preamps... what for?

Discussion in 'Amps and Cabs' started by GeorgeSunset, Apr 12, 2008.

  1. GeorgeSunset

    GeorgeSunset Member

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    Hey

    Not sure if this is the right place to post this… but,

    I have always asked myself, what do you use preamps for? Where do you put them in your signal pathway? What do they do?

    Do some of you have some examples?

    Thank you for your patience
     
  2. hamfist

    hamfist Member

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    A pre-amp describes the part of a guitar amplifier which shapes the tone (for example causes any clipping or distortion, or compression, or allows the user to change the tone using EQ). Once the "raw" tone from your guitar is shaped into something new, then the signal is fed to the power amp which simply makes it louder.
    It's often not quite a simple as that in a guitar amp because many amps are designed to also get clipping (break up) in the power amp too, as they are cranked.
    A pre-amp can be all tube or solid state (analog or digital). Similarly power amps can be tube or SS (don't want to be clipping SS power amps though -sounds very nasty).
    Hope that helps.
     
  3. StompBoxBlues

    StompBoxBlues Member

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    Nice explanation by hamfist!

    Mainly also, just to add...the guitar signal is a very fragile, easily lost signal. Preamps preamplify it...you know that, but it helps to think of it this way...

    preamps help get over the most of the signal, and boost the voltage (peak to peak) of that signal. Voltage is potential, which is linked to current, but current is what really does real work...so the designers of guitar amps knew that it was best to do the real amplification in two stages at least...

    Preamp- take that fragile voltage (and that voltage would be very hard to immidiately convert to "work" like pushing an electro-magentic driven speaker) and boost it up so that is is not so fragile....then use that stronger signal to go into...

    Power amp- which takes that refined, stronger signal and uses it as a guide to how to push real-world speakers in and out. The power amplifier is sort of a "current amplifier" in a way...

    Each preamp (stage) imparts its characteristics on the signal (sound) so someone may want to use a pre-preamp (another stage...sometimes these are built into amps) because some want to build up that fragile guitar signal in stages instead of just all at once...so each stage doesn't get near their overload point (or maybe so each stage IS over, making OD...).
     
  4. rob2001

    rob2001 Member

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    Well, there are several types of preamps... kinda. Theres the preamp section of your amplifier that is used to shape tone and gain structure before the power section of the amp. This is a common layout for master volume amps. The configuration is different in non-master volume amps.

    Then you have preamp pedals... I guess those would be considered more as signal boosters with possible tone control. Examples would be the MXR Micro-amp or the DOD Bi-fet Preamp. Those can be used as boosters at the end of a long chain of pedals to bump the signal back up after going thru many pedals. Or just as a volume boost.

    Then you have on board preamps that are built into the guitar. Very much like the pedal version and most require a battery to operate. That makes for a much hotter guitar and some circuits provide tonal changes. Eric Clapton uses a mid-boost type preamp and Jackson guitars have models with the J-2000 preamp, like in the Randy Rhodes V's and some Soloists.

    Then you have rack preamps like the Marshall JMP-1, ADA MP-1 and the latest incarnation would be the Axe-FX modeling unit. These are similar to the preamp built into the master volume amps but usually have much more control over tone and gain shaping. Like a master volume amp, they need to run a power amp, it's just a seperate power amp, not built in like a regular amp.

    THEN you have preamps that are used in recording. Thats for plugging a mic into and getting the best signal with as little noise as possible... but thats another thread!!

    So when you say preamp, it's gotta be in the right context. There are many circuits or pieces of gear that are called a preamp.
     
  5. StompBoxBlues

    StompBoxBlues Member

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    But I think in all cases, it is still referring to an amplifier designed to boost a signal (with or without distortion) prior to power amplification.
     
  6. rob2001

    rob2001 Member

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    I would agree. Prior to an amp or recording medium.
     
  7. GeorgeSunset

    GeorgeSunset Member

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    Sounds good…
    Thanks for the replies

    I was mostly interested in the extern preamps – so they are like a “sound-colouring” type of “effect”?


    Where do you put those things? I mean in the fx loop – (send-return), only via the return (by-pass the amps preamp) or as a effect in front of the amp?

    If you have some sound clips – please post them (or a link would be nice)


    thanks
     
  8. Jim S

    Jim S Silver Supporting Member Supporting Member

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    Usually the preamps like rackmount premps go into the fx return.

    But some rack preamps and stompbox type preamps go straight into the front end of the amp.

    You can't really harm your amp either way so experiment and have fun.

    I had a MESA Studio preamp which I ran into the front of a Twin Reverb combo with good results. Then I ran into into a dedicated MESA power amp with fine results too. I know run my rack Soldano x99 preamp into the (modiifed) fx return of my Soldano head.
     
  9. Sleepo

    Sleepo Member

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    I know of someone with a Vetta II who uses a tube preamp (Art MT Pro or something) in his loop to give it extra warmth.

    Having not heard this rig with/without it, I can't comment on how it helps or not.
     

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