# Designing an audio amplifier, problems with bandpass filter

I am trying to build a Audio amplifier for a university project, I think that most of my circuit is done but I have an issue with my filter as my sine signal is not symmetrical after filtering to eliminate the offset, Instead of having an interval of aprox. +-5V I get an interval of +5 -> -7. I have used 3 opamps for amplifying the signal and a potentiometer at the first one so that I can increase my interval to 8V(maximum that I need to output).

As my input data I have:

• Ai = 700µV
• Bandpass freq = 32 -> 512 Hz

• Ao = 5-8 V

• Rl = 40 Ohm

Below is the circuit and simulation(transient of 10ms w/ step of 10u) I've tried using Chebyshev filter or a Active Butterworth Filter using some online calculators but I am getting from the computations two resistors with negative values and that's not ok.

Bode plot:

I've tried using a active band pass filter but that also didn't work out; I can provide the project if needed.

• just a general recommendation: Analog Device's Filter design tool is actually excellent, and won't give you negative resistor values. – Marcus Müller May 22 '19 at 17:10
• Oh, and get rid of the µ741 opamp. That is like the grand-grandfather of modern opamps and really shouldn't be used anymore when it comes to filtering. It's terrible. – Marcus Müller May 22 '19 at 17:11
• Oh, these are all from the same generation. Seriously, a university should have access to better opamps – modern opamps cost insignificantly more than these dinosaurs. Hey, if you're a student, you might even ask Analog Devices to send you a free sample of their opamps :) Just use the Filter design tool that I've linked to above, and they will suggest an opamp that works with your frequencies, your supply and signal voltages, etc. You can also tell it to only use the most common capacitor and resistor values (E6 series / E12 series), so that you definitely find the right parts at university! – Marcus Müller May 22 '19 at 17:22
• Run an AC analysis, not transient, then you can look at all frequencies at once, not just one. Show a bode plot, not transient – Voltage Spike May 22 '19 at 17:28
• If you are going to run a transient analysis, run it for much longer than 10ms. I don't think your circuit has reached steady-state. – Elliot Alderson May 22 '19 at 17:33