As part of some ongoing work earlier this year I needed to test the performance of a high performance 200W/channel class D amp (more on the amp in some future post). To get accurate SNR and THD+N measurements a low pass filter is needed in front of the measurement system as the high frequency switching noise of the amplifier, even though it is far removed from the audio band, can confuse the measurement instrument inputs (an AP515 in this case). While AP makes such a filter box, it didn’t seem like it would be hard to develop my own as the desired feature set I wanted was a bit different from the AP unit.
Turns out there are some things that became issues that weren’t properly accounted for, specifically the impact of proximity effect on the coils. Also it was discovered that some of the load resistors were not linear, more details of that are documented here. The attached report details the analysis and development. Full construction details are provided so you can build your own.