Cassette deck audio limiter, how does it work?

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Here's something based around the Sony circuit but using an opamp rather than the discrete stuff. It works :) Single rail, it has a gain of around 1 , however when the input signal rises the level is clamped. You could easily experiment with something like this to get the levels you want. It seems to work best (the distortion profile of the limited signal) by running the opamp at highish gain.
 

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It looks OK, this is 100Hz. There is a time constant coming into play on the release of the limiting but that is pretty much how I remember it all worked on the discrete Sony. If you whacked the level up and then returned it back to normal then it seemed to take a few seconds to recover to normal gain. Which I guess is as it should be subjectively.
 

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PRR

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10uFd into three diodes looks like a "fast" recovery which will distort bass audibly.

The amplifier probably "has" to work at high gain because the maximum clean signal on the BJT is dozens of mV (whatever your sim models may claim). Which suggests a fixed pad in front of the thing (in addition to your 3:1 pad already).
 
This is the Sony limiter. It takes its input from the headphone amp driver transistor and the control transistor is used to shunt the audio. The line input feed appears to go to the wiper of the input slider pot (top left) via an 82k which is off the page.
That´s the system I mentioned :) : a bipolar transistor working as a straight voltage(current) controlled attenuator, where control signal comes from simplest of the simple single diode rectifier charging a large capacitor for long time constants.

Millions were made using discrete parts, latest versions included the VCA transistor inside a dedicated "cassette recorder" chip, but exact same working principle.

Works reasonably well as long as signal level *at the transistor collector* is kept around 25mV RMS; definitely not more than 50mV RMS.

It´s "soft assymmetrical" and produces lots of even harmonics.

Sounds better than it measures.
 
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The Sony sounded very acceptable as I recall, and I certainly wasn't aware of any obvious distortion at the time. I imagine there would be some transistors that would suit this much better than others.

The values I picked look to take around two to three seconds to recover from a large transient overload.
 

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PRR

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What about using a jFET as a variable resistor instead of BJT?

There have been *hundreds* of limiters made since the 1930s. Many-many of these have been published and are easily found.

The BJT limiter is notable mostly because it is CHEAP. But even in the world of cheap guitar pedals, the JFET was more often used.

I posted a list of links above. Skimming that and following where it goes will give you far more background than would fit in any Forum.

An iconic FET limiter is the UREI 1176:
1176 Peak Limiter - Wikipedia
 

PRR

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....I wonder what happened to this guy D. R. G. Self, B.A.?

Hey! He wrote a book!

And expands greatly on the subject of limiters. (It seems to be a special interest of his.) And notes "I produced a compressor/limiter design for Wireless World when the application of FETs in this way was relatively new; it was published some years after I designed it."

I STRONGLY suggest anybody interested in the topic get his Small Signal Audio book and digest the chapter "Level Control and Special Circuits".
 
Another solution it to use a light dependent resistor (CdS) and a LED. The photoresistor is used as a gain control element in a NFB loop. The LED is controlled by fast rise, slow decay DC voltage derived from the output signal. This arrangement has pretty low distortion in steady state.
 

PRR

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> use a light dependent resistor (CdS)

Done that a LOT. Some advantages. Increasing disadvantages.

The compelling issue today is that CdS is now a Controlled Substance in Europe, and use is dubious. For this and other reasons, all the Old-Time CdS cell makers quit the business some years ago. You can get them from China. Uniformity is reportedly fair to awful.

Note that when CdS was a big deal, 99% of uses were dusk-dawn light switches and flame-detect in burners. These had no requirement on response speed, and loose requirement on R/light, only in a narrow range (dusk, or good-flame). An audio LDR for limiting must work over a wide range, either "fast" or with a controlled "tail", and we like to have two alike for stereo.

Still perhaps the cheapest way. At an extreme you run a rectifier on your power amp output to run an LED to shine on the LDR in an L-pad at the amp input. That works, though the THD at power-amp input level can be audible.
 
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