In 1985, rather than reinvent the wheel, I outsourced an auto-patch board from a kit by Hamtronics—the repeater people. It was a very simple circuit but it had a serious problem—it didn’t work. There was an ALC circuit on the board that introduced high audio distortion on its output. Rather than dance through the routine of a call for help, I sketched out something simple based on what parts were in stock at the time (my Junque Box). Because audio came from several sources, including the phone line, a tape machine, and the repeater receiver, ALC was required so that the audio level into the DTMF decoder was within certain limits for proper operation and minimum falsing. Not precision leveling, but it worked without a hiccup.

Vin p-p	Vout p-p	dB	Gain
0.04	1.2	29.5	30
2.0	2.4	1.2	1.3