Noob question: guitar pedals...

[bsas]

Hi all,

I am learning electronics for a while and I finally got a "nice" oscilloscope and function generator. I have a Rigol DS1054Z (unlocked) and a Siglent SDG2042X (unlocked).

So, I am trying to make a very simple guitar pedal test and I just don't understand the outputs. Probably is something related to impedance that I just don't know.
It is a "overdrive pedal", which just means that it suppose to give a small amplification and a smooth distortion on the signal.

Setup:

I am taking the Signal output from the function generator, connecting to a BNC T, and that T connects to the pedal input and the Oscilloscope Channel 2. The pedal output connects to the oscilloscope Channel 1. Very simple setup. Signal is 2kHz (very common guitar level) at 750mVpp.

What I don't understand:
1) Why the input signal moves in phase a little when the pedal is on?
2) Why the output signal is "weaker" than the input signal? Being an overdrive I was expecting the opposite.
3) Why, if I change the "gain" on the pedal, the wave just don't change?

Thanks!
Bernardo Silva

[Audioguru]

I think that overdrive causes severe clipping distortion. the distortion is reduced by integrating the signal with a lowpass filter that cuts high frequency harmonics which adds a small delay (phase shift).
Poor circuit design causes the output level to be reduced.
Changing the gain does not affect the severely clipped signal.

[Shock]

What I don't understand:
1) Why the input signal moves in phase a little when the pedal is on?
2) Why the output signal is "weaker" than the input signal? Being an overdrive I was expecting the opposite.
3) Why, if I change the "gain" on the pedal, the wave just don't change?

I'll give it a stab.
1. Is the pedal true bypass? Is it analog, digital, buffered? Perhaps when it is on there is a slight delay (as in lag) in the signal.
2. The output could be limited in order not to drive the input of the next pedal hard. You could probably test for this by varying the input a little and playing with the controls on the pedal.
3. The input signal is pretty flat, so gain between 0-10 does nothing? What if you use a distorted signal does that change it? How about other frequencies? Square wave?

You might want to try the FFT function of the scope and look up how to perform a sweep test also, they might reveal more than you're seeing.

[Brumby]

It is a "overdrive pedal", which just means that it suppose to give a small amplification and a smooth distortion on the signal.

The key to this is in the name: "overdrive pedal".  In a general sense, "overdrive" means a signal that is greater than can be handled without distortion.  This pedal is designed to distort the input signal and while there are many ways to do this, perhaps the simplest is to amplify the signal to a point where it clips.  Where this clipping level occurs will affect the amount of distortion and subsequent filtering will change the tone.

Don't get too caught up with a minor difference in amplitude between the input signal and output signal.  While these pedals should (ideally) provide a unity gain path, they are often close enough and guitarists aren't usually that fussed and will simply adjust a volume somewhere else.

[Bassman59]

2) Why the output signal is "weaker" than the input signal? Being an overdrive I was expecting the opposite.

It’s odd, but many fuzz boxes have outputs which are quieter when the fuzz is on. This is why, at sound check, I have guitarists go back and forth between clean and distorted sounds, and have them adjust the fuzz output level so the two sounds are the same level. Or maybe the fuzz is louder.

3) Why, if I change the "gain" on the pedal, the wave just don't change?

Because it’s a clean gain control that comes after the distortion stage. It allows for the level matching I describe above. You don’t want the output gain to distort more, because that just makes mush.

Many guitar amps have a preamp gain control, which allows you to dial in the overdrive, and a master volume, which sets the actual output level.

[bsas]

What I don't understand:
1) Why the input signal moves in phase a little when the pedal is on?
2) Why the output signal is "weaker" than the input signal? Being an overdrive I was expecting the opposite.
3) Why, if I change the "gain" on the pedal, the wave just don't change?

I'll give it a stab.
1. Is the pedal true bypass? Is it analog, digital, buffered? Perhaps when it is on there is a slight delay (as in lag) in the signal.
2. The output could be limited in order not to drive the input of the next pedal hard. You could probably test for this by varying the input a little and playing with the controls on the pedal.
3. The input signal is pretty flat, so gain between 0-10 does nothing? What if you use a distorted signal does that change it? How about other frequencies? Square wave?

You might want to try the FFT function of the scope and look up how to perform a sweep test also, they might reveal more than you're seeing.

It is a very common pedal, Ibanez mini tubescreamer. So, answering your questions:

1. - It is true bypass and analog. But when it is on, it is obviously buffered (TS-808 circuit if I am not mistaken);
2. and 3. - I tried to sweep from 100Hz to 20kHz and obviously there is a big difference in curve (bode plot changes because of the EQ that the pedal implies), but the amplitude doesn't change. Even with volume and gain at max!

[bsas]

I think that overdrive causes severe clipping distortion. the distortion is reduced by integrating the signal with a lowpass filter that cuts high frequency harmonics which adds a small delay (phase shift).
Poor circuit design causes the output level to be reduced.
Changing the gain does not affect the severely clipped signal.

It suppose to distort, that is fine
What I really don't understand is why the amplitude reduces, and why the pedal changes phase on the "input". Changing on the output is OK, depends on the design of the circuit, I get it. But why the input is modified? Isn't the input buffer suppose to isolate it?

[bsas]

It is a "overdrive pedal", which just means that it suppose to give a small amplification and a smooth distortion on the signal.

The key to this is in the name: "overdrive pedal".  In a general sense, "overdrive" means a signal that is greater than can be handled without distortion.  This pedal is designed to distort the input signal and while there are many ways to do this, perhaps the simplest is to amplify the signal to a point where it clips.  Where this clipping level occurs will affect the amount of distortion and subsequent filtering will change the tone.

Don't get too caught up with a minor difference in amplitude between the input signal and output signal.  While these pedals should (ideally) provide a unity gain path, they are often close enough and guitarists aren't usually that fussed and will simply adjust a volume somewhere else.

It makes sense, and I know that an overdrive pedal suppose to distort the output signal. But the also suppose to increase the volume of it, so, it suppose to be at least higher than unity gain, am I right? That is what bothers me, the fact that the output signal is lower in amplitude...

[Bassman59]

What I really don't understand is why the amplitude reduces, and why the pedal changes phase on the "input". Changing on the output is OK, depends on the design of the circuit, I get it. But why the input is modified? Isn't the input buffer suppose to isolate it?

Looks to me like the input signal (blue in your traces?) amplitude doesn’t change.

The distortion output (yellow) looks phase shifted with respect to the input (blue) trace because of the distortion circuit. The clipper circuit (IC 1A here) is highly non-linear and the wave shape (and hence your scope trigger point!) depends on when the clipping diodes turn off and on.

[bsas]

Hmmm, I noticed some dumb error from my side. Looks like a guitar pickup output is far weaker than I thought.

I just measured my own guitar with full volume and it never crossed 200mVpp... When I use a weaker signal (like the guitar) the pedal "kind of behaves as expected, increasing the output. But, now the "new challenge" is that when I plug my guitar straight to the pedal, and that to the oscilloscope, the pedal amplifies that signal far more than the signal that comes from the function generator. Now, I am suspecting of an impedance mismatch since guitars are famous to be high impedance (high Z) sources...

So, is there any way to force my function gen to be high Z?

[Bassman59]

Hmmm, I noticed some dumb error from my side. Looks like a guitar pickup output is far weaker than I thought.

I just measured my own guitar with full volume and it never crossed 200mVpp... When I use a weaker signal (like the guitar) the pedal "kind of behaves as expected, increasing the output. But, now the "new challenge" is that when I plug my guitar straight to the pedal, and that to the oscilloscope, the pedal amplifies that signal far more than the signal that comes from the function generator. Now, I am suspecting of an impedance mismatch since guitars are famous to be high impedance (high Z) sources...

So, is there any way to force my function gen to be high Z?

Try setting the signal generator to have an output on the order of the 200 mV you see from the guitar.

[bsas]

Hmmm, I noticed some dumb error from my side. Looks like a guitar pickup output is far weaker than I thought.

I just measured my own guitar with full volume and it never crossed 200mVpp... When I use a weaker signal (like the guitar) the pedal "kind of behaves as expected, increasing the output. But, now the "new challenge" is that when I plug my guitar straight to the pedal, and that to the oscilloscope, the pedal amplifies that signal far more than the signal that comes from the function generator. Now, I am suspecting of an impedance mismatch since guitars are famous to be high impedance (high Z) sources...

So, is there any way to force my function gen to be high Z?

Try setting the signal generator to have an output on the order of the 200 mV you see from the guitar.

That is what I did.

[newbrain]

What I really don't understand is why [...] the pedal changes phase on the "input". Changing on the output is OK, depends on the design of the circuit, I get it. But why the input is modified? Isn't the input buffer suppose to isolate it?

While you had a number of good answers for the other points, it seems this one has not been directly addressed.

As many said, when the filter is switched on some phase shift (delay) will be added to the output signal.

Now, look at the settings of the scope, especially which channel is used for triggering, can you now see why the output waveform appears to stay where it is, and the input one moves (left, showing it it has got an earlier phase)?

Hint: the input signal is not really changing.

[isobig]

Hey bsas! I have the same o-scope and gen as you... and I also play guitar.  ;-)   I just got the generator and plan on using it to align the IF stages of an AM/FM radio kit I am building. Also plan on using for diagnostics on guitar amps and such.  Good luck with your pedal project!

[bsas]

What I really don't understand is why [...] the pedal changes phase on the "input". Changing on the output is OK, depends on the design of the circuit, I get it. But why the input is modified? Isn't the input buffer suppose to isolate it?

While you had a number of good answers for the other points, it seems this one has not been directly addressed.

As many said, when the filter is switched on some phase shift (delay) will be added to the output signal.

Now, look at the settings of the scope, especially which channel is used for triggering, can you now see why the output waveform appears to stay where it is, and the input one moves (left, showing it it has got an earlier phase)?

Hint: the input signal is not really changing.

A ha! Got it!
Awesome everyone, thanks!!!

[bsas]

Hey bsas! I have the same o-scope and gen as you... and I also play guitar.  ;-)   I just got the generator and plan on using it to align the IF stages of an AM/FM radio kit I am building. Also plan on using for diagnostics on guitar amps and such.  Good luck with your pedal project!

Awesome! I am right now just using pedals as a good start for simple analog electronics learning tools. And since I play guitar (obviously) I can get something useful out of my learning process
Best of luck with your radio kit!

[Nerull]

It is a "overdrive pedal", which just means that it suppose to give a small amplification and a smooth distortion on the signal.

The key to this is in the name: "overdrive pedal".  In a general sense, "overdrive" means a signal that is greater than can be handled without distortion.  This pedal is designed to distort the input signal and while there are many ways to do this, perhaps the simplest is to amplify the signal to a point where it clips.  Where this clipping level occurs will affect the amount of distortion and subsequent filtering will change the tone.

Don't get too caught up with a minor difference in amplitude between the input signal and output signal.  While these pedals should (ideally) provide a unity gain path, they are often close enough and guitarists aren't usually that fussed and will simply adjust a volume somewhere else.

It makes sense, and I know that an overdrive pedal suppose to distort the output signal. But the also suppose to increase the volume of it, so, it suppose to be at least higher than unity gain, am I right? That is what bothers me, the fact that the output signal is lower in amplitude...

An overdrive pedal is supposed to replicate the sound of an overamplified signal. There are many ways to do that other than actual amplification.



The output signal does not come directly from the overdrive stage.