While it’s technically a compressor, the 500-series is not a box that one actually uses to limit the dynamic range of an audio signal, but rather, it allows the user to add a little (or perhaps a lot) of aggression to an audio signal.


The simple, classy, vintage-esque 545 Optical Disrupter is essentially an optical compressor that applies gain reduction via a pair of VACTROL opto-isolators. The module only compresses the top half of the waveform, which creates second-order harmonic distortion without clipping the signal. The result is an asymmetrical waveform, and while compression is occurring, distortion is the primary function. The 545 is well made and features a 10-year warranty.

The XQP 545’s controls are simple and easy to use. The In switch activates the disruption (i.e. compression) circuit and the Disruption pot, along with the X4 switch that increases the side-chain sensitivity by 4, adjusts the amount of disruption with a red LED illuminating when the signal is being disrupted. The Makeup pot allows compensation for the signal lost to gain reduction. The In switch allows the unit to be bypassed. The box has a set 10 ms attack time and 18 ms release time with a 43KΩ balanced input impedance and a 50Ω balanced output impedance. The frequency response is 4 Hz – 55 kHz (-3 dB) with a maximum output of 22 dBu.

In Use

I’ve been using the 545 for several weeks now, and I’ve found it to be an exceptional tool for adding innovative distortion to various sound sources. When used subtly, it thickens and fattens the sound source. I’ve had good results using it in this manner on vocals, keyboards and guitars.

Where it really shines, though, is when it is used more radically on low-frequency prominent sound sources like toms, kick drum and bass guitar. On these sound sources, it’s magic. When used on bass guitar, it allows the instrument to speak and pop out of a mix without having its level increased.

My favorite application, though, is inserting it on a mono parallel compression drum bus during mix and routing toms and kick drum through it. I push it to the extreme and then mix it in under the normal drum mix and it works magic fattening the entire kit and adding a wonderful level of aggression.

I was initially turned off by the lack of a gain-reduction meter. After spending some time with the device, I’ve come to the realization that this is not an issue since distortion is the 545’s primary function and adding a meter to the design would increase the cost without any real benefit.


While “one-trick pony” is the phrase that initially comes to mind when describing the XQP 545, the device does its one trick, creating second-order harmonic distortion, pretty dang well making the module a winner in my book. Anyone interested in adding some aggression to a tracking or mixing session should give the XQP 545 top consideration. You won’t be disappointed.

Price: $399

Contact: XQP | xqpaudio.com