Hello Patrick:
I have always wondered about this question but from another perspective.
Let's say I connect a my 8 Ohm speakers on a Dynaco ST-70 to the proper 8
Ohm tap. Doesn't the feedback loop now intruduce a series inductance
produced by the connection of the feedback to the 16 Ohm tap?
Doesn't this now become a series RL feedback loop?
Or is the inductance of the left over winding negligible?
If one uses the 4 Ohm tap then the value of L increases even further, would
this change the feedback any by causing the loop to open-circuit at high
frequencies?
Just curious....
"Patrick Turner" wrote in message
...
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.