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I would like to obtain an absolute value of input signal with TL072 or MC4558 op amps (or other amp) with single supply.

When I connect the circuit upper one follows the signal and adds noise and lower one has constant value on output. Input signal is <-9,+9> V, up to 20 kHz (guitar).

I use R = 10 kΩ, C = 1 nF.

What do I miss? Should I use another op amp? Or did I omit something from documentation? Maybe there are better circuits?

Edit: Should I put input signal through transistor to get high resistance?

I will be grateful for any help or information.

analog.com fig.2.

fig. 1 abs(x)

ti.com fig 5

fig. 2 abs(x)

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    \$\begingroup\$ Your 1st circuit is only likely to work with that particular IC or one of its very close cousins. Your 2nd circuit will not work with a single supply - but should work with a split Vcc/2 if you bias your input signal to that value and connect A1's non-inverting input there too. \$\endgroup\$
    – brhans
    Commented Aug 22, 2021 at 21:46
  • \$\begingroup\$ Thanks very much. Now it works but I think that diode is too slow. But now I can test different circuits. \$\endgroup\$
    – Jomonger
    Commented Aug 29, 2021 at 23:13

1 Answer 1

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The TL072 (note capitals in part numbers) doesn't get anywhere near the negative rail. The devices chosen in your schematic might but I'm not familiar with them. For this to work on a single-rail supply you would need an opamp that doesn't latch up when the input signal goes below the negative supply.

For guitar you can create a VCC/2 to use as an artificial ground for your 9 V supply. This is the common approach used by the majority of guitar effects pedals. DC bias is blocked on the input and output using capacitors.

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  • \$\begingroup\$ So I need to pull up signal by Vcc/2 as well? Thx for reply. \$\endgroup\$
    – Jomonger
    Commented Aug 22, 2021 at 21:38
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    \$\begingroup\$ See if my answer to electronics.stackexchange.com/a/432704/73158 helps. It has a circuit. Thanks for accepting my answer but I recommend that you unaccept until the Earth has turned at least once to give the whole of humanity a chance to answer your question. You'll get other insights which may help you further. Then pick the best one. \$\endgroup\$
    – Transistor
    Commented Aug 22, 2021 at 21:41
  • \$\begingroup\$ Works very well on MC4558. Thanks. \$\endgroup\$
    – Jomonger
    Commented Aug 29, 2021 at 23:14

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