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  #1   Report Post  
west
 
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Default Active Crossover

Anyone know or can recommend a 3-way active (electronic) tube crossover at
least 3rd order? Thank you.
Cordially,
west


  #2   Report Post  
Arny Krueger
 
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"west" wrote in message
om

Anyone know or can recommend a 3-way active (electronic)

tube
crossover at least 3rd order? Thank you.


"know of":

http://www.marchandelec.com/xm126.html


  #3   Report Post  
Scott Dorsey
 
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west wrote:
Anyone know or can recommend a 3-way active (electronic) tube crossover at
least 3rd order? Thank you.


No, it would be enormous if you did it active.

You might be able to chain several Krohn-Hite filter modules together
to increase the slope, but it's going to weigh a ton and probably take
up more space than the amp rack.

Why do you want to do this? Active filters and tubes are usually a very
bad combination, even with wideband video pentodes.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #5   Report Post  
Arny Krueger
 
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"Scott Dorsey" wrote in message


Tubes are no good. Solid state electronics are no good
either. Only live music is any good at all.


Now, if we could get live musicians to work for the price of
playback electronics...




  #6   Report Post  
Paul Stamler
 
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"Scott Dorsey" wrote in message
...
In article znr1120047168k@trad, Mike Rivers wrote:
In article

writes:

Anyone know or can recommend a 3-way active (electronic) tube crossover

at
least 3rd order?

Heathkit made one. Actually, a bunch of folks have made them, using
passive filters. All the ones I have seen, though, were only 6 dB/octave.
And kind of noisy.

If you want sharper slopes, you're looking at instrumentation filters
like Krohn-Hites.


Marchand Electronics (
www.marchandelec.com) sells the XM126, which can be
configured as a 2-, 3- or 4-way crossover, 24dB/octave (4th order). It's
available as a kit if you like. Convenience factor isn't huge: crossover
frequencies are selected by plugging in different passive-component boards.
It's also unbalanced I/O, which may or may not be important to the poster.
Looks like a decent design, though, and Marchand's reputation for electronic
crossovers is good.

Peace,
Paul


  #8   Report Post  
Pooh Bear
 
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Paul Stamler wrote:

"Scott Dorsey" wrote in message
...
In article znr1120047168k@trad, Mike Rivers wrote:
In article

writes:

Anyone know or can recommend a 3-way active (electronic) tube crossover

at
least 3rd order?

Heathkit made one. Actually, a bunch of folks have made them, using
passive filters. All the ones I have seen, though, were only 6 dB/octave.
And kind of noisy.

If you want sharper slopes, you're looking at instrumentation filters
like Krohn-Hites.


Marchand Electronics (
www.marchandelec.com) sells the XM126, which can be
configured as a 2-, 3- or 4-way crossover, 24dB/octave (4th order). It's
available as a kit if you like. Convenience factor isn't huge: crossover
frequencies are selected by plugging in different passive-component boards.
It's also unbalanced I/O, which may or may not be important to the poster.
Looks like a decent design, though, and Marchand's reputation for electronic
crossovers is good.


" Uses Vacuum Tube technology for superior sound quality " !!!!!!

Have any of these guys ever drawn a load line ?

Graham

  #9   Report Post  
Pooh Bear
 
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Scott Dorsey wrote:

west wrote:
Anyone know or can recommend a 3-way active (electronic) tube crossover at
least 3rd order? Thank you.


No, it would be enormous if you did it active.

You might be able to chain several Krohn-Hite filter modules together
to increase the slope, but it's going to weigh a ton and probably take
up more space than the amp rack.

Why do you want to do this? Active filters and tubes are usually a very
bad combination, even with wideband video pentodes.


One can only assume he heard the " toobs are bettter " nonsence and swallowed
it.

Graham

  #10   Report Post  
Scott Dorsey
 
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Pooh Bear wrote:

" Uses Vacuum Tube technology for superior sound quality " !!!!!!

Have any of these guys ever drawn a load line ?


I've drawn load lines on the 2N3055... and it makes a pentode look
really good...
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."


  #11   Report Post  
Pooh Bear
 
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Scott Dorsey wrote:

Pooh Bear wrote:

" Uses Vacuum Tube technology for superior sound quality " !!!!!!

Have any of these guys ever drawn a load line ?


I've drawn load lines on the 2N3055... and it makes a pentode look
really good...


I didn't actually have an undegenerated emitter follower stage in mind
for comparison - nor indeed a 2N3005 - there but....

Ohhh - talking of which you'll easily get an open loop gain of around
4000 with a transistor operating off 250V. It's kinda necessary to use
feedback to tame the gain.

Reminds me, I once substituted a 2N3055 for a BC109 once in a pre-amp
'just to see'. It worked.


Curiously, despite what I ( and indeed one of my colleagues ) had
previously imagined to be the case, I came across about a discussion
about toob linearity in r.a.t probably and to my surprise when comparing
an EF86 to an ECC83 for example, the load line on the triode was
significantly more linear.

Graham

  #12   Report Post  
Scott Dorsey
 
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Pooh Bear wrote:
Ohhh - talking of which you'll easily get an open loop gain of around
4000 with a transistor operating off 250V. It's kinda necessary to use
feedback to tame the gain.


This is a major irritation to my mind, and you can hardly even _get_
small signal semis at that voltage. On Semiconductor makes a nice line
of bipolars intended for CRT drive, but they aren't exactly selected
for precise beta.

Reminds me, I once substituted a 2N3055 for a BC109 once in a pre-amp
'just to see'. It worked.


I saw some folks who made a preamp for moving coil phono cartridges,
who used the 2N3055 as a front end, because the large area allowed them
to better match the very low output impedance of the cartridge. As I
recall the noise numbers weren't all that great, but they were better
than some circuits using low noise semis.

Curiously, despite what I ( and indeed one of my colleagues ) had
previously imagined to be the case, I came across about a discussion
about toob linearity in r.a.t probably and to my surprise when comparing
an EF86 to an ECC83 for example, the load line on the triode was
significantly more linear.


Why does this surprise you? Pentodes are designed for gain rather than
linearity. You want serious linearity, drop the ECC83 entirely and go
with a frame grid triode. You want a lot of gain in a small footprint
for cheap, you go 6AU6 and try to ignore the noise. And don't clip it
whatever you do... 6AU6 clipping behaviour is no fun, whereas most triode
circuits clip less obnoxiously.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #13   Report Post  
Chris Hornbeck
 
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On Thu, 30 Jun 2005 00:31:27 +0100, Pooh Bear
wrote:

Curiously, despite what I ( and indeed one of my colleagues ) had
previously imagined to be the case, I came across about a discussion
about toob linearity in r.a.t probably and to my surprise when comparing
an EF86 to an ECC83 for example, the load line on the triode was
significantly more linear.


Perhaps surprisingly, an ECC83 is one of the most linear
devices ever made. And over *very* large parts of the
transfer curve.

Valves have a bad rep these days because of unsavory
associations, but the devices themselves can be amazingly
linear. Implementation can be expensive and impedances
limit compatibility, but for linearity, they're tough
to beat.

Chris Hornbeck
"Chief Assistant to the Assistant Chief" -F&S
  #14   Report Post  
Arny Krueger
 
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"Chris Hornbeck" wrote in
message
On Thu, 30 Jun 2005 00:31:27 +0100, Pooh Bear
wrote:

Curiously, despite what I ( and indeed one of my

colleagues )
had previously imagined to be the case, I came across

about a
discussion about toob linearity in r.a.t probably and to

my
surprise when comparing an EF86 to an ECC83 for example,

the
load line on the triode was significantly more linear.


Perhaps surprisingly, an ECC83 is one of the most linear
devices ever made. And over *very* large parts of the
transfer curve.

Valves have a bad rep these days because of unsavory
associations, but the devices themselves can be amazingly
linear. Implementation can be expensive and impedances
limit compatibility, but for linearity, they're tough
to beat.


Triodes are relatively linear as devices because they have
a great deal of local feedback, built-in. As a consequence,
they have relatively low amounts of gain.

From the standpoint of building a working amplifier that is
highly linear, both the gain and the linearity need to be
considered. No way are small amounts of amplification more
desirable when they have less linearity considering the
relatively low amount of gain that they have.


  #15   Report Post  
Chris Hornbeck
 
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On Wed, 29 Jun 2005 21:54:16 -0400, "Arny Krueger"
wrote:

Triodes are relatively linear as devices because they have
a great deal of local feedback, built-in. As a consequence,
they have relatively low amounts of gain.


I would say instead that triodes are *amazingly* linear
devices compared to conventional modern devices. The
concept of "local feedback" gets a lot of newsgroup ink
these days, but still seems totally wacky to me.

Perhaps you're the guy to convince me otherwise...

From the standpoint of building a working amplifier that is
highly linear, both the gain and the linearity need to be
considered. No way are small amounts of amplification more
desirable when they have less linearity considering the
relatively low amount of gain that they have.


Good point, and perfectly appropriate for input bandwidth
limited situations. We need to include both elements, just
as we would for an A/D conversion or a sample rate reduction.
Often (usually?) forgotten real-world.

Thanks, as always,

Chris Hornbeck
"Chief Assistant to the Assistant Chief" -F&S


  #16   Report Post  
Paul Stamler
 
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"Scott Dorsey" wrote in message
...

Curiously, despite what I ( and indeed one of my colleagues ) had
previously imagined to be the case, I came across about a discussion
about toob linearity in r.a.t probably and to my surprise when comparing
an EF86 to an ECC83 for example, the load line on the triode was
significantly more linear.


Why does this surprise you? Pentodes are designed for gain rather than
linearity. You want serious linearity, drop the ECC83 entirely and go
with a frame grid triode.


Is the 6SN7 a frame-grid triode? Cuz that's about the most linear device
without feedback that I've found.

There's a problem using triodes for electronic crossovers, though; you can't
just plug in numbers from the Active Filter Cookbook the way you can with
opamps. The cookbook numbers assume an active device (or block) with very
high open-loop gain, which triodes -- especially the really linear ones like
the 6SN7 -- ain't got. The filters' curves come out wrong if you use the
cookbook numbers on, say, a 6SN7 cathode follower, and to get them right you
actually have to do some work.

Peace,
Paul


  #17   Report Post  
Pooh Bear
 
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Scott Dorsey wrote:

Pooh Bear wrote:
Ohhh - talking of which you'll easily get an open loop gain of around
4000 with a transistor operating off 250V. It's kinda necessary to use
feedback to tame the gain.


This is a major irritation to my mind, and you can hardly even _get_
small signal semis at that voltage.


MPSA 42 and 92 ( 43 / 93 ) to name a couple ( of complementary pairs ) !
Cheap and wickedly good. Low Cob etc etc .... etc ! Blah blah blah. And
they're 'mercun and yet you don't know them !!!!

I've used them *everywhere* ! You find a gem - you use it !

On Semiconductor makes a nice line
of bipolars intended for CRT drive, but they aren't exactly selected
for precise beta.


You're looking in the wrong place.

Also the Japanese and Koreans make some interesting **** !


Reminds me, I once substituted a 2N3055 for a BC109 once in a pre-amp
'just to see'. It worked.


I saw some folks who made a preamp for moving coil phono cartridges,
who used the 2N3055 as a front end, because the large area allowed them
to better match the very low output impedance of the cartridge.


Noooooo ! Not 'match' ! They were purely looking for low Rbb and Ree. Another
story entirely and one that a 2N3055 might do ok at, but that line of
approach has been *very* discredited ! There are far better devices that fill
that bill without being power semis ! The 'nice devices' even have an hfe
worth talking about !

As I
recall the noise numbers weren't all that great, but they were better
than some circuits using low noise semis.

Curiously, despite what I ( and indeed one of my colleagues ) had
previously imagined to be the case, I came across about a discussion
about toob linearity in r.a.t probably and to my surprise when comparing
an EF86 to an ECC83 for example, the load line on the triode was
significantly more linear.


Why does this surprise you? Pentodes are designed for gain rather than
linearity.


Indeed. But somewhere along the line I got the idea they were acceptably
linear and my colleague's comment shows I wasn't alone in thinking this.

Sure... I know that things maybe erroneously understod - but even so....

You want serious linearity, drop the ECC83 entirely and go
with a frame grid triode. You want a lot of gain in a small footprint
for cheap, you go 6AU6 and try to ignore the noise. And don't clip it
whatever you do... 6AU6 clipping behaviour is no fun, whereas most triode
circuits clip less obnoxiously.


Well..... I'd actually be quite interested in pursuing the discussion about
thermionics but I reckon only a few ppl frankly care ! More are simply into
the 'toobs are best' ****wit mentality and I'd hate to be associated with
them !


Graham

  #18   Report Post  
Mark & Mary Ann Weiss
 
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"west" wrote in message
om...
Anyone know or can recommend a 3-way active (electronic) tube crossover at
least 3rd order? Thank you.
Cordially,
west



Not a tube type, but I can heartily recommend the Behringer DCX-2496, which
is a crossover that possessess several major slope types in just about any
order you can imagine, in addition to dynamic compressors, limiters and EQs,
all with the advantage of save and restore settings from memory and PCMCIA
card reader.
I added one to the big house system in December and oh, what an improvement
over my Linkwitz-Riley that I built in 1983.


--
Best Regards,

Mark A. Weiss, P.E.
www.mwcomms.com
-



  #19   Report Post  
Arny Krueger
 
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"Chris Hornbeck" wrote in
message

On Wed, 29 Jun 2005 21:54:16 -0400, "Arny Krueger"
wrote:


Triodes are relatively linear as devices because they

have
a great deal of local feedback, built-in. As a

consequence,
they have relatively low amounts of gain.


I would say instead that triodes are *amazingly* linear
devices compared to conventional modern devices.


With all that local feedback, they can hardly miss.

However, if you properly design a transistor circuit with
enough local or loop feedback to reduce its gain to triode
levels, it can be even more linear than the triode.

The concept of "local feedback" gets a lot of newsgroup

ink
these days, but still seems totally wacky to me.


There's nothing wacky about local feedback at all. It is
relatively simple to implement, and it pretty much works as
designed.

Perhaps you're the guy to convince me otherwise...


Convincing people about the validity of basic electronics is
not what I do if I can avoid it.

Here's a fairly sane article about tubes and feedback, local
and otherwise:

http://www.normankoren.com/Audio/FeedbackFidelity.html

From the standpoint of building a working amplifier that

is
highly linear, both the gain and the linearity need to be
considered. No way are small amounts of amplification

more
desirable when they have less linearity considering the
relatively low amount of gain that they have.


Good point, and perfectly appropriate for input bandwidth
limited situations. We need to include both elements, just
as we would for an A/D conversion or a sample rate

reduction.
Often (usually?) forgotten real-world.


There is one other figure of merit in a design, and that's
the degree to which available power supply voltages are
exploited to produce relatively high output voltages and
improve dynamic range. Because of their relatively high
saturation voltages, tubes are not efficient users of
available power supply voltages.


  #20   Report Post  
Adrian Tuddenham
 
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Scott Dorsey wrote:


I saw some folks who made a preamp for moving coil phono cartridges,
who used the 2N3055 as a front end, because the large area allowed them
to better match the very low output impedance of the cartridge. As I
recall the noise numbers weren't all that great, but they were better
than some circuits using low noise semis.


I seem to remember a circuit which used several printer-hammer driver
transistors in parallel to achieve low input impedance (they may have
been from the ZTX series).

It was in the [UK] Wireless World (or Electronics World?) about 10 or 15
years ago and was intended as a moving coil cartridge pre-amp without a
transformer. I think a pretty good noise figure was claimed.

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk


  #21   Report Post  
Scott Dorsey
 
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Paul Stamler wrote:
"Scott Dorsey" wrote in message

Curiously, despite what I ( and indeed one of my colleagues ) had
previously imagined to be the case, I came across about a discussion
about toob linearity in r.a.t probably and to my surprise when comparing
an EF86 to an ECC83 for example, the load line on the triode was
significantly more linear.


Why does this surprise you? Pentodes are designed for gain rather than
linearity. You want serious linearity, drop the ECC83 entirely and go
with a frame grid triode.


Is the 6SN7 a frame-grid triode? Cuz that's about the most linear device
without feedback that I've found.


No, it predates all the frame grid stuff by many years. It is a really
linear device, though. The problem is that the gain is low.

There's a problem using triodes for electronic crossovers, though; you can't
just plug in numbers from the Active Filter Cookbook the way you can with
opamps. The cookbook numbers assume an active device (or block) with very
high open-loop gain, which triodes -- especially the really linear ones like
the 6SN7 -- ain't got. The filters' curves come out wrong if you use the
cookbook numbers on, say, a 6SN7 cathode follower, and to get them right you
actually have to do some work.


Every attempt I have seen to build active filters with tube op-amp networks
has turned out badly, usually due to slew rate limitations. But there
is no reason you can't just build a bunch of passive RLC networks, with
tube gain stages between them. Other than S/N of course.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #22   Report Post  
Paul Stamler
 
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"Scott Dorsey" wrote in message
...

Every attempt I have seen to build active filters with tube op-amp

networks
has turned out badly, usually due to slew rate limitations. But there
is no reason you can't just build a bunch of passive RLC networks, with
tube gain stages between them. Other than S/N of course.


S/N could probably be made acceptable. What's tough is that at impedances
high enough for tubes to be comfortable with, the inductors need to be very
big and expensive. Unless you transformer-couple the tubes and make the
impedances low, but then you're paying for expensive transformers instead of
expensive inductors.

Peace,
Paul


  #23   Report Post  
Scott Dorsey
 
Posts: n/a
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Paul Stamler wrote:
"Scott Dorsey" wrote in message

Every attempt I have seen to build active filters with tube op-amp

networks
has turned out badly, usually due to slew rate limitations. But there
is no reason you can't just build a bunch of passive RLC networks, with
tube gain stages between them. Other than S/N of course.


S/N could probably be made acceptable. What's tough is that at impedances
high enough for tubes to be comfortable with, the inductors need to be very
big and expensive. Unless you transformer-couple the tubes and make the
impedances low, but then you're paying for expensive transformers instead of
expensive inductors.


Yes, absolutely. And you also now wind up having to deal with inductor
nonlinearity and non-ideal overload behaviours.

I will say that I have a source now for large-value audio inductors at
reasonable prices, though.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #24   Report Post  
Arny Krueger
 
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"Paul Stamler" wrote in message

"Scott Dorsey" wrote in message
...

Every attempt I have seen to build active filters with

tube
op-amp networks has turned out badly, usually due to slew
rate limitations. But there is no reason you can't just
build a bunch of passive RLC networks, with tube gain

stages
between them. Other than S/N of course.


S/N could probably be made acceptable. What's tough is

that at
impedances high enough for tubes to be comfortable with,

the
inductors need to be very big and expensive. Unless you
transformer-couple the tubes and make the impedances low,

but
then you're paying for expensive transformers instead of
expensive inductors.


What would stop one from doing state-variable filters with
tube op amps, patterned ofter the Philbrick modules of the
days of tubes?


  #25   Report Post  
Scott Dorsey
 
Posts: n/a
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Arny Krueger wrote:

What would stop one from doing state-variable filters with
tube op amps, patterned ofter the Philbrick modules of the
days of tubes?


Slew rate and bandwidth. Those Philbrick and HP tube op-amps
really did not have the bandwidth for anything approaching audio.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."


  #26   Report Post  
Arny Krueger
 
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"Scott Dorsey" wrote in message


Arny Krueger wrote:


What would stop one from doing state-variable filters

with
tube op amps, patterned ofter the Philbrick modules of

the
days of tubes?


Slew rate and bandwidth. Those Philbrick and HP tube

op-amps
really did not have the bandwidth for anything approaching
audio. --scott


I didn't know that tubed op-amps were that bad.


  #27   Report Post  
Scott Dorsey
 
Posts: n/a
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Arny Krueger wrote:
"Scott Dorsey" wrote in message


Arny Krueger wrote:


What would stop one from doing state-variable filters

with
tube op amps, patterned ofter the Philbrick modules of

the
days of tubes?


Slew rate and bandwidth. Those Philbrick and HP tube

op-amps
really did not have the bandwidth for anything approaching
audio. --scott


I didn't know that tubed op-amps were that bad.



Bob Pease has data sheets!

http://www.national.com/rap/images/BBB1.gif
http://www.national.com/rap/images/BBB2.jpg

This one actually isn't bad, with an open loop DC gain of 15,000
and a gain-bandwidth product of 100 KHz. That's about a tenth the
speed of a 741.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #28   Report Post  
Paul Stamler
 
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"Scott Dorsey" wrote in message
...
Arny Krueger wrote:

What would stop one from doing state-variable filters with
tube op amps, patterned ofter the Philbrick modules of the
days of tubes?


Slew rate and bandwidth. Those Philbrick and HP tube op-amps
really did not have the bandwidth for anything approaching audio.


Well, one isn't limited to those designs. Fred Forssell designed tube-based
op-amps that had a lot better audio specs. The maximum open-loop gain was
still a lot less than with IC op-amps, though, so one still has to do some
jiggering to make the filters come out right. It is, however, a lot more
feasible than with a simple cathode follower.

Also, you're looking at eight tubes for a 2-way 4th-order crossover, and
you're starting to generate some serious heat there.

Peace,
Paul


  #29   Report Post  
Chris Hornbeck
 
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On Thu, 30 Jun 2005 06:28:43 -0400, "Arny Krueger"
wrote:

With all that local feedback, they can hardly miss.

However, if you properly design a transistor circuit with
enough local or loop feedback to reduce its gain to triode
levels, it can be even more linear than the triode.

The concept of "local feedback" gets a lot of newsgroup

ink
these days, but still seems totally wacky to me.


There's nothing wacky about local feedback at all. It is
relatively simple to implement, and it pretty much works as
designed.


http://www.normankoren.com/Audio/FeedbackFidelity.html


Oh, OK. I thought you meant the "internal feedback"
concept that's going around on rec.audio.tubes.

So, why must a triode design inherently have more local
feedback than any other?

Chris Hornbeck
"Chief Assistant to the Assistant Chief" -F&S
  #30   Report Post  
Arny Krueger
 
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"Scott Dorsey" wrote in message


Arny Krueger wrote:
"Scott Dorsey" wrote in message




Arny Krueger wrote:


What would stop one from doing state-variable filters

with
tube op amps, patterned ofter the Philbrick modules of

the
days of tubes?


Slew rate and bandwidth. Those Philbrick and HP tube

op-amps
really did not have the bandwidth for anything

approaching
audio. --scott


I didn't know that tubed op-amps were that bad.


Bob Pease has data sheets!


http://www.national.com/rap/images/BBB1.gif
http://www.national.com/rap/images/BBB2.jpg


This one actually isn't bad, with an open loop DC gain of
15,000 and a gain-bandwidth product of 100 KHz. That's

about
a tenth the speed of a 741.


When I was an undergraduate back in the 60s, I did some time
*programming* analog computers. My recollection is the
Philbrick parts posted at
http://www.national.com/rap/vacuumtubes.html were
bottom-of-the barrel parts.

The sort of vacuum tube op amps I'm thinking of were more on
the order of this one:

http://ed-thelen.org/dc_amp.gif

There's really no excuse for vacuum tube amplifiers to be
terribly slow, after all Tektronics made some pretty fast
'scopes out of bottles....





  #31   Report Post  
Arny Krueger
 
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"Chris Hornbeck" wrote in
message
On Thu, 30 Jun 2005 06:28:43 -0400, "Arny Krueger"
wrote:

With all that local feedback, they can hardly miss.

However, if you properly design a transistor circuit with
enough local or loop feedback to reduce its gain to

triode
levels, it can be even more linear than the triode.

The concept of "local feedback" gets a lot of newsgroup

ink
these days, but still seems totally wacky to me.


There's nothing wacky about local feedback at all. It is
relatively simple to implement, and it pretty much works

as
designed.


http://www.normankoren.com/Audio/FeedbackFidelity.html


Oh, OK. I thought you meant the "internal feedback"
concept that's going around on rec.audio.tubes.


So, why must a triode design inherently have more local
feedback than any other?


It has to do with the space charge around the plate feeding
signal back to the region around the grid. They invented
tetrodes and pentodes to keep that from happening.


  #32   Report Post  
Chris Hornbeck
 
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On Thu, 30 Jun 2005 18:51:01 -0400, "Arny Krueger"
wrote:

So, why must a triode design inherently have more local
feedback than any other?


It has to do with the space charge around the plate feeding
signal back to the region around the grid. They invented
tetrodes and pentodes to keep that from happening.


Say it ain't so, Arny. Just say no to the Dark Side. Don't
let the pod people get ya, please.

Stay strong,

Chris Hornbeck
"Chief Assistant to the Assistant Chief" -F&S
  #35   Report Post  
Pooh Bear
 
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Adrian Tuddenham wrote:

Scott Dorsey wrote:

I saw some folks who made a preamp for moving coil phono cartridges,
who used the 2N3055 as a front end, because the large area allowed them
to better match the very low output impedance of the cartridge. As I
recall the noise numbers weren't all that great, but they were better
than some circuits using low noise semis.


I seem to remember a circuit which used several printer-hammer driver
transistors in parallel to achieve low input impedance (they may have
been from the ZTX series).

It was in the [UK] Wireless World (or Electronics World?) about 10 or 15
years ago and was intended as a moving coil cartridge pre-amp without a
transformer. I think a pretty good noise figure was claimed.


If you want a decent noise figure at low imedance you need a device with low
intrinsic resistance. That means a power / switching type device may be
suitable if you don't have access to a specialist part.

2N4401 and 3s did quite well in that respect. The Japanese devices intended
for the job are better though and also have a decent hfe.

Graham



  #36   Report Post  
Pooh Bear
 
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Arny Krueger wrote:

"Chris Hornbeck" wrote in
message
On Thu, 30 Jun 2005 06:28:43 -0400, "Arny Krueger"
wrote:

With all that local feedback, they can hardly miss.

However, if you properly design a transistor circuit with
enough local or loop feedback to reduce its gain to

triode
levels, it can be even more linear than the triode.

The concept of "local feedback" gets a lot of newsgroup
ink
these days, but still seems totally wacky to me.

There's nothing wacky about local feedback at all. It is
relatively simple to implement, and it pretty much works

as
designed.


http://www.normankoren.com/Audio/FeedbackFidelity.html


Oh, OK. I thought you meant the "internal feedback"
concept that's going around on rec.audio.tubes.


So, why must a triode design inherently have more local
feedback than any other?


It has to do with the space charge around the plate feeding
signal back to the region around the grid. They invented
tetrodes and pentodes to keep that from happening.


Indeed !

So-called screen or suppressor grids.

Graham


  #37   Report Post  
Chris Hornbeck
 
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On Fri, 01 Jul 2005 02:26:47 +0100, Pooh Bear
wrote:

If you want a decent noise figure at low imedance you need a device with low
intrinsic resistance.


Perzactly. Impedance matching is a red herring here. The input
device's intrinsic resistance (reciprocal of transconductance)
determines (thermal) noise voltage.

Noise factor and noise figure have well defined meanings in
related fields, and shouldn't be mis-used casually in our work.
What we care about is best defined as noise voltage in relation
to signal voltage.

Chris Hornbeck
"taking the cure, so I can be quiet,
wherever I want, so leave me alone.
You outta be proud, that I'm getting
good marks." -Elliott Smith
  #38   Report Post  
Chris Hornbeck
 
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On Fri, 01 Jul 2005 02:30:13 +0100, Pooh Bear
wrote:

It has to do with the space charge around the plate feeding
signal back to the region around the grid. They invented
tetrodes and pentodes to keep that from happening.


Indeed !

So-called screen or suppressor grids.


Well, screen (G2) and suppressor (G3) grids are different critters,
doing very different gigs.

But that ain't my gripe with the whole argument. And I still
feel uncomfortable airing it here. Ain't nobody's bidness...
etc.

Thanks,

Chris Hornbeck
"taking the cure, so I can be quiet,
wherever I want, so leave me alone.
You outta be proud, that I'm getting
good marks." -Elliott Smith
  #39   Report Post  
Pooh Bear
 
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Chris Hornbeck wrote:

On Fri, 01 Jul 2005 02:26:47 +0100, Pooh Bear
wrote:

If you want a decent noise figure at low imedance you need a device with low
intrinsic resistance.


Perzactly. Impedance matching is a red herring here. The input
device's intrinsic resistance (reciprocal of transconductance)
determines (thermal) noise voltage.

Noise factor and noise figure have well defined meanings in
related fields, and shouldn't be mis-used casually in our work.
What we care about is best defined as noise voltage in relation
to signal voltage.


You are indeed 100% correct. I find it mildy amusing ( under recent circumstances
) that your post on the matter came immediately after my comment to Adrian.

You should pop over to s.e.d and see what nonsence he's been posting there (
along with a couple of others ) about decibels.

See the thread - 'the truth about decibels'.

Graham

  #40   Report Post  
Paul Stamler
 
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"Mike Rivers" wrote in message
news:znr1120170690k@trad...

I was wondering about that term "local feedback" too. It sounds like
one of those terms coined by the same folks who talk about LDC and
SDC. Is it some hip name for not bypassing the cathode resistor?


In standard parlance, yes. I'd call the single-stage "anode follower"
(resistor from the output side of the plate coupling capacitor back to the
grid, another resistor from the grid to the input) global feedback even
though it's around a single stage, whereas an unbypassed cathode resistor is
local.

Peace,
Paul


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