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I'm trying to make a guitar pedal with atmega328 in Arduino but I've had trouble with input stage. I found this circuit from electrosmash.com, but I can't find the TL972. Are there any replacement for TL972? Can I use TL082 for that circuit? Or any circuit for guitar signal input stage to ADC. I'm an IT student so I only know a little about electronic. Thank you so much. enter image description here

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A little background:

enter image description here

Figure 1. An ancient 741 op-amp output stage.

To drive the output of the opamp high Q6 turns on connecting the output to the positive supply rail. To drive low Q13 turns on connecting the output to the negative supply rail. The older devices were limited in how much these devices could turn on and the output voltage could only get to within 2 or 3 V of the supply rails. As a result they were generally run from > 12 V.

Your Arduino is powered from 5 V or less. To use this voltage a modern opamp is required and the primary specifications are that (1) it works from low voltage and (2) that the output is rail-to-rail. i.e., It can swing from V- to V+.

You can pick any opamp that meets these basic requirements. The LT1677, for example and chosen at random, appears to fit the bill. You probably want an 8-pin DIL (dual-in-line) style opamp for ease of soldering or inserting into a socket.

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  • \$\begingroup\$ Thank for your answer. I only have LM358, TL082, LM324 and LM386. In my town, this is difficult to buy LT1677 IC. Are there another circuit? I can use another power supply, even dual power supply. \$\endgroup\$ – TucoMoc Aug 2 '16 at 9:20
  • \$\begingroup\$ Yes, you can do that but you will have to add protection after the input stage to prevent it driving the Arduino input > 5V or < 0 V. A pair of diodes as in post #3 on Arduino forum would do the trick. You already have the R and C. \$\endgroup\$ – Transistor Aug 2 '16 at 10:03
  • \$\begingroup\$ transistor,I think you are one of the most active users on EE-SE.You must have a moderate amount of free time for this :) \$\endgroup\$ – Daniel Tork Aug 2 '16 at 13:08
  • \$\begingroup\$ @DanielTork: I was surprised to find that I was top last month. It's good fun and I actually learn a lot by checking my facts or attempting an answer to a subject I'm not familiar with. The "free-time" problem is solved. I have none now. \$\endgroup\$ – Transistor Aug 2 '16 at 16:03
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Can I use TL082 for that circuit?

The schematic you have runs from a 5 V supply. A TL082 needs a minimum supply of +/-5 V and this makes it useless in your application. The TL972 runs for a supply as low as +/- 1.35 V up to +/- 6 V so is a good choice.

Look for an op-amp that has rail-to-rail capabilities (input and output preferably) and can run from a single 5 V supply or +/- 2.5 volts. The TL972 is also has quite a decent gain-bandwidth-product (GBP) of 12 MHz but I suspect an op-amp with a GBP of 1 MHz or more will be suitable because 20 kHz x max gain (20) is a GBP of 400,000.

It's got quite a low noise voltage but I don't think this will be a factor. Anything less than 25 nV/\$\sqrt{Hz}\$ should be fine. Good luck.

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