Unleashing the LM386

From Wikipedia Commons, author 'Samba pa ti'

The LM386 must be one of the most popular audio output amplifiers among radio amateurs, despite having been around for a long time. It's an IC which even has its own Wikipedia page. The LM386 can be obtained in both dual-in-line and surface-mount packages and outputs 325 mW in the standard version that runs from 4-12 Volts supply voltage.

Its voltage gain of 46 dB is in many cases too little, especially in direct conversion receivers. When I built the Pixie 2 with the LM386 audio stage, it struck me how the sensitivity of the receiver was limited by the audio gain. I asked myself if it would be possible to increase the gain and add some filtering in a simple way. The result is a gain of more than 70 dB and an audio bandwidth of a few hundred Hz by only adding 5 passive components to the standard circuit: 3 resistors, 1 capacitor and an inductor.

Inductance and capacitance adapters for a digital voltmeter

The adapters have been built from the drawings in chapter 26 of the ARRL handbook for radio amateurs, 2000, using the ARRL PCB templates. They convert capacitance/inductance to mV to read on a digital voltmeter: Capacitance is in two ranges: 2.2-1000 pF or 1 – 2200 nF, and inductance in these two ranges: 3-500 uH and 0.1-7 mH. The projects can be found here.

Errata for the capacitance adapter:

• The PCB template is incorrect. There is a connection between pin 7 of U1 (ground) and the minus connection of the DVM that needs to be cut away. In the template, the trace from pin 7 going down for a few mm and then to the right should only go to the next hole where the strap is soldered. Then it should be cut, and not be allowed to connect to the junction of the parallel capacitors C5 and C7 (whose sum make up 0.2 uF).

Recommendation for the inductance adapter:

• There are sharp pulses in the supply voltage unless a couple of decoupling capacitors are added as in the capacitance adapter. The values are not critical, e. g.: 10 uF/10V on the 9V supply, and 1.5 uF/10 V on the 5V supply.

Using pin 7 of the LM386 to reduce BCI and add side tone to Pixie 2

The Pixie is a full break-in transceiver which is attractive for its simplicity, yet it performs well enough to be used for real contacts. The idea of using the power amplifier transistor as a mixer comes from George Burt - GM3OXX – whose FOXX was described in 1983 in SPRAT. The basic design of the oscillator, PA/mixer and the simple keying has been more or less unchanged since RV3GM – Oleg Borodin - described the Micro-80 in 1992 in SPRAT (Pixie 1). The LM386 audio amplifier was introduced by WA6BOY - Dave Joseph - in the Pixie 2 (QRPp 1995). Most subsequent versions are variants of these designs. In parallel the FOXX has evolved into the FOXX-3.

Keyer-in-a-mouse

I use the K1EL K10+ chip as a morse trainer. The chip has more features built-in than I'll ever get use for.

The keyer is built into an old three button mouse. The center button is the command key and the two others are the paddles. The keyer mouse is completely self-contained: It has a 3 Volt battery, and a piezo speaker located under the mouse in the hole where the mouse ball used to be.

However, normally I connect a paddle and often also a set of headphones.