{"id":10495,"date":"2013-09-18T18:26:00","date_gmt":"2013-09-18T08:26:00","guid":{"rendered":"https:\/\/www.homebrewradio.us\/blog\/2013\/09\/18\/bitx-build-update-8\/"},"modified":"2025-07-22T04:36:52","modified_gmt":"2025-07-21T18:36:52","slug":"bitx-build-update-8","status":"publish","type":"post","link":"https:\/\/www.homebrewradio.us\/blog\/2013\/09\/18\/bitx-build-update-8\/","title":{"rendered":"BITX Build Update #8"},"content":{"rendered":"
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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. <\/strong>
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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. <\/strong>
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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? <\/strong>
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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. <\/strong>
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Our book: “SolderSmoke — Global Adventures in Wireless Electronics” http:\/\/soldersmoke.com\/book.htm<\/a> Our coffee mugs, T-Shirts, bumper stickers: http:\/\/www.cafepress.com\/SolderSmoke<\/a> Our Book Store: http:\/\/astore.amazon.com\/contracross-20<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

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 … Continue reading “BITX Build Update #8”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":10496,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[84,28],"tags":[],"class_list":["post-10495","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-amplifier-theory","category-bitx20"],"_links":{"self":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/10495","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/comments?post=10495"}],"version-history":[{"count":1,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/10495\/revisions"}],"predecessor-version":[{"id":10497,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/10495\/revisions\/10497"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/media\/10496"}],"wp:attachment":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/media?parent=10495"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/categories?post=10495"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/tags?post=10495"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}