MPX 96 FM Stereo Transmitter

I like to listen to audio off the Internet (NPR, C-Span) and being tied to the computer to do this is a bit of a hassle. I figured a low power transmitter for the FM broadcast band would be just the item, but did not find anything commercially built that had good enough customer reviews to buy.

I then saw the book Building Your Own Low Power Transmitters at Fry's and took a look at it and it had a circuit that looked like it would fit the bill. The circuit was the MPX 96 PLL Synthesized Stereo FM transmitter available from North Country Radio.

This kit is a nicely designed 8 IC circuit that generates about 10 mW of output (150 mW if wired for export use). It's not a difficult kit to put together, with the exception of 1 tapped coil that took me a couple of tries to get the tap on the coil correctly. A picture of my completed board is below.

The kit works great. Hooked up about 18 inches of wire as an antenna, and checked it out with a portable CD player and audio from the computer. Much to my delight it sounds great. Covers the whole house and the yard without problem. Does not carry much beyond the house and the yard, listened to it as I drove around the house and lost the signal after a block which is just what I wanted, as I was not trying to become a local FM broadcast station.

The circuit seems quite stable. Left it on overnight and it was still locked on frequency according to my HP frequency counter. As can be seen in the picture below, the output signal looks nice and clean as well.

I give this kit a thumbs up. The circuit is well documented, and fairly easy to debug/trace. Good circuit to learn how a stereo signal is generated and how a PLL frequency synthesizer works. Only down side on this circuit is that it does not have a limiter or pre-compensation, but I think only audiophiles would notice that.

I mounted this kit in a Ten Tec enclosure. A picture of it is below.

Rear view showing power, audio input and antenna connections.

And one more picture with the cover off.

Another FM stereo kit that looks interesting is the Ramsey FM25B. This kit is a bit pricier than the North Country MPX 96. It appears to be based on a ROHM BH1415F wireless audio link IC. As this chip needs a serial programming stream, the Ramsey kit adds a PIC microcontroller. I may build the Ramsey kit to see how it compares to the North Country Kit.

Comments to: Bill Tracey (

Last Updated: 5 June 2004

Copyright © Bill Tracey 2004