Multiple Feedback Bandpass Filter for Woofer

Band Pass filter is one which can pass only a specified band of frequencies. For example a band pass filter for woofers have the lower cutoff frequency as 200Hz and upper cutoff frequency as 2KHz, that is the mid range audio frequencies. I have made this bandpass filter which is suitable for woofers, that is the Mid Range speakers.

This bandpass circuit is an active filter circuit. This circuit is made using the Multiple Feedback technique with OPAMP. A single OPAMP section can make up the circuit for bandpass action. This technique is somewhat more advantageous than Sallen Key topology, since that would require two OPAMP sections to make up the filter.

We will proceed step by step, from design to simulation then finally testing the hardware.

1. LM358 / JRC4558 / NE5532 or similar opamps
2. 5 3.3k 0.25w resistor
3. 3 4.7k 0.25w resistor (not value critical, anything from 1k-10k will work fine)
4. 1 10k ohm potentiometer/trimmer
5. 2 100nF ceramic capacitor
6. 2 100uF, 16v capacitor
7. Male berg headers
8. Solid core wires
9. PCB, Soldering materials, breadboard (optional)
10. 9v/12v power supply

Multiple feedback is a very popular configuration used to develop active filter circuits using opamps. Opamp is used here in an inverting opamp configuration. For making bandpass filter, we have to use 3 resistors and 2 capacitors. The circuit diagram is provided here. I used Kicad software to design the circuit diagram and PCB.

Here is a article from analog devices which might be helpful in understanding the theory https://www.analog.com/media/ru/training-seminars/...

Now there are various online calculators available to calculate cutoff frequencies and corresponding RC values. I used the following calculator tool, recommend you to use the same :

http://sim.okawa-denshi.jp/en/OPtazyuBakeisan.htm

Now we have lower cut off as 200Hz, and upper cutoff as 2KHz, so centre frequency is Square Root (200.2000) = 632Hz. So choosing R as 3.3k and C as 100n can give a close value of 682Hz. You can also choose R as 33k and C as 10n, it will give same result.

The first opamp in the circuit diagram provides a maximum gain of approx (-4). This Gain stage has a 10k potentiometer to vary gain (though I have used a fixed 10k for the time being). The second stage is the inverting multiple feedback bandpass, hence overall polarity of the output is +ve, obviously output will have a phase shift wrt input due to RC components, but that doesn't create any problem.

Since this circuit runs on dual supply, so I created a simple resistor divider, the center point being the Ground.

I used Multisim online to simulate the circuit. Circuit link https://www.multisim.com/content/eZVUQzD8mxj68ijC9nvoB5/multiple-feedback-bandpass-filter/

Then I build the circuit on breadboard and tested with different frequencies. You need a DSO for this task. The signal being fed from my Mobile (if you have a function generator go for it). Use the app https://www.keuwl.com/FunctionGenerator/ install it and apply different frequencies to the circuit & observe the output on DSO. Take readings as I have done and used Excel to plot values, both from the simulated and actual hardware. The two results were similar as I have expected. So I'm now ready to make a PCB for it.

As mentioned before, I have used Kicad to design the Circuit and PCB. I have used copper fill to make ground plane. If you don't want to use ground plane then use Star Grounding method. For my PCB, I needed two hookup wires. Drilled holes and soldered all the components. Use a DIP socket for easy opamp replacement.

Now my circuit is ready to use, Use any amplifier at the next stage and attach a woofer at the output to get the clean mid range sound.