A 1kHz Test Tone Oscillator
From time to time, you'll need a 1kHz tone to line up recording equipment. Sometimes, the necessary tone is available from the gear itself, whilst more elaborate mixing consoles feature test tone generators, although these are rarely very accurate. If you do not have access to a 1kHz tone though, the circuit here may fit the bill. Its high accuracy (1kHz +/-3½%) is down to using 2½% tolerance polystyrene capacitors, which are both cheap (most high tolerance capacitors cost an arm, and a leg) and readily sourced. For lower spot frequencies, you should scale the resistors (160k gives a 100Hz tone), as capacitor values above 10nF won't readily come in a 2½% tolerance. The frequency determining resistors must be 1% tolerance types, as marked on the diagram.
The only expensive part, the RA53 thermistor is, sadly, not readily substituted: it offers high purity of tone, which helps when you're trying to listen for/track down distortion by ear. The LED is to help remind you not to flatten the battery, which should last 12-18 months with prompt yet regular use. The LED current, incidentally is set at a pretty minute 8mA, so as not to accelerate the power consumption unduly. If you intend to drive the tone generator into long cables - greater than ten feet say - replace the TL071 or LF351N op-amp with an NE5534N, and reduce the output resistor from 330R to 47R.
Feature by Ben Duncan
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