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The stereo version of the FM sender

This can in fact be accomplished with a single chip - the BA1404, or the newer NJM2035. But I say - why do it with one chip when you can use five? :) ( - or four chips, if you use the dual version of the opamp - the CA3240).

Seriously, this circuit works well as the sender end of a wireless headphone solution, in conjunction with a personal FM radio. The power is sufficient to service the author's house, though stereo loss occurs if I venture off down the garden. Alternatively, it can be used to hear various MP3 players, iPods, etc. on the car radio, where there is rarely a facility for external input.

The circuit just fitted into a case with integral 9V battery compartment (the sort that comes with a clip so that you can attach it to your belt, pocket, etc. though I didn't use this). Externally, the case has just a switch, a short lead to a 3.5mm stereo jack plug, and a two foot length of aerial cable. A small hole is drilled to allow access to the tuning trimmer without having to open the case.

All capacitors around the transmitter section must be ceramic. Leads must be kept short here. A self-supporting coil is used (because it's cheap), but to stop 'twangs' being transmitted when the case of the unit is bumped, the whole transmitter area, including the coil, was caked in candle wax. One must, of course, leave the trimmer accessible for tuning purposes.

If you have the equipment to set this circuit up precisely, then I probably don't need to explain how to do it! Set-up can be accomplished without any equipment, provided a stereo signal source is available, and a stereo FM radio with headphones connected (headphones will enable you to easily hear when stereo sound is being decoded).

The set-up procedure is roughly as follows:

1. Adjust the transmitter section tuning trimmer to place the sender's signal in a an empty part of the FM dial (if such a space can be found these days!).

2. A good starting point for the Pilot tone level pot is about 30% of setting.

3. Apply power and connect the stereo sound source to the circuit. Set the modulation level pot to minimum for the moment (i.e. maximum resistance).

4. Listening with the stereo radio, adjust the 76kHz frequency pot until the (albeit quiet) received signal suddenly resolves into stereo. This may be indicated on the radio by a 'Stereo' LED illuminating. Ensure that the radio is able to lock and resolve stereo every time by tuning it to another station momentarily, and then returning it to the sender's signal. You will find there is a range of settings of the frequency pot over which stereo can be obtained reliably. When the bounds of this range have been determined, set the 76kHz frequency pot to the centre of this range.

5. If stereo cannot be obtained at all by these methods, try repeating your efforts after increasing the Pilot tone level pot a little.

6. Finally, adjust the Modulation level pot so that the received sound has the same subjective volume as the commercial stations on the dial. It might be best to set the pot a little lower than this, as commercial FM stations are often subjected to compression so that they appear louder for a given modulation limit.

The Modulation level pot enables the circuit to be adapted to the particular level on the 'Line out' socket provided of most CD players, etc. If it is intended to connect several devices at different times, then it may prove necessary to use the headphone socket in each case, so that the volume control can then be used as a modulation level control instead.

FM radio employs 'treble pre-emphasis' by which signal frequencies above a certain frequency are lifted, so as to overcome noise. The 'time constant' emplyed for this purpose differs from country to country. In the US this time constant is 75us. In the UK it is 50us. It will be necessary to select an appropriate resistor in the opamp circuits so that the required pre-emphasis in your country is applied. Suitable values for the UK and US are given in the circuit.

Do make sure you don't annoy neighbours by jamming their favourite station! These circuits are not illegal in many countries so long as they do not interfere with legitimate services. Regardless of their legality, remember that you might just need that neighbour's help one day, and he may be less inclined to give it if you've ruined his favourite classical listening!

Finally, the 10mA current drain allows some fifty hours of use from a duracell battery. If a shorter range can be tolerated (e.g. if the circuit is only ever used in the car where the circuit can be sited close to the radio aerial), then greater economy can be achieved by increasing the emitter resistor (1k) in the transmitter circuit.