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A Stereo Audio Compressor

Do you hate the way the sound level on your TV suddenly jumps during the advert breaks? Or do you find that the sound levels vary widely when switching between digital TV stations? Or maybe you have problems listening to CDs or MP3s in your car or against the background din during a party? Are the soft parts too soft and the loud parts too loud? This Stereo Compressor will solve that problem. It reduces the dynamic range of the signal while still maintaining clean sound. The unit is also ideal for use with PA systems.

By John Clarke

Features & Specifications

Click for larger image

Main Features

• Stereo compression

• Input level & volume controls

• Power switch & indicator LED

• Several power supply options


Signal-To-Noise Ratio........ -75dB (20Hz - 20kHz filter) and -79dB “A”
weighted with respect to 1V in and 1V out

THD+N........ 0.005% with compression disabled; 0.007% @ 10kHz & 2:1
compression; 0.17% @ 1kHz & 2:1 compression; 1.6% @ 100Hz & 2:1 compression

Channel Separation........ 58dB (unweighted)

Frequency Response ........ -1.5dB at 10Hz, -3dB at 33kHz

Compression Ratio ........ typically 2:1 from +20dB to -20dB input with
respect to 0.318V RMS at the compressor input – see Fig.3

Power Consumption........ 17mA at 15VDC; 40mA for supplies over 15V;
(±40mA for supplies over ±15V)

COMPACT DISC PLAYERS and many MP3 players give great sound quality but they usually have a wide dynamic range. That means that the sound level can range from almost inaudible through to very loud, all without touching the volume control.

That can be a problem in noisy environments. For example, in a car, while the loud passages can be heard, the soft parts may well be lost due to road and engine noise. A similar problem can occur with PA systems, where crowd noise can drown out quiet passages in the sound.

In those situations, simply turning up the volume does not solve the problem. While the quiet bits may then be more audible, the loud sections can be ear-shattering and may even overload the amplifier, causing audible distortion.

What we need to do instead is “compress” the dynamic range of the signal so that the loud parts are not quite so loud and the soft parts are not nearly so quiet. And that’s what this Stereo Compressor does – it continuously adjusts the signal level by amplifying the quiet passages and attenuating the louder passages, so that the overall volume range is much reduced.

Listening to TV

A common annoyance for TV viewers is the way the average sound level suddenly jumps during advertising breaks or when you switch between digital stations. Some stations have quite low sound levels and so you have to turn up the volume. Then you switch channels and you get blasted! That’s bad enough but it’s much worse if you’re listening via headphones.

Again, an audio compressor is the answer, assuming that you’re using an external amplifier. By making the volume more constant, it will enable you to set the volume to a level that’s comfortable at all times. It sure beats having to hurriedly hit the “mute” button each time there’s an ad break.

PA systems & mood music

Apart from its use in cars and for listening to TV via headphones, an audio compressor is a “must-have” item when it comes to PA systems and mood music. That applies whether you want to provide background music at a dinner party or you want to pipe music into a PA system at a restaurant. In each case, the problem is the same – all those people talking at once creates a high level of ambient noise which drowns out the soft passages in the music.

Once again, an audio compressor is the answer to this problem.

Not all audio compressors are as effective as this design though. One problem with some units is that they markedly increase the noise at low signal levels due to the much increased gain at those levels. However, this problem is largely avoided in our unit because it features a “downward expander”. This reduces the gain once the incoming signal drops below a certain level (or threshold point).

As a result, the noise produced is considerably less than that from units that lack downward expansion.


As shown in the photos, the Stereo Compressor is housed in a small slim­line plastic case. It has two rot­ary controls, one to adjust the input level (which sets the amount of compression) and the other to adjust the volume (or output level). A power switch and an indicator LED are also included on the front panel. Four RCA connectors on the rear panel are used for the inputs and outputs.

Various power supply options are available for the Stereo Compressor. It can be powered from AC or DC supplies, eg, a DC or AC plugpack, a 12V battery in a car or from the supply rails of a power amplifier. Table 2 shows the various options.

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