Power attenuator (guitar)
Encyclopedia
In conjunction with an electric guitar amplifier, a power attenuator
is used to divert and dissipate some or all of the amplifier's excess or unneeded power in order to reduce the volume of sound produced by the speaker.
by overdriving the output stage. Under such a condition, the amplifier will produce at or near its maximum output power. The resulting volume level, however, may be considered unsuitable for many playing conditions. By reducing the amount of power delivered to the speaker, power attenuators reduce to desired levels the volume of sound produced by the amplifier, without altering the quality of the tone that is being sought.
The most common approach to power attenuation is the 'power soaker' approach. In a 'power soaker', a portion of the power is absorbed by the power attenuator (which is placed in circuit between the output transformer and the guitar speaker), while the remaining portion of the output power is directed to the speaker. An example of a common type of attenuator circuit is the L pad
. A variable L pad is a power divider circuit that is designed to operate with a load in such a way as to provide an adjustable level of power to a speaker while maintaining a constant load impedance on the amplifier.
A different approach, using a Variac or a power scaling circuit, reduces the B+ supply voltage available to the power tubes thus producing power tube distortion at a reduced level such that all available output power is sent to the guitar speaker. The Variac method is not without risk, as the reduced heater voltage applied to the tubes can damage the filament and/or cathode
if not operated within the manufacturer's specifications. In a power scaling circuit, by decreasing just the B+ plate voltage, the cathode bias and screen grid voltage decrease proportionately, while the filament voltage stays constant. However the term power attenuator
may be a misnomer for this type of power control because lowering B+ voltage tends to increase distortion, whereas according to convention, an attenuator should not introduce distortion.
Power Damping is a method of output level control that involves the phase inverter and a potentiometer. The potentiometer allows that stage to be overdriven at reduced output similar to a Post Phase Inverter Master Volume (PPIMV) control.
to select taps on a toaster
coil with low inductance windings. Another early model, simply called 'Power Attenuator' by Active Guitar Electronics, used continuously variable power rheostats. Other models, such as the Marshall Power Brake, add some electrical inductance
or capacitance
to the electrical load (including fans
, light bulbs and coils). There is debate about whether reactive attenuators do a better job of preserving a guitar amplifier's tone.
A guitar amp power attenuator may also offer a line-level output jack for sending the distortion-processed signal through an effects chain, to a recording console, or for the purpose of re-amplifying signal through a larger or smaller amplifier.
If the amplifier is designed to accommodate being driven continuously at full power, the use of an attenuator will neither increase nor reduce the potential of damage to the amplifier. An early guitar amplifier to include a power attenuator was the Jim Kelley amplifier
, which came with its own L-pad type attenuator. Some production attenuators are the Scholz Power Soak, the Marshall
Power Brake, THD Hot Plate, and Weber MASS.
Attenuator (electronics)
An attenuator is an electronic device that reduces the amplitude or power of a signal without appreciably distorting its waveform.An attenuator is effectively the opposite of an amplifier, though the two work by different methods...
is used to divert and dissipate some or all of the amplifier's excess or unneeded power in order to reduce the volume of sound produced by the speaker.
Explanation
With tube guitar amplifiers, it is often desirable to produce distortionDistortion
A distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...
by overdriving the output stage. Under such a condition, the amplifier will produce at or near its maximum output power. The resulting volume level, however, may be considered unsuitable for many playing conditions. By reducing the amount of power delivered to the speaker, power attenuators reduce to desired levels the volume of sound produced by the amplifier, without altering the quality of the tone that is being sought.
The most common approach to power attenuation is the 'power soaker' approach. In a 'power soaker', a portion of the power is absorbed by the power attenuator (which is placed in circuit between the output transformer and the guitar speaker), while the remaining portion of the output power is directed to the speaker. An example of a common type of attenuator circuit is the L pad
L pad
An L pad is a network composed of two impedances that typically resemble the letter capital "L" when drawn on a schematic. It is commonly used for attenuation and for impedance matching.- Speaker L pad :...
. A variable L pad is a power divider circuit that is designed to operate with a load in such a way as to provide an adjustable level of power to a speaker while maintaining a constant load impedance on the amplifier.
A different approach, using a Variac or a power scaling circuit, reduces the B+ supply voltage available to the power tubes thus producing power tube distortion at a reduced level such that all available output power is sent to the guitar speaker. The Variac method is not without risk, as the reduced heater voltage applied to the tubes can damage the filament and/or cathode
Hot cathode
In vacuum tubes, a hot cathode is a cathode electrode which emits electrons due to thermionic emission. In the accelerator community, these are referred to as thermionic cathodes. The heating element is usually an electrical filament...
if not operated within the manufacturer's specifications. In a power scaling circuit, by decreasing just the B+ plate voltage, the cathode bias and screen grid voltage decrease proportionately, while the filament voltage stays constant. However the term power attenuator
Attenuator (electronics)
An attenuator is an electronic device that reduces the amplitude or power of a signal without appreciably distorting its waveform.An attenuator is effectively the opposite of an amplifier, though the two work by different methods...
may be a misnomer for this type of power control because lowering B+ voltage tends to increase distortion, whereas according to convention, an attenuator should not introduce distortion.
Power Damping is a method of output level control that involves the phase inverter and a potentiometer. The potentiometer allows that stage to be overdriven at reduced output similar to a Post Phase Inverter Master Volume (PPIMV) control.
Description
A power attenuator can be either purely resistive, or mostly resistive and partly reactive. The original guitar amp power attenuator, the Altair Attenuator, was primarily resistive, used a rotary switchRotary switch
A rotary switch is a switch operated by rotation. These are often chosen when more than 2 positions are needed, such as a three-speed fan or a CB radio with multiple frequencies of reception or "channels"....
to select taps on a toaster
Toaster
The toaster is typically a small electric kitchen appliance designed to toast multiple types of bread products. A typical modern two-slice toaster draws anywhere between 600 and 1200 W and makes toast in 1 to 3 minutes...
coil with low inductance windings. Another early model, simply called 'Power Attenuator' by Active Guitar Electronics, used continuously variable power rheostats. Other models, such as the Marshall Power Brake, add some electrical inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
or capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
to the electrical load (including fans
Fan (mechanical)
A mechanical fan is a machine used to create flow within a fluid, typically a gas such as air.A fan consists of a rotating arrangement of vanes or blades which act on the air. Usually, it is contained within some form of housing or case. This may direct the airflow or increase safety by preventing...
, light bulbs and coils). There is debate about whether reactive attenuators do a better job of preserving a guitar amplifier's tone.
A guitar amp power attenuator may also offer a line-level output jack for sending the distortion-processed signal through an effects chain, to a recording console, or for the purpose of re-amplifying signal through a larger or smaller amplifier.
If the amplifier is designed to accommodate being driven continuously at full power, the use of an attenuator will neither increase nor reduce the potential of damage to the amplifier. An early guitar amplifier to include a power attenuator was the Jim Kelley amplifier
Jim Kelley Amplifiers
Jim Kelley Amplifiers is the trademark for the vacuum tube guitar amplifiers manufactured by Active Guitar Electronics of Tustin, California between the years of 1978 and 1985. Approximately 600 of these amps were built during that time...
, which came with its own L-pad type attenuator. Some production attenuators are the Scholz Power Soak, the Marshall
Marshall Amplification
Marshall Amplification is a British company, founded by drummer Jim Marshall, that designs and manufactures music amplifiers, brands personal headphones/earphones , and, after acquiring Natal Drums, drums and bongos. Marshall amplifiers, and specifically their guitar amplifiers, are among the most...
Power Brake, THD Hot Plate, and Weber MASS.
See also
- Power scalingPower scalingPower scaling of a laser is increasing its output power without changing the geometry, shape, or principle of operation. Power scalability is considered an important advantage in a laser design....
- Guitar amplifierGuitar amplifierA guitar amplifier is an electronic amplifier designed to make the signal of an electric or acoustic guitar louder so that it will produce sound through a loudspeaker...
- Attenuator (electronics)Attenuator (electronics)An attenuator is an electronic device that reduces the amplitude or power of a signal without appreciably distorting its waveform.An attenuator is effectively the opposite of an amplifier, though the two work by different methods...
- Distortion (guitar)Distortion (guitar)Distortion effects create "warm", "dirty" and "fuzzy" sounds by compressing the peaks of a musical instrument's sound wave and adding overtones. The three principal types of distortion effects are overdrive, distortion and fuzz. Distortion effects are sometimes called “gain” effects, as distorted...
- Isolation cabinet (guitar)Isolation cabinet (guitar)The characteristic sound of a tube guitar amplifier as heard on the majority of professional recordings is achieved by playing the amplifier at high volumes, and using one or more microphones to capture the sound. Turning the volume up causes the pre-amplifier to drive the power amplifier into...
- SoundproofingSoundproofingSoundproofing is any means of reducing the sound pressure with respect to a specified sound source and receptor. There are several basic approaches to reducing sound: increasing the distance between source and receiver, using noise barriers to reflect or absorb the energy of the sound waves, using...