Audio Output Level Indicator

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 meter will read full scale with about 1 Watt onto a 3R speaker, although the calibration (and indeed linearity) are not important for the intended use.


Circuit Description Circuit Diagram

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 negative 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.

Germanium diodes are used because of their low forward drop voltage. If silicon diodes were used the unit would be less linear.


Construction Circuit Board

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 completed circuit may be fitted into a small plastic case. The circuit board may be retained by using short rigid connecting wires to the meter.

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 Maplin Meter In Use

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.

Unless stated otherwise in the service information, the correct alignment point is that which gives the greatest reading on the meter.


Parts List

R1

18K 0.25W Resistor

C1

10uF 25V Electrolytic Capacitor

C2

2.2uF 25V Electrolytic Capacitor

D1

OA47 Germanium Diode

D2

OA47 Germanium Diode

M1

250uA Panel Meter

 

Plain matrix board

 

Case

 

Speaker wire

 

Croc. clips (2 off)

 

3R3 2.5W wirewound resistor (optional dummy load)

Note: If a 100uA meter is used, C2 should be 1uF and R1 should be 47K.


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).

One constructor, Charlie B. Cade said:

I followed your online article using 10uF for C1, 2.2uF for C2, silicon diodes 1N914 (I think) for D1 & D2, 15K for R1, and a 200uA meter from and old Heathkit Vacuum Tube Voltmeter. It works just fine.

So, if you cannot obtain the germanium diodes easily, try silicon diodes. 1N914 is electrically almost identical to the more common 1N4148 diode. I would have expected silicon diodes to make the scale more non-linear, but this does not really matter much for this unit. Charlie certainly seems happy with his unit.




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No part of this website may be reproduced in any form without prior written permission from Paul Stenning.
All details are believed to be accurate, but no liability can be accepted for any errors.
The types of equipment discussed on this website may contain high voltages and/or operate at high temperatures.
Appropriate precautions must always be taken to minimise the risk of accidents.

Last updated 14th April 2006.