Well, it becomes important, because very high inductor values are much harder to get while also keeping the Q high. For example, 1 mH is easy to implement air wound, while 35 mH is harder and might require a core.
No filter can be perfectly flat within a finite range, so I'm not sure what you're going for here. Would you actually be okay with a modest say 0.5dB ripple?
The main measurement I want to do is THD, so I guess it's ok if the filter isn't very flat. low noise and distortion are much more important than measuring the signal level.
Oh -- great! Then low distortion cores (bobbin and rod ferrite, powdered iron toroid) will be fine, and Q factor won't be a big deal in the first place as filters only need high Q (over 5 or 10) for much sharper cutoff.
Lower Q parts limit the sharpness at cutoff, and introduce loss in the transition band. But if you don't need terrific flatness in the, say, 20-100kHz range, who cares?
Indeed, you usually want loss in the filter, to help isolate its response from the load, which can be highly variable. A speaker is very inductive at high frequencies, so does not act to terminate the filter; conversely, a piezo tweeter is very capacitive, and can do nasty things to a poorly compensated amplifier, or to the response of a filter! This is accomplished by adding R+C dampers in parallel with capacitors, and R||L dampers in series with inductors.
The goal is to have the filter's nominal impedance somewhat higher than the amplifier's ideal load, so that reactive current into the filter does not have a big impact on SOA and losses (it consumes only some of the amplifier's peak current capability). Then, the load impedance is isolated from the filter, using a series R||L damper. Finally, the filter is terminated with an R+C, where C is several times the filter's total capacitance.
Also, you need an LCLC filter, so an L is facing the switching inverter. (Assuming it's a conventional constant-voltage inverter. It is possible to build it so that a C-input filter is required, but that takes more effort, so it's almost never done.)
For an 8 ohm amplifier, you might choose a 16 ohm filter. The filter prototype must be a one-side-shorted, singly terminated type. I don't know of any online calculators that offer this type, but it's tabulated in Zverev and others. The mean inductance will be on the order of (16 ohm) / (2*pi*(50kHz)) = 51uH, with individual inductors taking values above and below this value as needed (the mean is actually the
geometric mean).
Another tip: you can always use a signal generator to test the response of the filter, and calibrate out its frequency response. This is easiest with spectrum analyzers, but I don't know if your intended test has such capability.
Tim
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