Amplifying the Acoustics
articles from Acoustic Musician Magazine
Amplifying the Acoustics #3
Here it is, an acoustic musicians purgatory. You walk in a room; at the door someone takes your guitar, drills holes in it and stuffs it with wires. He tells you how great it will sound, hands you a cable and gives you a little shove into a room filled with... amplifiers. (... got sound?)
What makes them different? What do they have in common? Which one should I use?
What all these amplifiers have in common are the signal path and the basic audio components that are used. So lets take a quick journey down the signal path.
First stop is the Pre-amplifier; this is the interface between the signal source (pick-up/ mic) and the amplifier. It conditions and boosts the signal. This component has your level (volume) and equalization (tone) controls. One of the functions that is unique to an acoustic instrument pre-amp is that it will accommodate the level and impedance of your signal source [transducer/ magnetic pick-up/ mic]. The better acoustic instrument amps do this, but if youre using anything else, plug into a direct box (DI) first. You dont have to understand; look at it like this, when you have a diesel engine, use diesel fuel.
Next is the effects loop; it isnt necessary but it gets you to the fun stuff: reverb, delay (echo),chorus, flange, etc. The key here is a natural sounding reverb; one that will sound like your acoustic instrument played in a concert auditorium.
Now we get to the power amp: this is the muscle, its what powers the speaker. The lingo for rating power is watts, i.e., a 100 watt or 200 watt amp. By the way 200 watts is not twice as loud as 100 watts. If youre looking at specifications, then SPL (sound pressure level) is the spec that can indicate how much volume of sound a system can deliver. The rule here is to look for a clean, quiet, transparent sound; with enough power to achieve the volume you need without clipping (overdriving the power amplifier) or distorting. Distortion not only sounds really bad with acoustic instruments, it is what usually destroys speakers.
At the end of the signal path we find the speaker system: This is the voice of your amplified instrument. Acoustic instruments and electric guitars require different types of speakers. Electric guitar speakers and amps distort in a certain way so as to create that tone. Presumably when you want that sound, you will play an electric guitar. Studio and stage monitors are also made for specific applications, either as music re-producers or to be very loud and directional (piercing). Usually acoustic instrument amps use two-way speaker systems. This means two different size speakers, i.e. a woofer and tweeter, because a speaker that will accurately reproduce the bass notes sounds unnatural in the treble range. Here are a few generalities about speaker design: horns are very loud and reproduce cymbals and horns (e.g. trumpets) well, but they arent usually the best choice for stringed instruments or vocals; open-back cabinets are used to help electric guitars get that sound (which I go for when I play electric guitar); and large speakers tend to sound unnatural when used for a wide frequency range. Efficiency is an important consideration; the maximum SPL rating of a speaker will tell you more about how loud it can be than the power rating. Remember that sound pressure level is measured in decibels (db) and a 3db increase requires a doubling of amplifier power (watts).
The signal path and its fundamental parts are common to both types of amplifier systems; the one piece amp and the component (PA type) system. By recognizing the different parts of the signal path, you can size up the differences between amps and better evaluate what you need from an acoustic instrument amplifier, as well as what you can expect from a certain model.
Acoustic Musician / February 1997