There's nothing quite like the sound of a well amplified band. With good musicians and sympathetic mixing, the sound can be exciting, clear and musically satisfying.
The same applies to a performance by unamplified acoustic instruments. Whether classical in nature or an acoustic jazz band, with good players and a warm acoustic, the music can sound great.
The problem comes when you try to mix acoustic and electric, or electronic, instruments. When this is done, the result rarely sounds good. There is something about the comparison between the two styles of sounds – the natural acoustic instruments make the electric instruments seem artificial and unmusical; the electric instruments make the acoustic instruments seem subdued and lackluster.
There are two approaches to successfully combining acoustic and electric instruments. Care, attention and thought is necessary whichever you choose.
Probably the immediate instinct is to mike up the acoustic instruments and put the whole ensemble through a PA system. This will work, but at the expense of taking away the natural acoustic sound. Even so, this is a workable approach, particularly if you would prefer the higher sound level that will of course be available.
When amplifying acoustic instruments, the very best thing you can do is to listen to each instrument unamplified. Then listen to the ensemble of acoustic instruments unamplified. When you have that sound in your mind, you can do your best to replicate it through the PA system. This won't be the work of a moment. It is an experience that you will gather each time you work with acoustic instruments, and every time you will get a little bit better. It's all down to mic selection, positioning and EQ.
The other way is to work at an acoustic level, and make the electric instruments sound more acoustic. Some of the advice given here will be counter-intuitive, but that is only because most PA systems are set up in a 'standard' configuration, without too much thought for the precise requirements of the music.
So here is how to make electric instruments, such as keyboard, more acoustic…
Don't use a PA system. That would take you in the opposite direction of the result you want to achieve.
Use a full-range cabinet for each electric instrument . If you play an electronic keyboard through a Fender Twin Reverb then you are guaranteed to get the electric sound. You will lose high frequencies and gain the distortion that is necessary for the guitar player who would normally use an amp like this. Also, don't mix two instruments through the same cabinet. This could never happen acoustically.
Don't use too high a level. It is so easy to turn up the volume. The art in sound engineering is to choose the right level. Any fool can put their foot down in a fast car, but an experienced race driver could take a less powerful car through the curves more swiftly. Use a level that is compatible with the acoustic instruments, and don't force them to play louder than they naturally would.
Use a hifi loudspeaker or nearfield monitor . It's amazing why this isn't done more often. But if you don't need a lot of level, then you don't need a PA cabinet that is compromised in sound quality. Few acoustic instruments are louder than the level a typical nearfield monitor can handle. If you really want to go the whole way on this point, use an electrostatic loudspeaker, which in loudspeaker terms can be compared to a capacitor microphone. A conventional moving coil loudspeaker works like a dynamic mic in reverse.
Don't point the loudspeaker at the audience. Now this is really counterintuitive. But when you consider the directional patterns of acoustic instruments, many do not radiate directly towards the audience – the violin for example mostly radiates upwards. Pointing the loudspeaker directly away from the audience is probably an angle too far, although it can work if a 'wash' of sound is what you require. The best compromise is to set up the ensemble so that they feel good playing together. If that sounds right, then it will sound right to the audience.
There is room for experimentation here, but the result can be extremely satisfying and not sound obviously amplified at all.