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Wednesday, September 18, 2013

BITX Build Update #8

Big progress on the BITX:  All the bidirectional amp stages are done.   A bag of 20 5 MHz crystals arrived from Mouser today.  I put one in the BFO/Carrier oscillator and it fired right up.  I'll soon be checking frequencies on these crystals, looking for four that are closest in frequency for use in the filter. 

While waiting for the mail I built the audio amplifier for the receiver (lower left corner in the picture above).   Here I need some advice/encouragement:  In an effort to keep this rig "all discrete" I decided to dispense with the LM386, and replace it with an AF amp using individual transistors.  I found a circuit in the 1980 ARRL Handbook that I liked.   It has two direct coupled transistors, one NPN, the other PNP.  I went with a 2N3904 and a 2N3906.   The Handbook said it would yield 40 db gain.  I figured this was a close enough replacement for the 46 db gain of the LM386.  

As usual, I'm not sure of the impedance matching.  I built the first AF stage from the BITX schematic (the stage that precedes the LM386).  It goes to a 10K pot.  The wiper of the pot would normally go into pin 3 of the LM386.   I have the wiper going through a 4.7 uF electrolytic into base of the first transistor.  The Handbook says the circuit has an input impedance of 1000 ohms.  Does my arrangement sound OK? 

Output impedance from this Handbook circuit is also 1000 ohms.  I tried it with some HI-Z headphones that I have, putting AF in from my Maplin AF sig generator.   It sounds OK.   Not a LOT of audio available.  But OK.  I may need one more stage to drive a speaker.     

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  1. I did a quick google-'round and found


    I guess maybe 2 more transistors would get you close to 2 watts, however the two 'finals' are TIP31 (NPN) and TIP32 (PNP). However that might have some robusto output.

    Mike Y, KM5Z
    Dallas, Texas

  2. Hi Bill,

    I'd recommend the 'tail-end' of Rick Campbell's R1 receiver (ARRL article found here: www.arrl.org/files/file/Technology/tis/info/pdf/9208019.pdf‎)

    I've cannibalized the discreet audio amp's. (stereo) from old T.V. sets. Sometimes it is even possible to simply hack the amplifier(s) away from the PC board, HA!

    73.......Steve Smith WB6TNL "Snort Rosin"

  3. Hi Bill,
    Congrats on your new rig you are building. I found a "primo" audio amplifier that I built recently that works great. It is a design by K8IWQ called "The Enhanced Manhattan Islander Audio Amp." Just look it up on google.

    Al, N8WQ

  4. Hi Bill,
    You could use the 3 transistor amp from my trx, it works very well.




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