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  #1   Report Post  
Kevin Killebrew
 
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Default Best way to connect multiple Speakers?

Here is my question:

I've got a tubed receiver (Sherwood S8000) that is going to be connected to
two sets of speakers via an external switch. One set is outside on the
patio, the other inside at the bar. Most of the time only one set of
speakers will be playing, but occasionally I forsee both sets being played
simultaneously. Both sets of speakers are 8 ohms so when I occasionally
run both pairs I will have four ohms (assuming they are paralleled) per
channel. Which receiver output should I run to the switch box? Four or
eight ohms?

Is it best to run an occasional four ohms on the eight ohm tap? or run
mostly eight ohms on the four ohm tap having a four ohm load only when both
sets of speakers are playing? I suspect the eight ohm tap is the right
choice because it will be correct most of the time, but I wanted some RAT
opinions of the effect of running mismatched impedances, i.e., four ohm load
on an eight ohm OT tap. I want to avoid an overly complex switch
arrangement.

Kevin Killebrew
Austin, Texas


  #2   Report Post  
DougC
 
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Kevin Killebrew wrote:
Here is my question:

I've got a tubed receiver (Sherwood S8000) that is going to be connected to
two sets of speakers via an external switch.....

Kevin Killebrew
Austin, Texas



This subject is treated in the RDH4 book, chapter 21, bottom of page
885: proper loading of two sets of two speakers, all of the same
impedance, using either pair or both at once. ---- Chapter 21 is only
569 kb so it's not much to download to take a look, if you don't have a
copy of RDH4 around.
  #3   Report Post  
Jim
 
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Kevin,

I was wondering the same thing as you just yesterday when I picked up two
speakers in someone's garbage. I live nearby in Round Rock Texas. I also
have a Sherwood S8000 that I restored, and then got carried away with
modifications on. The wires were really bad with insulation falling off
everywhere, and the power transformer wires were all bare! You might laugh,
and then sigh, when you see the pictures at the following link:


http://pages.prodigy.net/jcandela/Sherwood_S8000/
I hope your transformers are in better shape than mine. I also had to
rebuild the audio output transformers in a similar way

Here is where the modifications began:

Proto boarding with 500 volts can be interesting! Always use a variac, ac
ammeter, use one hand, and stand on a piece of dry plywood!:
http://pages.prodigy.net/jcandela/Sh...rotoboard1.JPG

Close up of the proto board:
http://pages.prodigy.net/jcandela/Sh...rotoboard2.JPG

Final circuitry with description of changes:
http://pages.prodigy.net/jcandela/Sh.../regulator.htm

As for your question, I personally would run on the 4 ohm tap. That way you
will have maximum power available when running both sets of speakers. Also
if you load the amplifier with 4 ohms when on the 8 ohm tap, the reflected
plate to plate impedance will be reduced, and if you crank the volume when
in this condition, expect higher peak currents drawn by the 7868 plate and
screen, as well as higher distortion. The poor "stock" power supply
regulation of the Sherwood will really show it's ugly head too and this will
muddy the bass, and cause the FM multiplex to switch in and out of lock in
response to the audio bass notes.

Let me know how you proceed, and how it works out..

Regards,
Jim Candela

"Kevin Killebrew" wrote in message
...
Here is my question:

I've got a tubed receiver (Sherwood S8000) that is going to be connected

to
two sets of speakers via an external switch. One set is outside on the
patio, the other inside at the bar. Most of the time only one set of
speakers will be playing, but occasionally I forsee both sets being played
simultaneously. Both sets of speakers are 8 ohms so when I occasionally
run both pairs I will have four ohms (assuming they are paralleled) per
channel. Which receiver output should I run to the switch box? Four or
eight ohms?

Is it best to run an occasional four ohms on the eight ohm tap? or run
mostly eight ohms on the four ohm tap having a four ohm load only when

both
sets of speakers are playing? I suspect the eight ohm tap is the right
choice because it will be correct most of the time, but I wanted some RAT
opinions of the effect of running mismatched impedances, i.e., four ohm

load
on an eight ohm OT tap. I want to avoid an overly complex switch
arrangement.

Kevin Killebrew
Austin, Texas




  #4   Report Post  
robert casey
 
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Default

Kevin Killebrew wrote:

Here is my question:

I've got a tubed receiver (Sherwood S8000) that is going to be connected to
two sets of speakers via an external switch. One set is outside on the
patio, the other inside at the bar. Most of the time only one set of
speakers will be playing, but occasionally I forsee both sets being played
simultaneously. Both sets of speakers are 8 ohms so when I occasionally
run both pairs I will have four ohms (assuming they are paralleled) per
channel. Which receiver output should I run to the switch box? Four or
eight ohms?

Is it best to run an occasional four ohms on the eight ohm tap? or run
mostly eight ohms on the four ohm tap having a four ohm load only when both
sets of speakers are playing? I suspect the eight ohm tap is the right
choice because it will be correct most of the time, but I wanted some RAT
opinions of the effect of running mismatched impedances, i.e., four ohm load


Running 4 ohms on the 8 ohm tap will not hurt the amp. Tube amps
don't mind lower impedance loads, what they don't like is no load.
The amount of undistorted audio power will be a bit lower with
the mismatch, but other than that it will work fine.
  #5   Report Post  
John Stewart
 
Posts: n/a
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Kevin Killebrew wrote:

Here is my question:

I've got a tubed receiver (Sherwood S8000) that is going to be connected to
two sets of speakers via an external switch. One set is outside on the
patio, the other inside at the bar. Most of the time only one set of
speakers will be playing, but occasionally I forsee both sets being played
simultaneously. Both sets of speakers are 8 ohms so when I occasionally
run both pairs I will have four ohms (assuming they are paralleled) per
channel. Which receiver output should I run to the switch box? Four or
eight ohms?

Is it best to run an occasional four ohms on the eight ohm tap? or run
mostly eight ohms on the four ohm tap having a four ohm load only when both
sets of speakers are playing? I suspect the eight ohm tap is the right
choice because it will be correct most of the time, but I wanted some RAT
opinions of the effect of running mismatched impedances, i.e., four ohm load
on an eight ohm OT tap. I want to avoid an overly complex switch
arrangement.

Kevin Killebrew
Austin, Texas


Whatever you do, don't wire the speakers in series. For a two speaker hookup,
that removes the amplifier damping from both speakers since they would be then
be sourced through each other, an intolerable situation.

Better a lower Z match than higher since PP pentode amps benefit from that. The
3rd harmonic is reduced. Most speaker loads are far too high anyway at their
resonance(s). A parallel hookup tends to smooth that out provided the speakers
are dissimilar.

Good Luck, John Stewart




  #6   Report Post  
Patrick Turner
 
Posts: n/a
Default



John Stewart wrote:

Kevin Killebrew wrote:

Here is my question:

I've got a tubed receiver (Sherwood S8000) that is going to be connected to
two sets of speakers via an external switch. One set is outside on the
patio, the other inside at the bar. Most of the time only one set of
speakers will be playing, but occasionally I forsee both sets being played
simultaneously. Both sets of speakers are 8 ohms so when I occasionally
run both pairs I will have four ohms (assuming they are paralleled) per
channel. Which receiver output should I run to the switch box? Four or
eight ohms?

Is it best to run an occasional four ohms on the eight ohm tap? or run
mostly eight ohms on the four ohm tap having a four ohm load only when both
sets of speakers are playing? I suspect the eight ohm tap is the right
choice because it will be correct most of the time, but I wanted some RAT
opinions of the effect of running mismatched impedances, i.e., four ohm load
on an eight ohm OT tap. I want to avoid an overly complex switch
arrangement.

Kevin Killebrew
Austin, Texas


Whatever you do, don't wire the speakers in series. For a two speaker hookup,
that removes the amplifier damping from both speakers since they would be then
be sourced through each other, an intolerable situation.


The DF issue is not always important You can have 10 speakers in series and if each
has the same enclosure,
and each driver is the same, the response will be identical to a single
driver driven with the same low Rout, and this proves the
amp damping doesn't reduce.
In fact it increases.
If you have Ro = 0.5 ohms and a 5 ohm speaker, DF = 10.
If you have 10 series speakers of 5 ohms for 50 ohms, same Ro, DF = 50.

People who build line array speakers using a dozen or more drives know
what I am saying to be true, or else they would be compelled to
connect all speakers in parallel, giving input impedances of very
low value.

However, where you have two different brands of speakers, different boxes
and drivers, series speakers will produce big variations in response from each
because the impedances of each are very dissimilar at different F.

So its for this reason you wouldn't want to series speakers.

Set the amps for 4 ohms where you want to have a pair of 8 ohms in parallel.

Pentode amps are a bit queer.
They have a load optimum where thd is lowest and each side of that thd
tends to rise and thd is very high when RL is very high.

But the amp NFB also becomes most effective when RL is high, when
the tube gain is high, and also the PP pentode amp works in class A when RL
is high, when it isn't producing power in class AB where RL is low.
This is an idiotic statement, but speaker Z varies for different F,
and either its working in AB or it isn't, but one has to
contemplate that the amplifier is
sending a voltage x current signal to a constantly changing load value.

Since its almost impossible to think of 3 things at once, I leave
you to contemplate the tube's operation knowing that
to win a game of chess against a naked woman and keep an erection
and do your tax return all simultaneously is downright impossible.




Better a lower Z match than higher since PP pentode amps benefit from that. The
3rd harmonic is reduced. Most speaker loads are far too high anyway at their
resonance(s). A parallel hookup tends to smooth that out provided the speakers
are dissimilar.


Using two brands of speakers in parallel with different box resonances at LF
will indeed tend to produce a more even impedance.
But also its likely that at some F the impedance will be lower then 4 ohms.

Some makers have deliberately different enclosure volumes and Fb
for a pair of bass drivers in the one speaker cabinet.
It makes the resonant Z peaks overlap, and a much flatter
Z is realised. Its all in RDH4, with a graph.

Patrick Turner.





Good Luck, John Stewart


  #7   Report Post  
Chris Hornbeck
 
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Default

On Mon, 25 Apr 2005 15:28:52 GMT, "Kevin Killebrew"
wrote:

I've got a tubed receiver (Sherwood S8000) that is going to be connected to
two sets of speakers via an external switch. One set is outside on the
patio, the other inside at the bar. Most of the time only one set of
speakers will be playing, but occasionally I forsee both sets being played
simultaneously. Both sets of speakers are 8 ohms so when I occasionally
run both pairs I will have four ohms (assuming they are paralleled) per
channel. Which receiver output should I run to the switch box? Four or
eight ohms?


Modern multiple-room home installations often use stepped autoformer
level controls, wall-mounted in each room.

Two advantages: the volume is adjustable locally, and the unused
speakers don't load the amplifier, proportionally.

Disadvantages: pentode output stages might well resist being so
treated, and decent-but-not-Nirvana level controls like the "Niles"
brand run about $70US.

And whatever you decide to do, remember to (be able to) turn off
the outside speakers, especially if your neighbors are Texans.

Good fortune,

Chris Hornbeck
"This has been an account for those who don't keep them"
J-LG, _Tout Va Bien_ 1972
  #8   Report Post  
John Stewart
 
Posts: n/a
Default

Patrick Turner wrote:

John Stewart wrote:

Kevin Killebrew wrote:

Here is my question:

I've got a tubed receiver (Sherwood S8000) that is going to be connected to
two sets of speakers via an external switch. One set is outside on the
patio, the other inside at the bar. Most of the time only one set of
speakers will be playing, but occasionally I forsee both sets being played
simultaneously. Both sets of speakers are 8 ohms so when I occasionally
run both pairs I will have four ohms (assuming they are paralleled) per
channel. Which receiver output should I run to the switch box? Four or
eight ohms?

Is it best to run an occasional four ohms on the eight ohm tap? or run
mostly eight ohms on the four ohm tap having a four ohm load only when both
sets of speakers are playing? I suspect the eight ohm tap is the right
choice because it will be correct most of the time, but I wanted some RAT
opinions of the effect of running mismatched impedances, i.e., four ohm load
on an eight ohm OT tap. I want to avoid an overly complex switch
arrangement.

Kevin Killebrew
Austin, Texas


Whatever you do, don't wire the speakers in series.For a two speaker hookup,


that removes the amplifier damping from both speakers since they would be then
be sourced through each other, an intolerable situation.


The DF issue is not always important


We can see that as some folks still choose to use pentode output amps with no voltage
NFB. For example, it may work well for some musicians in order to get the sound they
want.

You can have 10 speakers in series and if each has the same enclosure,
and each driver is the same, the response will be identical to a single
driver driven with the same low Rout, and this proves the
amp damping doesn't reduce.


Within the limits of the similarities of the enclosures & drivers, true enough. As well
as the minor differences between enclosures & drivers, in actual practice there will be
a spread of response since the boxes will all have to be in physically different space.
As a group they do not constitute a point source. So each speaker it it's box will have
a unique response curve, unlike any of the others. Your statement doesn't prove
anything. It simply makes an assumption & a poor one at that.

In fact it increases.
If you have Ro = 0.5 ohms and a 5 ohm speaker, DF = 10.
If you have 10 series speakers of 5 ohms for 50 ohms, same Ro, DF = 50.


In your example the DF for the group is 50. However, for each speaker the source
impedance would be the amp Ro of 0.5 ohms plus the entire series complex impedance's of
the other nine speakers in the group. That is a condition I would prefer to avoid. An
even more important consideration is, how will we deliver power to the load? In your
example the load reflected to the output tubes (or transistors) is now 10X nominal. So
power available is drastically reduced. As well, if a pentode output, distortion goes
way up due to a gross mismatch.

People who build line array speakers using a dozen or more drives know
what I am saying to be true, or else they would be compelled to
connect all speakers in parallel, giving input impedances of very
low value.


Not at all. Systems are easily built using series/parallel connexions. A set of four 5
ohm speakers is still 5 ohms when series paralleled. The same is true for 16 speakers &
these kinds of things have been built by their fans for many years. I recall well a
friend building something called the 'Sweet 16' more than 45 years ago. It used a set
of sixteen 6X9 speakers in a single enclosure. It sounded pretty good too. In a recent
magazine article I see a project using four speakers per enclosure, again connected
series/parallel in order to maintain a useable impedance level.

However, where you have two different brands of speakers, different boxes
and drivers, series speakers will produce big variations in response from each
because the impedances of each are very dissimilar at different F.


That is what I recommended against in the first place. Why do we need to hear that
again from you? I certainly don't need your confirmation.

So its for this reason you wouldn't want to series speakers.

Set the amps for 4 ohms where you want to have a pair of 8 ohms in parallel.

Pentode amps are a bit queer.
They have a load optimum where thd is lowest and each side of that thd
tends to rise and thd is very high when RL is very high.


True enough. But how does that make a pentode a 'bit queer'? It is simply a property of
pentodes, which can be exploited. That is why I recommended using the 8 ohm tap. By
paralleling the load resulting in a mismatch on the low side, the 3rd harmonic is
reduced. If we were to go the other way as you have recommended, 3rd harmonic instead
goes way up. The choice IMO is obvious to anyone who carefully considers how a pentode
works.

But the amp NFB also becomes most effective when RL is high, when
the tube gain is high, and also the PP pentode amp works in class A when RL
is high, when it isn't producing power in class AB where RL is low.


When the Rl is higher (or lower) than optimum in a pentode amp, distortion increases.
Than more FB is required to get the distortion down to a reasonable level. In a PP amp
the even order (2nd, 4th, 6th...Etc) harmonics are reduced by the circuit. All the
other noise & distortion products are still in there. In all the ordinary amp
topologies, voltage NFB is applied around the entire amp to reduce the D. However, by
doing that there is a danger of manufacturing some intermodulation products that were
not originally present. NFB of this kind does not fix the D in the output stage. It
simply tries to correct what is already there while at the same time it is causing
other problems. Better no miss match in the first place.

This is an idiotic statement, but speaker Z varies for different F,


What is idiotic about that? It is a set of complex impedances, a property of the
speaker in it's enclosure & dependent on the space it is in, as well.

and either its working in AB or it isn't, but one has to
contemplate that the amplifier is
sending a voltage x current signal to a constantly changing load value.

Since its almost impossible to think of 3 things at once, I leave
you to contemplate the tube's operation knowing that
to win a game of chess against a naked woman and keep an erection
and do your tax return all simultaneously is downright impossible.


What has any of that got to do with the original query? Or is it simply another 'Fog of
Bull****' to cover something else you are unsure of?

Better a lower Z match than higher since PP pentode amps benefit from that. The
3rd harmonic is reduced. Most speaker loads are far too high anyway at their
resonance(s). A parallel hookup tends to smooth that out provided the speakers
are dissimilar.


Using two brands of speakers in parallel with different box resonances at LF
will indeed tend to produce a more even impedance.
But also its likely that at some F the impedance will be lower then 4 ohms.

Some makers have deliberately different enclosure volumes and Fb
for a pair of bass drivers in the one speaker cabinet.
It makes the resonant Z peaks overlap, and a much flatter
Z is realised.


And easy to simulate for anyone with simulation software.

Its all in RDH4, with a graph.


And many other references, as well.

If one thinks about what is happening here, this whole exercise (the debate) is
ridiculous. Kevin K asked a simple question & several responded with straight forward &
too the point answers. Kevin told us the speakers are in different locations ( And
probably different speakers). As one of my points, I simply recommended against a
series connexion for the speakers.

But our Patrick T knows better & proceeds to go into another of his many long winded
explanations, much of which was completely unrelated to the query. Worse still, parts
of his response are in error. It seems Patrick T is for some unknown reason is driven
to respond to every post that shows up on the NG. He is far from the expert he pretends
to be. Others should be aware of that. Be sure to get a second opinion!

John Stewart

Patrick Turner.



Good Luck, John Stewart


  #9   Report Post  
Ian Iveson
 
Posts: n/a
Default

"John Stewart" wrote

...
In your example the DF for the group is 50. However, for each
speaker the source
impedance would be the amp Ro of 0.5 ohms plus the entire series
complex impedance's of
the other nine speakers in the group. That is a condition I would
prefer to avoid.
...


Seems to me this is the crucial point, John. Could be seen as
another example of current sources in series.

cheers, Ian


  #10   Report Post  
Patrick Turner
 
Posts: n/a
Default



John Stewart wrote:

Patrick Turner wrote:

John Stewart wrote:

Kevin Killebrew wrote:

Here is my question:

I've got a tubed receiver (Sherwood S8000) that is going to be

connected to
two sets of speakers via an external switch. One set is outside

on the
patio, the other inside at the bar. Most of the time only one

set of
speakers will be playing, but occasionally I forsee both sets

being played
simultaneously. Both sets of speakers are 8 ohms so when I

occasionally
run both pairs I will have four ohms (assuming they are

paralleled) per
channel. Which receiver output should I run to the switch

box? Four or
eight ohms?

Is it best to run an occasional four ohms on the eight ohm tap?

or run
mostly eight ohms on the four ohm tap having a four ohm load

only when both
sets of speakers are playing? I suspect the eight ohm tap is

the right
choice because it will be correct most of the time, but I wanted

some RAT
opinions of the effect of running mismatched impedances, i.e.,

four ohm load
on an eight ohm OT tap. I want to avoid an overly complex

switch
arrangement.

Kevin Killebrew
Austin, Texas

Whatever you do, don't wire the speakers in series.For a two

speaker hookup,


that removes the amplifier damping from both speakers since they

would be then
be sourced through each other, an intolerable situation.


The DF issue is not always important


We can see that as some folks still choose to use pentode output amps
with no voltage NFB. For example, it may work well for some musicians
in order to get the sound they want.


Many old radios used a 6V6 with no NFB.
Above a very low level they sounded attrocious.

Better than no radio.

And sure, with muso amps, DF doesn't matter much but many guitar amps
do have some NFB.



You can have 10 speakers in series and if each has the same
enclosure,
and each driver is the same, the response will be identical to a
single
driver driven with the same low Rout, and this proves the
amp damping doesn't reduce.


Within the limits of the similarities of the enclosures & drivers,
true enough.


I didn't mean it any other way.

As well as the minor differences between enclosures & drivers, in
actual practice there will be a spread of response since the boxes
will all have to be in physically different space.


Nope, you can have a line away of speakers and they all share the same
box.
The dozen+ drivers used act like one big long driver, with a dozen times
the
box volume.
You could even replace all the cones with a single long
lightweight diaphram glues to the voice coils, and curved in one
direction,
like a pipe cut down the centre, and with a suspension rubber along the
edges and at the ends.
That would make all the drivers improve their team work.


As a group they do not constitute a point source.


That doesn't matter to the builders of large line arrays.

So each speaker it it's box will have a unique response curve, unlike
any of the others. Your statement doesn't prove anything. It simply
makes an assumption & a poor one at that.


You are not proving anything either, except you are giving a lesson in
side tracking.
Muliple 2 way bookshelf speakers can be hooked up in series without
changing the near field response of each or how they operate.
They just have to be the same brand, model number, and fairly well
matched.
Its routine to make speakers closely matched.
Such matching gives the best imaging where a stereo pair is used.
But we are talking about DF and whether or not is is decreased, ie, DF
number
is lower, just because we series connect speakers.
Clearly I have made my point that if one connects a dozen
of the *same* bookshelf speakers in series, the DF is actually improved
if the same amp with the same Ro is used.


In fact it increases.
If you have Ro = 0.5 ohms and a 5 ohm speaker, DF = 10.
If you have 10 series speakers of 5 ohms for 50 ohms, same Ro, DF =
50.


In your example the DF for the group is 50. However, for each speaker
the source impedance would be the amp Ro of 0.5 ohms plus the entire
series complex impedance's of the other nine speakers in the group.
That is a condition I would prefer to avoid.


So would I, so line array builders connect groups of say 4 speakers in
series,
so 4 give 32 ohms, then they might have 4 groups of 4 in parallel,
pulling the
Z to 8 ohms, and the same as one single driver.
DF then is the same as one driver.


An even more important consideration is, how will we deliver power to
the load? In your example the load reflected to the output tubes (or
transistors) is now 10X nominal. So power available is drastically
reduced. As well, if a pentode output, distortion goes way up due to a
gross mismatch.


See above.

When multiple drivers are seriesed and paralleled for the correct Z
wanted for a good amp
match there are no problems.

If somebody wants to ties multiple speakers together more tightly
rather than hooking them up in series and expecting them all
to behave with the same parameters then I suggest
one use a tapped auto transformer with say 4 equal windings in series
and one could have 4 x 8 ohm speakers across each winding for 2 ohms,
then with 4 windings loaded by 2 ohms each you have 8 ohms input
for 16 drivers, and they are all locked together magnetically.






People who build line array speakers using a dozen or more drives
know
what I am saying to be true, or else they would be compelled to
connect all speakers in parallel, giving input impedances of very
low value.


Not at all. Systems are easily built using series/parallel connexions.


Ah, but as I said you have to series speakers in line arrays.

A set of four 5 ohm speakers is still 5 ohms when series paralleled.
The same is true for 16 speakers & these kinds of things have been
built by their fans for many years. I recall well a friend building
something called the 'Sweet 16' more than 45 years ago. It used a set
of sixteen 6X9 speakers in a single enclosure. It sounded pretty good
too. In a recent magazine article I see a project using four speakers
per enclosure, again connected series/parallel in order to maintain a
useable impedance level.


So, what's wrong with seriesing speakers?





However, where you have two different brands of speakers, different
boxes
and drivers, series speakers will produce big variations in response
from each
because the impedances of each are very dissimilar at different F.


That is what I recommended against in the first place. Why do we need
to hear that again from you? I certainly don't need your confirmation.

So its for this reason you wouldn't want to series speakers.

Set the amps for 4 ohms where you want to have a pair of 8 ohms in
parallel.

Pentode amps are a bit queer.
They have a load optimum where thd is lowest and each side of that
thd
tends to rise and thd is very high when RL is very high.


True enough. But how does that make a pentode a 'bit queer'? It is
simply a property of pentodes, which can be exploited. That is why I
recommended using the 8 ohm tap. By paralleling the load resulting in
a mismatch on the low side, the 3rd harmonic is reduced. If we were to
go the other way as you have recommended, 3rd harmonic instead goes
way up. The choice IMO is obvious to anyone who carefully considers
how a pentode works.


But if you palce two 8 ohms speakers in parallel across a 8 ohm outlet
from a pentode amp,
RLa-a is halved, and all the measurements i HAVE EVER CONDUCTED indicate
thd rises heaps,
and at high sustained levels the tubes will glow red hot and expire.

The gain of pentodes is about proportional to their loads, so if load is

halved, so is the output tube gain and so is the amount od applied NFB
so the thd rises because of this fact and the fact that the lower load
increases thd anyway.
I am suggesting ppl try to keep the amp working with its
recommended correct RLa-a, ie, with a pair of parallel 8 ohm speakers
= 4 ohms, and connected to a 4 ohm outlet if there is one.

But if ppl connect 8 ohms to the 4 ohm outlet, the pentode AB amp
will make marginally less maximum power, and its output tube gain will
near double, and the slight increase in thd due to a higher than ideal
load match
will be offset by a near doubling of the applied NFB.

Applied NFB depends on open loop gain
Without any load, and with NFB, pentodes don't have an alarming
increase in thd.






But the amp NFB also becomes most effective when RL is high, when
the tube gain is high, and also the PP pentode amp works in class A
when RL
is high, when it isn't producing power in class AB where RL is low.


When the Rl is higher (or lower) than optimum in a pentode amp,
distortion increases. Than more FB is required to get the distortion
down to a reasonable level. In a PP amp the even order (2nd, 4th,
6th...Etc) harmonics are reduced by the circuit. All the other noise &
distortion products are still in there.


But as I said, the NFB is increased hugely with the worst case
situation, no load.

A properly designed pentode amp with 20 Db of NFB only has 20 dB
of FB when RL = the rated load, say 8 ohms.
output tube gain is perhaps 25. But with no load at all, tube gain leaps

to near µ for the tubes at mid frequencies, so gain = 120 for EL34.
So the applied NFB with no load leaps about 5 times, or 15 dB to 35 dB.
So the thd is controlled.


In all the ordinary amp topologies, voltage NFB is applied around the
entire amp to reduce the D. However, by doing that there is a danger
of manufacturing some intermodulation products that were not
originally present.


IMD is more likely to be made in amps with low NFB due to NFB
application.
Where NFB application is high, there is less IMD, period.


NFB of this kind does not fix the D in the output stage.


Yes it does, where the tubes are not driven into clipping, and have
enough gain and are not
driven into grid current.

It simply tries to correct what is already there while at the same
time it is causing other problems. Better no miss match in the first
place.


Sure, but speakers present loads which are always going to provide a
mismatch
at some F.



This is an idiotic statement, but speaker Z varies for different F,


What is idiotic about that? It is a set of complex impedances, a
property of the speaker in it's enclosure & dependent on the space it
is in, as well.

and either its working in AB or it isn't, but one has to
contemplate that the amplifier is
sending a voltage x current signal to a constantly changing load
value.

Since its almost impossible to think of 3 things at once, I leave
you to contemplate the tube's operation knowing that
to win a game of chess against a naked woman and keep an erection
and do your tax return all simultaneously is downright impossible.


What has any of that got to do with the original query? Or is it
simply another 'Fog of Bull****' to cover something else you are
unsure of?


Speakers present a dynamically varying load with the signal
whose frequency content varies dynamically along with amplitude.

Should one contemplate what is happening at their amp's output
and within the amp, one should remember that the load is not a
resistance of one value.
Because all speakers supply a degree of mismatch, their power ceiling
is somewhat restricted below the ideal resistance value that gives the
ideal power.

Series speakers are OK if well considered, but not if they
differ widely in specs.

Nearly all pentode power amps are set up to cope with twice the
average rated load value quite well, but certainly not with half the
recommended
load value.


Better a lower Z match than higher since PP pentode amps benefit

from that. The
3rd harmonic is reduced. Most speaker loads are far too high

anyway at their
resonance(s). A parallel hookup tends to smooth that out provided

the speakers
are dissimilar.


Using two brands of speakers in parallel with different box
resonances at LF
will indeed tend to produce a more even impedance.
But also its likely that at some F the impedance will be lower then
4 ohms.

Some makers have deliberately different enclosure volumes and Fb
for a pair of bass drivers in the one speaker cabinet.
It makes the resonant Z peaks overlap, and a much flatter
Z is realised.


And easy to simulate for anyone with simulation software.

Its all in RDH4, with a graph.


And many other references, as well.

If one thinks about what is happening here, this whole exercise (the
debate) is ridiculous. Kevin K asked a simple question & several
responded with straight forward & too the point answers. Kevin told us
the speakers are in different locations ( And probably different
speakers). As one of my points, I simply recommended against a series
connexion for the speakers.


Let us grant Kevin that he has adequate intelligence to draw from what
we have said
what he needs to know.

The conclusive simple recommendation is series speakers are OK
if they are exactly the same. This won't hurt the amp.

If the speakers are different, paralleling them is all you should do.
But the combined nominal load may be too low for the amp
and cause heat stress, unless you can move the speakers from the
8 ohm connection point to the 4 ohm connection point if there is one.

Other matters raised about series/parallel speakers are just
salient matters raised, and should not distract from the
simple answer.



But our Patrick T knows better & proceeds to go into another of his
many long winded explanations, much of which was completely unrelated
to the query. Worse still, parts of his response are in error. It
seems Patrick T is for some unknown reason is driven to respond to
every post that shows up on the NG. He is far from the expert he
pretends to be. Others should be aware of that. Be sure to get a
second opinion!


But I don't recommend paralleling speakers just to get the 3rd harmonic
down,
which won't occur as you claim, and I won't recommend loading an amp
adversly with a lower impedance load that it was designed for.

Feel free to get as many opinions as you want.

I am not worried by 100 different opinions.

And at least I take no notice of your consisten complaints about my long
posts.

I reserve the right to talk around and through a subject in detail,
and if you don't like to have to read what people post here, then
you are not interested in details, and the nitty gritty.

This is a discussion group, and nobody has the right to
tell me how much I should say.

If its awkward for you to post an answer then so be it.

And BTW, I don't reply to every post, that's another thing you got
wrong.

Patrick Turner.




John Stewart

Patrick Turner.



Good Luck, John Stewart





  #11   Report Post  
John Stewart
 
Posts: n/a
Default

Ian Iveson wrote:

"John Stewart" wrote

...
In your example the DF for the group is 50. However, for each
speaker the source
impedance would be the amp Ro of 0.5 ohms plus the entire series
complex impedance's of
the other nine speakers in the group. That is a condition I would
prefer to avoid.
...


Seems to me this is the crucial point, John. Could be seen as
another example of current sources in series.

cheers, Ian


Nature (And Physics) abhor certain conditions. One of them is current
sources in series. By way of 'The Principle of Duality' another would
be voltage sources in parallel. The definition of each should tell us
something. A perfect current source would deliver the same current to
the load no matter what the load was from a short circuit & on up.
Similarly, a voltage source would deliver a constant voltage to the
load from open circuit & on up to ever increasing currents.

Fortunately for us, most of the circuits we have to deal with include
some positive resistance in parallel with a current source & a positive
resistance in series with a voltage source. That helps us quite a lot
to establish an operating point. Without those resistors the circuit Q
point becomes indeterminate. So a simple current source while not
perfect would be able to source a reasonably constant current over a
wide range of loads. The max load is simply limited by the max voltage
available.

If a negative resistance were to be introduced into the circuit than we
have the potential for an oscillator.

So, I would sure agree with your point Ian, in particular at the
resonance F of the speakers.
Z tends to get quite high at that (those if a bass reflex) point. Even
at mid-frequencies far from the speaker resonance, nine speakers would
add up to 45 ohms. The source for each speaker if all were simply
series connected as Patrick T remarked in the first paragraph of his
original post of April 27th would than be 45.2 ohms. For each
individual speaker, particularly at resonance the remaining speakers
form a rather good current source. If Patrick didn't mean them to be
all wired in series he should have made that clear in his post. The
second paragraph of his original post has the speakers all in parallel.
What next?

I see in Patrick's later response to my critic of his post that he has
suddenly got the clue to series/parallel the speaker array. I wonder
where he got that idea!

As always with anything, including technical advice, Caveat Emptor.

More to go on this yet. And,
Cheers John Stewart

PS- I guess I will have to post the graph showing individual (2,3,4..)
harmonics for pentodes. It is not it in RDH4, but it does show up
elsewhere. That way one can see why a steeper load line is a better
condition for pentodes, again opposite Patrick's opinion. Best example
without having to look it up would be the steeper loadline applied to
PP amps. That helps to reduce the 3rd harmonic & one of the reasons we
do it. Depending on speaker resonance & program material, the tube is
always well loaded, both above & below the knee. Below the knee at
speaker resonance & above the knee at
mid-frequencies. JLS


  #12   Report Post  
Patrick Turner
 
Posts: n/a
Default



John Stewart wrote:

Ian Iveson wrote:

"John Stewart" wrote

...
In your example the DF for the group is 50. However, for each
speaker the source
impedance would be the amp Ro of 0.5 ohms plus the entire series
complex impedance's of
the other nine speakers in the group. That is a condition I would
prefer to avoid.
...


Seems to me this is the crucial point, John. Could be seen as
another example of current sources in series.

cheers, Ian


Nature (And Physics) abhor certain conditions. One of them is current
sources in series. By way of 'The Principle of Duality' another would
be voltage sources in parallel. The definition of each should tell us
something. A perfect current source would deliver the same current to
the load no matter what the load was from a short circuit & on up.
Similarly, a voltage source would deliver a constant voltage to the
load from open circuit & on up to ever increasing currents.

Fortunately for us, most of the circuits we have to deal with include
some positive resistance in parallel with a current source & a positive
resistance in series with a voltage source. That helps us quite a lot
to establish an operating point. Without those resistors the circuit Q
point becomes indeterminate. So a simple current source while not
perfect would be able to source a reasonably constant current over a
wide range of loads. The max load is simply limited by the max voltage
available.

If a negative resistance were to be introduced into the circuit than we
have the potential for an oscillator.

So, I would sure agree with your point Ian, in particular at the
resonance F of the speakers.
Z tends to get quite high at that (those if a bass reflex) point. Even
at mid-frequencies far from the speaker resonance, nine speakers would
add up to 45 ohms. The source for each speaker if all were simply
series connected as Patrick T remarked in the first paragraph of his
original post of April 27th would than be 45.2 ohms. For each
individual speaker, particularly at resonance the remaining speakers
form a rather good current source. If Patrick didn't mean them to be
all wired in series he should have made that clear in his post. The
second paragraph of his original post has the speakers all in parallel.
What next?

I see in Patrick's later response to my critic of his post that he has
suddenly got the clue to series/parallel the speaker array. I wonder
where he got that idea!


You have quoted me out of context and you alleges that I got the idea of
series and parallel combos of speakers from you.

Not so; known about all this for years before ever hearing what you had to
say.

But where you have say 4 speakers in series and each is 45 ohms at Fo,
then then one could say each one sees a drive impedance of the Ro of the
amp
in series with 135 ohms, and the DF is indeed very crook.

But it isn't, its very good.

Where the impedance of all the series speakers is the same, there is
an equal division of power in each one, and at all F,
and the response from each will be the same for the lot.
There is no manifestation of a speaker seeming to be driven by an impedance

of 4 times its own, just because you have series speakers.
This was the point I have been trying to make.

I have series speakers in a Dapollito arrangement, and there
is no DF problem.




As always with anything, including technical advice, Caveat Emptor.

More to go on this yet. And,
Cheers John Stewart

PS- I guess I will have to post the graph showing individual (2,3,4..)
harmonics for pentodes. It is not it in RDH4, but it does show up
elsewhere. That way one can see why a steeper load line is a better
condition for pentodes, again opposite Patrick's opinion.


One could have a very low RL, akin to a short circuit, say 500 ohms for
an EL34.
The distortion will be an unhappy picture.

IN fact the distortion with low loads becomes dominated purely
by the gm variation at varying Ia, and boy, do most pentodes vary!
Triodes have varying gm with Ia.

Triodes are virtual pentodes with a shirtload of internal NFB,
so that with no Ia change there is maximum NFB applied electrostatically,
hence they are very linear with no current change, as when loaded with a
ccs.

Pentodes on the other hand are attrociously non linear when no
NFB is applied internally because the screen stops it.
Gain when loaded by a CCS is extremenly high, about 130 for EL34,
and maybe 3,000 for a 6AU6.
Fortunately, the huge gain gives us an ability to apply
NFBV in external loops.
But still you end up with the same spectra of thd products plus a few
extras
from imd, although at low levels. Still not as clean as triodes.
But with either a too low load, or a too high load, pentodes
and beam tetrodes are disgusting.



Best example
without having to look it up would be the steeper loadline applied to
PP amps. That helps to reduce the 3rd harmonic & one of the reasons we
do it.


Next time you measure a class AB pentode amp
try reducing the RL to 1/4 of the load needed for maximum pure class A.
Then you find the amp works in class AB, and the 3H is far greater at all
power levels.

Beam tetrodes fare better than pentodes, if that makes you feel any better.

6550 can be configured to make under 1% thd
at 40 watts in pure beam class A.
Reducing RL isn't going to reduce the thd.



Depending on speaker resonance & program material, the tube is
always well loaded, both above & below the knee. Below the knee at
speaker resonance & above the knee at
mid-frequencies. JLS


But music has multiple frequencies, During bass notes where the
speaker Z = 40 ohms, the pentodes are loaded with a class A load, large v
swing,
low current swing.
But *simultaneously*, at some other F, say 300Hz, the tubes are loaded by a
Z at say a crossover F
between bass and mids where Z = 3 ohms.
What sort of load line appears then?

Speaker load lines are dynamically variable, so a
an elipse coloured grey for its whole enclosed area is a better idea of a
speaker load.
Resistances offer the only black line load.

And in class AB amps the load line changes from 1/2 RLa-a to 1/4 RLa-a
during each cycle which involves transition from A to AB.

So with a fixed R load value at the output, the load seen by each of a pair
in PP
is a bent line, or a curved one.

The changes in pentode gain between A and AB causes a lot of odd order thd.

PP triodes exhibit the gentlest transition from A to AB, so they
have the least odd order generation due to crossover,
offset by the way gm increases with Ia, so as load
drops with class AB, the gm increases, and so does gain......

Patrick Turner.






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