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OT: feedback resistors in PAs

Your post makes me think about a common problem in many PA implementation: the feedback resistors (wired as a divider).
Noise coming from thess resistors is in serial with the feedback path, and so adds its noise without possible reduction by the FB loop.

So, it's a good idea to keep them low, in the 100 ohm range. Furthermore, it helps to maintain good high-frequency characteristics.
As a result, current flow and dissipation won't be insignifiant.
For a well-designed PA, 100 to 200mW at 100W PA power is common.

Here comes the issue:
To avoid thermally-induced distorsion at low frequencies, these resistors should be oversized types, at least 10 times the peak dissipation.
And on a real musical program, which most of its power in the bass range, you will have only peaks of dissipation.
So a resistor with high peak-to-average power tolerance is required.
Which bans metal-film resistors.
Since it has to be the less inductive possible, standard wirewound resistor is banned.

So, the FB resistor of choice should be:

  1. bulk metal resistors (manganin is the better alloy in this matter): the best choice or
  2. crenelled-wound wire resistor (much less inductive than standard wirewound,
  3. mounted on an heatsink, or it will be a high spot (best is TO220 shaped resistors)

Unhappily, I scarcely saw these refinements used in so-called high-end equipment.
Often, this resistor goes hot when the PA is used at full power. To avoid burning the PCB, the manufacturer often lifts the resistor a few centimeters above the PCB, on standoff if he's serious, or with the resistors' wires if he's clueless about mechanical vibration tolerance. But anyway the open area under the resistor's body creates a loop, in which any induced voltage will be ...in serial with the feedback...Jeeeezzz. Often, this loop is vertical, so that it can get flux from the Earth's magnetic field if the PA is "correctly" set. So, any vibration will translate into a signal.
(btw that's not the only issue that makes an equipment microphonic. Other issues exist too, but they are O.T on this thread. All these issues can be solved, and a SS equipment should not be microphonic at all if correctly designed)
Well, it makes the life easier for vibration control manufacturers, but, wouldn't it be easier and more respectful of the customer to have the equipment correctly designed from the beginning?

So, as you see (and John C knows), many issues are intermixed.
FB resistors can act as a noise generator, a distorsion generator at low frequencies, and can make the PA microphonic.
But you can avoid it. Just a question of knowledgeable and serious design.

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