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I'm new to electronics, so I apologize if this is simple.

I'm trying to create a simple amplifier mainly for a guitar input, but testing it with an output and input from my sound interface.

The circuit produces a weird distortion that I don't know what is, but it is not clipping.

Here is the signal I'm sending Sent signal

And here is the signal I'm receiving enter image description here

And here is the circuit

schematic

simulate this circuit – Schematic created using CircuitLab

I would love to know why this is happening and how to fix it. Thanks a lot in advance

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  • \$\begingroup\$ What's the value of R5? \$\endgroup\$ – Russell Borogove Jul 3 '18 at 23:06
  • \$\begingroup\$ R5 is important to know, for dc bias. Further, if you take a look at the datasheet ti.com/lit/ds/symlink/tl082.pdf, you will see on page 5, that Vcc+ - Vcc- minimal has to be 10V! You have 9V. Why is R3 so high? I would make it a little bit smaller - 50k -100k. R5 I think should be 10 kohm, right? \$\endgroup\$ – abu_bua Jul 3 '18 at 23:15
  • \$\begingroup\$ @abu_bua Sorry, new to the site and forgot to change the opamp model, it's actually a TLC072(I've fixed it now), which according to the data sheet should have a max voltage rating of 36v. I've added R5's value, but yes, it's 10 kohm. I didn't have a lot of reason for making R3 that high, just worked of a reference schematic, Ill try changing it to 100k. Thanks \$\endgroup\$ – SomeNorwegianGuy Jul 3 '18 at 23:23
  • \$\begingroup\$ Your "received" waveform looks like a differentiated square wave. What is the waveform at the output of the op-amp (before C3)? \$\endgroup\$ – Peter Bennett Jul 3 '18 at 23:31
  • \$\begingroup\$ I'm missing a joint from the output to the feedback path! It seems to be a drawing fault. Make a joint or redraw the feedback You don't have a connection \$\endgroup\$ – abu_bua Jul 3 '18 at 23:35
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Actually, it is clipping. You are apparently driving a speaker, or perhaps a scope with the input set to 50 ohms input impedance, rather than an amplifier. Your complete circuit looks like

schematic

simulate this circuit – Schematic created using CircuitLab

You are feeding the circuit with a large signal, 1 V rms would be my guess, and this has a pk-pk of 2.8 volts. Multiplied by your gain of 10 gives about 28 V p-p, and this is clipped to about 9 V p-p. This (roughly) square wave then passes through a high-pass filter produced by the cap and speaker, and the speaker impedance is so low that the time constant is well below the period of the signal. This gives the characteristic steep rise followed by an exponential decay.

Either reduce your input level by about a factor of 10, reduce your amplifier gain, or increase your load impedance.

EDIT -

In comment you say you're feeding a 2k impedance, and I believe I mis-calculated the high-pass effect. 2k/0.1 uF will give a corner at about 800 Hz (time constant of 0.2 msec). This will have a noticeable effect on any audio you play through your amplifier. and it accounts for the waveforms you show, assuming you used about a 1 kHz signal. You'll need to increase your C3 from 0.1 uF to about 10 uF to get the break point down to 80 Hz, and 100 uF would be even better in terms of audio quality.

END EDIT

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  • \$\begingroup\$ Thanks a lot, I was actually driving the opamp with 1v(or slightly over) rms. I had made an assumption thinking that the line level input would be ≈1v peak-to-peak. The output was actually driving an input on my sound interface, which lists it's input impedance at 2kOhm, but the 28v p-p should account for the clipping by itself, right? \$\endgroup\$ – SomeNorwegianGuy Jul 3 '18 at 23:55
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    \$\begingroup\$ The 28Vpp output will cause the clipping. The relatively low input impedance of the next stage causes the capacitor to form a high-pass filter that changes the clipped signal into what you're seeing. You'll probably want to fix both issues, unless the cutoff of the high-pass filter is low enough for your liking. \$\endgroup\$ – Persona Jul 4 '18 at 6:55
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    \$\begingroup\$ @SomeNorwegianGuy - See edit. \$\endgroup\$ – WhatRoughBeast Jul 4 '18 at 11:15

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