Capacitance multipliers are known to designers.
Yes, there are a lot of examples.
Section III.72 of the Philbrick book has the same circuit:
https://proaudiodesignforum.com/images/ ... Part_3.pdf (~23 MB)
A lot of design ideas are (or were) circuits which have been published previously but lost or "rediscovered" by each new generation of engineers.
The bootstrap op amp articles are one that keeps re-appearing every few years.
Another thing to keep in mind with many of the Design Ideas are that they solve very specific problems and aren't necessarily being used in mass-produced products.
Sometime the quickest way to achieve a result might not be to use the least-expensive or use the fewest parts but the project winds up costing less due to the time savings.
Test and measurement gizmos are one that immediately come to mind.
Sometimes you only need to produce one.
One guy's solution to a very unique one-of-a-kind problem may give another guy an idea in a totally unrelated field.
A third problem are that the original publications no longer have these archived.
If they do, they often lack the figures and are worthless.
I'm posting this stuff because a lot of these articles are simply being lost.
The "Lefferts" LED voltage reference article I posted from 1975 is one of this site's most popular downloads:
https://www.ka-electronics.com/images/p ... 5_1975.pdf
I had a problem with companding the output from playing records with too much very LF content (like not centered record holes, or warped records). The compander's compressor stage would happily compress the extra rogue LF content but the tape recorder would scrape off that extra very LF content. Upon playback the expander was not getting the same full signal envelope to establish accurate playback gain tracking. The missing LF content would cause a phantom modulation to the rest of the remaining audio signal.
My fix was an added input cap at the very input to the compressor stage (which was inverting overall), with a resistor from the inverted compressor output. As the compressor gain varied with signal (higher gain at low signal level, lower gain for high signal levels) the pole created by the RC shifted from full LF response at high levels, to significant LF cut at low levels (when the compressor gain was max). This effectively reduced the impact of rogue LF content on low level signal passages while still delivering wideband response for high signal levels. With high level signals the envelope was dominated by real music so the lost LF content from the tape response didn't cause audible artifacts.
That's very clever. Makes me think about ways to make a program-dependent HPF for a compressor sidechain.
