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  • Removing feedback question.

    Hi.
    When doing troubleshooting on tube amps, I often remove feedback from the speaker output to the PI.
    This helps me when searching for output stage faults.
    My question is, can this be done with SS amps as well?
    If not what are the reasons?

  • #2
    NFB might (sometimes) be necessary for DC and AC stability?
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    • #3
      I think it's almost always required for DC stability. In the sense that everything is usually DC coupled so you just can't do it without throwing everything (DC) off. And there is no simple feedback path to be disconnected like in tube amps.
      Even in early SS circuits where you have capacitor coupling between stages, the power amp section is still all DC coupled.
      "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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      • #4
        It should be possible to disable the AC NFB while leaving the DC NFB intact by shorting the NFB signal at the inverting /NFB input to ground by a cap.
        Result will be vastly increased gain (much more than with tube amps).

        But my question would be: Why? What problem is this going to solve?
        SS amps typically don't tend to oscillate when probing circuit points because they are low impedance/low voltage circuits.
        Last edited by Helmholtz; 02-27-2021, 11:56 PM.
        - Own Opinions Only -

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        • #5
          I've never given much thought to this, only because I've never had the need to do this with any SS amp. Whilst NFB in a tube amp can be considered optional, with SS amps it's essential for the proper, stable operation and I can't see how normal signal levels and DC conditions could be maintained to enable effective troubleshooting. In other words, I think it would create more problems rather than simplifying matters.

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