n.b. The abbreviations in the published document (below) are in upper case and have no full stops after each letter (DC, RMS, LF and HF). In my orignal manuscript submitted to the magazine they were written correctly, i.e. in lower case letters with a full stop after each one, ( d.c., r.m.s, l.f. and h.f. ) as I was taught in English lessons at school.
Charles Bradshaw AMIMechE kindly pointed out a few errors which have now vbeen corrected.
Note: In Fig. 10, below, the output of IC16 should be connected, through the two series resistors, the 22k fixed and the 10k potentiometer to pin 3 of the XR-2206. The connection to 0V at their junction is an editorial error.