There are times that we may not have the specific PFM module you may want to make your system just the way you want it! This article will give you a breakdown of how you can obtain the correct parts and build your own PFM often for a very low cost.
Understanding the PFM module
Some AudioControl products use resistor modules for frequency programming (crossover or PFM frequency). On AudioControl products, we derive the high-pass and low-pass values from one single PFM.
All modules are constructed the same, except the actual value of the resistors used. In each module, ALL resistor values are equal.
The PFM's used in AudioControl devices are 14-Pin headers and 16-Pin DIP style headers.
18dB/Octave modules require 6 resistors mounted in the 14-Pin DIP Header.
24dB/Octave modules require 8 resistors mounted in a 16-Pin DIP header
The resistor value for any given frequency is chosen according to the following equation:
Resistor (kilohms) = 7200 / frequency (Hz) that you choose.
Example: To find the resistors needed for 1000 Hz.
R (kilohms) = 7200 / 1000
R (kilohms) = 7.2
R = 7.2 K = 7200 ohms
Resistor Table
Five-Percent resistors are available in 24 standard values per decade. In English, this means that from 10 Ohms to 100 Ohms there are 24 total different resistor values. The table below will give you a breakdown of the resistors you will need pertaining to the frequency of the PFM you wish to build. Remember, all of the resistors are the same value corresponding to the frequency you want.
PFM Module Parts
Resistors can be found on many platforms such as Amazon, Digikey, Mouser, Etc.
Here is the link to the 16-Pin DIP header: https://www.digikey.com/en/products/detail/on-shore-technology-inc/SA163000/3313525
Here is the link to the 14-Pin DIP header: https://www.digikey.com/en/products/detail/on-shore-technology-inc/SA143000/3313545
AudioControl recommends using 1/4 Watt 5% tolerance carbon-film resistors. If you want the best you can get, use 1% metal-film resistors. These are harder to obtain but well worth it for your system. Richter Scale modules ALWAYS use 1% resistors
Remember all resistor values are the same on the PFM. You cannot overlap frequencies on the PFM with AudioControl devices as our circuitry derives high and low pass from the same PFM. If over/underlapping is what you are looking to do, you would be better off using an equalizer
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