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The Leach ... odd to see that many op amps, and current mirrors
Everything has a context.
This Leach paper mentions "broadcast". So not live sound, not 30ips tape or 24-bit. Ultimate performance does not have to be impeccable. But it should be predictable and reliable.
"Many opamps" is relative. Two opamps for a full-wave rectifier is about par. (Half-wave would misbehave on some speech/music waveforms.) One to buff-up CV and one to buff audio does not seem like overkill.
This basic-limiter paper should be read as a sub-part of the Big Design:
A Four-Stage FM Broadcast Audio Peak Limiter.
This includes an exhaustive derivation of FM preemphasis as pertains to pre-limiting before the final preemphasis in the transmitter (exciter).
Reading between lines: Georgia Tech had a student-run FM radio station. Student radio does not over-indulge in "loudness wars". But radio always needs a limiter (overmodulation is too disruptive, and not-much undermodulation is disappointing). It is entirely reasonable that a tech-school student-staff would DIY the station limiter.
And turn to Dr Leach. Who would plan the lesson.
In that milieu, it would be favorable to use ONLY parts deeply stocked in the EE Lab. Dual/quad opamps, BJTs, diodes, caps, and FETs. Vacuum tubes, no way. LDRs, perhaps not. dBx/THAT chips, not stocked and the best use is to copy the Application Notes, nothing to learn there.
In all it uses _26_ opamps, twice for stereo, 52! So the full FM limiter may be a major dent in the EE Lab inventory. But the base limiter is just four opamps, two TL072 or one TL074, you can have every student build this part and report on it.
Note that if you build the base 4-opamp limiter, then add the input/output sections from the larger paper to get "balanced" interfaces, you double the opamp count. (He uses the invert/invert diff-input, not the 1-opamp diff in.)
The UA 1176 is an FET limiter, well regarded, and runs the equivalent of about four opamps (plus a meter driver, not done in Leach's). And transformers to get balanced interfacing.
https://media.uaudio.com/assetlibrary/1/1/1176ln_manual.pdf