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Don[_4_] Don[_4_] is offline
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Default 1st try kt88 amp

please see the schematic in alt.binaries.schematics.electronic
under "1st try kt88 amp"
it is a Mullard 5-10, Pat Turner, and www.diytube.com inspired mash-up.
-Don

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Default 1st try kt88 amp

In article ,
domain says...
please see the schematic in alt.binaries.schematics.electronic
under "1st try kt88 amp"
it is a Mullard 5-10, Pat Turner, and
www.diytube.com inspired mash-
up.
-Don


And tell us your comments and suggestions!


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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default 1st try kt88 amp


"Don"

please see the schematic in alt.binaries.schematics.electronic
under "1st try kt88 amp"
it is a Mullard 5-10, Pat Turner, and www.diytube.com inspired mash-

up.

And tell us your comments and suggestions!



** Very few people can see that group as most news servers have blocked all
" alt.binaries".



..... Phil


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Patrick Turner Patrick Turner is offline
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Default 1st try kt88 amp

On Sep 3, 1:29*pm, "Phil Allison" wrote:
"Don"

please see the schematic in alt.binaries.schematics.electronic
under "1st try kt88 amp"
it is a Mullard 5-10, Pat Turner, andwww.diytube.cominspired mash-

up.


And tell us your comments and suggestions!


** Very few people can see that group as most news servers have blocked all
" alt.binaries".

.... *Phil


Someone associated my name with a circuit schematic somewhere with a
KT88 but I can't see the schematic and it is most unlikely to be a
"comispired mash-up" I designed. I don't follow all the posts on the
web relating to me because there are so many. There is 100MB
downloaded from my site each day, this is neither good or bad, and
could be just bots searching the site; I don't much care about it. But
a Mullard 5-10 would not have a KT88 used in it and I don't have a
Mullard 5-10 schematic at my site, mainly because there are better
ways to build an amp than following Mullard's lead of the parsimonious
bean counter dominated 1950s.

One man's mash up is another man's dog's breakfast.
And a dingo's breakfast is just a **** and a lookaround, whatever.

Patrick Turner.i
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Default 1st try kt88 amp

On Sep 2, 8:03*pm, Don wrote:
please see the schematic in alt.binaries.schematics.electronic
under "1st try kt88 amp"
it is a Mullard 5-10, Pat Turner, andwww.diytube.cominspired mash-up.
-Don


Can't see it... could you post it somewhere else that is universally
accessible?
That said, and from memory, the 5-10 (I built two back in the day and
I'm still a "5-10 fan") used EL84 (6BQ5) o/p tubes driven directly
from a 12AX7 LTP phase splitter for 11 to 14 watts maximum output
depending on OPT connections (ultralinear or simple pentode,
respectively.) Your KT88's could give over twice that output power
but will likely need more drive than the LTP can provide - hence the
extra driver stage in the Williamson design. BTW, there was a Mullard
5-20 (20 watts) using EL34 o/p tubes around the same time, also with a
direct LTP phase splitter drive, but I don't know if that was pushing
the LTP drive limit.
If I had two KT88's (I wish!) I'd build the Williamson design, but
stabilize it with the "low frequency shelf" discussed here at length a
short while ago.
Cheers,
Roger


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Don Pearce[_3_] Don Pearce[_3_] is offline
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Default 1st try kt88 amp

On Sat, 03 Sep 2011 11:21:52 -0400, Don
wrote:

In article 2edeb104-6024-43e8-9749-
, says...
On Sep 2, 8:03*pm, Don wrote:
please see the schematic in alt.binaries.schematics.electronic
under "1st try kt88 amp"
it is a Mullard 5-10, Pat Turner, andwww.diytube.cominspired mash-

up.
-Don


Can't see it... could you post it somewhere else that is universally
accessible?


try
http://mysite.verizon.net/vze11z1rm/


You won't be hearing much until you sort out the connections to that
output transformer.

d
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Don[_4_] Don[_4_] is offline
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Default 1st try kt88 amp


If I had two KT88's (I wish!) I'd build the Williamson design, but
stabilize it with the "low frequency shelf" discussed here at length a
short while ago.
Cheers,
Roger


Regarding the "low frequency shelf" and high frequency
roll-off filters:
why are they usually after the voltage amp?
Why not a band-pass filter before the V1 tube?

o-----||---/\/\/\/\--------------------o
| |
===
|
|
| |
| |
o---------------------------------------o

-Don

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Don Pearce[_3_] Don Pearce[_3_] is offline
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Default 1st try kt88 amp

On Sat, 03 Sep 2011 19:27:48 -0500, flipper wrote:

On Sat, 03 Sep 2011 15:34:47 GMT, (Don Pearce) wrote:

On Sat, 03 Sep 2011 11:21:52 -0400, Don
wrote:

In article 2edeb104-6024-43e8-9749-
,
says...
On Sep 2, 8:03*pm, Don wrote:
please see the schematic in alt.binaries.schematics.electronic
under "1st try kt88 amp"
it is a Mullard 5-10, Pat Turner, andwww.diytube.cominspired mash-
up.
-Don

Can't see it... could you post it somewhere else that is universally
accessible?

try
http://mysite.verizon.net/vze11z1rm/


You won't be hearing much until you sort out the connections to that
output transformer.

d


Well he has, sorta. Just not a 'speaker' connected.

He doesn't have a suitable transformer model. What he needs is a 5 tap
primary

A-Plate, A-UL
CT
B-Plate, B-UL

and 4 tap secondary

GND, 4 Ohm, 8 Ohm, 16 Ohm

So he's taken two 1-1 transformer symbols and jury rigged the wiring
to 'sorta look like' what he doesn't have.

Imagine the core extends through both so it's 'one' transformer. The
1-2 CT connection should be clear. Then imagine another set of coils
connecting the middle 5 to 6 so it's a continuous winding from the
bottom 6 to the top 5, which should be renumbered so they don't
duplicate.

He's taking feedback from the 8 ohm tap and his zobel is on the 16 ohm
tap. The 4 ohm tap is 'open', unless one were using 4 ohm speakers in
which case that's where they'd connect.


The open circuit output tap needs a connection to the other half of
the transformer - otherwise there is no circuit.

d
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Default 1st try kt88 amp

On Sun, 04 Sep 2011 00:16:25 -0400, Don
wrote:


If I had two KT88's (I wish!) I'd build the Williamson design, but
stabilize it with the "low frequency shelf" discussed here at length a
short while ago.
Cheers,
Roger


Regarding the "low frequency shelf" and high frequency
roll-off filters:
why are they usually after the voltage amp?
Why not a band-pass filter before the V1 tube?

o-----||---/\/\/\/\--------------------o
| |
===
|
|
| |
| |
o---------------------------------------o

-Don


I think it is a sort of masochism - a craving for low and high
frequency instability. Large phase shifting mechanisms within a loop
are never a great idea. There should be ideally a single pole that
doesn't let up until beyond the unity gain frequency.

d


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Arny Krueger[_4_] Arny Krueger[_4_] is offline
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Default 1st try kt88 amp


"Don Pearce" wrote in message
...
On Sat, 03 Sep 2011 11:21:52 -0400, Don
wrote:

In article 2edeb104-6024-43e8-9749-
, says...
On Sep 2, 8:03 pm, Don wrote:
please see the schematic in alt.binaries.schematics.electronic
under "1st try kt88 amp"
it is a Mullard 5-10, Pat Turner, andwww.diytube.cominspired mash-

up.
-Don

Can't see it... could you post it somewhere else that is universally
accessible?


try
http://mysite.verizon.net/vze11z1rm/


You won't be hearing much until you sort out the connections to that
output transformer.


Looks to me like there are two separate identical output transformers, which
has the disadvantage of not cancelling out the partial saturating effect of
the output tube's quescient current, and not maximizing the cancellation of
even order harmonics.


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Don Pearce[_3_] Don Pearce[_3_] is offline
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Default 1st try kt88 amp

On Sun, 4 Sep 2011 05:01:12 -0400, "Arny Krueger"
wrote:


"Don Pearce" wrote in message
...
On Sat, 03 Sep 2011 11:21:52 -0400, Don
wrote:

In article 2edeb104-6024-43e8-9749-
, says...
On Sep 2, 8:03 pm, Don wrote:
please see the schematic in alt.binaries.schematics.electronic
under "1st try kt88 amp"
it is a Mullard 5-10, Pat Turner, andwww.diytube.cominspired mash-
up.
-Don

Can't see it... could you post it somewhere else that is universally
accessible?

try
http://mysite.verizon.net/vze11z1rm/


You won't be hearing much until you sort out the connections to that
output transformer.


Looks to me like there are two separate identical output transformers, which
has the disadvantage of not cancelling out the partial saturating effect of
the output tube's quescient current, and not maximizing the cancellation of
even order harmonics.


It looks like the schematic is from a Spice simulation, which won't
model a tapped transformer without special fiddling. Even so, those
two closest secondary taps should be connected to each other.

d
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Default 1st try kt88 amp


"Arny Krueger" wrote in message
...

"Don Pearce" wrote in message
...
On Sat, 03 Sep 2011 11:21:52 -0400, Don
wrote:

In article 2edeb104-6024-43e8-9749-
, says...
On Sep 2, 8:03 pm, Don wrote:
please see the schematic in alt.binaries.schematics.electronic
under "1st try kt88 amp"
it is a Mullard 5-10, Pat Turner, andwww.diytube.cominspired mash-
up.
-Don

Can't see it... could you post it somewhere else that is universally
accessible?

try
http://mysite.verizon.net/vze11z1rm/


You won't be hearing much until you sort out the connections to that
output transformer.


Looks to me like there are two separate identical output transformers,
which has the disadvantage of not cancelling out the partial saturating
effect of the output tube's quescient current, and not maximizing the
cancellation of even order harmonics.


Whoops! I didn't notice that the secondary of one output transformer is
basically connected to the air. That basically negates the existance of the
one half of the output stage!

Hopefully, this is all a consequence of a overly-simplistic modelling
program, as others have pointed out.


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Default 1st try kt88 amp

On Sep 4, 12:16*am, Don wrote:
If I had two KT88's (I wish!) I'd build the Williamson design, but
stabilize it with the "low frequency shelf" discussed here at length a
short while ago.
Cheers,
Roger


Regarding the "low frequency shelf" and high frequency
*roll-off filters:
why are they usually after the voltage amp?
Why not a band-pass filter before the V1 tube?

o-----||---/\/\/\/\--------------------o
* * * * * * * * * * * * *| * * * * |
* * * * * * * * * * * * === * * * *
* * * * * * * * * * * * *| * * * *
* * * * * * * * * * * * *| * * * *
* * * * * * * * * * * * *| * * * * |
* * * * * * * * * * * * *| * * * * |
o---------------------------------------o

-Don


They have to be inside the NFB loop. They are there to stabilize the
loop gain in the face of three phase leads (up to 90 deg each) at VLF
from the two RC couplings and one OPT.
Cheers,
Roger
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Default 1st try kt88 amp

In article ,
says...
"Don Pearce" wrote in message
...
On Sat, 03 Sep 2011 11:21:52 -0400, Don
wrote:

In article 2edeb104-6024-43e8-9749-
,
says...
On Sep 2, 8:03 pm, Don wrote:
please see the schematic in alt.binaries.schematics.electronic
under "1st try kt88 amp"
it is a Mullard 5-10, Pat Turner, andwww.diytube.cominspired mash-
up.
-Don

Can't see it... could you post it somewhere else that is universally
accessible?

try
http://mysite.verizon.net/vze11z1rm/


You won't be hearing much until you sort out the connections to that
output transformer.


Looks to me like there are two separate identical output transformers, which
has the disadvantage of not cancelling out the partial saturating effect of
the output tube's quescient current, and not maximizing the cancellation of
even order harmonics.


Flipper is correct- I didn't have a symbol for a ultralinear OPT.
Pretend that there is one UL transformer on the schematic.
-Don

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John Byrns John Byrns is offline
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Posts: 1,441
Default 1st try kt88 amp

In article ,
(Don Pearce) wrote:

On Sun, 04 Sep 2011 11:24:20 -0500, John Byrns
wrote:

In article ,
(Don Pearce) wrote:

On Sun, 04 Sep 2011 00:16:25 -0400, Don
wrote:


If I had two KT88's (I wish!) I'd build the Williamson design, but
stabilize it with the "low frequency shelf" discussed here at length a
short while ago.
Cheers,
Roger

Regarding the "low frequency shelf" and high frequency
roll-off filters:
why are they usually after the voltage amp?
Why not a band-pass filter before the V1 tube?

o-----||---/\/\/\/\--------------------o
| |
===
|
|
| |
| |
o---------------------------------------o

-Don

I think it is a sort of masochism - a craving for low and high
frequency instability. Large phase shifting mechanisms within a loop
are never a great idea. There should be ideally a single pole that
doesn't let up until beyond the unity gain frequency.


The use of a dominant pole to stabilize the feedback loop is transistor
thinking, not always practical in tube circuits.


Just good engineering practice and pretty much standard in op amps.
And the first op amps were valve, not transistor.


That's all pretty much true, however we aren't talking op amps in this thread,
were talking valve audio amps, and more specifically the particular design Don
posted.

Can you give us a worked example of how you would use a dominant pole to
stabilize Don's amplifier design, specifying all relevant time constants in the
amplifier. I would be especially interested in how you would do the low
frequency stabilization were a number of irritating tradeoffs often come into
play, and any assumptions about the effect of the OPT on phase shift are likely
to be more accurate than at high frequencies. The object is for the amplifier
to remain stable with a speaker load, no load, and any other load it might be
likely to encounter during its lifetime.

--
Regards,

John Byrns

Surf my web pages at,
http://fmamradios.com/
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Default 1st try kt88 amp

On Sun, 04 Sep 2011 12:20:16 -0500, John Byrns
wrote:

In article ,
(Don Pearce) wrote:

On Sun, 04 Sep 2011 11:24:20 -0500, John Byrns
wrote:

In article ,
(Don Pearce) wrote:

On Sun, 04 Sep 2011 00:16:25 -0400, Don
wrote:


If I had two KT88's (I wish!) I'd build the Williamson design, but
stabilize it with the "low frequency shelf" discussed here at length a
short while ago.
Cheers,
Roger

Regarding the "low frequency shelf" and high frequency
roll-off filters:
why are they usually after the voltage amp?
Why not a band-pass filter before the V1 tube?

o-----||---/\/\/\/\--------------------o
| |
===
|
|
| |
| |
o---------------------------------------o

-Don

I think it is a sort of masochism - a craving for low and high
frequency instability. Large phase shifting mechanisms within a loop
are never a great idea. There should be ideally a single pole that
doesn't let up until beyond the unity gain frequency.

The use of a dominant pole to stabilize the feedback loop is transistor
thinking, not always practical in tube circuits.


Just good engineering practice and pretty much standard in op amps.
And the first op amps were valve, not transistor.


That's all pretty much true, however we aren't talking op amps in this thread,
were talking valve audio amps, and more specifically the particular design Don
posted.

Can you give us a worked example of how you would use a dominant pole to
stabilize Don's amplifier design, specifying all relevant time constants in the
amplifier. I would be especially interested in how you would do the low
frequency stabilization were a number of irritating tradeoffs often come into
play, and any assumptions about the effect of the OPT on phase shift are likely
to be more accurate than at high frequencies. The object is for the amplifier
to remain stable with a speaker load, no load, and any other load it might be
likely to encounter during its lifetime.


Short answer - no, it isn't possible with this design. Capacitive and
transformer AC coupling combine to demand low frequency compensation
to get anything reasonably stable. Motor boating has always been a
plague to this kind if amp. As for high frequencies, there are too
many uncontrolled sources of phase shift for that to work. And of
course the severe limitation in open loop gain puts the cherry on the
top.

Dominant pole is what you use when you have everything under control.

As for different loads, if the amp had decently low output impedance
(which demands high OL gain and feedback), there would be issues with
stability into odd loads.

The design is a compromise, but that is fine - it is clearly an
experiment with something a little historic.

d


  #21   Report Post  
John L Stewart John L Stewart is offline
Senior Member
 
Location: Toronto
Posts: 301
Smile

The drive requirements of the OP stage are not as critical as mentioned in earlier posts. The KT88, EL34 & 6550 all have G about double that found in the original Mullard circuits where we find 6L6, 5881, Etc.

A single dominant pole is a safe way to go in this cct. There are a couple of worked samples in RDH4 worth looking at.

Also strongly recommended by none other then Norman Crowhurst. You can read about how to implement all in a couple of his articles.

I like the cct proposed by Alex Pogossov recently. The attachment shows how it is connected in an SE Amp. The frequency selective network is in the cathode of the first stage rather than between stages.

My opinion, anyway, having done it.

Cheers, John Stewart

PS- The bicycle went more than 1000 KM in August (622 miles). Pedalling again today. Hey Patrick, hows it going in OZ? J
Attached Images
 
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Default 1st try kt88 amp

On Sun, 4 Sep 2011 22:05:12 +0000, John L Stewart
wrote:


The drive requirements of the OP stage are not as critical as mentioned
in earlier posts. The KT88, EL34 & 6550 all have G about double that
found in the original Mullard circuits where we find 6L6, 5881, Etc.

A single dominant pole is a safe way to go in this cct. There are a
couple of worked samples in RDH4 worth looking at.

Also strongly recommended by none other then Norman Crowhurst. You can
read about how to implement all in a couple of his articles.

I like the cct proposed by Alex Pogossov recently. The attachment shows
how it is connected in an SE Amp. The frequency selective network is in
the cathode of the first stage rather than between stages.

My opinion, anyway, having done it.

Cheers, John Stewart

PS- The bicycle went more than 1000 KM in August (622 miles). Pedalling
again today. Hey Patrick, hows it going in OZ? J


+-------------------------------------------------------------------+
|Filename: Alex Pogossovs Solution.jpg |
|Download: http://www.audiobanter.com/attachment.php?attachmentid=245|
+-------------------------------------------------------------------+


Why would you want a circuit that drops the gain by about 16dB at
100Hz? What is its actual purpose?

d
  #23   Report Post  
John L Stewart John L Stewart is offline
Senior Member
 
Location: Toronto
Posts: 301
Smile

Quote:
Originally Posted by Don Pearce[_3_] View Post
On Sun, 4 Sep 2011 22:05:12 +0000, John L Stewart
wrote:


The drive requirements of the OP stage are not as critical as mentioned
in earlier posts. The KT88, EL34 & 6550 all have G about double that
found in the original Mullard circuits where we find 6L6, 5881, Etc.

A single dominant pole is a safe way to go in this cct. There are a
couple of worked samples in RDH4 worth looking at.

Also strongly recommended by none other then Norman Crowhurst. You can
read about how to implement all in a couple of his articles.

I like the cct proposed by Alex Pogossov recently. The attachment shows
how it is connected in an SE Amp. The frequency selective network is in
the cathode of the first stage rather than between stages.

My opinion, anyway, having done it.

Cheers, John Stewart

PS- The bicycle went more than 1000 KM in August (622 miles). Pedalling
again today. Hey Patrick, hows it going in OZ? J


+-------------------------------------------------------------------+
|Filename: Alex Pogossovs Solution.jpg |
|Download: http://www.audiobanter.com/attachment.php?attachmentid=245|
+-------------------------------------------------------------------+


Why would you want a circuit that drops the gain by about 16dB at
100Hz? What is its actual purpose?

d
Hi Don-

The cct posted is given only as an example of the concept posted earlier by Alex P. You can select any RC combination to yield the frequency that fits your requirement. Also pick the R to determine the depth of the step.

You can also control the max phase shift. Very useful.

Rather than using attenuation between stages this cct uses degeneration to get the required LF step. Probably don't need either in most 2-stage FB amplifiers.

But I had to use a between stages step network in a FB pair UL amp using a 6LU8 I built a few years back to get rid of an LF hump. It showed up in an AudioXpress publication.

Cheers, John
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Default 1st try kt88 amp

On Sep 5, 1:46*am, (Don Pearce) wrote:
On Sun, 4 Sep 2011 22:05:12 +0000, John L Stewart









wrote:

The drive requirements of the OP stage are not as critical as mentioned
in earlier posts. The KT88, EL34 & 6550 all have G about double that
found in the original Mullard circuits where we find 6L6, 5881, Etc.


A single dominant pole is a safe way to go in this cct. There are a
couple of worked samples in RDH4 worth looking at.


Also strongly recommended by none other then Norman Crowhurst. You can
read about how to implement all in a couple of his articles.


I like the cct proposed by Alex Pogossov recently. The attachment shows
how it is connected in an SE Amp. The frequency selective network is in
the cathode of the first stage rather than between stages.


My opinion, anyway, having done it.


Cheers, John Stewart


PS- The bicycle went more than 1000 KM in August (622 miles). Pedalling
again today. Hey Patrick, hows it going in OZ? * * * * * * * *J


+-------------------------------------------------------------------+
|Filename: Alex Pogossovs Solution.jpg * * * * * * * * * * * * * * *|
|Download:http://www.audiobanter.com/attachment.php?attachmentid=245|
+-------------------------------------------------------------------+


Why would you want a circuit that drops the gain by about 16dB at
100Hz? What is its actual purpose?

d


Ans: To get the loop gain (forward gain times NFB gain) well below
unity at the low frequency where the phase lead becomes 180 degrees
(due to two RC couplings and one OPT.) Very low frequency oscillation
in the range 0.5 to 2 Hz can result if there in no low-frequency shelf
inside the loop.
Cheers,
Roger
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Default 1st try kt88 amp

On Tue, 6 Sep 2011 09:03:54 -0700 (PDT), Engineer
wrote:

On Sep 5, 1:46*am, (Don Pearce) wrote:
On Sun, 4 Sep 2011 22:05:12 +0000, John L Stewart









wrote:

The drive requirements of the OP stage are not as critical as mentioned
in earlier posts. The KT88, EL34 & 6550 all have G about double that
found in the original Mullard circuits where we find 6L6, 5881, Etc.


A single dominant pole is a safe way to go in this cct. There are a
couple of worked samples in RDH4 worth looking at.


Also strongly recommended by none other then Norman Crowhurst. You can
read about how to implement all in a couple of his articles.


I like the cct proposed by Alex Pogossov recently. The attachment shows
how it is connected in an SE Amp. The frequency selective network is in
the cathode of the first stage rather than between stages.


My opinion, anyway, having done it.


Cheers, John Stewart


PS- The bicycle went more than 1000 KM in August (622 miles). Pedalling
again today. Hey Patrick, hows it going in OZ? * * * * * * * *J


+-------------------------------------------------------------------+
|Filename: Alex Pogossovs Solution.jpg * * * * * * * * * * * * * * *|
|Download:http://www.audiobanter.com/attachment.php?attachmentid=245|
+-------------------------------------------------------------------+


Why would you want a circuit that drops the gain by about 16dB at
100Hz? What is its actual purpose?

d


Ans: To get the loop gain (forward gain times NFB gain) well below
unity at the low frequency where the phase lead becomes 180 degrees
(due to two RC couplings and one OPT.) Very low frequency oscillation
in the range 0.5 to 2 Hz can result if there in no low-frequency shelf
inside the loop.
Cheers,
Roger


So it is a horribly bodged solution to a problem that has no business
being there in the first place? The idea of implementing negative
feedback and then defeating it by killing the open loop gain it
mediates is beyond ludicrous.

d


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On Sep 6, 12:37*pm, (Don Pearce) wrote:
On Tue, 6 Sep 2011 09:03:54 -0700 (PDT), Engineer









wrote:
On Sep 5, 1:46*am, (Don Pearce) wrote:
On Sun, 4 Sep 2011 22:05:12 +0000, John L Stewart


wrote:


The drive requirements of the OP stage are not as critical as mentioned
in earlier posts. The KT88, EL34 & 6550 all have G about double that
found in the original Mullard circuits where we find 6L6, 5881, Etc.


A single dominant pole is a safe way to go in this cct. There are a
couple of worked samples in RDH4 worth looking at.


Also strongly recommended by none other then Norman Crowhurst. You can
read about how to implement all in a couple of his articles.


I like the cct proposed by Alex Pogossov recently. The attachment shows
how it is connected in an SE Amp. The frequency selective network is in
the cathode of the first stage rather than between stages.


My opinion, anyway, having done it.


Cheers, John Stewart


PS- The bicycle went more than 1000 KM in August (622 miles). Pedalling
again today. Hey Patrick, hows it going in OZ? * * * * * * * *J


+-------------------------------------------------------------------+
|Filename: Alex Pogossovs Solution.jpg * * * * * * * * * * * * * * *|
|Download:http://www.audiobanter.com/attachment.php?attachmentid=245|
+-------------------------------------------------------------------+


Why would you want a circuit that drops the gain by about 16dB at
100Hz? What is its actual purpose?


d


Ans: To get the loop gain (forward gain times NFB gain) well below
unity at the low frequency where the phase lead becomes 180 degrees
(due to two RC couplings and one OPT.) *Very low frequency oscillation
in the range 0.5 to 2 Hz can result if there in no low-frequency shelf
inside the loop.
Cheers,
Roger


So it is a horribly bodged solution to a problem that has no business
being there in the first place? The idea of implementing negative
feedback and then defeating it by killing the open loop gain it
mediates is beyond ludicrous.

d


Not a bodge... steering the Nyquist plot around the "minus 1" point
for all frequencies is established practice in feedback control
systems. It's also perfectly acceptable here. BTW, the higher break
point (of the two) for the VLF shelf is well below 100 Hz... around 15
to 20 Hz is typical to stop a 1 Hz or so VLF oscillation. I'll agree
that the removal of one of the RC couplings in the forward path would
be good, but hard to do when you have a separate driver stage after
the phase splitter. There are direct-coupled DC-amplifier designs, but
they are far more difficult to implement than a VLF shelf and, anyway,
not needed. Also, a massively larger primary inductance in the OPT
primary would be next to godliness... but most of us (in NA, at least)
cannot afford that so we use the best Hammond iron we can afford or
find a decently large OPT from a recycled high grade amplifier!
Cheers,
Roger
  #27   Report Post  
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Don Pearce[_3_] Don Pearce[_3_] is offline
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Posts: 2,417
Default 1st try kt88 amp

On Wed, 7 Sep 2011 15:58:48 -0700 (PDT), Engineer
wrote:



So it is a horribly bodged solution to a problem that has no business
being there in the first place? The idea of implementing negative
feedback and then defeating it by killing the open loop gain it
mediates is beyond ludicrous.

d


Not a bodge... steering the Nyquist plot around the "minus 1" point
for all frequencies is established practice in feedback control
systems. It's also perfectly acceptable here. BTW, the higher break
point (of the two) for the VLF shelf is well below 100 Hz... around 15
to 20 Hz is typical to stop a 1 Hz or so VLF oscillation. I'll agree
that the removal of one of the RC couplings in the forward path would
be good, but hard to do when you have a separate driver stage after
the phase splitter. There are direct-coupled DC-amplifier designs, but
they are far more difficult to implement than a VLF shelf and, anyway,
not needed. Also, a massively larger primary inductance in the OPT
primary would be next to godliness... but most of us (in NA, at least)
cannot afford that so we use the best Hammond iron we can afford or
find a decently large OPT from a recycled high grade amplifier!
Cheers,
Roger


Don't know where you got those numbers. It does this:

http://www.soundthoughts.co.uk/look/shelf.png

d
  #28   Report Post  
Posted to rec.audio.tubes
John Byrns John Byrns is offline
external usenet poster
 
Posts: 1,441
Default 1st try kt88 amp

In article ,
(Don Pearce) wrote:

On Tue, 6 Sep 2011 09:03:54 -0700 (PDT), Engineer
wrote:

On Sep 5, 1:46*am, (Don Pearce) wrote:
On Sun, 4 Sep 2011 22:05:12 +0000, John L Stewart

wrote:

The drive requirements of the OP stage are not as critical as mentioned
in earlier posts. The KT88, EL34 & 6550 all have G about double that
found in the original Mullard circuits where we find 6L6, 5881, Etc.

A single dominant pole is a safe way to go in this cct. There are a
couple of worked samples in RDH4 worth looking at.

Also strongly recommended by none other then Norman Crowhurst. You can
read about how to implement all in a couple of his articles.

I like the cct proposed by Alex Pogossov recently. The attachment shows
how it is connected in an SE Amp. The frequency selective network is in
the cathode of the first stage rather than between stages.

My opinion, anyway, having done it.

Cheers, John Stewart

PS- The bicycle went more than 1000 KM in August (622 miles). Pedalling
again today. Hey Patrick, hows it going in OZ? * * * * * * * *J

Why would you want a circuit that drops the gain by about 16dB at
100Hz? What is its actual purpose?


Ans: To get the loop gain (forward gain times NFB gain) well below
unity at the low frequency where the phase lead becomes 180 degrees
(due to two RC couplings and one OPT.) Very low frequency oscillation
in the range 0.5 to 2 Hz can result if there in no low-frequency shelf
inside the loop.
Cheers,
Roger


So it is a horribly bodged solution to a problem that has no business
being there in the first place? The idea of implementing negative
feedback and then defeating it by killing the open loop gain it
mediates is beyond ludicrous.


On Sunday you suggested "a single pole that doesn't let up until beyond the
unity gain frequency", isn't that equally ludicrous as it also "kills the open
loop gain"?

--
Regards,

John Byrns

Surf my web pages at,
http://fmamradios.com/
  #29   Report Post  
Posted to rec.audio.tubes
Don Pearce[_3_] Don Pearce[_3_] is offline
external usenet poster
 
Posts: 2,417
Default 1st try kt88 amp

On Thu, 08 Sep 2011 09:31:41 -0500, John Byrns
wrote:

In article ,
(Don Pearce) wrote:

On Tue, 6 Sep 2011 09:03:54 -0700 (PDT), Engineer
wrote:

On Sep 5, 1:46*am, (Don Pearce) wrote:
On Sun, 4 Sep 2011 22:05:12 +0000, John L Stewart

wrote:

The drive requirements of the OP stage are not as critical as mentioned
in earlier posts. The KT88, EL34 & 6550 all have G about double that
found in the original Mullard circuits where we find 6L6, 5881, Etc.

A single dominant pole is a safe way to go in this cct. There are a
couple of worked samples in RDH4 worth looking at.

Also strongly recommended by none other then Norman Crowhurst. You can
read about how to implement all in a couple of his articles.

I like the cct proposed by Alex Pogossov recently. The attachment shows
how it is connected in an SE Amp. The frequency selective network is in
the cathode of the first stage rather than between stages.

My opinion, anyway, having done it.

Cheers, John Stewart

PS- The bicycle went more than 1000 KM in August (622 miles). Pedalling
again today. Hey Patrick, hows it going in OZ? * * * * * * * *J

Why would you want a circuit that drops the gain by about 16dB at
100Hz? What is its actual purpose?

Ans: To get the loop gain (forward gain times NFB gain) well below
unity at the low frequency where the phase lead becomes 180 degrees
(due to two RC couplings and one OPT.) Very low frequency oscillation
in the range 0.5 to 2 Hz can result if there in no low-frequency shelf
inside the loop.
Cheers,
Roger


So it is a horribly bodged solution to a problem that has no business
being there in the first place? The idea of implementing negative
feedback and then defeating it by killing the open loop gain it
mediates is beyond ludicrous.


On Sunday you suggested "a single pole that doesn't let up until beyond the
unity gain frequency", isn't that equally ludicrous as it also "kills the open
loop gain"?


Not at all. It operates as a local feedback loop within the overall
loop. At lower frequencies, where it really matters to have
particularly the output stage included, the loop stretches from
beginning to end. As the frequency rises the loop transfers steadily
to just the voltage amplifier. This does result in a slight rise in
harmonic distortion above about 10kHz, but frankly by then who cares?

And this pole is going to have to be present in some or other form
anyway because high frequency instability needs to be tamed - so why
not make it simple, predictable and controllable?

d
  #30   Report Post  
Posted to rec.audio.tubes
John Byrns John Byrns is offline
external usenet poster
 
Posts: 1,441
Default 1st try kt88 amp

In article ,
(Don Pearce) wrote:

On Thu, 08 Sep 2011 09:31:41 -0500, John Byrns
wrote:

In article ,
(Don Pearce) wrote:

On Tue, 6 Sep 2011 09:03:54 -0700 (PDT), Engineer
wrote:

On Sep 5, 1:46*am, (Don Pearce) wrote:
On Sun, 4 Sep 2011 22:05:12 +0000, John L Stewart

wrote:

The drive requirements of the OP stage are not as critical as
mentioned
in earlier posts. The KT88, EL34 & 6550 all have G about double that
found in the original Mullard circuits where we find 6L6, 5881, Etc.

A single dominant pole is a safe way to go in this cct. There are a
couple of worked samples in RDH4 worth looking at.

Also strongly recommended by none other then Norman Crowhurst. You can
read about how to implement all in a couple of his articles.

I like the cct proposed by Alex Pogossov recently. The attachment
shows
how it is connected in an SE Amp. The frequency selective network is
in
the cathode of the first stage rather than between stages.

My opinion, anyway, having done it.

Cheers, John Stewart

PS- The bicycle went more than 1000 KM in August (622 miles).
Pedalling
again today. Hey Patrick, hows it going in OZ? * * * * * * * *J

Why would you want a circuit that drops the gain by about 16dB at
100Hz? What is its actual purpose?

Ans: To get the loop gain (forward gain times NFB gain) well below
unity at the low frequency where the phase lead becomes 180 degrees
(due to two RC couplings and one OPT.) Very low frequency oscillation
in the range 0.5 to 2 Hz can result if there in no low-frequency shelf
inside the loop.
Cheers,
Roger

So it is a horribly bodged solution to a problem that has no business
being there in the first place? The idea of implementing negative
feedback and then defeating it by killing the open loop gain it
mediates is beyond ludicrous.


On Sunday you suggested "a single pole that doesn't let up until beyond the
unity gain frequency", isn't that equally ludicrous as it also "kills the
open
loop gain"?


Not at all. It operates as a local feedback loop within the overall
loop. At lower frequencies, where it really matters to have
particularly the output stage included, the loop stretches from
beginning to end. As the frequency rises the loop transfers steadily
to just the voltage amplifier. This does result in a slight rise in
harmonic distortion above about 10kHz, but frankly by then who cares?

And this pole is going to have to be present in some or other form
anyway because high frequency instability needs to be tamed - so why
not make it simple, predictable and controllable?


So if I follow you, you are not talking about a single overall negative feedback
loop containing a dominant pole within, but instead you are talking about nested
feedback loops where the dominant pole within the overall feedback loop is
created by a second shorter internal feedback loop that rolls off the high
frequencies?

--
Regards,

John Byrns

Surf my web pages at,
http://fmamradios.com/


  #31   Report Post  
Posted to rec.audio.tubes
Don Pearce[_3_] Don Pearce[_3_] is offline
external usenet poster
 
Posts: 2,417
Default 1st try kt88 amp

On Tue, 13 Sep 2011 11:41:11 -0500, John Byrns
wrote:

In article ,
(Don Pearce) wrote:

On Thu, 08 Sep 2011 09:31:41 -0500, John Byrns
wrote:

In article ,
(Don Pearce) wrote:

On Tue, 6 Sep 2011 09:03:54 -0700 (PDT), Engineer
wrote:

On Sep 5, 1:46*am, (Don Pearce) wrote:
On Sun, 4 Sep 2011 22:05:12 +0000, John L Stewart

wrote:

The drive requirements of the OP stage are not as critical as
mentioned
in earlier posts. The KT88, EL34 & 6550 all have G about double that
found in the original Mullard circuits where we find 6L6, 5881, Etc.

A single dominant pole is a safe way to go in this cct. There are a
couple of worked samples in RDH4 worth looking at.

Also strongly recommended by none other then Norman Crowhurst. You can
read about how to implement all in a couple of his articles.

I like the cct proposed by Alex Pogossov recently. The attachment
shows
how it is connected in an SE Amp. The frequency selective network is
in
the cathode of the first stage rather than between stages.

My opinion, anyway, having done it.

Cheers, John Stewart

PS- The bicycle went more than 1000 KM in August (622 miles).
Pedalling
again today. Hey Patrick, hows it going in OZ? * * * * * * * *J

Why would you want a circuit that drops the gain by about 16dB at
100Hz? What is its actual purpose?

Ans: To get the loop gain (forward gain times NFB gain) well below
unity at the low frequency where the phase lead becomes 180 degrees
(due to two RC couplings and one OPT.) Very low frequency oscillation
in the range 0.5 to 2 Hz can result if there in no low-frequency shelf
inside the loop.
Cheers,
Roger

So it is a horribly bodged solution to a problem that has no business
being there in the first place? The idea of implementing negative
feedback and then defeating it by killing the open loop gain it
mediates is beyond ludicrous.

On Sunday you suggested "a single pole that doesn't let up until beyond the
unity gain frequency", isn't that equally ludicrous as it also "kills the
open
loop gain"?


Not at all. It operates as a local feedback loop within the overall
loop. At lower frequencies, where it really matters to have
particularly the output stage included, the loop stretches from
beginning to end. As the frequency rises the loop transfers steadily
to just the voltage amplifier. This does result in a slight rise in
harmonic distortion above about 10kHz, but frankly by then who cares?

And this pole is going to have to be present in some or other form
anyway because high frequency instability needs to be tamed - so why
not make it simple, predictable and controllable?


So if I follow you, you are not talking about a single overall negative feedback
loop containing a dominant pole within, but instead you are talking about nested
feedback loops where the dominant pole within the overall feedback loop is
created by a second shorter internal feedback loop that rolls off the high
frequencies?


That is exactly what a dominant pole is - a nested feedback system in
which the voltage amplifier is controlled by a single capacitive
feedback element. The overall feedback which surrounds it is flat, and
composed of two resistors, the ratio of which sets the gain of the
amplifier.

d
  #32   Report Post  
Posted to rec.audio.tubes
John Byrns John Byrns is offline
external usenet poster
 
Posts: 1,441
Default 1st try kt88 amp

In article ,
(Don Pearce) wrote:

On Tue, 13 Sep 2011 11:41:11 -0500, John Byrns
wrote:

In article ,
(Don Pearce) wrote:

On Thu, 08 Sep 2011 09:31:41 -0500, John Byrns
wrote:

In article ,
(Don Pearce) wrote:

On Tue, 6 Sep 2011 09:03:54 -0700 (PDT), Engineer
wrote:

On Sep 5, 1:46*am, (Don Pearce) wrote:
On Sun, 4 Sep 2011 22:05:12 +0000, John L Stewart

wrote:

The drive requirements of the OP stage are not as critical as
mentioned
in earlier posts. The KT88, EL34 & 6550 all have G about double
that
found in the original Mullard circuits where we find 6L6, 5881,
Etc.

A single dominant pole is a safe way to go in this cct. There are a
couple of worked samples in RDH4 worth looking at.

Also strongly recommended by none other then Norman Crowhurst. You
can
read about how to implement all in a couple of his articles.

I like the cct proposed by Alex Pogossov recently. The attachment
shows
how it is connected in an SE Amp. The frequency selective network
is
in
the cathode of the first stage rather than between stages.

My opinion, anyway, having done it.

Cheers, John Stewart

PS- The bicycle went more than 1000 KM in August (622 miles).
Pedalling
again today. Hey Patrick, hows it going in OZ? * * * * * * * *J

Why would you want a circuit that drops the gain by about 16dB at
100Hz? What is its actual purpose?

Ans: To get the loop gain (forward gain times NFB gain) well below
unity at the low frequency where the phase lead becomes 180 degrees
(due to two RC couplings and one OPT.) Very low frequency oscillation
in the range 0.5 to 2 Hz can result if there in no low-frequency shelf
inside the loop.
Cheers,
Roger

So it is a horribly bodged solution to a problem that has no business
being there in the first place? The idea of implementing negative
feedback and then defeating it by killing the open loop gain it
mediates is beyond ludicrous.

On Sunday you suggested "a single pole that doesn't let up until beyond
the
unity gain frequency", isn't that equally ludicrous as it also "kills the
open
loop gain"?

Not at all. It operates as a local feedback loop within the overall
loop. At lower frequencies, where it really matters to have
particularly the output stage included, the loop stretches from
beginning to end. As the frequency rises the loop transfers steadily
to just the voltage amplifier. This does result in a slight rise in
harmonic distortion above about 10kHz, but frankly by then who cares?

And this pole is going to have to be present in some or other form
anyway because high frequency instability needs to be tamed - so why
not make it simple, predictable and controllable?


So if I follow you, you are not talking about a single overall negative
feedback
loop containing a dominant pole within, but instead you are talking about
nested
feedback loops where the dominant pole within the overall feedback loop is
created by a second shorter internal feedback loop that rolls off the high
frequencies?


That is exactly what a dominant pole is - a nested feedback system in
which the voltage amplifier is controlled by a single capacitive
feedback element. The overall feedback which surrounds it is flat, and
composed of two resistors, the ratio of which sets the gain of the
amplifier.


I beg to differ, a "dominant pole" does not require nested feedback loops, it
can also be a simple pole within a single overall feedback loop controlled by
the same two resistors you describe.

--
Regards,

John Byrns

Surf my web pages at,
http://fmamradios.com/
  #33   Report Post  
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Don Pearce[_3_] Don Pearce[_3_] is offline
external usenet poster
 
Posts: 2,417
Default 1st try kt88 amp

On Tue, 13 Sep 2011 12:08:14 -0500, John Byrns
wrote:

So if I follow you, you are not talking about a single overall negative
feedback
loop containing a dominant pole within, but instead you are talking about
nested
feedback loops where the dominant pole within the overall feedback loop is
created by a second shorter internal feedback loop that rolls off the high
frequencies?


That is exactly what a dominant pole is - a nested feedback system in
which the voltage amplifier is controlled by a single capacitive
feedback element. The overall feedback which surrounds it is flat, and
composed of two resistors, the ratio of which sets the gain of the
amplifier.


I beg to differ, a "dominant pole" does not require nested feedback loops, it
can also be a simple pole within a single overall feedback loop controlled by
the same two resistors you describe.


Oh jeez. OK. But the way it is done 99%, no, 100% of the time is by
putting the pole around the voltage amplifier. That way the dominant
pole reduces the voltage amplifier distortion at high frequencies. If
you just stick in a pole it can't. Why would anyone compromise their
HF open loop gain for nothing?

d
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