Extensive compressor reviews and FAQ

 
How to use your amp's effects loop, and pedals vs. rackmount:
 
Generally speaking, pedals are meant to go between your instrument and the main input of your amp, while rackmount units are meant to go in an effects loop, or between a preamp and a power amp. Please see this article for more explanation about preamps.
 
"Line level" is usually designated as +4dB on professional studio gear. Older gear, as well as inexpensive consumer gear, often runs at -10dB. "Instrument level" is more widely variable, but is usually somewhere around -20dB. And "mic level" has the widest range, but is typically lower than instrument level, perhaps in the neighborhood of -30dB. Ideally you want the devices you are connecting to match up in their ideal range of input and output levels, otherwise you may get distortion, noise, or a weak signal.
 
Pedals are usually designed to run at instrument level, which means they will operate best with the input signal from your bass or other pedals, and the output needs to go into a preamp (or amp head/combo). The output of most pedals is not the right level or impedance to work well going directly into a power amp or the effects loop of most amps.
 
Rack units are usually designed to run at line level, which means they operate best taking the output of a preamp. An effects loop typically runs at line level (either +4dB or -10dB, with -10dB being more common on amp heads), and is a direct connection between the preamp and power section of that amp. The signal from most basses is not the right level or impedance to go directly into most rack processors. Technically you can plug your bass straight in, but it will not work nearly as well as if you put a preamp between them, and it may not work at all depending on the particular gear.
 
There are exceptions: Some rack units have a "Hi Z" input, which is designed for you to be able to plug your instrument straight in; and some rack processors (especially older guitar-oriented units) are designed to operate at instrument level, or at least have enough input gain available to be used that way. There are also some effects loops which are able to operate fine with pedals in the loop- you'll need to check the manual for your specific amp, or just experiment. And finally, some pedals can handle a line-level signal. One fairly common example would be a loop that operates at -10dB, paired with an fx pedal designed to work well with active basses -that combination can often work out fine. Whatever you put in an effects loop, be sure it is able to put out a signal at least at high in level as the output of the preamp.
 
When experimenting, listen carefully and ask yourself these questions: Do you hear any distortion? Does the signal seem weak? Does the compressor seem to react too strongly, or not enough? How bad is the hiss, and does the hiss change in level or "quality" when you switch positions? Does the comp have meters to indicate signal levels or compression amount, and do those meters seem to read the same or differently in each position? If the levels are not matched up well between devices, you may hear distortion, noise, or a weak signal. Those are your signs to adjust the levels or change the placement of your effects and the amp. Fortunately, you won't damage either the amp or the effects in this process.
 
Another set of factors to consider:
-Pedals almost always have 1/4" unbalanced (AKA regular instrument cable) inputs and outputs;
-Rack processors may have either unbalanced or balanced (AKA XLR, mic cable, 1/4" TRS/stereo) in/outputs;
-The jacks for connecting a preamp to a power amp may be balanced or unbalanced;
-And an amp head's effects loop is typically unbalanced.
 
Note that 1/4" balanced and unbalanced jacks look the same from the outside. So you'll need to identify which types of plugs and cables are needed for optimal signal quality and connection of your specific gear. It won't hurt either your amp or your effects to experiment with connecting them different ways though, and sometimes experimentation is the best way to find the results that sound best to you. The only thing you cannot do is place your compressor or other fx after the output of a power amp, as that would kill your gear.
 
Effects loops can be "series" or "parallel". Series means one device leads straight into the next, with no signal splitting. Generally speaking, a serial connection is best for a compressor (although there are esoteric exceptions). Parallel means the signal from your preamp is split into a "clean" channel and an "effected" channel, and then those two are blended back together. Parallel loops typically have a "blend" knob (wet/dry). If your fx loop is parallel, you may occasionally find that the wet and dry signals interfere with each other, causing spikes or dropouts of signal level at different frequencies. A loss of lows is common. If that happens, you can solve it by either setting the blend knob to 100% wet, or using a device to adjust the phase of one of the signals.
 
Rackmount gear is not necessarily better quality than pedals. There are good and bad units in either format. That said, rackmount gear sometimes offers more controls over the compression than you'll find on most pedals, and good-quality rack gear may offer a better signal-to-noise ratio.

 
 
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