To be quite honest, i think that "stereo" isn't exactly the most appropriate method of live sound reproduction, especially when working with a small ensemble. If you picture a typical small band i.e. one guitar, bassist, drummer, singer and maybe a keyboardist, the "soundstage" would get pretty dull and boring. While adding another guitarist on the opposite side would help to balance things out somewhat, you would still be stuck with the vocals and drums centered with the bass and / or keyboards stuck to one side of the presentation or the other.
In this respect, i've always tried to get bands to "stagger" their speaker cabinets on stage. That is, if you've got two guitar players each playing a "full stack", each side would have a cabinet from each player on it. The guitarist would place his amplifier on his cabinet with the cabinet from the other guitarist underneath that. The end result would look like a normal "stack", but each guitarist really had a cabinet on each side of the stage. This specific set up meant that each guitarists' own cabinet was at ear level, allowing the sound from his cabinet to take precedent over the sound of the other guy's cabinet. At the same time, he could still hear the other player's cabinet too, which was right below his. Splitting bass cabinets to each side of the drummer also helps to fill out the sound for the musicians themselves and improves the sonics for those sitting up front and center of the stage.
In effect, this gives you a "wide mono" signal*, which works much better for electrically amplified music. Not only can each guitar player hear the other now, which helps them work with and off of each other better, the sound is more uniform in every aspect. At some of the smaller shows that i did, i would even supply the guitar bass cabinets for the bands. This negated set-up / break down times, making the shows go much smoother and faster.
With this approach, i used four very large cabinets on the stage for the instruments. Each cabinet housed two 18's stacked vertically with two 10's stacked horizontally on top of the 18's. This gave us a total of eight 18's and eight 10's. If i had two guitar players, they would each get two tens on each side of the stage. Otherwise, the one guitarist fed all eight of the 10's. The bass was fed through all of the 18's. The solid bottom end of the bass provided a great amount of "drive" to the music with great dispersion / sound off of the stage for each guitarist.
Using such an approach though, one would have to resort to specific panning tricks on the mixing board, which is what they do in the studio. Unfortunately, they don't get the basic right / left mix right most of the time, which is why they need to play with the balance control so much. When playing live though, you don't typically have such effects. Another nice factor about using the approach that i did is that no side-fill's were really needed as each player could now easily hear the others i.e. they weren't getting drowned out by standing in front of their own individual "stack" of speakers.
The opposite is true of large groups of densely packed acoustically based musicians. One can obtain a very good sense of soundstaging / instrument placement using rather simple microphone placement and a standard "stereo" presentation. With very large groups i.e. orchestra's, i personally think that "three channel" would have worked best i.e. a left, right and a center to blend it all together. Paul Klipsch was an advocate of this approach, but then again, this may have had to do with the fact that his speakers were designed to be spaced in each corner of the room. In effect, using a center channel filled in the gap that existed due to the wide spacing of the left / right speaker when using K-horns. Sean
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* Anyone remember the early bass on the left / guitar on the right ( or vice-versa ) "stereo" recordings? While this may be more accurate of what you see on stage, having all of the drums & vocals in opposing channels is what screwed things up. If they would have centered the drums and panned them "just slightly" off to each side as they are set up on stage, and kept the vocals centered, we might have been down main street. To be quite honest, i typically end up listening to most of these "early stereo" recordings in mono ( i.e. "wide mono with dual speakers" ) as the totally separated presentation of each instrument tends to drive me crazy. Then again, going "mono" with these early recordings tends to degrade the sonics somewhat, so your "damned if you do, damned if you don't", etc...
