freestompboxes.org

Hello to you all,
I'm opening this thread to ask for a specific question...It's been a lot of time I started to wonder about the opamp stage involved in distortion capabilities of many, many popular guitar pedals...
Let me just say that I'm not a tech by any means, but I'm trying to figure out this object from a "player" point of view.
So....When you have a distortion stage with a non-inverting opamp at its heart, like in a Tube Screamer, for example;
Does the opamp itself clip when the gain potentiometer is at its higher settings?
I know that the diodes in the feedback loop of the opamp clip the signal before it reaches the output of the opamp, and that they do that "frequency selective".

I was specifically wondering IF, when the gain knob is high, the diodes clip and limit the waveform that is ALREADY being distorted full bandwidth for the peaks of the input signal that are amplificated from the opamp, because of the high-gain of the opamp together with not enough voltage supply, OR NOT.
I'm asking because of this article:
http://www.muzique.com/lab/boost.htm

The article claims that, even if there aren't diodes involved here, the opamp could clip the peaks of the input waveform if the gain knob is high, when there is one, like in the Orman booster, because of the scarce voltage supply.
Please, sorry for my bad English, I really hope that what I'm trying to ask is somewhat understandable...
Thank you very much in advance!!

I would say, (caution, here comes some amateur opinions) in most pedals, the op amp itself is clipping the signal as well as the clipping diodes. I've run through some mods on a mxr dist plus. When it came stock, the op amp was biased so it produced a lot of distortion, and the clipping diodes didn't have much to do with it. It is fairly easy to bias the distortion plus pedal so the op amp is not clipping so much, (or at all) and the diodes can do their job, it turns the pedal into more of an overdrive, but it sounds way better than a stock distortion plus. I can play my distortion plus with the distortion knob at noon, and get some great tube type overdrive, and even at about 3'o'clock it's still not really into "distortion" territory.

I would say also, (more amateur opinions) the tubescreamer is doing the same thing. at lower gain settings, you're hearing the clipping diodes, and when you turn the gain up, and start hearing the wasp nest get disturbed, that's the op amp doing some of it's own clipping. Of course the major difference is the frequency sweep in the tubescreamer that makes it totally unique, (LOL, I said tubescreamer was unique), but it is really.

I really wish MXR would lay off of the "hot bias" of the op amp in the distortion plus, that circuit has so much potential when that chip is running "cooler". But if they did that, they might have to charge more that $49 for it. If you know anyone that has one lying around, it's WELL WORTH IT to mod it "cooler", it turns it into a boutique $199 overdrive.

Another amateur weighing in here....

I understood that if the diodes are in the feedback loop, the gain of the opamp is effectively limited once the diodes "turn on" and start conducting: Distortion, or clipping of the waveform is caused by the diodes, not by exceeding the headroom of the opamp itself in a tubescreamer (I.e. opamp clipping). In the MXR D+, on the other hand, there are no diodes in the loop, so opamp clipping can and does occur, and the diodes to ground that follow the opamp will further clip this signal, and possibly smooth out the slightly less than musical sound of pure opamp clipping. From "technology of the tubescreamer by rg keen:

"the diodes have no effect until the signal at the output is greater than the threshold voltage of the diode. In the stock TS series, these are silicon signal diodes, with a turn on voltage of about 0.5 to 0.6v. As a diode turns on, the equivalent resistance of the diode goes down as the diode turns on harder. Effectively, there is a small range from about 0.4V to 0.7V (the exact voltages depending on the type of diode, the package, the doping, etc, etc.) where the diode resistance goes from an open circuit to a very low value, perhaps a few ohms for signal diodes. So as the diode turns on, the gain of the opamp stage changes, going down to just over 1 if the diode can be considered a short circuit compared to the 4.7 K resistor on the (-) input. Even if the Drive control is set for a gain of 100, the diodes conducting cause the gain to drop to 1 for only those portions of the signal where the product of the input signal and the gain exceeds the diode threshold. This means that the signal is "clipped" at the forward voltage of the diode; because there are two opposing diodes, this happens for both signal polarities."

http://www.geofex.com/article_folders/T ... sxfram.htm

In the following I'm assuming the op-amp in question is ideal, for the sake of clarity

The voltage differential between the positive and negative input terminals on an ideal op-amp is always 0V
A non-inverting amplifier will always try to maintain this relationship by changing the output voltage corresponding to the signal present on the positive input.

When there is no diodes in the feedback loop, the op-amp will amplify a given signal until it hits the supply rails. This is the definition of op-amp clipping.

With diodes in the feedback loop, the op amp output terminal will never be able to swing further away from the negative input than +/- one diode drop.
Because of the 0V potential between the input terminals, this means that the output will only be able to swing +/- one diode drop from the bias voltage on the positive input terminal, thus never being able to hit the supply rails unless the input is biased to close to the supply rails or if the supply voltage is lower than two diode drops.
Effectively this means that if the amplifier stage is biased properly, the op amp will never hit the rails, and no op-amp clipping will occur.

I hope that made sense, Otherwise just ask, and I'll explain further to the best of my abilities

grrrunge wrote:In the following I'm assuming the op-amp in question is ideal, for the sake of clarity
With diodes in the feedback loop, the op amp output terminal will never be able to swing further away from the negative input than +/- one diode drop....
Effectively this means that if the amplifier stage is biased properly, the op amp will never hit the rails, and no op-amp clipping will occur.

This is true for an inverting op-amp stage.
The Tubescreamer and variants use a non-inverting stage. In this case, the minimum gain is 1, not 0. So, you can hit the supply rails if your input signal is hot enough - more specifically the output voltage will be input +/ diode forward voltage. This is more likely to be a problem (if you see it that way) if you use LEDs or MOSFETs (think Zen Drive) as the clipping elements.

Yes. My bad Thanks for that..

Nocentelli wrote: From "technology of the tubescreamer by rg keen:
"the diodes have no effect until the signal AT THE OUTPUT is greater than the threshold voltage of the diode.

http://www.geofex.com/article_folders/T ... sxfram.htm

Thank you all guys!! Much appreciated!!
I was asking myself this...the "limiting" effect of the diodes, isn't maybe something that's involved for almost every input signal, for sure, but that's operating on a signal that's already clipped for the peaks of the input signal?
I mean....from the sentence above, "the diodes have no effect until the signal AT THE OUTPUT is greater than the threshold voltage of the diode": couldn't it be that you'll never have a signal greater than around 1 after that stage because of that threshold voltage of the diodes, yes, BUT that maybe those diodes operate on a signal that has ALREADY got some clipped qualities when the gain pot is high enough?

I'm asking because, as I already said before, I'm talking just from a "player" point of view, and I'll say this: I definitely hear some slightly muddy, intermodulated harmonics with a Tube screamer, ONLY WHEN the gain knob is high enough.
It's definitely similar to those sounds of light fuzzes, with their gain set low, i.e. And what those circuits do, is to basically clip almost all the bandwidth of the input signal, or its bass content, at least.

I can't explain why the Tube screamer should have this, let's say, "almost fuzz quality" to it when the gain is high enough, because of the BIG mid boost it has got at that same stage.
I never encountered this quality when you have that kind of mid boost in a circuit BEFORE or WITHIN the section that is producing distortion, and in this case we have around 15 dB of boost at 1000 Hz !!(or you could see that as a bass cut at around 100 Hz, maybe? but still, that's the point...)

Could you please tell me why? Do you hear it too?
Please don't tell me that is because of the sound of the diodes, because of symmetric distortion versus asymmetric, or other things....As I said before, please believe me if I say that I have some experience with "sound manipulating"....And those kind of muddy, "farty" harmonics I hear are definitely similar to those that you have WHEN YOU ARE CLIPPING A FULL BANDWIDTH SIGNAL, or at least A SIGNAL WITH SOME BASS CONTENT.

So, that's why I was trying to make up my mind that the diodes are operating in a frequency selctive manner for sure, BUT that maybe the input signal is already being clipped, at its peaks at least, because of that relation involving the high gain of the opamp and the voltage supply...
And even if this isn't the case, why I hear those "almost fuzz quality" at those conditions, when in a Tube screamer you have all that mid boost that should make for a tight overdrive tone even when at higher gain?

I hope I better explained the point I'm trying to keep in evidence......and please, sorry for my bad English...!
Thank you all!!