Simple interference fix for the Chinese Pixie

The Chinese Pixie transceiver operating at 7023 kHz has become very popular. It often costs less than 5 USD on Ebay. Like most Pixies it is susceptible to broadcast breakthrough and intermodulation. Much of this is caused by the keying circuit of the audio amplifier, the LM386. The cure is to move the muting diode from the power supply pin (no. 6) to the bypass pin (no. 7). I have described this in another blog post with title: "Using pin 7 of the LM386 to reduce BCI and add side tone to Pixie 2".

Here are two pictures that show how this can be done for the Chinese Pixie. One needs an additional resistor in the range 10 - 51 ohms. If you can fit it, then use the large 51 ohms resistor that come with some of the kits (I think it is meant for a dummy load). I have used 10 ohms in the picture. It replaces the old R3 of 1 k. The diode D3 is not mounted in the holes provided, and instead it is mounted under the PCB with the minus (denoted by the ring) connected to where D3's minus was, and the plus side connected to pin 7 of the LM386.

R3 is indicated by the lower left arrow, and the old
placement of D3 is shown with the upper arrow

The LM386 Pixie challenge

The Pixie 2 is this minimal transceiver which I and many others have played around with and had lots of fun with. My 80 m version is shown below, but right now it is very popular with some incredibly cheap Chinese ones on sale on Ebay and other places.

The Pixie 2 uses the versatile LM386 amplifier for its audio output. I have shown previously on this blog how its gain can be boosted and how it can implement a CW filter, and also how the muting can be improved. However, during transmission, the LM386 just sits there idle, although it can be used to amplify a sidetone from an external oscillator.

But I'm sure the old 70's LM386 can do better than that. Despite its age, recently some pretty amazing uses of this chip have been demonstrated. It can be used as a regenerative receiver at least up to medium wave frequencies and it can also be used as an envelope detector/demodulator.

The LM386 challenge is this: Is is possible to implement a sidetone oscillator for the Pixie using only the LM386 with as few other components as possible? The output level needs to be controllable in order to make it comparable to that of the Pixie in the receiver mode.

The best data sheet for the LM386 seems to be the one for NJM386 from New Japan Radio Co. It is, as far as I know, the only one which shows the various muting circuits including the one using pin 7 which I have explored. It also shows the LM386 as an oscillator: both a sinusoidal and a square wave one.

In order for the LM386 to be useful as a sidetone oscillator, I believe that the oscillation must take place in the input circuitry. That seems to be the only way to ensure that the output doesn't come out at a blasting full rail-to-rail swing as in the square wave oscillator example in the data sheet.

By the way, the data sheet referred to above is also the basis for the improved Spice model for the LM386 that just was developed. It came partly as a response to my complaint over how poor the present one was. Maybe the new Spice model, developed by EasyEDA, could help solve the LM386 challenge?

Regenerative receiver based entirely on the LM386

I got a tip the other day that there is an interesting circuit over at the RadioBoards Forum where an LM386 IC is used as a regen receiver for the medium wave band. It is the user 'Selenium' who has come up with that circuit. I think it is quite an amazing application of this IC - so here it is:

LM386 as a medium wave regen receiver by user Selenium on RadioBoards Forum

Interestingly, both the + and - inputs are tied together (pins 2 and 3). It is also quite unusual to connect pin 7 to anything but capacitors (for bypass or extra input as I have done), so that may change the bias of the input stage. Further, the smaller the impedance from pin 1 to ground, the larger the gain (here 10 uF in series with 1k for low frequencies and 100 pF for high frequencies).

If you want to read more about the regen circuit, go to the RadioBoards Forum here.

Disappointment with Spice and the QRP-er's favorite, the LM386

The trusty old LM386 audio amplifier from the 70's is still used a lot in low power and portable equipment. Recently some ultra high gain circuits have been recommended that I wanted to simulate with Spice. I started with the datasheet examples for checking the quality of the model. The result was surprising.

In the data sheet one can find a minimum parts circuit, a high gain circuit, and a bass boost circuit:

Gain = 20 (26 dB), minimum parts

Gain = 200 (46 dB)

Bass Boost

Practical Wireless with the LM386 unleashed

"Improving the LM386 Audio Amplifier" is the title of always inspiring Rev. George Dobbs G3RJV's column in this month's Practical Wireless. George talks about the basic amplifier with a voltage gain of 20 (26 dB) and how it is straight forward to increase gain to any value up to 200 (46 dB). The article then shows how it can be built dead-bug style.

Then he goes on to talk about various ways to improve the circuit such as for hiss reduction and even higher gain. In that connection he also mentions my article "Unleashing the LM386" from SPRAT in the Autumn of 2003. That circuit was an enhanced version of the high-gain circuit of Kazuhiro Sunamura JF1OZL. Basically it increases the gain to up to more than 70 dB, enough for it to drive a loudspeaker. My circuit also adds bandpass filtering. All this is achieved by adding only 5 passive components to the basic amplifier. The circuit was recently revisited in SPRAT by Johnny Appel SM7UCZ. He added a 40 dB Darlington pair preamplifier for even further gain.

The bandpass filtering (not low pass as the article says) is due to the series resonance of 100 uF and 1 mH which happens at about 500 Hz making it a nice CW filter.

QRPp: Ultra low power operation with the Pixie 2

QRPp is radio amateur jargon for communication using a transmitter with less than 1 Watt power output. That’s less than a handheld GSM mobile phone (max wireless range ~35 km) or a flash light. My Pixie 2 transceiver has so far contacted 15 different countries on 3.5 MHz CW. The first version from 2002 was made in an Altoids tin (picture later), but in 2010 I repacked it in one of the nice clear top tins from the 4SQRP (picture below).

My repackaged Pixie 2

Pixie sidetone oscillator and reduced broadcast interference

This circuit provides a sinusoidal sidetone oscillator for the Pixie. As an added benefit, it will also cause reduced broadcast interference, since the muting circuit of the Pixie is a major source of broadcast interference as the LM386 goes into some sort of nonlinear mode and acts as an old-fashioned crystal detector.

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.

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.