Audio Amplifiers

The "Gain Adjust" cap adjusts the gain of the audio pre-amplifier. In recent experiments, the best value for the gain adjust is a 470 ohm resistor in series with a 2.2mfd electrolytic. See schematic above.

The image below shows the mounting of the resistor and capacitor. The pre-audio amplifier was modeled in LTSpice to find the best value for maximum output with no distortion.

The pre-amplifier was added to the circuit because there were some complaints the volume from the Beginner's and Experimenter's Receiver Kit was not adequate for some people, and articles in ham radio literature talking about deficiencies in audio volume of homebrew receivers.

In a very quiet setting of a ham shack, the "Gain Adjust" will not be needed. In high background noise environments, like hamfests, the "Gain Adjust" cap/resistor will almost double the volume.

The audio amplifier is a standard 8 watt automobile AM/FM receiver audio amplifier (TDA2002). Some kits will have a LM383, which is identical to the TDA2002. Another audio amplifier will be included in the newer kits with a house number "9779", an identical part to the TDA2002.

My favorite speaker is a 6" with an 8 watt rating. I have found the bigger the speaker the better. The TDA2002/LM383 likes 4 ohm impedance better than 8 ohm. Both will work, but the 8 ohm will not produce as much volume as the 4 ohm.

If there is considerable noise output, check the capacitors in the output filter of the product detector. If the .05 caps are bypass caps instead of .047 mono, the noise level will be very high.

For additional information about the audio amplifier, check out "Audio Amplifier IC Notes", QST, August 1989, Page 41.

The TDA2003 datasheet can be found at http://home.eunet.cz/rysanek/pdf/tda2003.pdf. The TDA2003 is identical to the TDA2002. This is a .pdf file and requires the Adobe Acrobat Reader 4.0.

Don't forget to check out "A Potpourri of Audio Amplifiers", by N1HFX. He has some really good information, including a very neat IRF510 1.5 Watt audio amplifier. Also, check out "Design Guidelines for Bipolar Transistor Audio Preamplifier Circuits" for the down and dirty details.

Most ham CW and SSB communication is between 300Hz to 3000Hz. The following is a chart made from the data of an SGS TDA2003 data sheet intended to aid the designer for fine tuning the audio amplifier.