What exactly is a preamp, and why would I want one?

[Poor Tom]

I play a passive bass, and have a cheapo amp head. I am trying to figure out what preamps actually do, like what is their purpose? Is it to change the sound? Is it supposed to be a cheaper alternative to just buying the amp head that already has the sound you are looking for? It all intrigues me and if anyone could enlighten, I would be grateful.

 

[bongomania]

Different preamps are designed to do different things. Most of them are meant for tone shaping. Some of them are meant for boosting your signal level. Of the ones that boost, some of them are designed to slam the input of a tube amp head for more distortion, and some of them are meant to drive a power amp directly (with any kind of tone, clean or dirty).

IOW you have to look at each so-called preamp on a case-by-case basis and see if it is actually meant for whatever function you need. In your case, with a cheapo amp head, you could benefit from a preamp in two ways:

1) Some tone-shaping pedals are known to improve small/cheap amp tone. The Sansamp BDDI is one example.

2) If your amp has an effects loop or a "power amp in" jack, you can usually use those to bypass the built-in preamp. That means you can buy an external preamp for whatever tone you want, and use it to drive the power section of your amp head.

In both of those cases though, you have to do some research to be sure the preamp you're looking at is actually designed for the specific purpose you want- for example different pre's have very different output levels.

 

[thehangingmist]

as always, bongo has told you more then i could

 

[BassmanPaul]

To take the good Bongo's post a little further. Any amp head, such as your own, consists of

(1) a pre-amp to boost the instruments signal level and to add tone shaping (treble and bass etc.) and
(2) a power amp that produces the power required to drive the speakers.

Some of us prefer to separate these functions into individual units thus a "Pre-amp and Power amp. It is not cheaper to do this but it is more versatile. You can couple the pre-amp that gives you the sound you are looking for to any make of power amp with whatever power output you might fancy.

My set up for one of my rigs is a BBE BMax-T preamp into a mixed bag of Carvin and Mackie power amps. The other is a Carvin DCM1000 ($325 500W/CH into 2&#937 stereo power amp driven by a tube pre-amp based on the front end of my Ampeg B15.

Paul

 

[Poor Tom]

thanks bongo and Paul! You guys gave me exactly the info I was looking for. My next question was going to be what a power amp is, and now I know ^_^

 

[wazdabaz]

Excuse me for using this thread. I just bought the ebs microbass which I think is going to be a realy nice tone shaping tool. What I am not sure of is how to bypass the heads. I was told that I need to use the head's RETURN fx loop to bypass the head's eq. But is that all? I just plug the bass to the microbass & then it's OUTPUT to the head's RETURN jack? I mean, plugging anything to the fx RETURN (&not using the fx SEND) isolates the poweramp of the head?
The microbass has an FX loop send/return jack set of it's own. Does it have something to do with the bypass thing or is it only usefull for ...er, well, Fx's...?
Please help me so that I don't do any harm to any head or my microbass