Small General Purpose
Audio Test Set
Circuit : Rodney Byne, UK
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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 meter is set at 100mV FSD and can be used for gain measurements and general testing. Current consumption is just under 4mA.

Circuit Notes
The oscillator is a phase shift based around Q1 and associated components. Frequency is set at 700Hz by C1, C2, C3, R1, R2 and R3. The sine wave 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 harmonic filter the oscillator signal is adjustable by preset RV1 and appears at the 3.5mm jack socket, J1.

Any gradual fall in battery voltage due to ageing usage, only reduces the oscillator output level which can be upwards corrected by RV1, but has no effect whatever on the Meter Driver calibration.

The following comments and caveats that come to mind, help the user of the finished item particularly 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.

WARNING:   Please be careful when applying any signal into a sound card. See notes 3 and 4 below and also the sites General Disclaimer.

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, can be usefully widened to 20khz for system or module testing purposes, by reducing C8 to 10p from 39p.

3) Be advised that computer and laptop soundcards have very sensitive inputs. Motherboard replacement costs of laptops are hugely prohibitive! Signal much over 150mV rms will result in almost certain destruction of the sound module, particularly if of a very high instantaneous pulse nature. The software microphone boost switch MUST always be off and fader kept to only about 1/3 up from the bottom.

4) It is strongly recommended that if the wanted signal to be viewed and analysed is very high, then that signal should be fed through a resistor attenuator before entering the pc. In addition, two reverse-wired silicon diodes are suggested to be included in a safety interface across the laptop microphone input socket. These will clip to about 0.6volts, but shouldn't have any effect on smaller test signals of around 80mV rms.

Free Software
The image below shows the spectrum of the prototype phase shift oscillator viewed on Visual Analyser. The signal was applied to the microphone input socket on my laptop, but can be applied to a line input if one is available. More free software for windows and linux available below.

Visual Analyser or VAanalyser. The program numerous facilities, download link here:

SpectrumLab, facilities too numerous to mention. Download link here:

No frills audio tools & toys in Techmind. Downloads here: The newest spectrum analyser version here:


Basic calibration of the test set meter driver internally paralleled with the oscillator output, is by connection to the probes of a small digital pocket multimeter such as a Beckman shown in the above photo. This particular instrument has useful auto-ranging sensitivity to read down from 0 to 2V AC.

Unless stated otherwise by the multimeter manufacturer, AC range readings are always displayed as Mean or Average form-factor, NOT RMS.

So assuming the multimeter is itself in calibration, the indicated reading must be converted to RMS by multiplying by 1.11.

Conversely, RMS converts to Average by multiplying by 0.9.

Therefore if the variable output setting of the oscillator to the multimeter and meter driver together is to show 100mV RMS full scale deflection, the Beckman must display "90".
ie. 90 x 1.11 = 99.9 which is close enough, so the meter scale FSD preset pot can then be set correctly.

To confirm linearity, the test set scale can then be tracked along its important points using the multimeter.

This concludes this calibration method, but alternatively use the known reliable output from another sine tone source such as included in pc audio software. The phase-shift oscillator can then have its preset output, set to read whatever level is required.

Finished Prototype
Click Images to Zoom

The above pictures which include an interior view are Rodney's prototype built into a small black ABS box.

Go ahead to build and enjoy using your test set in conjunction with installed free analysis software. If you are a novice, it will give bountiful technical education experience.

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