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Patrick Turner
 
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Jim Gregory wrote:

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?


The NFB resistor, and its compensation phase advancing cap do not have any
effect
on the open loop gain of the amp at HF, which should be limited by the zobel
across the whole of the sec winding, so an R load is is effectively connected at
HF
above 20 kHz where instability will occur.
Most speakers are inductive with rising impedance as F rises, so
without a zobel, virtually no load is connected to the amp at 100 kHz,
where excessive output tube gain may cause instability due to rapid phase shifts

due to poor OPT quality with large shunt C and leakage inductance.



Or do the NFB components, effectively in parallel with the secondary
winding, also react like a Zobel R+C out of band?


No.

But then there is not
usually a low-value resistor in the NFB loop.


Yes.



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?


Mnay OPTs do have more than one winding for various load matches.
See my pages about OPTs at
http://www.turneraudio.com.au/htmlwe...utputtrans.htm



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


It is indeed harder to cater for 4,8 and 16 ohms.

But the best amp will have no wasted windings on OPTs, and
current densities in each wire of the secondaries is equal when each impedance
selection is made, and leakage inductance remains constant for all load matches
when referred to the primary.

Hardly any amps are configured like this, but its *the* right way to go.

Patrick Turner.