The Steel Guitar Forum

Hi Everyone,

Please forgive this embarrassingly basic question...

In the context of amplification for a standard Pedal Steel guitar:

What is the difference between a PREAMP (like the sarno revelation tube preamp) and an AMP (like a VOX AC15)? Is is strictly the power of the signal out?

If you have a Preamp, do you always need an amp downstream to generate adequate power for the speaker(s)?

And if you do need an amp downstream, why do you need/want a preamp upstream?

Any discussion of the thought process around when a preamp +/- an amp would be used with a pedal steel would be appreciated!

Thanks

Gary

An amp like an AC15 essentially has two stages of amplification: the preamp, where the tone is shaped, effects are introduced, and the volume is set; and the power amp, which takes the output of the preamp and converts it to a signal that can drive the speakers.

This is a bit of an oversimplification, but it works for describing the Sarno, which cannot drive speakers on its own. You can run it into a full amplifier or just into a power amp, but either way you need a power amp stage after it to drive speakers.

People use preamps because they offer additional tone shaping capabilities. Nowadays, it's also popular to mix the preamp of your choice with a Class D power amp; these are physically small but can still be very powerful, resulting in a compact but loud amp. But since an amp like an AC15 already has a built-in preamp, it's not necessary to have a separate one - some players just like the resulting sound one way or the other.

I often wonder this same thing. A player buys an amp and just raves about it. Then he or she adds a bunch of pedals and preamps in front of it to make what they like sound better.

Hi Gary,
Don't worry about the question, we all have to learn sometime.... Anyway, the terms are used in relation to the signal level out, like you said.

A normal amp's input signal is about 1/2 a volt (or something like that...) A microphone, or guitar pickup's output is much lower than that. Hooking up the guitar or mike directly to the amp would produce insufficient volume for proper use. (Plug your guitar directly into the "Main Amp Input" bypasses the pre-amp circuitry....and...not enough volume.)

The pre-amp brings the low signal level up so it can be used by the main amp which is what drives the speaker. So "Yes" to the other question "Do you always need an amp downstream...", because each amp (pre vs. main) serves a different purpose.

The 'pre-amp' itself is built into most guitar amps, and that may be why it is not seen as a separate unit.

In addition to what was already mentioned, I hope this helps. .........Pat.

That is helpful! I didn't realize that most "guitar amps" actually have a built-in "pre-amp" built in.

It also seems like it's the pre-amp that is doing the shaping of the sound, not the amp...correct?

Thank you!

Gary

The preamp is the tone controls and what you plug into.

The amp takes the preamp signal and amplifies it(makes it louder).

A Fender Twin instrument amplifier contains both...as most other makes do too.

Old stereo systems might have had them as separate components.

A modern day studio might have *favorite* preamp for microphones...with each having their own unique *sound*.

Unpowered speakers will have separate amplifiers to power thwm.

Gary, consider that each stage of the process is a stage in a system…and each stage of the system adds a certain usually unique sonic element.

Preamp takes the signal generated by detecting variations of a magnetic field from the pickup and boosts them, adding some EQ control. That preamp has a specific sound.

Then the boosted and EQd signal goes to a power amp, which also has a specific influence on tone, and boosts that signal to an audible level. It’s the combination of preamp and power amp that allows us to hear what’s being played.

If recorded, the instrument preamp/power amp system goes thru a mic, then it’s own preamp to the recording medium (tape, digital, whatever) so that signal chain has its own system. The system concept is applied at each stage of the end result, your playing to what you hear.

Quoting most successful recordists : everything matters. If the instrument system is good but recording system isn’t, then tone suffers. If the instrument system is less than wonderful, the tone suffers.

Thank you to all who replied...that really helps!

Gary

Just to beat the dead horse

Most "amps" actually have several stages of amplification in them - guitar amps are like this -- each stage does a certain job, like maybe amplifying just the voltage or just the current (because each circuit can really only do one thing well), or flipping the polarity, or rejecting noise, or driving an output -- for ex., a "power amp" still has many stages inside of it, with the last stage being a large current amplifier to provide current to the speaker.

The term "preamp" I believe came about with the idea that some signals, like microphone signals, are very small, microvolts, and need to be boosted when they come in to get them to the general level of the the rest of the system, that's the genesis of the term -- it was generalized later to include *any* initial processing stage before the power amp, so it became a more generic term over the years.

Thanks George,

I appreciate your help!

Gary

Preamps don't provide AC.. speakers run on AC.

The “amp” is really two parts - the “preamp”, and the “power amp”. As others have said, most guitar amps combine both into one device.

The preamp’s job is to shape and condition the tone, starting from the pickup. Guitar pickups (including steel pickups) do not produce much current. Current is voltage divided by resistance (or impedance, which is effectively the same for this discussion). Pickups have high impedance - 15-20k ohms, in the case of a typical steel. So they may produce useful voltage, but very little current.

So the first job of a preamp is to sense that very low current and amplify it to much higher current levels - similar voltage, but much lower impedance. This lets it drive long cables without signal loss, and drive lossy circuits like equalizers. Standalone preamps put out a pretty standard signal that works with most power amps. They also do things like equalization, and providing musically useful coloration or distortion. So the “color” is what makes preamps most interesting.

Power amps, in theory, are supposed to take the line-level signal from a preamp and amplify it to very high current levels - enough voltage to get lots of power from the 4 to 8 (to 16) ohm speaker load. Now, this is actually a hard thing to do, and the theory of perfect amplification falls in practice - power amps also color the sound, just less obviously than preamps. And speakers color the sound! And different power amp/speaker combinations work differently.

Anyway, we often wind up feeding preamps into preamps into preamps. For example, in my signal chain, my pickup is first plugged into a Sarno Freeloader - a preamp that doesn’t do anything but provide impedance matching for the pickup, putting out a higher-current signal for the rest of the system. It doesn’t even have a volume knob, just a subtle tone control. This then goes to my volume pedal, then to my Fender amp. In theory, I could run the pickup directly into the volume pedal, and then into the Fender, but this doesn’t sound as good. It’s hard for the pickup to push even through 15 feet or so of cable! The Freeloader makes everything sound better. The Fender, in turn, just provides some overdrive/coloration and tone controls, and a reverb, and then drives the power amp.

It gets better. In theory, I could run pickup > Freeloader > volume pedal > power amp, but this would suck, because I’d be using the volume pedal to control my maximum volume rather than shaping my tone. So I want a global volume somewhere after the volume pedal. In theory, that’s the “volume” knob on my Fender Tone Master. But “volume” is also “gain”, and it strongly changes the coloration of the sound, adding overdrive. So I set it to where it sounds best, and my amp has ANOTHER volume control on the back, which scales the output of the power amp. That’s my actual master volume - the Fender preamp volume is all about tone, not volume, for me at least.

I could go even farther and suggest that the reverb pedal I use is also a preamp, but you get the idea. There’s nothing wrong with having layers of different preamps, each shaping the tone in a different way.

Dave,

That's super helpful. I have a lot to think about from your post. I appreciate you outlining different setups and the "whys" of each.

Thank you!

Gary