# Class D amplifier output filter calculation

I want to build a simple amplifier with TI's TPA3116D2, which is "filterless" Class-D amplifier. This device can be run with a traditional LC filter at the output, or, if the speaker wires are short, simply a ferrite bead with a 1nF capacitor.

I'd like to test with a traditional LC filter. The example values in the datasheet are 10uH and 680nF.

I don't happen to have any 10uH coils, but I have plenty of 22uH ones. What happens if I use a larger inductance and smaller capacitance?

According to my calculations, the suggested filter has a 61KHz cutoff frequency and 3.8 ohms impedance. A filter made with 22uH and 330nF will have a 59KHz frequency but the impedance will be 8.16 ohms

What does this mean in this case? Will it affect the performance of the device?

I don't happen to have any 10uH coils, but I have plenty of 22uH ones

Wire two in parallel to get 11 uH - this is near enough 10 uH so I don't see a big issue given that inductors are usually toleranced at greater than +/- 10% (yes I know there are exceptions)

• Yep that was my plan B and in a way I guess it's better. Heat will be distributed between the two coils as well.
– hjf
Sep 1, 2014 at 13:04
• @hjf - depending on output current into speaker you are significantly further from saturation because the current is halved into each (although there will be about 1.5 more turns on a 22$\mu H$ coil than the 10$\mu H$ coil). Do you have a spec for the inductor? Sep 1, 2014 at 13:51
• Yes, it's the DR127-220. Datasheet here: cooperindustries.com/content/dam/public/bussmann/Electronics/…
– hjf
Sep 1, 2014 at 13:59
• Yeah you should be OK with two of those in parallel. Sep 1, 2014 at 16:46

Yes, it may affect the performance depending on what the load is. Higher impedance of the circuit at upper frequencies is one problem. The resistance of the coil, which will determine the impedance at low frequencies, is another. The frequency response will be affected and electrical damping reduced. On the low end the response may be weaker overall but due to damping "boomier", in the sense that the speaker resonance will be more pronounced. So, in short, follow Andy aka's advice and put 2 coils in parallel.

• Excellent! My guess was that higher resistance (caused by longer thinner wire) would be a factor but I didn't know how the filter impedance would have affected the sound.
– hjf
Sep 1, 2014 at 13:06