Why do Dynaco Amps and even Uncle Ned's schematic of a Hi Power Williamson
Amp use the 16ohm OPT tap instead of the 8ohm for feedback? What would
happen if you took a stock ST-70 or MKIII and moved the feedback over to the
8ohm? Thanks.

west

"west"

Why do Dynaco Amps and even Uncle Ned's schematic of a Hi Power
Williamson
Amp use the 16ohm OPT tap instead of the 8ohm for feedback?


** The former industry standard for hi-fi loudspeakers was 16 ohms - so
most designs were optimised for this load. It only became 8 ohms ( or
lower ) with the advent of output transformer-less SS amplifiers. The 16
ohms tap also gives the highest voltage level.



What would
happen if you took a stock ST-70 or MKIII and moved the feedback over to
the
8ohm?


** You will get about 3dB more gain and 3dB more THD.




............ Phil

Phil Allison wrote:

"west"

Why do Dynaco Amps and even Uncle Ned's schematic of a Hi Power
Williamson
Amp use the 16ohm OPT tap instead of the 8ohm for feedback?

** The former industry standard for hi-fi loudspeakers was 16 ohms - so
most designs were optimised for this load. It only became 8 ohms ( or
lower ) with the advent of output transformer-less SS amplifiers. The 16
ohms tap also gives the highest voltage level.

What would
happen if you took a stock ST-70 or MKIII and moved the feedback over to
the
8ohm?

** You will get about 3dB more gain and 3dB more THD.

True, but if the global feedback resistor was reduced by about 30%,
the gain, ie, the sensitivity of the amp could be kept
constant, and with it the thd.

The trouble is that 30% of the secondary winding is then unused, and leakage
inductance
will have risen, so stability could be affected.

The connection of the NFB to the actual point of speaker take off is
better imho, because the speaker signal is fed back to the amp more directly,
and better HF response is obtained.
Its ok if the amp is then stabilised properly.

But many makers don't like the effort and complexity of
changing matching taps *and* NFB networks because
so many of the ignorant general public make mistakes with such things,
and the smoke gives a maker a bad reputation...

Patrick Turner.




........... Phil

west wrote:

Why do Dynaco Amps and even Uncle Ned's schematic of a Hi Power Williamson
Amp use the 16ohm OPT tap instead of the 8ohm for feedback? What would
happen if you took a stock ST-70 or MKIII and moved the feedback over to the
8ohm? Thanks.

Its a very fair question.

The arrangement where FB is taken off the 16 ohms tap, and usually at the end of

a tapped secondary winding is that it allows free movement of loads from one
tap to any other without having to re-arrange the values of feeback R and
compensation cap often needed in other amps to keep them stable.

The 16 ohm FB point works when 8 ohms is on the 8 ohm tap,
at 0.7 of the S turns, and when 4 ohms is on 0.5 of the S turns.
The signal voltage at the 16 ohm tap is the same for all 3 load values.

But a tapped S winding is an appalling practice imho, since the
leakage inductance rises badly, 4 times, when 4 ohms is connected to the
CT of the S winding.
The HF response can be less than when 16 ohms is used at the 16 ohm connection.
HF stability can be a problem...

Patrick Turner.



west

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

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.

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.

I've been following with interest this thread about the use of the 16 ohm
tap as a FB point.

I once rebuilt an Eico ST-70 that had poor bass response and a lot of
instability and distortion. It had a peculiar output configuration where
the 4 ohm tap was grounded, and FB came off of the 16 ohm tap.

I don't know if the configuration caused the problems (there were a few
other things going on) but when I grounded the "C" tap and lifted the 4 ohm
off of ground, things improved markedly. I don't recall if I also shifted
the FB point to the 8 ohm tap, but I think I did.

For what it's worth . . . .

Jon

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?

Fair question indeed.

In my experience, the response at the speaker sags while that at the 16 ohm tap
looks better.

For best correction of the sag in speaker response the FB should be taken from
the speaker terminals, not via the extra winding which must introduce
some effective extra series L.
The model for the OPT at HF gets rather complex, and the "unused"
portion of the sec winding in this case remains magnetically locked to the rest
of the
tranny, but nevertheless the response sags.
One might expect the NFB take off from the end of the sec might make the
response correction best, but it doesn't, much in the same way
as taking an anode signal back to some previous stage
for FB.
This last option is sometimes used, (EAR509), to avoid the
phase shift caused by the leakage inductance, since its not included in the
NFB loop.
Careful OPT design is required to minimise these concerns,
and a tertiary dedicated FB winding wound close to the sec
is perhaps the best option, so that the speaker sec is slightly isolated from
the
FB network.




Doesn't this now become a series RL feedback loop?

Slightly, yes.



Or is the inductance of the left over winding negligible?

Its a small L, and the series R of the NFB and this series L
have a very high F pole.
Phase shift occurs before the pole, and as I said,
exactly what the model is for a given OPT for the actual
L and C between each section is a very complex model.




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?

It would open circuit, but by the time one gets to the F pole the
amp open loop gain has already sagged, usually so that the phase shift caused
doesn't
make the amp oscillate due to this cause.
Other things might cause the amp to play up though.



Just curious....

Curiosity never killed too many people's brain cells, and the more you
allow yourself to figure out OPTs, the more questions are raised than can be
answered.

Leak had a range of ways to connect the 4 secondary windings on their
TL12 amps. Each impedance match also required different NFB R and compensation
cap.
The ordinary man in the street could never ever have guessed how to alter the Z
match
without a tech.

And some arrangements of secs in a Leak give quite different
stability margins. Usually the 4 or 16 ohms are the lowest loss,
best response, highest stability compared to the 8 ohm match.
But in 1955, many speakers were 16 ohms.

Patrick Turner.


"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.

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/

In article , "west"
wrote:

Why do Dynaco Amps and even Uncle Ned's schematic of a Hi Power Williamson
Amp use the 16ohm OPT tap instead of the 8ohm for feedback? What would
happen if you took a stock ST-70 or MKIII and moved the feedback over to the
8ohm? Thanks.

Taking the feedback signal from the 16 Ohm tap provides improved high
frequency stability when either the 8 or 4 Ohm tap is in use. This is a
result of the "unused" end of the secondary winding providing some of the
stability benefits of a tertiary feedback winding, although obviously
since this tertiary winding is in series with the "active" portion of the
secondary, the advantage of the tertiary for feedback is diluted.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/