Audio Output Level Indicator

Posted on Feb 7, 2014

This unit is designed for monitoring the audio output level across a loudspeaker when carrying out alignment of radios. As no great precision is required, a simple passive circuit arrangement has been used. The circuit is a voltage-doubling rectifier driving a panel meter. The input signal is AC coupled via C1. D1 holds the signal so that the nega

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tive peaks are at ground potential. The mean level is above ground, so the polarity of C1 is important. The peak level is rectified by D2 and stored in reservoir capacitor C2. The value of R1 has been selected to give the required calibration with a 250uA meter movement. If a 100uA is used, the alternative component values should be used. The prototype was constructed using a small piece of plain matrix board. Tag strip or stripboard could be used if preferred. A PCB would be overkill for such a simple design! The meter used on the prototype was a low cost ( £3. 20) 250uA signal strength meter obtained from Maplin (Order Code LB80B). This is marked "SIGNAL" and has an arbitrary scale marked 0 to 5, making it ideal for the purpose. A higher quality meter could be used, but this would offer no real advantage. The input may be bought in via a length of two-core speaker cable. The free end may be fitted with a pair of small crocodile clips for easy connection to the speaker tags in the radio being aligned. The unit is designed for connection across a loudspeaker. If you wish to disconnect the loudspeaker because of the annoying noise, replace it with a suitable wirewound resistor. Two people have contacted me regarding the availability of the OA47 germanium diodes. These are listed in the current Maplin catalogue (Order Code QH70M, price 49p each). I followed your online article using 10uF for C1, 2. 2uF for C2, silicon diodes...

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