I'm trying to design an amplifier, which amplifies line level audio to 5 Vpp, so I can sample it with a microcontroller. My plan is to do this in two steps: amplify the signal to 5 Vpp and then add a 2.5 V offset. I'm stuck at the first step.
I put together a simple inverting amplifier like so:
I'm using the LM324N op-amp. I'm powering it using a PC power supply (-12 V for negative rail and 12 V for positive rail). Ground, pictured in the circuit is 0 V.
I'm testing the setup with a sine wave (from my phone) as input. The amplifier works normally at higher frequencies, but the gain starts dropping at around 300 Hz. At 20 Hz, the gain is basically 1. For example:
What am I doing wrong? Is this normal behaviour for this op-amp? As I understand, op-amps start behaving abnormally at high frequencies, not at low. Is something wrong with my input?