Extensive compressor reviews and FAQ

Buffers, boosters, effects chains, long cables, and "tone":
In order to get the least signal degradation between your instrument and your amp, you want the lowest amount of capacitance in the cables, and you want the last device before any long cable to have low output impedance. I'll explain those ideas in a minute, but first it's important to acknowledge that sometimes "signal degradation" can sound really good, subjectively. For one example, cables that have high capacitance are often described as sounding "warmer" or "less harsh" than cables with low capacitance--this is because the higher-cap cable has a gentle rolloff of extra-high frequencies. For another example, people who are accustomed to the sound of an old passive instrument (high output impedance), into their favorite amp, may not like the sound of an active instrument (low output impedance) because it's not what they are used to hearing as a "classic tone". So this article will discuss the ways to reduce signal degradation, but please remember that not everyone will need (or want) to follow these guidelines.
Impedance ("z") is like efficiency. The more efficient the connection, the less work your signal has to do to get from A to B, and the less signal-loss happens along the way. The ideal impedance relationship is a low output impedance (z out) going into a high input impedance (z in). One example would be 1 K ohms z out from a buffer, 1 Meg z in at the amp (1,000 out going to 1,000,000 in). The z in should be at least 10 times the z out, and the bigger the difference between them, the more efficient the system will be.
On the flip side of efficiency is brute force. A Ford Super-Duty pickup truck has poor MPG (efficiency) but excellent torque (applied force). In the same way, some instrument or effect signals will be strong enough to overcome any problem with inefficiency. It won't always sound/work exactly the same, but it may work just as well.
A buffer is a powered device that lowers the output impedance, for greater efficiency, and isolates the signal from interference by other parts of the system. Active pickups and most preamps, boosters, or effects (switched on) contain a buffer; and many effect pedals (like Boss) use a buffer for their bypass switch. But whether any one of those is a good buffer is whole other story. A good buffer for avoiding signal degradation has a very low z out, like under 100 ohms, and is designed for the best sound quality; but too often gain stages and switching stages are designed for low cost rather than good sound, and some are followed by a volume pot or other components that raise the impedance back up! So first identify whether you have a buffer; then ask around to find out its output impedance.
Capacitance is the quality of a cable that determines its frequency range when combined with the other circuit components connected to it. The longer the cable, the more its capacitance adds up to reduce the high frequencies. A reduction of highs, even super-high frequencies that a bassist (for example) would normally never care about, can be perceived as a loss of tone. So lower-capacitance cable works best for long cables. Medium to high capacitance is perfectly fine for short cables. As a very loose guideline, the dividing line between long and short is somewhere around 15 feet. "Medium" capacitance is about 35 pF/ft (picoFarads per foot). Check out this article about cheap vs. expensive cables for much more info about that. If you tend to roll off the highs anyway for the tone you like, then you really don't need to concern yourself about cable capacitance.
Generally, when you have a long cable you should put a buffer in front of it, because the increased efficiency given by the buffer will cancel out any tone loss from the high capacitance of the long cable. If you have a large number of "true bypass" pedals, when they are all switched off they act like one long cable--so in that one instance it can also make sense to put a buffer at the very beginning of the effects chain. If one of your pedals will be "always on" then the signal is buffered at that point. The only reason to put a buffer at the end of a pedal chain is if your pedals have high output impedance when switched on, so that a long cable from pedalboard to amp will load the signal down. Also, it's a balancing act because every device you add to the chain introduces the possibility of distortion, muddy sound, or added noise. So if you are thinking of putting buffers at the beginning and end of the pedalboard, be sure you have picked ones with the lowest-possible noise and distortion.
The bottom line is: if you want the most high-frequency content, and the least "tone loss", you will use low-capacitance cables, and put a clean low-noise buffer in front of any cable longer than 15 feet. Just remember, as I said at the beginning, some people will find that they prefer the tone without those "improvements"!

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