Playing the much-appreciated role of "poor man's plate reverb", spring reverb units have been enjoying a renaissance as of late. Thankfully, they've taken their place among a growing set of tools that battle the cold, unforgiving world of digital. I've had a Tube Works Real Tube stereo unit for years and have come to rely on it for its lush, analog reverb trails and its ability to avoid sounding "spitty" on vocals.
Zerotronics has stepped up to the plate with a smart, cost-saving design they call CoolSprings. The unit is intended to be used mainly during mixdown with a console's aux send, and thus has one line input and two discrete mic- level outputs. That's right, I said mic-level. There's no gain stage or power supply. In fact, there are no controls whatsoever; the CoolSprings design is completely passive. The user plugs the outputs into a mic preamp or console mic input to bring the reverb level up to line level. Works like a charm, and for the thrifty-minded like myself, it's a way of using an otherwise neglected mic preamp during mixdown.
How does it sound? Very lush, smooth, and dense for a spring reverb. Very 3D. As is usually the case with spring reverb, it is not always a first choice for percussive sounds (think "boing"), but it shines on vocals, guitars, keyboard pads, etc. In fact, I would go so far as to say it sounds quite "plate-like" if used on these sources. The Zerotronics website (an appealingly snarky site, I may add) has some handy MP3 and AIFF files that you can check out, as well as a diagram and description of what's actually inside the chassis.
It bears mentioning that each unit is made to order. You can specify whether you want short, medium, or long decay times. You can even order a mixture of different decay times for an extra dense response. Also, the unit can be configured for conventional horizontal rack-mounting, or vertically for potentially saving space. ($499 direct, discounts for multiple orders; www.zerotronics.com)