Audio Amplifier

This audio amplifier is described in "A High-Performance Receiver for 160 Meters", Solid State Design for the Radio Amateur, by Wes Hayward and Doug DeMaw, Page 138. This audio amplifier is rated at 3.5 watts and uses a pair of hefty transistors in a push-pull output. I use this amplifier in the receiver.

The high performance ability of this amplifier is described on page 138: "There has been no aural evidence of distortion at any signal level while using the circuit of Fig. 40". This includes very weak DX signals for which this audio amplifier really does its best.

The LM301 can be ordered from BG Micro at 29 cents each. This part may be difficult to find. They also carry a complimentary pair of output transistors that are used in the output stage of car audio amps - 2N6488/2N6491. Their part number is TRN2N6488/TRN2N6491. Cost is $1.25 a piece.

From my experience, this is the best audio amplifier that I have ever built. It is not picky about the type of output transistors. Any PNP-NPN pair in a TO-3, TO-220, TO-040 or TO-043 package advertised for audio circuits work very well.

Use landscape setting on your printer when printing this diagram. Right click on the diagram and select View Image, then when it opens in a new browser window, click on Print Settings, select Landscape layout in the appropriate folder, select Fit to Page, and then click on Print.

This amplifier works best for SSB. The audio pre-amplifier was added for additional gain and the ability to adjust the gain to suit different people's requirements for adequate audio volume.

C28 is the gain adjust capacitor for the pre-amp. For quiet hamshacks, this capacitor is not needed. For noisy locations, a 2.2mfd electrolytic will boost the volume significantly.

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

A good place to check out on the internet for audio amplifier information is "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.