| Author |
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Wessel Dirksen
Guest
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 Posted:
Wed Sep 21, 2005 12:06 pm Post subject:
Value of coupling cap's |
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Hi Friends,
Gee, after being quite plugged in, I've been away from the tube hobby
and this group for a few months now. Successful inguinal hernia
operation, new shrink job, 2 speaker projects, new girlfriend (could be
a keeper), and looking into selling the house. My musical heart has
been growing a bit cold though from the lack of filament glow and high
voltage. Nice to be able to tune back in and ask my semi-novice
questions.
If when using coupling cap's between stages, and there is no wish to
have a strategic LF cutoff in that particular stage, does it make any
sense to use an overkill cap value to get f-3 way low (to 1 Hz or so)
as an attempt to keep the whole audio band reasonably within the same
time constant? Is a bigger cap "dirtier" due to more dielectricum
material in the path?
Wessel Dirksen
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Patrick Turner
Guest
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| Posted:
Wed Sep 21, 2005 9:51 pm Post subject:
Re: Value of coupling cap's |
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Wessel Dirksen wrote:
| Quote: | Hi Friends,
Gee, after being quite plugged in, I've been away from the tube hobby
and this group for a few months now. Successful inguinal hernia
operation, new shrink job,
|
New shrink job?
Does that mean you had your head seriously examined, re-programmed,
and turned back on, or that you went on a diet, or that you became a
psychiatrist?
| Quote: | 2 speaker projects, new girlfriend (could be
a keeper),
|
Game keeper?, house keeper? green keeper? What on earth is the kind of
keeper?
.....but whatever she is, its easier to please her without the problems of a
hernia.
That only leaves your back elbows, knees, and brain to fall apart
trying to please her..... :-)
| Quote: | and looking into selling the house.
|
Man, that's drastic.
Sharing the costs of the stamp duty are you?......
| Quote: | My musical heart has
been growing a bit cold though from the lack of filament glow and high
voltage. Nice to be able to tune back in and ask my semi-novice
questions.
|
Be careful, its being nerdy again, and women get jealous of a
a man's alternative interests.....
| Quote: |
If when using coupling cap's between stages, and there is no wish to
have a strategic LF cutoff in that particular stage, does it make any
sense to use an overkill cap value to get f-3 way low (to 1 Hz or so)
as an attempt to keep the whole audio band reasonably within the same
time constant? Is a bigger cap "dirtier" due to more dielectricum
material in the path?
|
Well part of the reason is men's tendencies to be able to say
Mine is bigger than Yours.
So bigger caps are a manly thing.
But seriously, bypassing with caps that are 470uF
but smaller than 4.7uF made 40 years ago
is only good sense.
So a cap which is 470uF now may be lots smaller than a
47 uF cap from 40 years ago.
And what matters is what we do with the available capacitance.
So size isn't all important, its how you use what is available.
And for guys who have a small one, use it twice a night instead of once,
and if she says its too small, then tell her she's too big down there.
This must be an utterly taboo concept; I have never ever heard
it mentioned, ever before.
There can be a row about this which may lead to door slamming
events and low frequency transients but its better that
such issues be out in the open.....
Some things cannot be bypassed......
Patrick Turner.
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Sander deWaal
Guest
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| Posted:
Wed Sep 21, 2005 10:37 pm Post subject:
Re: Value of coupling cap's |
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"Wessel Dirksen" <wdirksen@gmail.com> said:
| Quote: | If when using coupling cap's between stages, and there is no wish to
have a strategic LF cutoff in that particular stage, does it make any
sense to use an overkill cap value to get f-3 way low (to 1 Hz or so)
as an attempt to keep the whole audio band reasonably within the same
time constant? Is a bigger cap "dirtier" due to more dielectricum
material in the path?
|
Depending on the cirsuit, a (too) large coupling cap may cause
motorboating and/or blocking of the power tubes.
--
"Audio as a serious hobby is going down the tubes."
- Howard Ferstler, 25/4/2005 |
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Guest
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| Posted:
Thu Sep 22, 2005 4:43 am Post subject:
Re: Value of coupling cap's |
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Deep Subject
I've always been taught that bandwidth (LF point partly determined by the
coupling caps) should get wider
as you travel through the amp toward the output. Taking a 2 stage amp, with
the same identical bandwiths in
each stage can be a problem. When you hit the -3dB point of the first stage,
you'll be -6dB down at the second
stage output. Now you have some really funny phase shift stuff happining
(multiple poles) an octave or so before the first -3dB point. And this is
worse for a 3 stage amp! I always try limit the bandwidth of the first stage
to a reasonable point.
And then an octave wider for the next stage and ect. With a reasonable
amount of negative feedback you will end up with a stable amp with a
bandwith wider than the first stage anyway.
RonL
"Sander deWaal" <nospam@wanadoo.nl> wrote in message
news:ug63j11bu2rgoch6nns513msdi3cg16lqv@4ax.com...
| Quote: | "Wessel Dirksen" <wdirksen@gmail.com> said:
If when using coupling cap's between stages, and there is no wish to
have a strategic LF cutoff in that particular stage, does it make any
sense to use an overkill cap value to get f-3 way low (to 1 Hz or so)
as an attempt to keep the whole audio band reasonably within the same
time constant? Is a bigger cap "dirtier" due to more dielectricum
material in the path?
Depending on the cirsuit, a (too) large coupling cap may cause
motorboating and/or blocking of the power tubes.
--
"Audio as a serious hobby is going down the tubes."
- Howard Ferstler, 25/4/2005 |
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west
Guest
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| Posted:
Thu Sep 22, 2005 4:43 am Post subject:
Re: Value of coupling cap's |
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"Patrick Turner" <info@turneraudio.com.au> wrote in message
news:43318EB3.61E3CA7C@turneraudio.com.au...
| Quote: |
huge snip for bandwith stewardship |
| Quote: | Wessel Dirksen wrote:
So size isn't all important, its how you use what is available.
|
Does that mean I can use my 12 picos after a long tail?
west
| Quote: |
Patrick Turner.
Wessel Dirksen
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Ian Iveson
Guest
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| Posted:
Thu Sep 22, 2005 4:43 am Post subject:
Re: Value of coupling cap's |
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<r.laury@comcast.net> wrote
Much deeper than it used to be!
Things have changed since Patrick's day. Driving some of today's
excellent but awkward speakers down to their depths poses a real
challenge now. I'm talking myself into a low-turns-ratio-or-bust
mentality.
| Quote: | I've always been taught that bandwidth (LF point partly determined
by the coupling caps) should get wider
as you travel through the amp toward the output. Taking a 2 stage
amp, with the same identical bandwidths in
each stage can be a problem. When you hit the -3dB point of the
first stage, you'll be -6dB down at the second
stage output. Now you have some really funny phase shift stuff
happining (multiple poles) an octave or so before the first -3dB
point. And this is worse for a 3 stage amp! I always try limit the
bandwidth of the first stage to a reasonable point.
And then an octave wider for the next stage and ect. With a
reasonable amount of negative feedback you will end up with a
stable amp with a bandwith wider than the first stage anyway.
|
Yes, but combination and permutation are important for different
reasons AFAIK.
Total function is a product of stage functions, so sequence doesn't
matter with a linear system, and wouldn't matter anyway if poles and
zeros were the only considerations.
Ignoring the sequence, there are two ways of choosing poles and
zeros. You can distance them all, frequency-wise, carefully from
each other to avoid conspiracy, or you can stuff them as high or low
as you can at the expense of bunching, and then slug one of them.
Slugging involves moving that pole or zero so that it is dominant,
and far enough from the others to make them insignificant.
There is a rule I've been looking for concerning slugging but I
can't find it. It expresses how the amount of feedback is related to
the number of octaves required between the slugged pole or zero and
the bunch. Perhaps an octave per 6dB? Should be easy to visualise
but I'm not very focussed at present.
Power supply and linearity issues determine the order, I assume. No
point in amplifying a frequency only to attenuate it later. Better
efficiency and lower IMD if you start narrow and get wider.
| Quote: | Depending on the circuit, a (too) large coupling cap may cause
motorboating and/or blocking of the power tubes.
|
What's blocking?
Motorboating is often a power supply thing, and is best dealt with
by improving PSRR of gain stages, not by slugging. Received wisdom
suggests a strategy of distributing poles and zeros through the PS
is just too hard to accomplish.
Voltage regulation for gain stages kills all PS related stability
issues.
cheers, Ian |
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Chris Hornbeck
Guest
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| Posted:
Thu Sep 22, 2005 4:43 am Post subject:
Re: Value of coupling cap's |
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On Thu, 22 Sep 2005 02:33:35 GMT, "Ian Iveson"
<IanIveson.home@blueyonder.co.uk> wrote:
| Quote: | Power supply and linearity issues determine the order, I assume. No
point in amplifying a frequency only to attenuate it later. Better
efficiency and lower IMD if you start narrow and get wider.
|
Here I must disagree strongly. For an incredibly
prescient and insightful discussion of this topic,
I can recommend nothing more highly than D.T.N.
Williamson's revision to his original amplifier
paper.
He discusses two too often overlooked issues,
blocking and the increase in primary inductance
with signal level.
| Quote: | Depending on the circuit, a (too) large coupling cap may cause
motorboating and/or blocking of the power tubes.
What's blocking?
|
You might know it as "hang time" or some other term
meaning the recovery time from grid forward conduction
re-biasing.
The term "blocking" obviously means a *really* bad
case. Comes from American ham radio usage, I believe.
Always the best,
Chris Hornbeck |
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Wessel Dirksen
Guest
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| Posted:
Thu Sep 22, 2005 8:51 pm Post subject:
Re: Value of coupling cap's |
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I'm a psychiatrist a few days a week and do speaker work on the other
few. I decided on a change in the psych job.
I don't do any fishing but I thought that a keeper is the one you don't
throw back. Looks like I'm interested in holding on to this new one.
The speakers and amp's are part of my package to keep, or not to keep .
.. .
In the scenario's I've been coming accross, depending on the load you
could choose between 0.1 and 1.5 micF to get a F-3 of 10hz or 1 hz.
Wessel |
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Wessel Dirksen
Guest
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| Posted:
Thu Sep 22, 2005 8:54 pm Post subject:
Re: Value of coupling cap's |
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The "start narrow and get wider" seems to be recommended and makes
sense to me
Thanks all |
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Ian Iveson
Guest
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| Posted:
Thu Sep 22, 2005 9:02 pm Post subject:
Re: Value of coupling cap's |
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"Chris Hornbeck" <chrishornbeckremovethis@att.net> wrote
| Quote: | Power supply and linearity issues determine the order, I assume.
No
point in amplifying a frequency only to attenuate it later. Better
efficiency and lower IMD if you start narrow and get wider.
Here I must disagree strongly. For an incredibly
prescient and insightful discussion of this topic,
I can recommend nothing more highly than D.T.N.
Williamson's revision to his original amplifier
paper.
He discusses two too often overlooked issues,
blocking and the increase in primary inductance
with signal level.
|
Got a link to the paper?
| Quote: | What's blocking?
You might know it as "hang time" or some other term
meaning the recovery time from grid forward conduction
re-biasing.
The term "blocking" obviously means a *really* bad
case. Comes from American ham radio usage, I believe.
|
Is blocking your only reason for your strong disagreement, or does
the variation in primary inductance come into it too?
The word I use to myself is "jacking", because I think of a pump and
a ratchet. I may be misunderstanding you though coz I haven't seen
the paper.
Practical circuit details can force good general principles to be
compromised. I got a bit carried away replying to a post about
general principles, which I still think are good. I fear sometimes
the universe isn't entirely on my side.
So if cap coupling is used to the output stage should it define the
dominant LF cut-off? Is direct or transformer coupling the only way
to avoid this compromise?
cheers, Ian |
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David R Brooks
Guest
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| Posted:
Fri Sep 23, 2005 3:46 am Post subject:
Re: Value of coupling cap's |
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Morgan Jones states it thus:
"The loop gain may be as large as the ratio of the two most dominant
time constants" (2nd edn., P284)
"Ian Iveson" <IanIveson.home@blueyonder.co.uk> wrote:
[snip]
:
:There is a rule I've been looking for concerning slugging but I
:can't find it. It expresses how the amount of feedback is related to
:the number of octaves required between the slugged pole or zero and
:the bunch. Perhaps an octave per 6dB? Should be easy to visualise
:but I'm not very focussed at present.
:
[snip]
-- Dave Brooks <http://www.iinet.net.au/~daveb>
PGP public key: servers daveb@iinet.net.au
fingerprint 20 8F 95 22 96 D6 1C 0B 3D 4D C3 D4 50 A1 C4 34
What's all this? see http://www.iinet.net.au/~daveb/crypto.html
"If anyone slew an innocent person it would be as if he slew the whole of mankind and if anyone saved a life it would be as if he saved the life of the whole mankind" [Al-Qur'an 5:32]
"Do treat your women well and be kind to them for they are your partners and committed helpers." [The last sermon of the Prophet Muhammad (pbuh).]
"An Arab has no superiority over a non-Arab, nor a non-Arab has any superiority over an Arab; also a white has no superiority over a black, nor a black has any superiority over white except by piety and good action." [ibid] |
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Ian Iveson
Guest
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| Posted:
Fri Sep 23, 2005 4:43 am Post subject:
Re: Value of coupling cap's |
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"David R Brooks" wrote
| Quote: | :There is a rule I've been looking for concerning slugging but I
:can't find it. It expresses how the amount of feedback is related
to
:the number of octaves required between the slugged pole or zero
and
:the bunch. Perhaps an octave per 6dB? Should be easy to visualise
:but I'm not very focussed at present.
Morgan Jones states it thus:
"The loop gain may be as large as the ratio of the two most
dominant
time constants" (2nd edn., P284)
|
Aha, right, thanks for taking the trouble to look it up. Sounds
mysterious the way he puts it, but it's clear with a little thought.
cheers, Ian |
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Chris Hornbeck
Guest
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| Posted:
Fri Sep 23, 2005 4:43 am Post subject:
Re: Value of coupling cap's |
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On Thu, 22 Sep 2005 16:02:47 GMT, "Ian Iveson"
<IanIveson.home@blueyonder.co.uk> wrote:
| Quote: | I can recommend nothing more highly than D.T.N.
Williamson's revision to his original amplifier
paper.
He discusses two too often overlooked issues,
blocking and the increase in primary inductance
with signal level.
Got a link to the paper?
|
I'm not really much of a web person. It may be out there
somewhere, though. Not too many years ago, Ed Dell's
group in Peterborough, New Hampshire republished it
for, I believe, $4.95 US or some similar small
amount. Their web address now is Audio Xpress in
some webby variant .com
Brilliant work from a quite young man, IIRC.
| Quote: | Is blocking your only reason for your strong disagreement, or does
the variation in primary inductance come into it too?
|
No, it's completely secondary.
| Quote: | So if cap coupling is used to the output stage should it define the
dominant LF cut-off? Is direct or transformer coupling the only way
to avoid this compromise?
|
The issue that DTNW summarizes so clearly is one
fundamental to feedback amplifiers: having two poles
near each other in the forward signal path.
Two poles = 180 degrees = trouble in River City, friends.
As you, and most all, know.
An amplifier with multiple poles *must* have a dominant
pole for unconditional stability. How to choose? In
conventional valve amplifiers we have one, two, maybe
even three RC poles and one RL pole.
The tricky part is that the RL pole moves with signal
level. At higher signal levels it moves down in frequency.
Cost considerations might dictate that the RL pole be
the dominant pole, and this is (sorta) possible with
large enough RC pairs. Downsides include exaggerating
power supply imperfections and poor overload recovery
("blocking", "jacking") time.
So DTNW argues for a large enough primary inductance
to move the small-signal RL pole below *one* RC pole.
The logical place to put this chosen RC pole is at
the grid of the stage most likely to be clipped.
Long essay. Sorry. Read the Man. He was great.
Always the best,
Chris Hornbeck |
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Ian Iveson
Guest
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| Posted:
Fri Sep 23, 2005 7:04 pm Post subject:
Re: Value of coupling cap's |
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Chris Hornbeck wrote [below, and...]
| Quote: | So DTNW argues for a large enough primary inductance
to move the small-signal RL pole below *one* RC pole.
|
Right. This leads to a conclusion that is almost the opposite of
what I said was true in principle. It's not just cost either, but a
matter of unfortunate physics. Nick Sheldon, once a regular here
into BIG valve amps, pointed out that some transformer parameters
depend on volume, some on area, and some on length. Hence they don't
scale simply. Rather like there is an optimum size for animals on
earth (my size, more or less) because smaller is too weak and bigger
is too cumbersome. Strength depends on cross-sectional area, and
weight is related to volume. That's why elephants have such
impossibly thick legs. Anyway, for a given power rating, there is a
limit to the bandwidth of a given transformer technology.
So in Williamson's day, even the best and most expensive OPT in
combination with the output valves would roll off so high that
there'd be only room for one pole above if he was lucky.
Conventionally, as a result of this and the blocking issue, amps
can't start narrow and get wider.
Now I'll have to do some reading to find out whether this is
desirable in principle, or only an accident of history. Is there
room above a good toroid to fit 2 poles?
| Quote: | The logical place to put this chosen RC pole is at
the grid of the stage most likely to be clipped.
|
Aaargh! Morgan Jones to name but one author does this. The logical
thing is fol-de-rol tra-la-la but where's the logic in saying it's
logical without saying what the logic is that makes it so?
| Quote: | Long essay. Sorry. Read the Man. He was great.
|
I will when I find him.
Thanks much. It's good to learn. Sat with Schaum's mildewed
"Feedback and Control Systems".
cheers, Ian
| Quote: | I can recommend nothing more highly than D.T.N.
Williamson's revision to his original amplifier
paper.
He discusses two too often overlooked issues,
blocking and the increase in primary inductance
with signal level.
Got a link to the paper?
I'm not really much of a web person. It may be out there
somewhere, though. Not too many years ago, Ed Dell's
group in Peterborough, New Hampshire republished it
for, I believe, $4.95 US or some similar small
amount. Their web address now is Audio Xpress in
some webby variant .com
Brilliant work from a quite young man, IIRC.
Is blocking your only reason for your strong disagreement, or does
the variation in primary inductance come into it too?
No, it's completely secondary.
So if cap coupling is used to the output stage should it define
the
dominant LF cut-off? Is direct or transformer coupling the only
way
to avoid this compromise?
The issue that DTNW summarizes so clearly is one
fundamental to feedback amplifiers: having two poles
near each other in the forward signal path.
Two poles = 180 degrees = trouble in River City, friends.
As you, and most all, know.
An amplifier with multiple poles *must* have a dominant
pole for unconditional stability. How to choose? In
conventional valve amplifiers we have one, two, maybe
even three RC poles and one RL pole.
The tricky part is that the RL pole moves with signal
level. At higher signal levels it moves down in frequency.
Cost considerations might dictate that the RL pole be
the dominant pole, and this is (sorta) possible with
large enough RC pairs. Downsides include exaggerating
power supply imperfections and poor overload recovery
("blocking", "jacking") time. |
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Chris Hornbeck
Guest
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| Posted:
Sat Sep 24, 2005 4:43 am Post subject:
Re: Value of coupling cap's |
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|
On Fri, 23 Sep 2005 14:04:31 GMT, "Ian Iveson"
<IanIveson.home@blueyonder.co.uk> wrote:
| Quote: | Now I'll have to do some reading to find out whether this is
desirable in principle, or only an accident of history. Is there
room above a good toroid to fit 2 poles?
|
I guess you'd have to start to deciding on the phase
margin you'd be comfortable with, which determines the
spacing between the poles (at least the top two;
open loop gain may be low enough between later
pole pairs to ignore them).
Then you'd have to pick a dominant pole. Personally,
I have no trouble with about 16 Hz.
Not really certain why you're wanting to do this,
though...?
| Quote: | The logical place to put this chosen RC pole is at
the grid of the stage most likely to be clipped.
Aaargh! Morgan Jones to name but one author does this. The logical
thing is fol-de-rol tra-la-la but where's the logic in saying it's
logical without saying what the logic is that makes it so?
|
Because overload recovery time is inversely related
to the RC pole frequency. The C bleeds down through
the same R that sets the pole, or close enough to it.
Haven't read Morgan Jones though.
Thanks,
Chris Hornbeck |
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