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I built (more or less) this opamp. I know normally the gain would be defined by the ratio of the feedback resistor to the input resistor but here there is no feedback resistor so how do I tweak the gain? enter image description here

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You can increase the gain at IC1a. In your schematic it is configured as a voltage follower buffer or an amplifier with a gain of 1.

enter image description here

Add in R1 and R2 as shown.

Note that if R2 = infinity as in your circuit then the gain AV = 1.

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Of course there is a feedback resistor...several of them in fact, along with some feedback capacitors. The RC network is used to change the frequency response (bass and treble) of the amplifier. You haven't shown enough of the the circuit to analyze the gain, but it can be done.

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  • \$\begingroup\$ Right, sorry I'm a little new to opamp design. I've added an image with the full preamp to the question \$\endgroup\$ – sean read Jul 19 '18 at 2:46
  • \$\begingroup\$ Wait, I think I get it. So is the feedback resistor in this case actually the combined resistors on the right of the tone circuit? \$\endgroup\$ – sean read Jul 19 '18 at 3:27
  • \$\begingroup\$ More correctly, you have a feedback impedance. The value of the impedance depends on frequency...you can't ignore the capacitors. \$\endgroup\$ – Elliot Alderson Jul 19 '18 at 11:55
  • \$\begingroup\$ Right, but I want to increase the overall gain for the circuit. I'm not sure which resistor values I need to change \$\endgroup\$ – sean read Jul 19 '18 at 12:09

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