Extensive compressor reviews and FAQ

Heavy Electronics Sonic Oppressor: This is an optical design with a couple of extra features not found on other common opto pedals.
 
The knobs include Output, Threshold, and Sustain. Threshold controls the sensitivity of the opto circuit, and Sustain controls the amount your signal is boosted into that circuit; in other words they do a similar job, but in different ways, and you use them together to find the response you want. There is also a toggle, labeled Gain, which introduces a huge fixed volume boost in front of the comp circuit. This increases both the compression and the output level. It also features an LED "eye" which glows to indicate the amount of effect happening.
 
The action is smooth and even, with no "dip and swell" or other artifacts. The ratio is fairly low even at the strongest settings, so it is no good for peak limiting. However it is excellent at providing extra thickness and punch to your sound. Basically the range of knob and switch settings determine just how much thickness and punch it adds, rather than the more literal or technical responses you would expect from a VCA-based rack comp's knob settings, for example. I recommend this pedal to people who want to add some fat color, and do not recommend it to people wanting clean precise control over their peaks.
 
It doesn't add much noise, and there is no loss of either highs or lows. At low settings the tone is mostly uncolored, and it gets more flavorful at stronger settings. Additionally, engaging the Gain switch can cause it to overdrive some. I had a hard time determining exactly how much distortion came from the pedal, and how much from my listening equipment. In general I would say it is best used with an amp that overdrives sweetly; in fact your amp's headroom and distortion flavor could determine whether you love or hate this pedal.
 
When I first received it, this unit was not operating correctly. I contacted Heavy and they had me send it in for service. It took several months before they finally returned the repaired pedal to me, and they did not respond to any of my emails or phone messages during those months. On the plus side, they fixed it at no charge, even though I was not the original owner.
 
The construction quality is pretty good. The housing is the "medium" size of Barber or Diamond. It runs on Boss-standard 9V DC. The footswitch is "true bypass". Price in USD: new $190, used $110 to $140
 
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