amp input sensitivity

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by prof2915, Jun 10, 2005.


  1. prof2915

    prof2915 Guest

    Hi, here´s some thoughts from a "Stratplayer".

    I suppose that most people agree to that amps usually sound much better with a cleanboost in front of them.
    I´m not talking about pushing a preamp into distortion - by adding an extra 20db or so to the input signal - I´m talking about getting a full and warm basic CLEAN sound.

    The "Hi"/"LO" input jacks in amps usually doesn´t do the proper job here...

    I can see that amp designers have to take in to consideration all the differencies in the output signal from pickups, and create some kind of "average amp inputstage headroom" that will make the amp work with "all" pickups.
    Still I have a feeling (yes a "feeling", I know NOTHING about amp design :rolleyes: ) that it should be possible to have some kind of adjustable circuit (before tube V1) in an amp, that does what a cleanboost do!?

    As there are so many great amps that I´ve never tried, maybe this already has been done by someone?
    This would be nice to know, as well as getting some feedback regarding my ideas here in general.

    /Peter
     
  2. loverocker

    loverocker Member

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    Yes it is possible, and yes it has been done. I seem to recall one amp that had a JFET input as well as a normal input...

    I added exactly this type of cleanboost to the front end of my Marshall 18W clone. It's just an extra stage (see cascaded thread), so that the High input uses the cleanboost and the Low input gives the normal unboosted 18W tone. You can see it at: http://www.firstmail.com/18w/cleanboost.gif - it's the same approach as the earliest Marshall Master Volume amps - the 2203/2204.

    Things get tricky if you want to footswitch it in and out though. You'd need to add a relay or opto switching, which adds considerable complexity and cost. I suppose that most ampmakers assume that most guys would use a pedal.
     
  3. prof2915

    prof2915 Guest

    I wasn´t thinking of anything switchable.

    I´m just thinking of a way to have amps sound "better" right from the start.
    To me a good clean sound is a sound that is very close to the point of breakup but without really crossing that border.

    With a cleanboost I can set my input signal just "right" for the V1 tube. Kinda´figured it would be better if things were that way right from the start.

    Just doesn´t sound as good to raise the amps "volume" to a certain level when compared to obtaining the same volume level (at a lower amp setting) by having a cleanboosted signal fed into the amp.



    Peter
     
  4. Swarty

    Swarty Member

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    One thought is to replace the 1m grid leak resistor with a 10m pot (you could mount it in the the lo input jack hole). This way you adjust the impedance of the amp to match the instrument.
     
  5. prof2915

    prof2915 Guest

    As I wrote before: I don´t know anything about amp design, and I have no idea of what you´re suggesting :confused: I just have a feeling that the output of most pickups seem to be to low for getting the best sound out of most amps.

    Hot/High output pickups sound bad in my ears, so that´s not the "right" answer however.
    I can live with having my RC Booster or on board Bartoloni preamp kicking the input of my amps, I´m just a little puzzled why this "little problem" haven´t been solved.

    But what do I know :rolleyes:

    There are great amps out there, constructed by scilled craftsmen doing a fantastic job :dude

    /Peter
     
  6. hasserl

    hasserl Member

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    Most people probably do not find this to be a "problem". I wouldn't want a boosted signal on all the time, but there are times when I do like that. I don't find it a problem to use an outboard device for this.

    One of the problems I see with this is everyone is so finicky about just what they want in this type of effect. It would be really hard for an amp maker to please everyone. So maybe the best thing is to leave them like they are and let people stick their pedal of choice in front.

    But everyone is different, and if you like a boost on all the time that is fine. You could probably hire a tech to add some type of boost to the front end that could even be footswitchable. If it pans out and enough people like it some amp maker will fill the market space. I suppose to this point the demand just hasn't been there.
     
  7. prof2915

    prof2915 Guest

    Yeah , you´re probably right. I just find that most amps sound fatter and most of all more "alive and breathing" (it´s not easy to describe sound...) when a slightly boosted signal is fed into them.

    /Peter
     
  8. r9player

    r9player Silver Supporting Member

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    My Sommatone amp seems to have that clean boost. Not sure how it is wired in but since it has only one input ... it could be anywhere and it sure fattens the tone up nicely.
     
  9. Richard Guy

    Richard Guy Member

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    Loverocker wrote; "Yes it is possible, and yes it has been done. I seem to recall one amp that had a JFET input as well as a normal input..."

    Dumble
     
  10. zachomega

    zachomega Member

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    Ampeg V series amps (VT22, VT40, V2, V4, and V4B) have input sensitivity selector switches.

    -Zach Omega
     
  11. drbob1

    drbob1 Silver Supporting Member

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    I wonder if the problem you're describing isn't so much that you don't like the volume you're getting but the tone? So it might work to buffer the output of the guitar (the pickups give the purest signal into a very high impedance load) then send that signal unboosted into the first tube? That could easily be done as a mod to an amp, but using a high quality buffered pedal in front is cheaper and easier (like the Klon for instance or the SHO, turned off but in circuit).
     
  12. Richard Guy

    Richard Guy Member

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    How about a Barber Launch Pad?
     
  13. bob-i

    bob-i Member

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    Well there's a few things here I don't know if we'll all agree on.

    1) Clean boost is not always better. For gritty tones it is but for pristine clean tones it's not.

    B) Preamp distrortion. I.E. clean boost is not always the best way to fatten up the sound. Power tube distortion, I.E. an attenuator and cranked amp, is sometimes more desirable.

    4) Some amps alreay have this built in.

    So a small education, and again not everyone will agree.

    "Gain" means to increase the signal, it doesn't mean to distort.

    most Fender type preamps use 2 gain stages.

    most Marshall type preamps use 3 gain stages, there are exceptions.

    Most folks would prefer to have that boost switchable, I know I do.

    To answer your question directly. yes that would be easy to build into any tube amp. One more tube and a few resistors and caps is all it would take.
     

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