Gain Stage Feedback Loop

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mbratch, Jan 30, 2006.


  1. mbratch

    mbratch Member

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    I noticed that in early Fender amp designs, such as BF princeton, BF deluxe, even earlier tweed models, feedback the output of some of the gain stages back to the input of the 2nd gain stage. Newer designs, such as the Blues Deluxe, do not do this.

    Can someone comment on the pros and cons of these different approaches in the pre-amp gain design? Is either superior for any tonal reasons? Etc.
     
  2. KLB

    KLB Member

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    The Deluxe Reverb negative feedback loop only includes the phase inverter.

    The PR doesn't use as much NFB compared with the Deluxe Reverb (2.7K resistor vs. 820 ohms, both with 47 ohm ground resistor). Also, it is possible that the PR NFB input goes to the next to last stage (3rd stage, actually) because of technical needs related to using only 1/2 of a 12AX7 for the cathodyne phase inverter.

    IMO, it is useful to be able to tune the amount of overall negative feedback. The Heritage Colonial has a "mood" knob on the front just for this purpose.
     
  3. avwalker

    avwalker Member

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    negative feedback is used on and off in fender designs since they started. basically, the negative feedback will smooth out the sound and reduce the gain somewhat. Sometimes it was used to tame a higher gain circuit so it would "behave" (not oscillate, motorboat, etc.). Amps without negative feedback tend to have a more raw, agressive, non-hifi sound.
     
  4. loverocker

    loverocker Member

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    Apart from NFB around the power stage, there are some very creative ways to use local NFB in the preamp, but I don't know many production amps that have done it. You can make some pretty effective tone controls this way.
     
  5. mbratch

    mbratch Member

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    OK, maybe I'm using the wrong term for what I see in the AA763 and AB763 s schematics.

    I see the output signal of the second and third gain stages sending to multiple places. One of those places is the input to the second gain stage. Is that a negative feedback loop?
     

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