TA7642 Power Resistor Rp

The value of the power resistor Rp is very critical as it provides a voltage drop for the TA7642 radio IC. According to its documentation, this IC will operate down to +1.3 V. Its absolute maximum rating is +6 V, whilst the typical operating voltage is +1.2 V. Originally, I had a 4.7 kΩ resistor which provided around 1 V to the output pin 3 of the radio IC. This was the recommended value by "radio experts", however, I was getting a very loud distorted audio, which sounded as if it was clipping. In addition, there was much feedback noise. I then decided to try other values for Rp. Decreasing the value did not help, however, increasing it did. I got a nice clear sound from all the stations, but the volume was slightly lower. I experimented with many other values and finally settled on 150 kΩ as the optimum for my area in Croydon. With this value, the voltage measured at pin 3 was 0.7 V, which is well below the recommended, however, not only was I getting a clear sound, but it was also nice and loud. Part of the reason for it being loud was because I was using the LM386 with a gain setting of 200. Another reason was that Croydon is located near a number of powerful stations, not to mention, Crystal Palace, so signal swamping is a huge problem with receivers such as this... If you are located in an area where the signal is weak, then you could try using another value that is the optimum for the signal strength in your location. You need the sound to be as loud as possible, without breaking up or clipping.

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