OK, you've just spent copious amounts of hard-earned cash on an acoustic transducer. Now you are told you need a preamp. What are these things? Why do you need one? What does it do? Which features do you need?

What is it?
Simply put, a preamp is a device which processes the signal from a transducer so that it is suitable to plug into a standard amplifier or a mixing desk. Even though amplifiers designed specifically for acoustic guitar transducers are now available (which have acoustic preamps built in), it is preferable to have an on-board preamp.

Why Do You Need It?
Without a preamp, the pickup will sound 'thin' , lacking in bass response and be overly percussive. Cables will need to be as short as possible, and will be prone to picking up EMF interference (Hum, buzz, etc.) Most have equalisation and gain (volume) controls, some even have a feedback reduction control which can be essential for a performance situation.

Many people use a floor pedal equaliser to correct the frequency response problem, but this is far from ideal as this is compensating for the problem rather than fixing it at the source.

What does it do?
The signal produced by an acoustic transducer is low current, very high impedence (Z), typically 2-10 Mohm. A typical input for a mixing desk or amplifier ranges from 150 Ohm to 1Mohm (many will only go as high as 500Kohm). The impedence mismatch between a transducer and a mixer or amplifier causes distortion of the frequency response.

A preamp converts the high impedence signal of the transducer to a lower impedence, more suited to the input device, as well as adding functionality such as volume and equalisation.

Horses for Courses
The large variety of preamp makes, models and options available ensure that, whatever your needs there is a preamp out there with your name stamped on it.

Take time to analyse your playing situation - do you need volume control, tone control, anti-feedback capabilities? Do you have a sound engineer on a mixing desk or are you plugging, playing and mixing yourself?

Do you want to leave your guitar as pristine and unmodified as possible? There are preamps whith no controls which are built into the jack socket of the guitar, needing no extra holes cut in the side of the instrument. Others still are offboard belt clip units, connecting to the guitar via a short lead (with the added advantage of  being able to plug into the guitar directly, bypassing the preamp, if your battery dies).

There are even models which allow you to mix the signals of two different transducers (such as a bridge transducer and an internal microphone).

Price is obviously an issue, but in the case of preamps "Buy cheap, Get Cheap" applies with force, as you may be permanently modifying your instrument with no chance for upgrade at a later date. Stick to reputable brands, as they are more likely to have a good  backup service.

Once you have decided what is appropriate for your situation go out and see what is available. Take your time to choose a model with the features you need that fits in your price range.

Final Word
Now that you have decided to spend large amounts on your dream preamp - don't use conventional batteries in it. The cheaper normal (manganese dioxide) batteries have a tendency to leak - and the acid will destroy your investment totally. Alkaline batteries not only last longer, but if they do by some chance leak, the acid crystallises on contact with the air, rendering it inert and harmless.