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Jim Gregory
 
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I notice the valved/tubed amps' negative feedback source node is usually at
the live leg of a secondary designed for feeding a 16-Ohm load.
But I also found that none of the various valve/tube schematics in these RAT
topics involved an obvious Zobel R+C network across their final o/p point.
These frequency-conscious shunts are often implemented in audio buffers/line
drivers coupled to o/p xformers.
When made prudently, its adoption restricts super upper bandwidth to give
reliable HF stability, thwarting probable misbehaviour arising with high
freq resonance in xformers.
In such power amps, assuming the load is always connected, was it omitted
because the loudspeaker coils + crossover network/s would band-restrict any
funnies beyond 22kHz anyway?
Or do the NFB components, effectively in parallel with the secondary
winding, also react like a Zobel R+C out of band? But then there is not
usually a low-value resistor in the NFB loop.

Incidentally, why don't any of these o/p transformers ever have two
secondaries for either parallel or series config to cater for 4 or 16 Ohm
loads (you might also then need 2 Zobel R+C shunts), delivering virtually
the same peak Power?
Though 8 Ohms would become the odd-one-out impedance in that event, you
would do away with 1/2 way tapping and its associated half-wasted energy
whose whole isn't a lot of Watts to start with, by today's standards.
Jim