Power Amps
Power Amp Fast Facts
There is a direct correlation between the guitar player’s amp and musical style. The two basic categories are clean sound and distorted. Usually, an amp that is used for clean, clear musical passages will have to be more powerful than a distorted amp at comparable volume levels. Distorted signals contain more harmonics and are easier to hear than the more fundamental notes of a cleaner signal. Also, an amp that is rated at 50 watts will put ut more than that when it is overdriven and distorted. The power rating is measured at a low distortion specification, usually less than 5%.
A Brief History of the Power Amp
What started out as a way to make musical instruments louder has become an industry that is responsible in part for the kind of music we hear today. The electric guitar is one of the mainstays of rock, country, fusion, and jazz. Without an amplifier there would be no electric guitar. This combination of guitar and amp has transcended the capabilities of acoustic instruments and has created ways to achieve new and exciting sounds. The creaming and powerful sound of rock, the crying and twanging notes of country, the sustain and blistering scales of fusion, and the staccato precise playing of jazz, all came about for the guitarist since the invention of the electric guitar amplifier.
Electronically, the guitar amplifier is a fairly simple, straightforward device. Guitar amps may have other goodies (reverb, tremolo, distortion, etc.) in addition to the basics, but always the preamp, power amp, power supply, and speaker will be there.
The initial signal processing is performed by the preamplifier. The preamp increases the signal level from the guitar, controls the output level via a volume control, and modifies the frequency response by the tone controls. The guitar’s output varies with pick attack, guitar volume setting, type of pickups, pickup height, and strings. The average level for a single-coil pickup such as a Fender-Stratocaster is about 0.1 volts wide open with a strong pick attack. A Gibson Humbucking Pickup is about three times more powerful, and a high-output custom wound pickup can put out a volt. When the maximum level of clean power is attained, the signal level at the output of the preamp should be about 1 volt for solid-state circuits and 8 to 10 volts for a tube amp. This signal then goes to the power amp which drives it to full power before the point of clipping.
For a 100-watt amplifier with a 4-ohm speaker load the voltage level at the output is 20 volts. Using the 0.1 volt level of a Stratocaster as an example, the voltage amplification from guitar to solid-state preamp output (1 volt) is by a factor of 10 (1 divided by 0.1). The amplification of the power amp is 1 volt increased to 20 volts at the speaker. The amount of amplification of voltage is called gain. So, the gain of the preamp is 10 and the power amp is 20. The overall gain required to drive the amp to full power is 200 times the input signal. Gain can also be expressed in dB (decibels). Amplification of voltage in the above example would be: 20 Log(200) = 46dB of gain. Guitar preamplifiers usually have much more gain available, initially, because pickups were a lot weaker.
View entire Power Amp Collection
Copyright © 2007
Musical Instrument Shopping Store
All Rights Reserved
