Announcement

Collapse
No announcement yet.

Negative Feedback Question

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Negative Feedback Question

    This may seem like a crazy question -- I understand the need to keep capacitance out of an amp's output stage in order to avoid an impedance mismatch at the speaker, but what is the danger of running something like a diode there? I've gotten some fun fuzz tones by running a diode in parallel to the NFB resistor, just wondering if this is going to damage the OT. Thanks!

  • #2
    What amp?

    Comment


    • #3
      It depends on the circuit. Can you provide a drawing. diagram or schematic??? Adding a diode to an NFB loop COULD send watts to circuit not designed to handle it. I'd need to see what's actually being done before endorsing it.
      "Take two placebos, works twice as well." Enzo

      "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

      "If you're not interested in opinions and the experience of others, why even start a thread?
      You can't just expect consent." Helmholtz

      Comment


      • #4
        This one is kind of a bastardized version of the Fender 5C3 schematic: http://www.davidsonamp.com/sf/images/deluxe5c3.gif

        Comment


        • #5
          Any voltage above the forward voltage drop of your diode for one half of the output waveform is going to be applied to the grid of the connected output tube. Would make an interesting scope pic to see what's going on.

          Comment


          • #6
            Interesting... so it's potentially taxing to the power tubes? Doesn't seem like it would generate more voltage, and in fact it's noticable quieter when engaged. I guess that's because half the waveform is getting lopped off!

            Comment


            • #7
              I don't see a diode in the above schematic so I'm still not clear about the circuit. But just speculating, a diode in the NFB shouldn't tax the power tubes because the relative impedance of the circuit is much higher than the speaker load. It may do funny things to the speaker damping due to NFB. And THAT may change the ideal load impedance. But I don't think there is any danger to the tubes.
              "Take two placebos, works twice as well." Enzo

              "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

              "If you're not interested in opinions and the experience of others, why even start a thread?
              You can't just expect consent." Helmholtz

              Comment


              • #8
                Originally posted by cryptozoo View Post
                Interesting... so it's potentially taxing to the power tubes? Doesn't seem like it would generate more voltage, and in fact it's noticable quieter when engaged. I guess that's because half the waveform is getting lopped off!
                It's out of phase to the grid voltage, so will reduce the drive to the output much more than the standard setup (on one half of the waveform). You can work out what peak voltage is getting fed back, from the amp's output and speaker impedance. You'll not only get asymmetric distortion, but a slight shift in frequency response due to increased NFB.

                I would imagine that the peaks on one half of the output waveform are severely attenuated and the 0.6v or so forward voltage drop will introduce a step in the leading and trailing edge. Bit like crossover distortion. You could make it even (...worse!) with anti-parallel diodes.

                I'm in agreement that the impedance of the circuit is unlikely to have any detrimental effect on the OT.

                Have you got a scope and signal generator? - I'd like to see a pic of the output.
                Last edited by Mick Bailey; 06-25-2013, 06:51 PM.

                Comment

                Working...
                X