National's 170w Class D amplifier Inductors

[KarlP]

I was interested in building National's 170w Class D amplifier with the LM4651 driver IC and the LM4652 mosfet IC. However, on their schematic it shows the output and ground having 50uH inductors on them. I checked inductor prices and this would be at least $10 an inductor. Is there any way to make or get these cheaper, or eliminate them from the design? I am afraid the inductors might be necessary in a switching amp though.

[BeanZ]

The output inductors are absolutely necessary in a class-D amp and the amplifier will not produce audio without them. The inductors and caps form a lowpass 2nd order or higher LC filter to remove the constant switching frequency and recove the audio. Ten bucks a piece are a pretty good deal and I dont think that you can make them for cheaper. You are lucky that you even found an off the shelf part to use for the application. Typically, for switching power applications a custom magnetic component must be designed and wound. I would buy the inductors.

BeanZ

[KarlP]

If the amp was only going to be used as a subwoofer, would it be possible to use inductors with lower uH?

[BeanZ]

The LC filter is to remove the switching frequency. If the switching frequency is not removed completely there will be all kinds of garbage all over the place. An option would be to use a lower value inductor but a higher capactior value. The cutoff frequency is: 1/(2*pi*sqrt(L*C)) where L is in Henries and C is in Farads. The cutoff frequency is chosen to be much less than the switching frequency but higher than the audio region. Make sure that the inductor that you choose is rated for adequete current. This should be at least 10Amps. This is enough current for the output plus a few amps of ripple current. If you are going to buy an inductor make sure that it can run at the switching frequency. Hope this helps.

BeanZ