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16 ohm Feedback Tap



 
 
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  #1  
Old March 17th 05, 10:45 AM
west
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Default 16 ohm Feedback Tap

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


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  #2  
Old March 17th 05, 10:56 AM
Phil Allison
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"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




  #3  
Old March 17th 05, 01:01 PM
Patrick Turner
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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


  #4  
Old March 17th 05, 01:28 PM
Patrick Turner
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Default



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


  #5  
Old March 17th 05, 04:21 PM
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




  #6  
Old March 17th 05, 04:48 PM
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.




  #7  
Old March 17th 05, 05:08 PM
John Byrns
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Default

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/
  #8  
Old March 18th 05, 07:33 PM
Rich Sherman
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Default

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.
>
>
>
>



  #9  
Old March 18th 05, 08:12 PM
Jon Yaeger
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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

  #10  
Old March 19th 05, 04:24 AM
Patrick Turner
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Default



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.
> >
> >
> >
> >


 




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