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  • Inverter question

    Hello. Here is how Ampeg designed their nfb network around paraphase inverter (top). If I move the common OT lead from the ground over shunt nfb resistor (bottom) the decoupling cap over cathode resistor is still necessary please ? And I talking here about possible positive feedback issue... Thanks.
    Click image for larger version  Name:	20230731_124911.jpg Views:	0 Size:	3.37 MB ID:	984726
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

  • #2
    Unless there is lots of shielding within a transformer, there will be some capacitance between windings. Even if the capacitance is small, because the Voltage is so large on the primary, significant current wants to flow. Normally this is not a problem when one side of the secondary is grounded. When you lift the ground on the secondary, it can be a problem because the Voltage from the parasitic coupling can easily be bigger than the Voltage across the secondary, not to mention it will be higher at high frequencies.

    Do you need any more reasons not to try this ? I'm guessing you don't own an oscilloscope.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
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    • #3
      I think the decoupling cap is still beneficial, as those common cathode stages are in cascade and share cathodes, and a (weak) positive feedback loop would form if unbypassed.
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      • #4
        At first glance and without knowing circuit values,
        I expect lower PI gain and poor balance.
        But could be wrong because of several interacting NFB paths.
        Last edited by Helmholtz; 07-31-2023, 04:48 PM.
        - Own Opinions Only -

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        • #5
          Originally posted by loudthud View Post
          Unless there is lots of shielding within a transformer, there will be some capacitance between windings. Even if the capacitance is small, because the Voltage is so large on the primary, significant current wants to flow. Normally this is not a problem when one side of the secondary is grounded. When you lift the ground on the secondary, it can be a problem because the Voltage from the parasitic coupling can easily be bigger than the Voltage across the secondary, not to mention it will be higher at high frequencies.
          Yes, that could cause stability issues.
          Primary to secondary capacitance can be between 300pF and 3nF.

          - Own Opinions Only -

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          • #6
            Thank you gents. Clearly understand. Just a additional question I wonder jn this case please: Even the common nfb return wil be linked to ground (like in top sample) then the grounding point choosed make sense to be close to power stage supply node more than the stage the nfb network it belong.This should be correct please ? Thanks.
            Last edited by catalin gramada; 08-01-2023, 07:59 AM.
            "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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            • #7
              From circuit logic the NFB return should use the same ground as the PI.
              - Own Opinions Only -

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              • #8
                Ok. I installed separate cathode resistors without bypass caps. 6db of global nfb used but did a small experiment using different shunt resistors into a string. To be honest there is a effective diference jn sound comparing 100k/2.1k than 470ohm/10ohm despite the fact the global nfb ratio is the same. I much prefer 470/10 ohm. Why such of difference please ? Thanks

                Click image for larger version

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Views:	202
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ID:	984848
                "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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                • #9
                  Originally posted by catalin gramada View Post
                  To be honest there is a effective diference jn sound comparing 100k/2.1k than 470ohm/10ohm despite the fact the global nfb ratio is the same. I much prefer 470/10 ohm.
                  Is that drawn on toilet paper?

                  I guess you measured/compared the closed loop gain?
                  Actually I wouldn't expect a difference.
                  In both cases the NFB signal reaching the cathode should be roughly the OT secondary voltage divided by 150.
                  Also the DCV at the cathode shouldn't change noticeably.

                  Might be a parasitic effect.
                  Did you mount the NFB series resistors directly to the output jack?

                  "Sound difference" is rather vague.
                  - Own Opinions Only -

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