Hi Rich,
You are worrying about the wrong inductance when you worry about the
inductance of the winding itself. If the winding inductance were a
problem think of the high frequency loss you would have just from the
inductance of the active portion of the secondary driving the low
impedance load of the speaker. But that isn't how transformers work, and
few people seem to realize how they really work, even many of the so
called transformer guys.
The real problem is the transformers leakage inductance which is
effectively in series with the load, and causes high frequency roll off
and phase shift when driving the speaker load. The phase shift
contributes to feedback instability, especially when driving a capacitive
load. If there is no load on the secondary, then the leakage inductance
causes relatively little phase shift, and the feedback loop will be more
stable. Many old high quality broadcast amplifiers from the tube era took
advantage of this effect by using a tertiary winding to provide the
feedback signal, eliminating or greatly reducing the phase shift caused by
the leakage inductance. One potential drawback of taking the feedback
from a tertiary winding is that the feedback does not compensate for the
leakage inductance between the primary and secondary resulting in some
high frequency roll off, but this can be minimized by not allowing an
excessively large leakage inductance between the primary and secondary.
The primary to secondary leakage inductance does serve a useful purpose
however in that it serves to isolate the amplifier and feedback loop from
the effects of capacitive loads, like the series coil often found in the
output circuit of solid state amplifiers.
The unused part of the winding in the common hi-fi amplifier where the
feedback is taken from the 16 Ohm tap provides some of the same beneficial
feedback stabilizing effect as a tertiary winding, only it is watered down
by the fact that this "tertiary" winding is in series with the active
portion of the winding driving the speaker, for feedback purposes.
A second disadvantage of using a tertiary winding for feedback, in
addition to the uncompensated primary to secondary leakage inductance is
that the resistance of the secondary winding is also uncompensated. This
is not a serious problem in professional applications, but it is a problem
for consumer hi-fi amplifiers because it degrades the "damping factor",
which is an important number for advertising purposes. The lower damping
factor is probably one reason consumer hi-fi amplifiers don't often take
advantage of a separate tertiary winding to stabilize the feedback.
A few consumer hi-fi amplifiers do use a tertiary feedback winding because
of the greater feedback stability it provides, in spite of the reduced
damping factor. The famed Marantz 8B is a particularly interesting
example of the use of tertiary feedback. The 8B uses a tertiary feedback
winding to provide improved feedback stability at high frequencies, but
retains the damping factor advantage of taking the feedback from the
secondary winding driving the speaker load, by incorporating a sort of
crossover network in the feedback circuit such that at high frequencies
the feedback comes from the tertiary winding and a low frequencies the
feedback comes from the secondary winding. This scheme provides the best
of both worlds, the greater feedback stability of the tertiary feedback
scheme, and the low damping factor of the secondary feedback scheme so
necessary in consumer hi-fi amplifiers for advertising purposes.
Regards,
John Byrns
In article , "Rich Sherman"
wrote:
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.
Surf my web pages at,
http://users.rcn.com/jbyrns/