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PREAMPLIFIERS
PREAMPLIFIER," or "preamp," is a term often used (especially by novice audiophiles) with no real idea of just what such a device is. The picture it all too often conjures up is a box that has a lot of knobs and switches and looks like the front part of an integrated amplifier. That isn't totally wrong, but what is being thought of is in actual fact a preamplifier -control unit, and preamplification is only one of its many functions.
Preamplifiers came into the home audio picture with the advent of the first commercially popular magnetic cartridges. Until then, crystal cartridges were in general use, and crystals generate an electrical signal of about the same strength as a tuner. The magnetic cartridge, though, was quite a different creature: if a magnetic cartridge were substituted for a crystal, playing a record would produce only a thin, weak sound since magnetics have far less output voltage than crystals and require bass boost for flat response. The few thousandths of a volt (millivolts) produced by a magnetic cartridge are just not enough to drive an amplifier originally meant for crystals to any where near full output. What is needed is additional amplification between the phono cartridge and the amplifier to compensate for the amplifier's inadequate gain, or the cartridge's inadequate output, depending on how you look at it.
The first home units for this purpose were little one -tube affairs made by G.E.
(they made the magnetic cartridges, so they obviously had to provide the equipment to make them usable). These units went into the circuit before the amplifier, hence preamplifier. The idea was by no means new; preamps were (and still are) used in a variety of commercial applications. But these new units had to do more than merely boost signal strength. The inherent response characteristics of the magnetic cartridge are quite different from those of a crystal, so equalization (bass boost and treble roll-off) to compensate was included. Immediately, then, these were (properly speaking) preamplifier -equalizers. These little out board preamps were only a stopgap measure for hi fi, however: they became redundant as soon as new amplifiers appeared with preamp functions already built in.
At about this time, hi-fi component design took two separate paths. One of them resulted in an "integrated" amplifier with a built-in preamp; the other removed the controls, switching, and first stages of amplification and put them into a separate, re mote unit (often powered by the main amplifier). The little magnetic -cartridge preamp was incorporated into these remote units (as it was into the single -chassis amps), thereby establishing for the home user the multipurpose module now known simply as a "preamp." Initially, the controls were just the basic ones then in use on most integrated amplifiers: volume, bass, treble, input selector, and, perhaps, a loudness switch. As home systems became more sophisticated and complex, more and more control functions became necessary or at least desirable.
Switching appeared for remote speakers, and facilities for tape decks were added when these became popular. Now it is possible to find units that can accommodate two or even three tape decks (as well as a couple of turntables) for a really elaborate system. Stereo, of course, added another batch of controls: channel balance, left -
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right reverse, mono -stereo, etc. In time, many units began sporting switches for choosing the turnover points of the tone controls, and some even added a third tone control for the midrange.
For a long while, if one wanted maximum flexibility and quality, the separate preamp control unit was the way to go, but the use of miniaturized circuitry changed this. It became practical to incorporate a respectable number of controls and a pair of power amplifiers on one chassis of relatively convenient size. Today, even receivers, once not even considered real "audiophile" equipment, are available with enough controls to keep the knob -happiest of us busy.
In a way, the size problem is still with us, though. On a receiver or integrated amplifier the number of controls will be proportional (mostly for marketing, not technical, reasons) to the power output and price of the unit. Anyone requiring only a low- or medium -power amplifier but a great variety of controls should therefore look for a separate preamp; an integrated amplifier or receiver with maximum control facilities could prove too large and/or expensive be cause it would likely be the most powerful in the manufacturer's line.
With the growing popularity of such out board units as equalizers (see page 72), noise -reduction units, time-delay (more correctly, signal-delay) devices, and other types of signal processors, there are now a number of "preamps" available that go well beyond the basic control functions. Some preamps now not only allow you to change the volume of your music but will alter the acoustic ambiance of your listening room and correct for compression in recordings; a few even include graphic equalizers. Also, lo and behold, we now have another kind of phono cartridge (moving coil) growing in popularity that usually requires a pre -preamplifier. These pre-preamps (also known as head amps) are now being built into preamplifiers. (You are following all of this, aren't you?) NOT to complicate matters, but there are those audiophiles who don't really hold with any of this. Their belief is that any interference with the audio signal, even that of a tone control, constitutes distortion. This is not the place to go into the philosophical complexities of this position except to point out that there are two controls we really can't do without: the volume knob and the mode selector, but even these might be open to argument. However, there are some few preamps available that have minimal control facilities, and this is a case of less not being cheaper: the units are generally of a quality that commands a high price.
"Preamplifier," then, has become a generic term covering a rather wide variety of units. What's right for you depends on your own situation. There is no stock solution, but here are a few factors to consider: all -in -one units cost less (per feature) than separates, but they may be less versatile; smaller separates (individual, non-integrated components) could be easier to handle physically than large all-in-ones, but may also represent considerably more money; per haps the particular functions you want are available only in separates. As usual, you'll have to find the point at which the scales balance best for you.
Also see: GOING ON RECORD, JAMES GOODFRIEND
Source: Stereo Review (USA magazine)