The dynamic range of a sound is the difference between its loudest and softest levels. For example, as you play your bass, you’ll probably find that some notes (such as notes played on the upper frets of the lowest string) are considerably louder than others. The function of the compression circuitry in the TX600 amplifier is to reduce overall dynamic range by automatically reducing the level of the loudest sounds you play so that they are closer in level to softer ones. The end result is a smooth, even sound.
The front-panel COMPRESSOR control determines the amount of compression (peak signal reduction) by adjusting the threshold. Threshold is the limit above which compression is applied. As you turn the COMPRESSOR control clockwise, the threshold level is lowered. The compression ratio is set to around 2:1, which means that any input signal over the threshold level will be halved.
The TX600 compression circuit follows the incoming signal, and will adjust the compression ratio down when you play notes with fast attack to avoid “squashing” your sound. This is useful when playing fast staccato passages, as it will cause all the notes to sound at the same level, even if the signal is above the threshold level.
The TX600 provides front-panel compression lights that visually indicate when the audio signal crosses the threshold level and activates the compressor circuit. When orange, no compression is being applied. When the indicator light yellows, compression is being applied to the incoming signal.
Compression has three main uses. First, as described above, it “evens” out the notes played by your bass so that they all are at virtually equal levels. Second, it adds “punch” to a sound. Since all levels are nearly the same, you can play with greater force without worrying about the loudest notes distorting. Finally, it serves to protect your loudspeakers from damage as a result of brief (transient) high output levels, which might otherwise be caused by slapping, finger-popping or other performance techniques.
Whether or not you need to use compression with your TX600 will be a matter of personal taste and playing style. Experiment and see if you like the effect. If you usually play at low volume levels, you’ll find that even with the COMPRESSOR knob turned up, the compression circuitry may have no audible effect. In general, if you don’t need compression, leave it off.