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Definition of a preamp
- Thread starter Mister Mo
- Start date
My simply-stated understanding is that a preamp takes the signal you send it and amplifies it sufficient for the power amp to grab.
and typically provides tone-shaping and other features you might want/need, like compression, effects, various filters, etc.
and typically provides tone-shaping and other features you might want/need, like compression, effects, various filters, etc.
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A preamp is a gain stage that amplifies the relatively low-level signal from a pickup to a level usable to drive a power amplifier. It also isolates the high output impedance of the pickup from loading by the following input stage. It's gets murkier when you start talking about stomp boxes that add just a little gain (along with maybe some other signal processing) that are designed to still be plugged into the onboard preamp of an amp. Technically, I think those should probably be called pre-preamplifiers, or something else to distinguish them from preamps that do provide sufficient gain to drive a power amp directly, or via an effects return on a conventional bass amp. I think that would be a lot less confusing for the average Joe.
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Way outside of musical electronics, in the world of measurement instruments (where I work by day), a "preamp" was the circuit that took care of the physical interface to the sensor. Many sensors don't produce nice output signals that can just be connected to a cable running across the room, because of things like high impedance, the need for additional power sources, and so forth. Some kinds of photon sensors need power supply voltages of hundreds or even thousands of Volts. The preamp took care of that stuff and produced a nice signal where a stimulus was represented by a voltage level. It simplified the job of the signal processing folks to work with relatively standardized voltage levels, and avoid messing with high voltages.
I designed a preamp that had to be cooled by liquid nitrogen, in a vacuum. But we didn't want to require the entire instrument to be in that environment.
Today, the output of the preamp is typically fed into an analog-to-digital converter (ADC), which can do a lot of the processing that you need, but the preamp is still necessary because of that physical interface. In many kinds of measurement instruments, preamps are the only analog circuits remaining in the system.
There are some circuits like that in audio. Things like microphones and phonograph cartridges still need preamps. To make things more convenient, the preamps can all be designed so that their output voltages and connectors obey some kind of standard and are interchangeable. This is super handy in the studio or at the PA console. Unplug one microphone and plug in another. There are circuits that adapt non-microphone signals to the studio / PA standard, such as the ubiquitous DI box.
In the hi-fi and guitar amp world, "preamp" took on a different connotation: The distinction between "preamp" and "power amp" was based on the power requirements for two portions of the system. The signal processing, such as volume and tone controls could be done with low voltage, low-current signals, and produced relatively little heat. Even the preamp section of a tube bass amp runs at a lower plate voltage than the power amp, and uses smaller tubes. The power amp has to manage high voltage and current levels, and deal with waste heat, thus it uses somewhat different circuit design techniques. It also manages a transducer interface, but in the opposite direction: Driving the speaker. Published bass amp schematics are often divided into preamp and power amp sections.
I think "preamp" has been used in the guitar effects world for something that performs one or more of the functions of a preamp found in an amp, such as gain control and EQ, but not much more. But I think it's a loose definition, and if you're looking for a specific functionality, you should search based on your criteria and not on a name that might exclude some beneficial options. As with any art form that has a history, the terminology is often rife with abuse, and asking "what does this thing actually do?" is a good way to cut through the confusion.
I designed a preamp that had to be cooled by liquid nitrogen, in a vacuum. But we didn't want to require the entire instrument to be in that environment.
Today, the output of the preamp is typically fed into an analog-to-digital converter (ADC), which can do a lot of the processing that you need, but the preamp is still necessary because of that physical interface. In many kinds of measurement instruments, preamps are the only analog circuits remaining in the system.
There are some circuits like that in audio. Things like microphones and phonograph cartridges still need preamps. To make things more convenient, the preamps can all be designed so that their output voltages and connectors obey some kind of standard and are interchangeable. This is super handy in the studio or at the PA console. Unplug one microphone and plug in another. There are circuits that adapt non-microphone signals to the studio / PA standard, such as the ubiquitous DI box.
In the hi-fi and guitar amp world, "preamp" took on a different connotation: The distinction between "preamp" and "power amp" was based on the power requirements for two portions of the system. The signal processing, such as volume and tone controls could be done with low voltage, low-current signals, and produced relatively little heat. Even the preamp section of a tube bass amp runs at a lower plate voltage than the power amp, and uses smaller tubes. The power amp has to manage high voltage and current levels, and deal with waste heat, thus it uses somewhat different circuit design techniques. It also manages a transducer interface, but in the opposite direction: Driving the speaker. Published bass amp schematics are often divided into preamp and power amp sections.
I think "preamp" has been used in the guitar effects world for something that performs one or more of the functions of a preamp found in an amp, such as gain control and EQ, but not much more. But I think it's a loose definition, and if you're looking for a specific functionality, you should search based on your criteria and not on a name that might exclude some beneficial options. As with any art form that has a history, the terminology is often rife with abuse, and asking "what does this thing actually do?" is a good way to cut through the confusion.
I must have got this wrong. I learned it was the comesouta goes to the goesinta.A preamp interfaces the comezinta to the goesoutta.
I must have got this wrong. I learned it was the comesouta goes to the goesinta.
I think mine came from the Navy.
