![]() ![]() The LM4500 draws about 40mA in mono mode, and about 50mA in stereo mode. The easiest solution is to use a 500-1000mA wall wart type supply, that can be found inexpensively at surplus electronics outlets. The LM4500 board can be used stand alone, powered by an unregulated supply, typically at +18V DC, as a minimum working configuration. I suggest everyone use the 0dB gain values. 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. This capability is detailed on pages 6 and 7, and is accomplished with a panel mounted 100k pot from pin 11 to ground. 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 is critical for best stereo separation especially with older tube tuners. Second, the optional stereo separation adjustment circuit as shown on page 6 is implemented. ![]() First, it has an LM317 adjustable type regulation circuit, with taps for power to other boards. There are a few extra options not shown in that schematic on my board. The schematic shown on page 1 details the heart of my LM4500 board. It was used in many popular tuners in the 80’s and 90’s, and many, including me, feel it is one of the best sounding MPX decoders available. How this is accomplished is explained on page 5. This MPX decoder chip was one of the later generation decoders, with 38kHz third harmonic cancellation built in, for adjacent channel noise rejection. Here’ s the LM4500 datasheet, which everyone should read to understand the circuit and operation.
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