Extensive compressor reviews and FAQ

ISP Beta Bass: This is a full-featured rackmount bass preamp that includes a compressor. ISP is known for their Decimator noise reduction system, and a few years ago they also began marketing amps, powered speakers, and preamps for guitar and bass.
 
The compressor has only one control knob, for threshold. However its other internal parameters (attack, ratio, etc.) are program-dependent, which means the envelope of the compression, and the amount of make-up gain, adjust automatically depending on the input signal. This type of automatic system doesn't always work so well, but in this case they did a very good job. The action is decently smooth and not obtrusive. The ratio is moderate--good for general control of levels, but not a hard limiter. It will limit big spikes if you set the threshold to an extreme low level, but then the whole signal gets extremely compressed, and kind of weird-sounding.
 
There is no loss of lows or highs, although under heavy compression the highs can get muddy sounding for a moment, just as with the dbx units. In fact overall I would say the ISP comp is fairly similar in tone and action to a typical dbx in Over-Easy mode. Not magical, but useful. There is little or no noise at most settings, and only noticeable noise at extreme threshold settings.
 
The Decimator function is essentially just a gate. From their advertising, I thought it would be more of a complex and subtle noise-filtering system, but really it just opens and closes like any other gate--and frankly I've heard better. The edge of each opening and closing is abrupt and unnatural. The Exciter function is almost exactly like the high-frequency process of the BBE Sonic Maximizer. It does add brightness and clarity to the high end, but it can also sound artificial if you over-do it. The EQ, including high and low shelving and two bands of semi-parametric mids, is well-designed and effective. The tone of the preamp overall is clean and full, and a bit dark unless you boost the treble some.
 
The various functions do not have any bypass switches. The preamp has relatively low gain overall which means the output is designed for power amps with a low input sensitivity rating, like around 0.75V. If your amp's input sensitivity is higher than that, you'll need to boost your signal in order to get full power from the amp. The headroom of the preamp is high, so the input stage does not distort easily. The construction seems quite rugged.
 
Price in USD: new $643, used about half that.
 
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