[2500] What is a Preamp?

Date: February 1st, 2019 | Comments : [3] | Categories: DIY.

In the last year or so, I have noticed a number of new pedals being introduced that are being called preamps. So what is a preamp, and can a pedal serve as one?

In audio applications, a preamp is a circuit that amplifies very weak audio signals to a level that will drive a power amplifier to its rated capacity.

The typical preamp usually features multiple inputs sometimes with input selection switching, a tone control network, as well as voltage gain. Also, something like a phono preamp will have equalization designed into the circuit to reduce the high frequency bands to counteract the treble boost that is processed into a vinyl recording. This restores the output to a flat response and helps cut noise, hiss and other unwanted artifacts.

The output of a guitar pickup is a weak signal that cannot drive the typical power amplifier to a usable level. It needs a preamp to boost the signal to a voltage level capable of driving a power amp. However, every guitar amplifier that I recall already has a preamp included in its design.

If we look at the classic Bassman amp schematic (shown above), the power amplifier is the circuit inside the blue area. The preamp is the remainder of the circuit to the left side, and it has multiple inputs, tone controls and signal gain.

If you were to build only the power amp circuit from the blue area (with the proper power supply), it would serve as an audio amplifier; however, a strong signal would be required to push the amp to its 50 watt capacity. A guitar signal is far too weak to do the job straight from the pickups into the power amp. Voltage amplification is required to boost the tiny signal up to a level that the power amp needs.

Most guitar pedals that have gain could drive the simplified Bassman power amp to make some noise, but would fall short of getting the most from the amp.

Because of the requirement to match the characteristics of the preamp to the needs of the power amp, most musical instrument amplifiers will have a preamp as part of their design. Audio hi-fi is different, and it is not unusual to find standalone power amps, but they are almost always designed for use with a separate hi-fi preamp that can be selected by the owner.

As was said, a guitar won’t drive the Bassman to a good listening level, but will a pedal? Probably not. Even though the pedal can have a proper input impedance to mate with the guitar pickups, and has some voltage gain, it is still inadequate to drive the power amp to full power. It will certainly be better than plugging the guitar straight into the power amp, but nonetheless, a pedal is probably not up to the job, especially if powered by only 9 volts.

With tube amps, the explanation above is mostly on target. However, solid-state power amps can be a bit different. It is not unusual for a solid-state power amp to only require 1 or 2 volts RMS for maximum output into a speaker. This is because SS amps work with a lower power supply voltage and have higher gain as part of their design. There are many pedals that can output 2vrms, and therefore able to drive a power amp to capacity. In fact, on my test bench, I have a class-D power amp being driven by one of my Tweed Stack pc boards that I use for pedal testing. The Tweed Stack actually has too much gain for the module that it is mated to, so the design had to be modded to lower the gain when powered with the 24v used in the setup. I do not recall ever referring to the Tweed Stack as a preamp, but it is serving as one in this application.

Including “Preamp” in the pedal name can be a way to indicate that the circuit in the pedal was derived from a vintage design, where a part of the original circuit was copied and adapted for pedal use. It does not mean that the pedal was designed to serve as an actual preamp.

Also, since almost every guitar amplifier already has a preamp included in its design, what is the point? Is there a need for a pre-preamp? Or is it just marketing wank? You decide.


3 Responses to “What is a Preamp?”

[786245] Howie Says: 9:36 am, February 6th, 2019

Ideally, I think the term “preamp” should be reserved for devices with a certain P-P or RMS output level. Not sure what those numbers would be, maybe enough to drive, say, a 15 watt tube power section. But in the real world, people will use “preamp” for whatever they want 🙂

[786292] The Eradicator Says: 12:40 pm, March 14th, 2019

Funny… Was just “browsing” some low end stompboxes on Amazon this morning and like 2/3’s of the items that kept showing up were some kind of “preamp” diddle-box, some with half a dozen buttons/knobs or more… I was thinking “wow! Am I missing something here?” Jeez… How do I choose which one is the one for me?? Hahaha there were 3 offerings from the same company just on one page… I would think that if they built one with 6 controls on the face that it ought to pretty much cover whatever possible shortcomings your rig is experiencing…. Is it just me???

[786335] admin Says: 10:57 pm, March 29th, 2019

If you are just putting the “preamp pedal” into the input of an existing guitar amplifier, then it is just an amp-in-a-box and nothing more.

The reason is that the guitar amplifier already has a preamp that is contributing its characteristics to the sound coming out of the speaker. Put an amp-in-a-box pedal before the existing preamp does not nullify the sound from that preamp, and it would require a specially designed circuit for each guitar amp to do so. I don’t know of any pedal that does this, but it could exist.

You would have to know what amp is being used before you could design the preamp-modification pedal to turn a Fender into a Vox, or a Vox into a Marshall, for example. Without taking that into consideration, the “preamp” pedal is just another overdrive/distortion driving the input of a guitar amp.


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