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Cassette deck audio limiter, how does it work?
Cassette deck audio limiter, how does it work?
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Old 15th September 2019, 02:03 PM   #21
Mooly is offline Mooly  United Kingdom
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Cassette deck audio limiter, how does it work?
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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Old 15th September 2019, 02:45 PM   #22
MarcelvdG is offline MarcelvdG  Netherlands
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How about the distortion at low audio frequencies?
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Old 15th September 2019, 05:15 PM   #23
Mooly is offline Mooly  United Kingdom
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Cassette deck audio limiter, how does it work?
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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Old 15th September 2019, 11:36 PM   #24
PRR is offline PRR  United States
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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).
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Old 16th September 2019, 01:59 AM   #25
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by Mooly View Post
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.
Thatīs 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.

Itīs "soft assymmetrical" and produces lots of even harmonics.

Sounds better than it measures.
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Old 16th September 2019, 07:07 AM   #26
Mooly is offline Mooly  United Kingdom
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Cassette deck audio limiter, how does it work?
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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Old 16th September 2019, 07:52 AM   #27
sesebe is offline sesebe  Romania
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Try simulate with a 47-100ohm series resistor with C5 and 100-330nF in place of original position.
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Old 16th September 2019, 08:38 AM   #28
Mooly is offline Mooly  United Kingdom
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Cassette deck audio limiter, how does it work?
Here you go, 82 ohm and 330nF:
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Old 17th September 2019, 11:12 PM   #29
sesebe is offline sesebe  Romania
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Thank's.
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Old 18th September 2019, 05:05 AM   #30
lcsaszar is offline lcsaszar  Hungary
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What about using a jFET as a variable resistor instead of BJT?
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