Audio frequency generator

My generator can produce sine- and squarewaves with frequencies between 1 Hz and 100 kHz and amplitudes ranging from zero to 1.55 Veff in 600 Ohms. Sinewave distortion is 0.1% or less between 20 Hz and 20 kHz, somewhat greater outside this region. The output voltage varies less than 0.1 dB within the audio range (20 Hz to 20 kHz); there is some rolloff (less than 1 dB) at the frequency extremes.
Audio frequency generator - schematic

An audio frequency generator is a very useful addition to the workbench. Common purposes of this instrument are: 1. Signal source, to check if amplifier stages actually work (requires additional signal tracer); 2. Measurements of AC gain at various frequencies, i.e. assessment of frequency curve and -3 dB bandwidth (requires AF millivoltmeter); 3. Examination of the transient response of a circuit: do squarewaves cause ringing or oscillation? (requires an oscilloscope); 4. Assessment of the overload margin of an amplifier, i.e. determining the input voltage required for onset of clipping (visible check on a scope); 5. Distortion measurements (please note: low levels of distortion, i.e. < 0.1%, cannot be determined with the circuit described here) The instrument consists of four different modules within a single enclosure, and is fed from an external (+ and - 15V) power supply. The modules are: Sinewave generator with amplitude stabilization circuit [1]; Schmitt trigger (sine-to-squarewave converter) [2]; 600 Ohm line driver (with stepped attenuator and volume control) [3]; Output indicator [4]. Most sinewave generators (including the circuit from publication 1) can be tuned continuously over a 1:10 range of frequencies, using a stereo potmeter (audio taper). Inspired by reference 4, I used a different approach. My generator is tuned in small steps (0.1 on a log scale) using a dual rotary switch with 12...

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