Extensive compressor reviews and FAQ

The Engineer's Thumb: This is a DIY project, not a ready-to-buy product from a brand, though several small-scale builders do offer completed versions for sale. It gets its name from a Sherlock Holmes story, and the fact that recording studios originally did not have compressors, instead relying on the engineer to adjust the volume by hand for every second of every track.
It uses an OTA circuit like the Dynacomp/Ross types, but it is not a clone of those. Here is the forum thread where the circuit's creator, "Merlin" AKA "ValveWizard", announces and explains it all with a schematic and several images. The basic version just has the standard Comp and Volume knobs, but the circuit can easily be expanded to include Attack and other controls.
The good news is that it does everything he claims: it has similar quality and amount of sustain and action as the Dynacomp, but with lower noise and wider frequency range. On the other hand, it does not reach the high audio quality and low noise of the Wampler or the Smoothie, for example. So it really is better than a Dynacomp, and it's a great and simple DIY project, but it is not the end-all of OTA designs. If you're looking for a DIY comp project, the Afterlife has much better sound quality. But the Engineer's Thumb is a good base for adding different controls and features, and there are lots of people that have built it who can help you troubleshoot the project.
There is no loss of either highs or lows. The frequency response is mostly flat, though it boosts the low and high ends a bit (leaving the mids slightly scooped) as you turn the Comp up higher. The tone is clean, not crystal transparent but not overtly colored either. The particular one that I am testing has an unfortunate glitch (tic sound) artifact every time my signal crosses the threshold. I don't see comments from other users having that experience, so it may be something in how just this one was assembled. But if I planned on building one myself, I would definitely ask around on the forums to find out if this glitch is something known that can be prevented.
The quality and nature of the construction and cosmetics are entirely up to you or whoever builds it. It runs on standard Boss-type 9V DC. The original Merlin design uses "Millennium bypass" which is a true bypass system that is intended to be superior to just throwing in a standard 3PDT switch, avoiding the pops that plague those switches. The price depends on whether you DIY, or buy one pre-made from someone else. It should be fairly cheap either way.

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