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A question about ~Sebatron~ mic pres...

Discussion in 'Pro Audio Equipment' started by alk509, Aug 11, 2003.

  1. alk509

    alk509 Guest

    Hey Seb,

    I'm just curious, do you pre-amps use negative feedback for gain control, or are they fixed gain with attenuation? I'm just trying to understand tube circuits better, learn the tricks of the pros, etc, so any other technical info you can share would be great. :)

  2. Sebatron

    Sebatron Well-Known Member

    Dec 22, 2002
    Hi Al,
    Yes , good question.

    Yes , with the vmp range the pad is configured with negative feedback.

    Basically, ,,…this means that the first gain stage of the 12AT7 triode is fed an inversion of itself …180 degree out of phase….relative to the position of the pad control.

    In my great wisdom I have set up the 0 db pad point to just be able to accept a hot condesor-type mic signal without severly clipping.This will get you the most coloured result.
    Engaging the pad to –15 db allows a bit of negative feedback into the first gain stage…
    Engaging the pad to –30 db feeds even more negative feedback …..

    Negative feedback :
    Decreases gain
    Decreases effect of variations of valves and other components on gain
    Decreases effective plate resistance
    Decreases harmonic distortion
    Decreases effect of load impedances on output voltage

    Increases bandwidth

    Most of this was taken from RADIOTRON DESIGNER’S HANDBOOK
    Fourth edition , page 315.
    Lent to me by Paul Perry of ‘Frostwave’…. I really should return it to him…..
    …I’ve had it for at least four years now……

    You see , if you have too much gain ( no headroom ) …one approach is negative feedback….it’s better than attenuation because it’s not wasting energy and is a lot quieter.

    My new ‘THORAX’ features both pad/attenuation and a colour control… which is negative feedback.

    …due out soon….. :p:
  3. alk509

    alk509 Guest

    Thanks for the info, Seb! :)


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