# Dual passive low-pass filter

I am building a class D amplifier and need to design passive low-pass filters for H-bridge with cut off frequencies at about 400 Hz. However, I am unsure about few things, this is the simplified equivalent circuit:

simulate this circuit – Schematic created using CircuitLab

Speaker values were measured using an RLC meter set to 100 Hz.

Lets say that the signal is coming into the L1 coil. Should I calculate the cut-off frequency only for L1, C1 pair? If L2 is grounded, do L2 and C2 have a noticeable effect on L1, C1 filter? How can I calculate it then?

I often see a capacitor in parallel with speaker what exactly is its purpose? I am thinking it is to lower reactive power of the speaker, right? To what $cos\phi$ should I lower it, and should I use middle range values for voltage and current?

• The parallel capacitor does the same as C1 and C2 May 6, 2015 at 20:30
• @Andyaka To create a higher order filter? May 6, 2015 at 20:31
• No it doesn't make a higher order filter - it's still a 2nd order filter. May 6, 2015 at 20:47

• Thanks, this clears up a lot! So the purpose of parallel capacitor (C3) is to increase the capacitance of the half filter, to allow for lower inductance values? How can I be sure what RLC meter showed (there is a setting for parallel/series measurement)? Even if the meter showed this as parallel impedence I get 600uH if I calculate it like this: $$Z=\sqrt{ESR^2-\frac{1}{\omega^2 L^2}}=7.72m$$ $$L=\frac{1}{\sqrt{\omega^2(ESR^2-Z^2)}}\approx 600uH$$ May 7, 2015 at 13:14