# TL072CP non inverting circuit doesn't amplify

I am using TL072CP op amp in inverting mode to amplify guitar signal. This is the circuit I have made:

I assembled it and even when changing resistors of different values and trying out different potentiometers I can't hear any amplification. I am sure I am doing a lot of newbie mistakes but if someone could point out some stuff that are wrong with my circuit it would be of great help. Thanks you!

• What equipment do you use to "hear" the amplification?
– Jens
Commented Jul 19 at 14:24
• I am using fender twin reverb amp Commented Jul 19 at 14:25

if someone could point out some stuff that are wrong with my circuit it would be of great help

The first thing to point out is that your circuit will have an input impedance of 1 kΩ due to the resistor connected to the op-amp's inverting input. The 2nd thing to point out is that most guitar amplifiers have an input impedance of at least 1 MΩ. This is required because of the large output impedance of the guitar pick-up: -

In short, you are totally wrecking the output signal of your guitar. Try using a non-inverting op-amp configuration.

• Thank you a lot for detailed response. I have tried to test it with a non inverting configuration but now no sound comes through at all... This is the circuit I have now imgur.com/a/X4lJygo Commented Jul 19 at 14:32
• @vjeranbach get rid of the ground connection on the pot cause it'll short the op-amp output. Make sure you have bipolar supplies and decoupling capacitors. If we are done here, please take note of this: What should I do when someone answers my question. If you are still confused about something then leave a comment to request further clarification. Commented Jul 19 at 14:45

I am using TL072CP

Excellent. You got two op-amps, one can be used to buffer the input signal with a fixed gain and high(ish) input impedance, and then another to add variable gain.

The circuit below has gain between 5x-101x, adjustable with VR1. The input impedance is 100x higher than in your circuit and should at least make the circuit functional. The output is not inverted, since the signal passes through two inverting amplifiers in series.

simulate this circuit – Schematic created using CircuitLab

The next thing we can do is to use non-inverting, high-impedance op-amp inputs. This is better suited to high-impedance sources. The gain range is 5x-100x. The amplifier is non-inverting overall:

simulate this circuit

Yet another thing to do would be to use a quadruple op-amp to have two op-amps as the input stage, working as a roughly differential-input stage. This has the benefit of reducing the DC offset drift by an order of magnitude. Thus, VR2 - the fixed potentiometer DC zero adjustment - becomes usable. The zero won't drift as much as it normally would.

This is a 3-op-amp instrumentation amplifier, but is not used for the same purpose and doesn't have the same trade-offs. It is used mainly to keep the input offsets of OA1 and OA2 tracking as temperature changes, since these op-amps are on the same die. OA2 is amplifying the DC ZERO offset voltage with the same gain as OA1 is amplifying the input voltage. OA3 is then taking the difference of the two. 1% resistors work fine in this application, because we don't care much about the CMRR of the overall amplifier. All we need is offset drift reduction, and for that the 1% resistors are fine.

simulate this circuit