Pre Amps/Power Amps/Amp Heads

[izdo89]

This totally sounds like a nube question but I just never got what the difference is between a pre amp and a power amp. And why should I buy the two separately instead of just getting a good amp head?

[Floyd Eye]

If you're talking about guitar, you shouldn't. That;s my opinion, but I know there are guys who prefer certain preamps. I play bass and I use a preamp and a power amp, but it's mostly because I could afford an RBI, but not an SVT.

The difference between the two is that a power amp is an amplifier. A preamp is not. The section of your amp head where the tone controls are is a preamp.

[Rex Anderson]

In hi-fi, a pre amp handles line level signals (like CD players etc). The power amp drives the speaker(s). A "package" that contains a pre amp and a power amp is called an integrated amp. Add a tuner (AM/FM) and you have a receiver.

In guitar land, the "pre amp" takes the guitar pickups and handles their impedance and output voltage and gives you tone controls (bass, mid, treble), reverb, tremolo etc. It then sends it to the power amp section which drives the speaker(s). Power is rated in watts.

An amp head is essentially a preamp stage and a power amp built in one cabinet.

There are many more manufacturers that make guitar "amps" (which are actually preamps and power amps in one package) than ones who make separate pre amps and power amps. Mesa is one who made separates.

[mrmatt1972]

For guitar, you can buy them separately, and they are called "rack gear" in that form, but every "normal" guitar amp has a preamp and a power amp section. In tube amps this matters because the kind of tubes in both sections greatly influence the sound. The preamp takes the signal from the guitar (a low impedance signal, aka low z) and basically boosts it up to the point that it can then be amplified by the power amp, which needs a high impedance signal (high z) to work right. I'm being as non-technical as possible, it gets complicated and weird in there, but basically what matters is the kind of sound you can expect from various preamp and power amp sections.

There are a few amps that are considered archetypal for guitar sound. Their preamp configuration, and especially their power amp configuration is what players tend to have a preference for. Vox AC30 is one of the big archetypes - and my preferred power section. It runs EL84 tubes in class A - which means a jangle and grit to the sound when the power section is turned up. On many tube amps there is a gain knob that controls the amount of signal (aka gain) from the preamp section and a volume knob (or master)that controls the amount of signal the power section puts out. Classic Marshalls ran KT88 or EL34 tubes in the power amp. They give that AC/DC kind of sound when pushed really hard (i.e., loud). 6L6 tubes are the power section found in Fender Twins, Bassmans and many other classic American amps. They give a warm, bass heavy but non-distorting sound when pushed.

Hope this helps clarify.

Matt

[izdo89]

Awesome guys! Thanks, that was very helpful. So, everyone seems to be talking about this Mesa/Boogie Triaxis but I can't seem to find it anywhere. I'm assuming it's not in production anymore. Is there anything else that stacks up? I've been looking into the Voodoo Labs Preamp but its solid state and I've heard it doesn't do the real "burning lead" thing that well. Any suggestions?

[Modulator]

marshall jmp-1 was the top teir in its day
I think the ADA-MP1 had a preamp tube. I could be mistaken.

[mrmatt1972]

There are a few rack preamps and power amps in current production. AMT, VHT, Mesa (I think?), ENGL are a few I know of off the top of my head. I'm going to buy a peavey rockmaster tonight! I'm excited to be getting back into rack!

[Rex Anderson]

I forgot to mention Lance Keltner and Retro Channel.

Check this out:

www.retro-channel.com