A passive audio low pass filter kit might be interesting for those AM radio operators who want to build their own analog audio processor or already have some kind of compressor or clipper. The filter is usually put at the output of the processor, before the transmitter/modulator.

Of course the filter could be made on a perf board, but a professional printed board looks better and shielding is built in. To that end I designed a board and selected a BOM for it. 

Response is designed to be sharp cut off but allow full voice range. Insertion loss is 6dB in passband. Choosing different capacitors allows some adjustment of frequency cutoff which is easily determined in LTspice. 
As designed (BOM) it is prety flat to 3.4KHz, -3dB @ 3.48KHz, and from there almost straight down to -45dB @ 4KHz. 
Cap values example for narrower bandwidth: flat to 2.7KHz, -3dB @ 2.9KHz, and -52dB @ 3.4KHz.

Input and output impedance is 500 Ohms and there is provision for on-board termination. Low-resistance 10mH inductors are used to obtain curve near ideal. Using only one inductor part number keeps cost down.

Filter is compatible with 500 Ohm 'studio' line levels and low power speech amplifiers. 
Input power limited to 36dBm (4W) by 500 Ohm termination resistor and 100V capacitors.
Input power up to 40.5dBm (11.4W) if terminated by matched 500 Ohm source and load (200V caps suggested). Alternate inductor Hammond 1539M32 with 1.25" lead spacing (manually bend leads/fit) allows up to 47dBm (48W) audio input (400V caps suggested).

This was done using LTspice and ExpressPCB. 

The board is only economical in a small quantity of 10 and up. The boards are made in the USA. 20 double sided plated-through boards with solder mask and silk screen (less components) can be made for $24.50 per board (20 pc. order is $483.48 delivered) or $38 per board (10 pcs. $368.96 delivered) The other parts are all in stock at Digikey and mouser. 

The BOM -less PCB is all the other parts, resistors, capacitors, and inductors. The inductors are about $2-4 each, the rest are very low cost parts.


Complete Information is here:http://www.bunkerofdoom.com/lit/audio_passive_LPF.zip


