Vin to (+), has a side route to ground via the 220/.22uF.  The 20k controls the amount dumped to ground, but doesn't affect the cut-off point.  Likewise, Vout runs through the 1K, to the (-) and has a side route to ground through the 220/.22uF, amount controlled by the 20k.  I believe this is because the main signal is not passing straight through the 20K resistor as it is in the 1k/.22uF low-pass prior to the tone control.  To test this I would disconnect the 20K from the (+) input.  If this is the case, I believe the Tone Pot would only INCREASE treble, because it can only cut the treble on the feedback leg, but not the input.  Just a theory.


I can, however, offer an explanation for why the response goes flat when you short the 220ohm resistor.  Recalculating what the cut-off would be, assuming a resistance of 1 for the shorted resistor, the cut-off frequency is now 723kHz!!!, well above the human range of hearing.  The 220oHm resistor controls the cut-off, put a pot in there and you've got a way to control the cut-off frequency as well.


For the record, I'm not some newb throwing out suggestions out of my rear.  I'm an experimental research physicist.  I work with electronics everyday and on during any other week I would love to breadboard this, document it, and give a definitive answer.  I just may do that for myself at some point, but I haven't as much as a free moment right now.  My apologies that all I can offer is discussion.

[Projectile]

Forgive me for trying to help, perhaps I miss-communicated.  I was just pointing out that its a Fixed Low Pass at 720Hz, not High Pass as you said in the post previous to mine.

I think what I said makes a lot of sense, and the Geo article is in line with that as well, I'm just stating that I agree with RG's explanation thus far.

The clipping stage increases everything over 720Hz. 
Between Op-Amps, everything over 720 is reduced (6dB/Octave).
The tone control has a gain of 1, and can either cut above 3.2kHz at the input, or cut in the Feedback loop (adding more highs to the output).

I never claimed that the tone control added the mid-hump on it's own.  In fact, I believe its the combination of the Clipping Amp's Filtering and the Fixed Low-Pass between stages, but I haven't had time to calculate that.  That's just my hunch.  I don't believe you'll find a mid-hump without the Clipping amp.

On to your actual question:
The easiest way to see what the 1k Resistor does its to put a pot in there.  My bet all along has been that its just setting the gain to 1, but I cannot be sure, because I don't recognize the 20K Pot between (+) and (-) inputs as any typical op-amp circuit.  You seem capable of doing this.  I can suggest Spice or 5Spice or LTSpice, a program where simulating would be very simple.  I'd do it myself, but I haven't learned the ins and outs yet.  It's coming in a week or two for the lab I'm teaching.

EDIT:  Thinking a little more, I'd like to suggest that the 220 and .22uF is not a typical low pass filter in that, its not a simple voltage divider with upper leg R, lower leg C.  I see it more as, a side path, or a dead end side street.  (Getting figurative here i suppose)  Vin to (+), has a side route to ground via the 220/.22uF.  The 20k controls the amount dumped to ground, but doesn't affect the cut-off point.  Likewise, Vout runs through the 1K, to the (-) and has a side route to ground through the 220/.22uF, amount controlled by the 20k.  I believe this is because the main signal is not passing straight through the 20K resistor as it is in the 1k/.22uF low-pass prior to the tone control.  To test this I would disconnect the 20K from the (+) input.  If this is the case, I believe the Tone Pot would only INCREASE treble, because it can only cut the treble on the feedback leg, but not the input.  Just a theory.


I can, however, offer an explanation for why the response goes flat when you short the 220ohm resistor.  Recalculating what the cut-off would be, assuming a resistance of 1 for the shorted resistor, the cut-off frequency is now 723kHz!!!, well above the human range of hearing.  The 220oHm resistor controls the cut-off, put a pot in there and you've got a way to control the cut-off frequency as well.


For the record, I'm not some newb throwing out suggestions out of my rear.  I'm an experimental research physicist.  I work with electronics everyday and on during any other week I would love to breadboard this, document it, and give a definitive answer.  I just may do that for myself at some point, but I haven't as much as a free moment right now.  My apologies that all I can offer is discussion.

Sorry, but much of what you have said is about the way this tone control circuit functions is completely wrong. I have done a lot of experimenting and I am 100% sure at this point.

First of all if you read what I posted earlier, I already HAVE disconnected the tone control pot from the positive input of the opamp, and like I said earlier, "It doesn't make a discernible difference in how the tone control functions when the pot is turned all the way up." I can disconnect the wire and barely hear a difference. So, we can ignore that.

Your explanation of how the signal can goes flat when I remove the 220 ohm resistor doesn't make any sense. I am really starting to think you are pulling my chain. If the first part of the tone control network rolls off 6db/octave from 720Hz up (before the feedback network), then how on earth could the signal be flat if the feedback network has a roll off point beyond the range of human hearing!??? Huh!?

I apologize if I am coming off as abrasive, but I am very frustrated. You see, I am not a theoretical research physicist. I am merely an unemployed college drop-out who decided to build a tube screamer for fun. I don't know calculus or physics, but when I tinker with things I like to have a good working model of how they function in my head, so I started reading about electronics. After a while I felt I had a pretty good understanding of how this stuff works, but some of the models in my head were directly contradicting what I was reading in the GeoFX article, among other places. So, I thought I would ask here to see if anyone could clear up the gaps in my understanding. Instead, what has happened is that it became very clear to me that the explanations I have been given are obviously wrong. In order to confirm my understanding I had to do some experimentation that further proved that the information circulating is bad. I would have thought that upon argument and examination all of the "experts" here would either clear up the issue for me, or simply realize their folly and fix the error.  I am a novice at this, but it is painfully obvious to me that my understanding is at least somewhat correct, and that the  "experts" here are completely wrong.  The data doesn't lie. You can't imagine how frustrating it must be for me as a novice to see very knowledgeable and highly educated people persist in making glaring errors. It's really quite shocking! What in the hell is a newbie like me supposed to do in that kind of situation? Over 200 people have apparently read this thread, but so far all I have gotten in response is more errors. It really bothers me that I could possibly be seeing gaping holes in a theoretical physicist's basic understanding of how electrical currents are controlled by an RC filter, when I'm just a freaking newb! But I know I'm not completely nuts, so what in the hell is going on!!!!? ARRGGGGGHHHH!!!!!
 

[MohiZ]

Before your mind blows up, I'm going to throw in my 2 cents again.

I can, however, offer an explanation for why the response goes flat when you short the 220ohm resistor.  Recalculating what the cut-off would be, assuming a resistance of 1 for the shorted resistor, the cut-off frequency is now 723kHz!!!, well above the human range of hearing.  The 220oHm resistor controls the cut-off, put a pot in there and you've got a way to control the cut-off frequency as well.

First of all, this doesn't seem right to me, aziltz. As you said, it's not a typical voltage divider filter, so you can't calculate the roll-off frequency like this? The roll-off should be Z1=Z2, where Z1=220R+1/(2*pi*0.22uF*f) and Z2=the resistance of the tone control pot. This equation should be solved for f (frequency) and it's made more complicated by the fact that it affects both the non-inverting and inverting sides of the op-amp. If the 220ohm resistor is shorted, then the tone control looks like a normal low pass filter with the tone control as a variable resistor. The roll-off seen from the (+) side should simply be: f=1/(2*pi*20k*0.22uF) ~ 36 Hz when the tone control is fully on. But this only concerns the frequencies from the (+) side of the op-amp to the (-) side. So it's actually more complicated than you might first think.

I tried to simulate this circuit in LTSpice, but I already deleted the results, so forgive me for not posting the graphs. The tone control only added a very slight hump at about 1 kHz when fully on, but I agree with aziltz that the hump is mainly caused by the combined efforts of the clipping stage and the tone stage. Shorting the 220ohm resistor produced a quite flat frequency response for the tone stage when the tone control was fully on, but with the tone down the treble was rolled down.

[Projectile]

You guys are killing me.

I have some pretty accurate data for the isolated tone control section now, and the mid hump IS mostly from the clipping section after all, but I'm still pretty sure my general idea about how the tone control section works and how the roll-off value is calculated are for the most part correct... at least more in the right general direction than the GeoFX article, which is completely wrong. The data seems to back that up although the actual interactions are a little bit more complicated than I first understood. You MUST account for the 1K resistor in the calculation, and the 220 resistor also. The 220ohm resistor's interaction is a lot more complicated because it doesn't just set the roll off. I don't know where you guys learned how to apply all of these formula, but your basic concepts about how the R-C filter works in this circuit leave me scratching my head. I'm not going to claim to fully understand this stuff, but I can definitely say that the way you guys analyze this stuff seems to contradict very basic foundational concepts about how the different components of a circuit interact with electrons. That's what's really bothering me.

I'll post some stuff when I get all my data collected, but I'm still working on the clipping section to figure out exactly how the caps in that filter work. It seems the AMZ article is also wrong. WTF?!!! Once again, I honestly don't know where you guys learned how to apply these formula, but I think everyone needs to go back and re-learn some of the basic concepts of how these things function, because there is obviously a lot of errors and misunderstanding floating around. I've done a ton of reading and research at this point and I'm 99% sure my general thinking is correct and the internet is freaking crazy. Weird.

[MohiZ]

The thing is, this tone circuit is not your basic RC-filter, so your basic formulas won't apply unless you formulate your own. I've never seen an op-amp with the inverting and non-inverting inputs connected together like that, either. I'd really like to hear you view on how these components interact with electrons. Maybe it would clear some things up. There have been many times that I've been sure I understand something perfectly only to be proven completely wrong with an even more logical explanation. I don't claim that I understand this circuit perfectly and I'm not claiming that you're wrong, I just hope someone like R.G. himself could swoop in here and explain his article.