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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 12th September 2019, 01:35 PM   #1
6V6dude is offline 6V6dude  Australia
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Default Cassette deck audio limiter, how does it work?

I got couple of cassette decks that have audio limiter, some had this feature in the early days. It's actually very useful, not just for recording cassettes but for using as a basic limiter. I use one deck to level TV sound for example. Anyone knows how these actually work? Is it diode based soft clipping or something like that? I don't see anything that would resemble compressor circuit in these decks. It would be great it it was possible to make this limiter as a stand alone.

Last edited by 6V6dude; 12th September 2019 at 01:38 PM.
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Old 12th September 2019, 01:55 PM   #2
Mooly is offline Mooly  United Kingdom
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Cassette deck audio limiter, how does it work?
They are often very simple affairs consisting of a control element such as FET (used as a variable resistance) and a means of driving it in relation to the audio level.

Have a look at some old service manuals to see how it was done. The Sony TC136SD was the first deck I owned and that had a limiter.

Straight from the web:
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File Type: jpg Annotation 2019-09-12 145248.jpg (45.9 KB, 191 views)
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Old 12th September 2019, 02:08 PM   #3
Galu is offline Galu  Scotland
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Further to Mooly's contribution, I attach a photo of the limiter switch on the Sony TC136SD along with the relevant schematic.
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File Type: jpg Limiter Switch.jpg (65.9 KB, 192 views)
File Type: jpg Limiter Circuit.jpg (483.6 KB, 180 views)
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Old 12th September 2019, 02:14 PM   #4
Mooly is offline Mooly  United Kingdom
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Cassette deck audio limiter, how does it work?
That brings back happy memories

So an npn transistor and rectifier for that one. It certainly worked well enough.
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Old 12th September 2019, 02:35 PM   #5
Galu is offline Galu  Scotland
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Quote:
Originally Posted by 6V6dude View Post
It would be great it it was possible to make this limiter as a stand alone.
This IC may be exactly what you are looking for:

https://www.njr.com/semicon/PDF/NJM2762_E.pdf
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Old 12th September 2019, 02:50 PM   #6
Galu is offline Galu  Scotland
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I found this diyAudio thread which gives a bit more information on using the IC:

Limiter with njm2762
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Old 12th September 2019, 06:49 PM   #7
MarcelvdG is offline MarcelvdG  Netherlands
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Quote:
Originally Posted by Mooly View Post
They are often very simple affairs consisting of a control element such as FET (used as a variable resistance) and a means of driving it in relation to the audio level.

Have a look at some old service manuals to see how it was done. The Sony TC136SD was the first deck I owned and that had a limiter.

Straight from the web:
As the collector is biased negative, shouldn't T2 be a PNP? An NPN will act as an emitter follower with very poor hFE because of it being connected in reverse.

Assuming T2 acts as a rectifying common-emitter stage, how can it be turned on by signals of the order of 25 mV when the base-emitter bias voltage is 0?
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Old 12th September 2019, 07:01 PM   #8
Mooly is offline Mooly  United Kingdom
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Cassette deck audio limiter, how does it work?
I agree it does look a bit strange when looked at more closely. Don't believe everything you see on the web

The image directs to here. It also mentions a BA741 which I assumed would be a 741 but the pinouts are weird:

https://www.electroschematics.com/32...uit-schematic/

Best we look for another example
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Old 12th September 2019, 07:16 PM   #9
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Try TDA7284, you can find it in many 16bit audio (SB16) cards.
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Old 12th September 2019, 07:31 PM   #10
PRR is offline PRR  United States
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> It would be great it it was possible to make this limiter as a stand alone.

1176 limiter
RNC
dBx-266
Top 20 compressors all time
UA-2a
Gates Sta-Level

Clipping diodes, shearing the peaks away, sound awful.

How would you do it manually?

Keep your hand on the Volume knob. When it gets too-LOUD, turn-down. When it is not-loud and has been turned-down, sneak back to original volume.

People don't do well at this task. Slow, and tend to miss peaks. This was a major problem in talking movies (light-valve clang) and radio broadcasting (overmodulation splatters into adjacent channels, unless the 25,000 Watt modulator blows-up).

What we need is a circuit to reduce gain according to an electrical signal. This signal compares output to a set level, quickly signals "less" when LOUD happens, and slowly signals "more" when the loud has passed.

There are dozens of ways to do this.

The most weekend-friendly plan is well exampled by the Orange Squeezer guitar pedal.
http://beavisaudio.com/schematics/Da...-Schematic.htm
A resistor and a JFET form a voltage divider. A gain stage boosts-up level, drives output and also a rectifier with separate attack and release time constants. R3 R5 are a cheap trick to reduce JFET (actually circuit) nonlinearity.

The OS is scaled for guitar level. For hi-fi line you may want some attenuation in front. (Yes, leave the gain stage at gain >20 or you will overdrive the JFET.)

Good stereo matching is difficult with JFETs (also LDRs, which are going out of style).

The excellent Nakamichi 550 limiter is the same idea with a fancier rectifier (integrated in the meter driver).

Sony used a BJT.

There are more elaborate methods.
Attached Images
File Type: gif OS-simple.gif (9.7 KB, 166 views)
File Type: gif Nakamichi550-limiter.gif (25.6 KB, 159 views)
File Type: gif Fairchild670-thumb.gif (68.8 KB, 40 views)

Last edited by PRR; 12th September 2019 at 07:34 PM.
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