Re: NFB101 part 3

Ian Thompson-Bell wrote:

It should be plain that, when sourced from a purely
resistive load, there is no difference between series-
and shunt-derived feedback. I don't understand why you
keep insisting there is, and you haven't bothered
explaining so it's just an empty assertion. And a wrong
one, to boot, I contend.

Let me see if I can explain. Suppose you have a typical
tube power amp that feeds a loudspeaker or resistive load
via an output transformer. The load is connected across
the secondary winding of the transformer. If you connect
one side of the secondary to ground and take feedback from
the other secondary terminal, the feedback is connected in
shunt with the output and senses the output voltage. It is
therefore shunt or voltage derived. No matter which way
you apply this feedback, deriving it this way will lower
the the output impedance seen by the load.

If instead of connecting one side of the secondary to
ground you connect it to ground via a small value series
resistor, then the voltage across that resistor is
proportional to the current in the load. If you take your
feedback from across this resistor it is therefore series
or current derived. No matter how you apply this feedback,
deriving it this way will raise the output impedance seen
by the load.

Hope that helps clarify things.

It makes clear exactly how received wisdom thoughtlessly
applied can be a barrier to progress, rather than an aid to
learning.

When sourced from a purely resistive load, there is no
difference between series- and shunt-derived feedback.

I don't want to help you write handouts, so that's it. It's
not rocket science.

Ian


.

Relevant Pages

  • Re: NFB101 part 3
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