Audio compressor-limiter is a device, which weakens audio loud signals and intensifies silent signals. On its output there is signal with constant level. Signal clipping on the limiter output allows to increase the signal level without exceeding maximum frequency deviation limit 75 kHz. It’s very suitable since preemphasis is used.

This Stereo Compressor-Limiter have signal overshooting protection and have no precise preemphasis with HF clipping option. With these devices it’s not possible to keep loud sound AND meet the frequency deviation limit. So there is no reason why to pay for them. The device should guarantee basic technical characteristics of the modulation signal, nothing more – no equalizers and other disutilities. Mono Compressor-Limiter Mono Version.

If your transmission chain does not include any similar device, you should build this one. You will be surprised by its quality, efficiency and simplicity.

Characteristics:
Supply voltage: 9-16 V
Quiescent supply current (12 V): 30 mA
Output voltage: linear adjustable 0-3.5 V p-p (0-1.2 V rms)
Lower cut-off frequency (3 dB): input: 25 Hz, output: smaller than 2 Hz
Upper cut-off frequency (3 dB): 14.5 kHz
Min. input voltage: 0.6 V p-p (0.2 V rms)
Input impedance: 5K ohm
Output impedance: 500 ohm
Signal-to-noise ratio: greater than 70 dB

Stereo Compressor-Limiter Schematic

Download Parts List

Tips

Following steps are recommended for the stereo version:

• If you have a possibility to measure static current gain of the transistor (h21e), find a pair of Q1’s with similar h21e from about 5-10 pieces.
• Place a temporary 3-pin IC socket piece instead of Q2’s. Set the same input level value for both channels. Find a pair of Q2’s from about 5-10 pieces which results in the same output level in both channels (you may follow the LED luminance if clipping is set). Then remove the sockets and solder the Q2’s found.

Adjusting Elements Description

Preemphasis on/off – Connect (close) the jumper to turn on the preemphasis – recommended just here.
Attack time, release time – Compressor/limiter time constants. The attack time should usually be as small as possible, the release time determines the signal processing intensity.
Clipping, HF clipping - Recommended adjustment procedure: 1) Set the HF clipping to the middle, set the Clipping closely below clipping level of low frequencies (1 kHz and lower). 2) Adjust HF clipping on a common program content. Check the sibilancy (hiss) reproduction – no distortion should be audible. The more clipping the more signal loudness.
Input level - Gate function analogy. If set right, background sound (noise, hum) present in original signal is not amplified and clipping is working on all common program content.

Stereo Compressor-Limiter Printed Circuit Board (PCB)

Source: Compressor/limiter/clipper – Stereo Version

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