preventing opamp from overdriving

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billonious
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Post by billonious »

Hides-His-Eyes wrote:A rat is a good example; the diodes are to ground and so there is a chance of the op-amp clipping itself;
yes, it the best example of opamp clipping and I have played with rat clipping methods. Once, I removed the diodes completely from Rat, clipping was more "barbarian" than of diodes (not due to the higher volume). I could say that diodes refine Rat's opamp clipping (appart from the obvious: control the volume down).
Hides-His-Eyes wrote:sorry if you were talking about diodes to ground and we diodes in the feedback loop. You're welcome here even if english isn't your first language, but please don't get angry if we misunderstand you.
Never mind :D , I just got angry (but not now) when people wrongly make me look out of tune. Really, no offense.
Hides-His-Eyes wrote: Sources of drive in a RAT:
however, I just did some maths and it does seem possible to clip the op-amp in a traditional rat circuit depending on the frequency: ie infinite frequency = caps are a short = gain ~300 at least; largest possible input voltage without op-amp clipping = 10mv, approx? If it was 100 I might be more cynical, but that seems somewhat conclusive to me.
I affirm you that not only the rat's opamp clips, but it clips producing a very agressive sound. You could play death metal with a bare Rat (= rat with no diodes). In your calculations, consider the strong pickups which put out 200-500mV from a power chord. The output buffer can;t clip as far as it is correctly biased. :thumbsup

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billonious
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Post by billonious »

DrNomis wrote: All amplifiers,including ss preamplifiers will distort if a large enough signal is fed into them,and this is because the output of an amplifier cannot excede the supply rails.... :)
so, we can prevent exceding the supply rails :D

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Gerry de la Sel
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Post by Gerry de la Sel »

billonious wrote: I affirm you that not only the rat's opamp clips, but it clips producing a very agressive sound.
Even if the Rat opamp did clip, it would clip at about 7.7Vp-p. However, this signal is immediately delivered to the diodes, which will clip at about 0.7Vp-p. Therefore, whatever bad sound the opamp might have created, it is immediately removed by the diodes. You never hear the opamp clipping. I don't know any distortion pedals where you can hear opamp clipping; it always covered by diode clipping.

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billonious
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Post by billonious »

Gerry de la Sel wrote: Even if the Rat opamp did clip, it would clip at about 7.7Vp-p. However, this signal is immediately delivered to the diodes, which will clip at about 0.7Vp-p. Therefore, whatever bad sound the opamp might have created, it is immediately removed by the diodes. You never hear the opamp clipping. I don't know any distortion pedals where you can hear opamp clipping; it always covered by diode clipping.
ok, the red-marked point is what is wrong, because there is this 1k resistor between diodes and lm386's output that "isolates" the signal in some degree. You may think that resistor's effect is completely unimportant, so,
so, I made a simulation in Tina to look it better.

I feeded input with a 50mV- 600Hz sine, pretty typical for guitar tone. Tone shaping & gain is the same as in real Rat, but I used tl071 (inaccuracy of simulation).
The escilloscope results are enlightening and interesting.

Image

The green waveform is taken from the "OUT" point. It is clipped by 1N4148 diodes, at 600mV (each grid is 200mV for green signal) and is almost square.
The brown waveform is taken from the "VM1" point. It is clipped by tl071 only, at 2V - 2,7Volts (each grid is 2V for brown signal) and is somehow sawtooth.
Last edited by billonious on 20 Dec 2010, 10:05, edited 1 time in total.

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Gerry de la Sel
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Post by Gerry de la Sel »

billonious wrote:
Gerry de la Sel wrote: The escilloscope results are enlightening and interesting.

The brown waveform is taken from the "VM1" point. It is clipped by tl071 only, at 2V - 2,7Volts (each grid is 2V for brown signal) and is somehow sawtooth.
There is an error in your simulation. R8 should be connected to a 4.5V battery, not to ground. Pin 4 of the opamp should be connected to ground. With these corrections you should see a larger signal (clipped) at the opamp output.

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billonious
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Post by billonious »

thank you for mention the error. Tina automatically splits supply voltage at half, and considers that 9V battery in opamp is +4,5V / -4,5V, if a ground is placed in the schematic. Although, I changed schematic to the usual symmetric supply and I replaced the image in the previous post

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Post by DrNomis »

billonious wrote:
DrNomis wrote: All amplifiers,including ss preamplifiers will distort if a large enough signal is fed into them,and this is because the output of an amplifier cannot excede the supply rails.... :)
so, we can prevent exceding the supply rails :D



To a certain extent,yes,but,once you get to the supply rail limits of the Op Amp IC you run the risk of destroying the IC itself..... :)



I posted some pics of screenshots of different diode clipping in the workbench,I think it was the "Bridge Rectifier As Clipper" thread,feel free to check them out.... :)
Genius is not all about 99% perspiration, and 1% inspiration - sometimes the solution is staring you right in the face.-Frequencycentral.

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Post by billonious »

sure, I am going to check "Bridge Rectifier As Clipper" right now.

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