FM radio circuit


Posted on Apr 1, 2012

Two conventional amplifier stages built with SF215 transistors are linked by ceramic 10.7MHz IF filters, and followed by an FM demodulator built with a TBA120T i.c. Straight from the databook. Output goes via a 100k log potmeter (not shown) to the audio stage. (NB SF215 transistors can be replaced by BF240, BF241, BF254, BF255, BF594 or BF595). Since my junkbox does not contain TIP41s, I made a Japanese version, using 2SC945, 2SA564A and 2SD1138s salvaged from damaged Oriental stereos. The amplifier sounds great and is unconditionally stable.


FM radio circuit
Click here to download the full size of the above Circuit.

The power supply is also made from junkbox parts. A 2 x 12V, 1.5A wall-wart (computer item) feeds a 6800 µF / 100V smoothing capacitor via two BY298 diodes. A Motorola MC78T12CK i.c. (3A regulator in TO-3 package) provides 12V DC. For stability’s sake I placed a 10µF tantalium electrolytic parallelled by a 10nF ceramic capacitor directly between the output terminal of the MC78T12CK and ground. Audio quality is strongly dependent on the employed loudspeaker. I tried various modern speakers but finally opted for an ancient model salvaged from an empty radio cabinet (Philips AD3700M/06). Valve radio specialist John Hupse (http://www.hupse.eu/radio/speakers/AD3700.htm) lists the following specs for this item: dual-cone, diameter 14 cm, power 6 W, impedance 5 Ohms, efficiency 6%, frequency 90-18,000 Hz, strength of the magnetic field 9,500 Gauss (26,200 Maxwell). It dates from 1961 and is still going strong after 46 years.




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