small general purpose audio

Posted on Feb 5, 2014

This is a small general purpose audio test set. It comprises a low 0. 3% distortion phase shift oscillator and a level meter. The level meteris set at 100mV FSD and can be used for gain measurements and general testing. Current consumption is just under 4mA. The oscillator is a phase shift based around Q1 and associated components. Frequency is set

small general purpose audio
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at 700Hz by C1, C2, C3, R1, R2 and R3. The sinewave appears at the collector of Q1 and the amplitude is stabilized by D1 and the voltage tap R8 and R9. The sine wave is buffered by Q2 which will also reduce output impedance of the oscillator. R7 and C4 form a low pass single pole harmonicfilter the oscillator signal is adjustable by preset VR1 and appears at the 3. 5mm jack socket, J1. The following comments and caveats that come to mind, help the user of the finished itemparticularly when used with pcs and laptops for which this test set was intended. Items 3 and 4 are not aimed at being alarmist, but should be treated with serious caution. 1) The applied/terminated oscillator level is always available to be seen on the meter. That remains so until a 3. 5mm mono plug is inserted into the audio IN socket. This breaks the monitoring and lets the user apply their own audio signal. 2) Two multiturn trimpots control both the required oscillator output level up to a few hundred millivolts rms and the meter driver sensitivity. Both circuit parts are obviously open to modification. Meter FSD is deliberately restricted to 100mV rms for the reasons stated in item 3. The 3k9 and 22k in parallel to the meter are also useful. If one end of the 3k9 is unsoldered, the meter will cope with external audio line level input, up to the order of a couple of volts rms. Also the meter driver audio bandwidth which currently is only for speech,...

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