Extensive compressor reviews and FAQ

FMR PBC-6A: My dislike for FMR's RNC and RNLA on bass is well-known, so you can imagine I was hesitant to spend twice as much money on their newer PBC-6A. But enough people requested it that I had to give it a shot. I'm glad I did!
This is a 1/3 rack-width unit like the others, but this one is mono only, and only has balanced XLR in and output (aside from the sidechain and link which are 1/4"). It's meant for line level use, but it has plenty of clean input and output gain for use at instrument level or for taking a low-level signal and boosting it to line level. To test this I used a unity-gain DI box to balance my instrument signal, and it worked very well, without adding much noise from the PBC's input gain being cranked up. The knobs include Drive (input gain), Knee (related to ratio), Attack time, Release time, and Output volume. The Attack, Release, and Output are quite straightforward, but the Drive and Knee need more explanation:
Drive controls a huge amount of available gain boosted into a fixed threshold. With a line-level input, setting Drive at 7:00 gets you light or moderate compression, and with Drive near 9:00 it gets you into heavy compression territory, with zero dynamics. Turning Drive up higher than that results in extreme compression effects, seriously atom bomb amounts of squash. With an instrument-level input signal I got moderate-to-heavy compression with the Drive near 12:00, so there was plenty of gain left to use. The higher you turn up the Drive, the more any noise in your signal chain will be amplified: at minimum Drive there is no extra noise; a moderate Drive level gets you about the same amount of noise as a typical comp pedal like the Keeley; and Drive turned high gets you sheer walls and oceans of noise. But again, this is just your existing line noise being amplified--the FMR does not produce much (if any) noise of its own. The noise ducks down during the compression envelope, so even with extreme settings it is possible to minimize any noise by ensuring the envelope does not "release", by either choosing the maximum release time, or keeping your input signal steadily over the threshold. At the light to moderate compression settings I prefer, there is no noise at all.
"Knee" describes a curve of ratio response--imagine a picture of a person's leg bent at the knee. The ratio of compression will increase, the higher your signal goes over the threshold, and the knee represents how fast it will increase to the maximum ratio. With a "soft" knee, meaning a shallow curve, the ratio increases slowly. So when your signal only goes over the threshold a little, a small ratio will be applied; when it goes higher over the threshold, a higher ratio will be applied; and eventually it will apply the maximum ratio designed into the device. This method provides the most natural and unobtrusive compression action, but it may act too slowly for some purposes. A "hard" knee means the maximum ratio is applied the instant your signal crosses the threshold. This makes it very good for peak limiting and extreme compression effects, but again it may not sound as "natural". Comps that even have a knee control usually only have a hard/soft switch, but the Knee knob on the PBC-6A selects a continuous range of knee curve. This is a great feature, and working with it really is the key to understanding and using this unit.
It doesn't lose any lows or highs, and it handles bass much better than the earlier FMR units. The tone is warm and somewhat colored, meaning it does have a bit of its own "flavor" even at the cleanest settings. It also has a switch labeled "Thick" which engages another level of thick fat tone. The Thick setting, plus a medium Knee, gives an addictive tube-like action and warmth. It becomes a little bit dark and mushy in this mode, which is exactly perfect for when you want a vintage-amp sound, but of course somebody looking for crisp snappy brightness would not be happy. So I would not recommend this unit to somebody who "wants it all", the full gamut from transparent to dirty. But if you are mainly looking for a thick warm furry tone, this is a great choice. It is also capable of some distortion when you crank both the Drive and the Knee--it's not like the unwanted bass clipping of the RNC and RNLA, but a pleasant light overdrive-like sound.
It has a multi-segment LED meter to indicate the amount of gain reduction, which is quite useful, especially when using this unit at lower than line level. The construction quality is very good. I don't know if the bypass switch is "true bypass", but it is an excellent transparent bypass. It is powered with an external wall-wart supply at 9V AC and 500 mA, so it won't run off daisy-chain supplies, and even most "brick" supplies won't do. Normally I don't mention that sort of thing with rack gear, but I could envision somebody wanting to use this on a pedalboard, in spite of the balanced inputs. Bottom line: this unit actually delivers what it promises, and I'd particularly recommend it to people who were considering buying a Distressor but needed something less expensive.

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