Audio Amplifiers

Board 2 || Second Mixer || Crystal Oscillator/Amplifier || IF Strip || AGC
S-Meter || BFO/Amplifier || Product Detector || Audio Amplifiers || Board 1

Adjustable Pre-Amplifier
8 Watt Audio Amplifier

K5OAI LTSpice Simulation of the Audio Pre-Amplifier || References

The audio pre-amplifier is patterned after the excellent web pages of N1HFX. N1HFX has excellent tutorials on audio amplifier design. His idea of not including an emitter bypass to stabilize an audio amplifier circuit and including it as a gain booster was incorporated into this circuit.

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. Below is reference information from the datasheet showing the purpose of all the elements in the design of the amplifier:

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.

C103 and C104 were changed to 330 mfd from the recommended values of 470 mfd and 1000 mfd noted below. This was an economic decision and not related to any improvement in the circuit. No simulation was run to evaluate the changes. C106 was not included in the circuit.

Part Value Purpose Larger Value Smaller Value
C105 2.2 mfd Input DC decoupling Noise at switch-on, switch-off
C103 470 mfd Ripple rejection Degradation of SVR
C106 0.1 mfd Supply bypassing Danger of oscillation
C104 1000 mfd Output coupling to load Higher low frequency cutoff
C101 0.1 mfd Frequency stability Danger of oscillation at high frequencies with inductive loads
C102 39 pf Upper frequency cutoff Lower bandwidth Larger bandwidth
R69 220 ohm Setting of gain Increase of drain current
R66 2.2 ohm Setting of gain and SVR Degradation of SVR
R68 1 ohm Frequency stability Danger of oscillation at high frequencies with inductive loads
R70 39 ohm Upper frequency cutoff Poor high frequency attenuation Danger of oscillation

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.

K5OAI LTSpice Simulation of the Audio Pre-Amplifier

Sam Morgan, K5OAI, is building the ELR and became interested in the Pre-audio amplifier. He entered the circuit into LTSpice and noted some problems with the output waveform. He posted to the EMRFD site and the results were some very useful improvements to the circuit as noted above with the capacitor/resistor modification to the audio gain location.

K5OAI posted a web page with all the files and a tutorial on using LTSpice. Click out "elr audio mod and RC Low-Pass Filter data".

K5OAI's page has all the information that is needed to learn LTSpice and model the pre-audio amplifier, which would be a good starting point for learning the program. You may find a better improvement than the one I incorporated into the circuit. Enjoy!


"Audio Improvements" (scroll almost to the bottom of the article) is included in the document "Tips for Improving Receiver Performance" on the Kiwa Electronics web page. An excellent tutorial on improving audio performance in any receiver.

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

The TDA2002 datasheet can be found at 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 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. Highly recommended!

Board 2 || Second Mixer || Crystal Oscillator/Amplifier || IF Strip || AGC
S-Meter || BFO/Amplifier || Product Detector || Audio Amplifiers || Board 1

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Last Update: 4/24/10
Web Author: David White, WN5Y