Spice Analysis of Tube Screamer - Frequency Response

[grolschie]

You could always increase the bass after the distortion like in the Bad Monkey - that sounds much better to me than the usual bass response mod which sounds fartier.

[johngreene]

http://www.greene-pedals.com/GEAD/newtech.html

[grolschie]

http://www.greene-pedals.com/GEAD/newtech.html

"The third plot shows what happens to the frequency response when a 100 ohm resistor is added in series to the first .22uF capacitor (after the 1K). This results in leveling off the midrange bump somewhat and allows more variance with adjustment of the tone control. It gives it a somewhat more full sound and less harsh."

Interesting. Thanks for the link.

[earthtonesaudio]

The thing that most bugs me about the post-clipping EQ in the TS is that it really seems like overkill.  A passive high-cut followed by a buffer could replace maybe a dozen parts and still work basically the same.

[johngreene]

http://www.greene-pedals.com/GEAD/newtech.html

"The third plot shows what happens to the frequency response when a 100 ohm resistor is added in series to the first .22uF capacitor (after the 1K). This results in leveling off the midrange bump somewhat and allows more variance with adjustment of the tone control. It gives it a somewhat more full sound and less harsh."

Interesting. Thanks for the link.

I put that page up in 1997. 

[johngreene]

The thing that most bugs me about the post-clipping EQ in the TS is that it really seems like overkill.  A passive high-cut followed by a buffer could replace maybe a dozen parts and still work basically the same.

I think it is pretty elegant. The passive high cut rolls off the harsh overtones at just the right spot. Then the buffer with the tone control goes from additional cut to boost. The TS808 had several designs before it with different EQs. So it wasn't exactly the first try...

[Cliff Schecht]

I'm one of those who tend to ignore the phase plots. But seriously, what does it matter? I thought the human ear can't hear a difference in phase anyway. I get it that phase differences can be used to advantage in phasers, etc. but in a distortion circuit such as this one, is there a difference anywhere else than the screen of the scope?

The human ear CAN hear the difference from phasing, even in distortion circuits. It causes all kinds of problems like smeared bass, nasally sound, ringing, even oscillation! Look up Barkhausen Stability Criterion, understanding it can help you understand why your distortion is self-oscillating. In a fuzz circuit, phase misalignment can cause those nice square waves to become ringy, although this typically isn't a problem with passive filters. With active filters, you sometimes get humps in the group delay that can reveal problems as well. For example, with a Butterworth filter type, you have a slight hump in the bass frequencies - this means that your high frequencies are going to get delayed more than your low frequencies are going to get delayed more than your high frequencies. In terms of audio, this means that your bass will get delayed enough to be not only audible, but highly annoying!

It still would be cool to take a look at the phase plots for the filter section, but I don't feel like simulating it .

[johngreene]

I'm one of those who tend to ignore the phase plots. But seriously, what does it matter? I thought the human ear can't hear a difference in phase anyway. I get it that phase differences can be used to advantage in phasers, etc. but in a distortion circuit such as this one, is there a difference anywhere else than the screen of the scope?

The human ear CAN hear the difference from phasing, even in distortion circuits. It causes all kinds of problems like smeared bass, nasally sound, ringing, even oscillation! Look up Barkhausen Stability Criterion, understanding it can help you understand why your distortion is self-oscillating. In a fuzz circuit, phase misalignment can cause those nice square waves to become ringy, although this typically isn't a problem with passive filters. With active filters, you sometimes get humps in the group delay that can reveal problems as well. For example, with a Butterworth filter type, you have a slight hump in the bass frequencies - this means that your high frequencies are going to get delayed more than your low frequencies are going to get delayed more than your high frequencies. In terms of audio, this means that your bass will get delayed enough to be not only audible, but highly annoying!

It still would be cool to take a look at the phase plots for the filter section, but I don't feel like simulating it .

Phase has more meaning/effect if it is referenced to something. The phasing of the harmonics is more meaningful if you run the waveform through a phaseshift network that doesn't attenuate. A filter, on the other hand, attenuates the harmonics significantly therefore significantly reducing the effect their phase will have on the resulting waveform. Also, the phase change through a tone control circuit is static, therefore you never really notice what it is doing. It all gets lumped into the 'character' of the various ways of implementing tone controls.

This is a pretty interesting read:
http://silvertone.princeton.edu/~john/monauralphaseexperiments.htm

--john

[johngreene]

Look up Barkhausen Stability Criterion, understanding it can help you understand why your distortion is self-oscillating.

http://web.mit.edu/klund/www/weblatex/node4.html
hmmmm. Someone from MIT seems to think there's a problem with good ol' mister Barkhausen. 

[Projectile]

The thing that most bugs me about the post-clipping EQ in the TS is that it really seems like overkill.  A passive high-cut followed by a buffer could replace maybe a dozen parts and still work basically the same.

I think it is pretty elegant. The passive high cut rolls off the harsh overtones at just the right spot. Then the buffer with the tone control goes from additional cut to boost. The TS808 had several designs before it with different EQs. So it wasn't exactly the first try...

There is no boost. When the tone control is maxed the opamp stage merely counteracts the cut that was made by the earlier passive high cut. The tone knob just goes from mild treble cut to extreme roll off. I have to agree with earthtonesaudio that it seems rather overkill. I don't know anyone who uses the tone knob on any setting less than about 60%. It's a waste. For what it is though, I'll agree that it's tuned rather well. I wasn't happy  with any of the other cap combinations I tried, but I'm still tempted to scrap the whole tone control circuit and try something different.

[Cliff Schecht]

I'm one of those who tend to ignore the phase plots. But seriously, what does it matter? I thought the human ear can't hear a difference in phase anyway. I get it that phase differences can be used to advantage in phasers, etc. but in a distortion circuit such as this one, is there a difference anywhere else than the screen of the scope?

The human ear CAN hear the difference from phasing, even in distortion circuits. It causes all kinds of problems like smeared bass, nasally sound, ringing, even oscillation! Look up Barkhausen Stability Criterion, understanding it can help you understand why your distortion is self-oscillating. In a fuzz circuit, phase misalignment can cause those nice square waves to become ringy, although this typically isn't a problem with passive filters. With active filters, you sometimes get humps in the group delay that can reveal problems as well. For example, with a Butterworth filter type, you have a slight hump in the bass frequencies - this means that your high frequencies are going to get delayed more than your low frequencies are going to get delayed more than your high frequencies. In terms of audio, this means that your bass will get delayed enough to be not only audible, but highly annoying!

It still would be cool to take a look at the phase plots for the filter section, but I don't feel like simulating it .

Phase has more meaning/effect if it is referenced to something. The phasing of the harmonics is more meaningful if you run the waveform through a phaseshift network that doesn't attenuate. A filter, on the other hand, attenuates the harmonics significantly therefore significantly reducing the effect their phase will have on the resulting waveform. Also, the phase change through a tone control circuit is static, therefore you never really notice what it is doing. It all gets lumped into the 'character' of the various ways of implementing tone controls.

This is a pretty interesting read:
http://silvertone.princeton.edu/~john/monauralphaseexperiments.htm

--john

Very interesting read! I was actually looking for an article about that exact thing - I've run experiments myself in the lab and have noticed the same effects on timbre from phasing. A great example of phasing is running two out of phase square waves at very close frequencies. The effect you hear is the harmonics going in and out of phase and it really adds a lot to the sound. I guess the point I was trying to make earlier is that phase has a big effect in certain places. I agree that a typical tonestack (nth order passive RC filters) isn't going to have much of a noticeable effect too, my example was specifically for active filters. We really don't see a lot of active filters in guitar distortions, but phase distortion can sound cool too! Best example I can think of is a fuzz type effect (think square waves) with a nearly self-resonant filter, that would sound gnarly!

[aziltz]

[
There is no boost. When the tone control is maxed the opamp stage merely counteracts the cut that was made by the earlier passive high cut. The tone knob just goes from mild treble cut to extreme roll off. I have to agree with earthtonesaudio that it seems rather overkill. I don't know anyone who uses the tone knob on any setting less than about 60%. It's a waste. For what it is though, I'll agree that it's tuned rather well. I wasn't happy  with any of the other cap combinations I tried, but I'm still tempted to scrap the whole tone control circuit and try something different.

i really think it depends on your view of the function of the tone knob.  some people view/hear 12 Noon as the "Voice" of the pedal, in which case, the Tone Knob can Boost Highs or Cut more.  Some view Full as Zero Cut.  It all depends on the view/ear of the user and not so much as the other factors as long as you don't care to see how the design was intended to work.

IMHO, seeing as full noon can be quite harsh, I think the pedal had an intended voice some where between 40% and 60%.  For the record, I can use a TS with low tone settings for smooth lead tones with single coils.  Not unusable, just not-optimal IMO.

I can HIGHLY suggest trying a W-Taper 20K replacement.  It events out the sweep much better than stock.  I just did this mod in my Route 66/TS9 Clone and it is miles above the regular tone control.

[johngreene]

The thing that most bugs me about the post-clipping EQ in the TS is that it really seems like overkill.  A passive high-cut followed by a buffer could replace maybe a dozen parts and still work basically the same.

I think it is pretty elegant. The passive high cut rolls off the harsh overtones at just the right spot. Then the buffer with the tone control goes from additional cut to boost. The TS808 had several designs before it with different EQs. So it wasn't exactly the first try...

There is no boost. When the tone control is maxed the opamp stage merely counteracts the cut that was made by the earlier passive high cut. The tone knob just goes from mild treble cut to extreme roll off. I have to agree with earthtonesaudio that it seems rather overkill. I don't know anyone who uses the tone knob on any setting less than about 60%. It's a waste. For what it is though, I'll agree that it's tuned rather well. I wasn't happy  with any of the other cap combinations I tried, but I'm still tempted to scrap the whole tone control circuit and try something different.

I was talking about the tone control circuit -after- the cut. It definitely has boost. Although the entire circuit together actually does provide some boost, but it is mostly in the mid range (because of the passive cut).

[owenjames]

Thats really useful.

Thing I notice with spice is that it ignores the efects of clipping (by measureing the output amplitude of the input frequency), which as we know adds lots of high end. It confused me for a while when i was studing the TS schematic, thinking why does it cut so much of the highs, forgetting that the clipping will have added alot of harmonic content that needs to be tamed. I think perhaps the designers were making the tone knob cut so much because if it recived a very high input level and the gain was set high, you would need extra cut to remove some of the highs.

[aziltz]

Thats really useful.

Thing I notice with spice is that it ignores the efects of clipping (by measureing the output amplitude of the input frequency), which as we know adds lots of high end. It confused me for a while when i was studing the TS schematic, thinking why does it cut so much of the highs, forgetting that the clipping will have added alot of harmonic content that needs to be tamed. I think perhaps the designers were making the tone knob cut so much because if it recived a very high input level and the gain was set high, you would need extra cut to remove some of the highs.

i noticed that too, but its important to leave the diodes in the simulations because they also limit level.

[ScottB]

...

That discovery is what led me to start this thread where I calimed that the GeoFX article on the Tubescreamer must be wrong. The arguments on that thread were what led to Aziltz conduct his spice analysis which cleared everything up, and I thank him for that. Apparently nobody was following my original post, which is okay, because most of it is just a lot of pointless arguing and mis-communications. I was wrong about some of my initial assumptions anyway, which were just my noob explanations for what I was seeing on frequency plots that directly contradicted what I was reading. If anyone is more curious about the details, I would suggest just skipping to the 3rd page of that thread.

[/quote]

Actually that thread was exactly what I was looking for. I posted to the end of it a similar paradox (in my mind) and wonder if you or anyone would like to help clear that up as well?

[brett]

Hi
my 2c regarding phase....

One could argue about humans hearing or not hearing phase, but I know that if a pedal with gain doesn't shift phase, or shifts 180 degrees, you can get mountains of feedback (expecially with simple old valve amps).  e.g. the Fuzzface with its 2 simple inverting stages.  Feedback is often quite audible ( ).
Of course, depending on what you want to achieve, stacks of feedback can be a good or bad thing.  Jimi did very well with it.
cheers

[siore]

Here's what happens when you increase C4.  Stock, 2x and 3x.  (Same values as the BYOC Mods).  It seems to pull the mid hump down a bit, into the 400Hz range.

Bump for a good thread.  Azilitz, thank you for confirming what I'm hearing.  I got two TS builds but one with the .1 cap instead of the .047 was louder.  Now with the graphs you posted, I see why along with less mid-hump, it's also louder.  Just saved me the trouble of tinkering with the 'stock' build (I built the modded TS first), second-guessing myself if I put it together right.   

[aziltz]

I'm glad it was helpful!

[siore]

Also, for someone using the TS to boost, IMO you want the sharp mid-hump to cut into a mix.  Seeing how the controls vary the freqs, I think the charts give a good idea of how to move that hump around when other instruments occupy the same sonic space.  But I bet those who use the TS to full effect already know.     It starts with the tone control, then with the gain knob to compensate for the dB's gained or lost.  For that, I think the tubescreamer is a really nice design, even with stock components.