The LM4500 Datasheet
Hereí s the LM4500 datasheet, which everyone should read to understand the circuit and operation. This MPX decoder chip was one of the later generation decoders, with 38kHz third harmonic cancellation built in, for adjacent channel noise rejection. How this is accomplished is explained on page 5. It was used in many popular tuners in the 80ís and 90ís, and many, including me, feel it is one of the best sounding MPX decoders available. The schematic shown on page 1 details the heart of my LM4500 board. There are a few extra options not shown in that schematic on my board. First, it has an LM317 adjustable type regulation circuit, with taps for power to other boards. Second, the optional stereo separation adjustment circuit as shown on page 6 is implemented. This is critical for best stereo separation especially with older tube tuners. Last, it has pads brought out that allow for panel mounted controls - a stereo LED indicator, a switch to engage mono operation, and stereo blend adjustment pot that let you decrease the separation, and thus noise, for fringe stations. This capability is detailed on pages 6 and 7, and is accomplished with a panel mounted 100k pot from pin 11 to ground. Note that the de-emphasis can be set up for either 50us or 75us depending on where you live, and those values are on page 6. I suggest everyone use the 0dB gain values.
Powering the board
The LM4500 board can be used stand alone, powered by an unregulated supply, typically at +18V DC, as a minimum working configuration. The easiest solution is to use a 500-1000mA wall wart type supply, that can be found inexpensively at surplus electronics outlets. The LM4500 draws about 40mA in mono mode, and about 50mA in stereo mode. The +18V DC power supply allows for a regulated voltage of 13.8V to power the LM4500, using standard resistor values of 240 and 2.k, and a LM317 regulator. If one wishes to use a lower voltage from an onboard power supply, thatís done by using different values for the adjustable regulator resistors. The minimum recommended voltage per the LM4500 data sheet is +8V. For instance, one may wish to add the board to a tube tuner, and use a voltage doubler circuit from the AC filament supply to power the board. An example is shown here. In that case, it may be convenient to use a low dropout regulator like the LT1086 (or cheaper LM1086) set up to supply about 12V. This is a much better solution than running the LM4500 directly from the doubler circuit. Depending on how the input and output to the chip is handled, one may also be using a quality +/- supply to power op-amps used in filter circuits. In this case, it may be appropriate to not load the LM317 regulator and associated parts, and simply power the board from the +/- supply. Take care that the LM4500 never sees a power supply voltage larger than +16V, the maximum voltage allowed per the datasheet.
Input and Output Options
One can also add options onto the input, i.e. gain for tuners with low output level, or filtering to remove IBOC noise. On the output, similar options can be added for passive de-emphasis, as well as filtering to remove the 19/38 kHz ultrasonic signals.