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HIGH POWER FM MIC by SMOYPO

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HIGH POWER FM MIC

by Harry Lythall - SMOYPO

My FM Wireless Microphone has bccn a very popular project with bcginncrs and e\pericnccd coastruetors alike. It has been used inside guitars and as the basis of a remote control system. I do howevcr, receive many requests for a liigjicr powcred circuit and better microphone scnsilivhy. Now I can introduce the new FM Wireless Microphone (v5). which also has a better frequency stability, over IKm rangę (under ideał eonditions) and is good on microphone sensitivity. fliis has been achieved by adding an RF amplifier bufler (with lOdB gain) and an AF preampliFicr to boost the modulation a little.

Construction is quite simple. LI is 3.25 tunis in spiral form and is an integral part of the PCB foil pattem. Ilie two BC547 transistors can be replaced with (ahnost) any small-signal NPN transistor. such as the 2N2222. 'Ilie finał stage is a BC557 PNP generał purpose device. If you use diłTerent devices then you should select the 1 MO resistor for 5-volts DC at the collector of the the first transistor. Select the 47K resistor for 3 - 4 volts on the collector of the third transistor. Here is the V5 component overlay drawing Notę that tliere is a modification:

Iliere used to be a InO 5mm cap for supply decoupling but ałter a eange of component supplier (manufacturer?) there dcvelopcd sonie form of RF instability whcn the gain of the PA transistor was a little above normal. Roplacingthe InO to an electrolytic capaeitor of 22uf cured this problem totally. .Any "radial" (the leads both come out of the same end) type electrolytic capaeitor lrom 0.47uf upwards curcs the problem, lite finished unit draws about 30mA which should vary as you touch the tuncd circuit. a good test that the unit is oscillating You should remove the 4K7 resistor if you use a dynamie microphone.

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2009-12-09 17 35