7.How to get 74dB by LM386 lm386.gif
LM386 is very popular IC also in Japan. It is used in Radio receiver. The power of it (500mW) is enough for the personal use. But sometimes, the gain of it (40dB) is not enough. Especially the direct conversion receiver depend on all of its gain on the audio amplifier. And therefore, we must stay sometimes the pre-amplifier before LM386. I will show you the way to get the 74dB only with LM386. Please watch the circuit ! Rf is the feedback control resister. By chanbeing the Rf you can tune the gain of this circuit as follows.
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Dear Sunamura-san, I like to read your web-site very much, and I repeated some of your constructions. I especially like your LM386 70 dB AF amplifier which I called =B3the Sunamura amplifier=B2 in our national Ham Magazine. Now, I have some news for you. The other day I found in our national ham magazine a new type of a rock stable VFO. They did not mention the first inventor, but I will try to find out who it was. It is a =8CColumbus= =B9 egg=B9 and it can substitute syntesizers. It uses a CERAMIC RESONATOR, and the frequency can be spread for about 100 kHz. I attach the electric diagram. The chief part s are the Ceramic Resonator CR and the variable capacitor C1. I used a standard CR for 3.579 kHz which is used as a clock in American digital devices. After a short warm-up, when you turn the capacitor C1 you can change the frequency for about 150 kHz - from 3440 to 3620 kHz. To cover the CW segnment of the 80 mteres band a variable capacitor of 140 pf is enough. And the frequency is rock stable. The capacitors C2 and C3 must have a capacity similar to the own capacity of the CR. In my case it was 600 pF. I also tried the ceramic filter for 10,7 MHz with three pins, but then I had to use 50 pF for C2 and C3, because that was the capacity of the filter. I used only two pins: the middle one and a side one - The frequency was between 10,9 and 11 MHz and the stability was very good. I also tried a 5,5 MHz filter, but it did not work well - it gave a frequency of 17 MHz with only 10 kHz of spread. I am sure that you will make use of this oscilator - with your tehcnical ingenuity. When I get some new CRs, I will make some more experiments. Meanwhile, I remain with best wishes, Bozidar Pasaric, 9A2HL, City of Rijeka, Croatia, Europe My E-mail: firstname.lastname@example.org
LM386 は，日本でもとても評判のよいIC です．それは，ラジオ受信機の製作等によく使われています．500mWの出力は，個人用には十分です．しかし、しばしば，その利得 ( 40dB )は十分では無い場合があります．特に直接の- 転換- 受信機（ダイレクトコンバージョン）ではオーディオの増幅器にすべてのゲインを依存します．そしてそれゆえに，我々は，LM386 の前に前置増幅器を時々設けなくてはいけなくなります．あなたにLM386で74dB 得る方法を示します.サーキットのRf は，フィード・バック- コントロール- 抵抗です．Ｒｆを換える事によって、全体の利得が上の表の様に調節できます。
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