To qualify as a legit golden-channel-box (gcb), a device must at least include a balanced mic preamp, an equalizer and a compressor. We would argue that a slap echo and reverb are minimum requirements as well but the bottom line is, it must make your voice sound great.
Mic Pre Amp
The mic pre amp is pretty standard stuff but it should support phantom power since there is a good chance you'll be using a condenser mic at one point or another. Not all mic preamps are created equal but this is rarely the weak link in the chain.
For most common use, the eq is just to make minor adjustments for the microphone you're using, to compensate for a crappy PA or as a special effect (telephone, megaphon, etc..). The eq can also help deal with feedback problems in some situations.
A compressor is like having a magic hand on the volume control turning it down if you get to loud and turning it up if you get to soft. One of the main reasons that tube amplifiers are so popular with guitarists is that they compress the sound of the instrument making it easier to play at a consistent level. Tube amps also distort the sound of the guitars as they compress which is not usually what you want with vocals (there are of course exceptions). A dedicated compressor does introduce distortion, especially when the level is changing but the distortion is usually not desirable and is often masked by the voice. Usually, the more compression you use, the more distortion you get. Less is definitely more so use compression sparingly.
Limiting is the militant cousin of compression. A compressor constantly adjusts the level of the sound whereas a limiter only prevents the volume from exceeding a defined limit. Many speakers have limiters built in to prevent them from being blown out by loud spikes or feedback.
Countless great vocal performances owe their appeal to a simple slap echo. Any singer knows that you belt it out with more intensity when there is a 60-140ms slap echo on your voice. The classic slap echo is 130ms due to the speed of the recording tape that was used to create echos before the digital audio technology arrived in the late 70s. There is probably a psycho-acoustic explanation why echos in that range are comforting to us but a softening of the echoed sound (roll of the high end) enhances the illusion of a wall 100 feet away. Of course, you need to shut off the slap echo if you are speaking which is why it's good to have a button the singer can press to turn this on an off during the performance.
At some point the music behind your voice is going to drop to the point where the lack of reverb is going to be obvious. Use of reverb needs to be subtle but any self respecting gcb will include some kind reverb even if the selection of effects is limited.
Not essential, but the current vocal harmonizers are excellent so, as a singer, you're going to want one in your gcb. Some of the current devices are silly with features but often a simple one voice harmony with formant correction and perhaps a little gender adjustment can make for a truly memorable performance.
A noise gate is basically a magic finger on the mute button when the sound level gets below a defined threshold. The classic use of a noise gate is to hide the hum of a noisy guitar when it is not being played. If you sing below the threshold, the gate will kick in an your voice will be muted. Not good. You can usually adjust the threshold which helps but many gated don't allow you to adjust home much to mute by which is a drag since dropping the level by 6db or so is often enough to hide the noise. Subtle changes in level tend to be less obvious to the listener when you hit the threshold unexpectedly.
One of the ways we like to use a noise gate is to control the audio that is fed into a harmonizer. Every sound the mic hears is harmonized including things like drums and acoustic guitars creating a wash of clashing harmonized sounds. With a noise gate in front of the harmonizer, the only sound that gets harmonized is the loud one made by the singer that exceed the gates threshold. Everything else is blocked. Not all devices support this configuration which is unfortunate since it expands the usability of a harmonizer dramatically.
In The Box
In terms of gear, it's a great time to be a musician. The sound quality of things like inexpensive microphones has gone way up and along with processing power of computer chips that make the magic. What used to require a whole rack of gear now fits into a single foot pedal or rack unit. As a bonus, the settings of all the components can now be saved and restored as a collection. Press one button to load a completely new sound configuration. A life saver in the confusing environment of a typical gig.
The concept of a golden-channel-box isn't exactly new but the horsepower of the current chips has taken this concept to a level that is changing live music forever. Harmonizers actually sound like humans and are smart enough to adjust for key and gender. The circuits that convert from analog to digital (mic to mp3?) have improved to the point of transparency. Now they actually sound too good for some situations which is where the equalizer comes to the rescue. You can still get the warmth of the old gear but there is much less distortion at high frequencies (even at low levels) so the illusion is more like being in the presence of the actual singer or instrument.
While it seems like the prices for a lot of the current gear has not especially gone down, the value of the dollar declines perpetually so the actual cost of the gear has gone down. When you consider how many pieces of gear are combined in a typical piece of audio gear, the price has actually gone down a lot.
It may seem like a hassle to drag around another piece of gear, (it is) but consider how many shows you've seen recently where you thought the vocals sounded good. Probably not many. A golden-channel-box won't solve all your problems but it's a big step in the right direction. If you are frustrated by the sound of your voice when you play out, one of these boxes may be your ticket to Valhalla.
The devices listed below are a good place to start looking for a golden-channel-box to suit your needs.