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west west is offline
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I built a typical phono amp with 3 12AX7s, the last tube a cathode follower
(low impedance). It has plenty of gain (70db) and can easily drive any amp
directly. I like using a 100k step resistor volume control instead of a line
amp.
My problem is that when I use any of my tube amps, the sound has limited
bandwidth. A SS amp does not exhibit this problem, if fact the SS sound is
rather impressive to me (for silicone).
With my limited know-how, I'm swagging that an impedance mismatch is the
culprit.
If my contention is correct, is there a simple cure besides using a line
amp? Perhaps some kind of buffer is called for. All comments welcomed and
thank you.

Cordially,
west


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Sander deWaal Sander deWaal is offline
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"west" said:

I built a typical phono amp with 3 12AX7s, the last tube a cathode follower
(low impedance). It has plenty of gain (70db) and can easily drive any amp
directly. I like using a 100k step resistor volume control instead of a line
amp.
My problem is that when I use any of my tube amps, the sound has limited
bandwidth. A SS amp does not exhibit this problem, if fact the SS sound is
rather impressive to me (for silicone).
With my limited know-how, I'm swagging that an impedance mismatch is the
culprit.
If my contention is correct, is there a simple cure besides using a line
amp? Perhaps some kind of buffer is called for. All comments welcomed and
thank you.



You already have a buffer in the form of the 12AX7 cathode follower.
Just put the pot in front of it, with a switch for phono/CD if you
like.
No need for another amplifying element in the chain, less is more.

Better yet, get a 6SN7 for that CF function and power the heater
separately, with DC offset to about cathode level.

--

- Maggies are an addiction for life. -
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Patrick Turner Patrick Turner is offline
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west wrote:

I built a typical phono amp with 3 12AX7s, the last tube a cathode follower
(low impedance). It has plenty of gain (70db) and can easily drive any amp
directly. I like using a 100k step resistor volume control instead of a line
amp.
My problem is that when I use any of my tube amps, the sound has limited
bandwidth. A SS amp does not exhibit this problem, if fact the SS sound is
rather impressive to me (for silicone).
With my limited know-how, I'm swagging that an impedance mismatch is the
culprit.
If my contention is correct, is there a simple cure besides using a line
amp? Perhaps some kind of buffer is called for. All comments welcomed and
thank you.

Cordially,
west


Seems like you don't know how to make your tube amps have full
bandwidth.

But all the well made tube amps have full BW, so make your tube amps
well and no problems, OK.

If the tube sound seems like there isn't full bandwidth
when in fact there is, then your statement about SS amps having more
subjective? BW than tube amps could be because
the SS has more distortion. Usually SS has less measured THD/IMD.


Have you conducted a proper AB test, set up two amp systems, one SS, the
other tubed,
and with same source from the same CD played feeding both amp systems,
then matched the level very
carefully, then switched the same speakers from one amp system to the
other?

Usually such a test reveals very little difference between tube or SS
especially if
the THD/IMD measurements are the same figures, Rout are equal, and BW is
-1dB down at 20Hz to 20kHz or better.

Many audiophiles hate this test because it tests their ability to tell
tube and SS apart,
and its a big blow to their ego when they find they cannot tell good
samples of SS and tube amps apart.
Similary, ppl prejudiced in favour of SS or who hate tubes especially
SET amps are also given an uncomfortable
lesson about reality when given the switch to switch the speakers, they
cannot tell to
which amp system the speakers are connected.

Patrick Turner.
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west west is offline
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"Patrick Turner" wrote in message
...


west wrote:

I built a typical phono amp with 3 12AX7s, the last tube a cathode

follower
(low impedance). It has plenty of gain (70db) and can easily drive any

amp
directly. I like using a 100k step resistor volume control instead of a

line
amp.
My problem is that when I use any of my tube amps, the sound has limited
bandwidth. A SS amp does not exhibit this problem, if fact the SS sound

is
rather impressive to me (for silicone).
With my limited know-how, I'm swagging that an impedance mismatch is the
culprit.
If my contention is correct, is there a simple cure besides using a line
amp? Perhaps some kind of buffer is called for. All comments welcomed

and
thank you.

Cordially,
west


Seems like you don't know how to make your tube amps have full
bandwidth.

But all the well made tube amps have full BW, so make your tube amps
well and no problems, OK.

If the tube sound seems like there isn't full bandwidth
when in fact there is, then your statement about SS amps having more
subjective? BW than tube amps could be because
the SS has more distortion. Usually SS has less measured THD/IMD.


Have you conducted a proper AB test, set up two amp systems, one SS, the
other tubed,
and with same source from the same CD played feeding both amp systems,
then matched the level very
carefully, then switched the same speakers from one amp system to the
other?

Usually such a test reveals very little difference between tube or SS
especially if
the THD/IMD measurements are the same figures, Rout are equal, and BW is
-1dB down at 20Hz to 20kHz or better.

Many audiophiles hate this test because it tests their ability to tell
tube and SS apart,
and its a big blow to their ego when they find they cannot tell good
samples of SS and tube amps apart.
Similary, ppl prejudiced in favour of SS or who hate tubes especially
SET amps are also given an uncomfortable
lesson about reality when given the switch to switch the speakers, they
cannot tell to
which amp system the speakers are connected.

Patrick Turner.


First thing Patrick ... tone down some, OK? If you have 150X an impedance
mismatch between a preamp out & amp's input, i.e. preamp out 10K; amp input
150K, does that not account for any aberrations in the overall bandwidth
passing on to the amp? Is this OK? My amps' bandwidths check out to specs
and the numbers are excellent. What is an advantage to a line stage over a
passive stage? Stop picking fly **** out of pepper and don't put me to the
ringer to ask perfect questions. If you do not understand me, then move on
to another thread. Thank you.

west


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west wrote:

I built a typical phono amp with 3 12AX7s, the last tube a cathode follower
(low impedance). It has plenty of gain (70db) and can easily drive any amp
directly. I like using a 100k step resistor volume control instead of a line
amp.
My problem is that when I use any of my tube amps, the sound has limited
bandwidth. A SS amp does not exhibit this problem, if fact the SS sound is
rather impressive to me (for silicone).
With my limited know-how, I'm swagging that an impedance mismatch is the
culprit.
If my contention is correct, is there a simple cure besides using a line
amp? Perhaps some kind of buffer is called for. All comments welcomed and
thank you.


You're using a 100k 'pot' effectively to drive a cable.. With as little as 1 nF
of cable capacitance that forms a low pass filter at 6kHz !

The transistor amps will have a lower input impedance that loads this which will
push the turnover frequency out of the audible area (just).

You should never drive a cable from a pot or attenuator of high value.

Try using a 10k stepped attenuator instead. The whole point of cathode followers
is to drive lower impedances.

Graham



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west wrote:

First thing Patrick ... tone down some, OK? If you have 150X an impedance
mismatch between a preamp out & amp's input, i.e. preamp out 10K; amp input
150K, does that not account for any aberrations in the overall bandwidth
passing on to the amp?


Firstly there is no such thing here as a 'mismatch' ! Matching in audio is only
an issue when you have kilometres of cable. All that matters here is the source
impedance, load impedance, cable capacitance and the resulting transfer
characteristic.

You built yourself a sloppy low pass filter. That's all. Get yourself a
modelling program and you can see the effect of cable capacitance on a 100k
attenuator for yourself. Plus the influence of load impedance on same.

Graham

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Eeyore wrote:

Get yourself a
modelling program and you can see the effect of cable capacitance on a 100k
attenuator for yourself. Plus the influence of load impedance on same.


Better still, learn how to calculate it yourself ! The modelling program just makes
it easy for you. A demo version will do the job.

Graham

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Ian Iveson Ian Iveson is offline
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west wrote

I built a typical phono amp with 3 12AX7s, the last tube a cathode
follower
(low impedance). It has plenty of gain (70db) and can easily drive
any amp
directly. I like using a 100k step resistor volume control instead
of a line
amp.
My problem is that when I use any of my tube amps, the sound has
limited
bandwidth. A SS amp does not exhibit this problem, if fact the SS
sound is
rather impressive to me (for silicone).
With my limited know-how, I'm swagging that an impedance mismatch is
the
culprit.
If my contention is correct, is there a simple cure besides using a
line
amp? Perhaps some kind of buffer is called for.


You don't say which part of the bandwidth goes missing.

A resistance ratio of at least 1:10 generally does the job, as a rule
of thumb.

The max output resistance of a 100k attenuator is 25k + source
resistance/4. Its minimum input resistance is its load resistance in
parallel with 100k.

What are the input resistances of the SS and valve power amps?

Bandwidth isn't hard to measure. Input resistance not so easy, but in
the case of the valve amps you can look at the circuits.

Incidentally, don't be conned by the AB test thing, West. A difference
cannot be heard, only inferred. For a meaningful inference, the
listener must be trained in AB testing. Training is a self-fulfilling
prophecy.

All comments welcomed and
thank you.


Hostage to fortune...you have failed to keep this promise.

cheers, Ian


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west wrote:

"Patrick Turner" wrote in message
...


west wrote:

I built a typical phono amp with 3 12AX7s, the last tube a cathode

follower
(low impedance). It has plenty of gain (70db) and can easily drive any

amp
directly. I like using a 100k step resistor volume control instead of a

line
amp.
My problem is that when I use any of my tube amps, the sound has limited
bandwidth. A SS amp does not exhibit this problem, if fact the SS sound

is
rather impressive to me (for silicone).
With my limited know-how, I'm swagging that an impedance mismatch is the
culprit.
If my contention is correct, is there a simple cure besides using a line
amp? Perhaps some kind of buffer is called for. All comments welcomed

and
thank you.

Cordially,
west


Seems like you don't know how to make your tube amps have full
bandwidth.

But all the well made tube amps have full BW, so make your tube amps
well and no problems, OK.

If the tube sound seems like there isn't full bandwidth
when in fact there is, then your statement about SS amps having more
subjective? BW than tube amps could be because
the SS has more distortion. Usually SS has less measured THD/IMD.


Have you conducted a proper AB test, set up two amp systems, one SS, the
other tubed,
and with same source from the same CD played feeding both amp systems,
then matched the level very
carefully, then switched the same speakers from one amp system to the
other?

Usually such a test reveals very little difference between tube or SS
especially if
the THD/IMD measurements are the same figures, Rout are equal, and BW is
-1dB down at 20Hz to 20kHz or better.

Many audiophiles hate this test because it tests their ability to tell
tube and SS apart,
and its a big blow to their ego when they find they cannot tell good
samples of SS and tube amps apart.
Similary, ppl prejudiced in favour of SS or who hate tubes especially
SET amps are also given an uncomfortable
lesson about reality when given the switch to switch the speakers, they
cannot tell to
which amp system the speakers are connected.

Patrick Turner.


First thing Patrick ... tone down some, OK?


What tone do I turn down? bass, treble, both perhaps?
Do I turn them up? That'd really mean turning the midrange down.


The phono amp with 12AX7 CF output and with only 0.7mA of idle current
should easily produce 0.4Arms into a load as low as 10k, which is 4Vrms.

So the 100k attenuator after the CF won't cause a problem because of its
value.

If you have 150X an impedance
mismatch between a preamp out & amp's input, i.e. preamp out 10K; amp input
150K, does that not account for any aberrations in the overall bandwidth
passing on to the amp?


It depends on the impedance character of the amp following the
attenuator and the attenuator resistance.

You have not given us any technical advice about the exact input-output
resistance impedance
networks which exist, but just say all your tube amps hve little bass
and treble.

Kindly submit the facts please when asking for a detailed technical
opinion.



For simplicity, let's assume you have cathode followewr, dc blocking cap
= 1uF
feeding 100k attenuator with output to the outside world taken directly
from the wiper of the attenuator.

There will be ab LF pole at xxxx Hz. Please work this out.
I refuse to spoon feed you by doing calculations you should be prepared
to do yourself.

Hint, F, -3dB = 159,000 / ( R x C uF ) Hz.



Assume the SS amp has "10k" input resistance, and dc blocking cap of
10uF
to keep out the DC from any previous amp.

What is the F pole?


Assume you base your calulations of a setiing of the attenuator at the
12oclock
position, ie, -20dB, so that the attenutaor is 90k from CF to wiper, 10k
from wiper to OV.

What is the actual network that you have? what is the F response at LF.
What is the source resistance looking back into the attenuator?

What happens to the answers to all questions as you move the wiper up
the attenuator?
how about downwards?

Now that you have answered all those very basic questions about your
gear's LF behaviour,
let's consider the HF.

Knowing that the following amp has 10k input resistance, we assume it is
shunt R, ie,
measured from input terminal after the blocking cap
to 0V, and we assume it is 10k.

Is there any R beyound this point that is in series after the blocking
cap
to the first base of the first input transistor? What is the R value?
Is there any C from this base to 0V? what is the C value?

If we assumed the signal source resistance was 100ohms, what is the F
pole
at HF caused by the series R and shunt C?

Knowing that you would have signal source R = 10k, what happens to the
pole, and why?
What is the actual R & C filter at the input that you have when source R
= 10k?
What happens to the HF pole when you change the source R by raising
or lowering the wiper along the attenuator track, or steps on seriesed
resistors?

I want you to spend more time working out anwers to your own questions.

"Work it out yourself" is a statement that all members of the group
should
not ask, but command themsleves to do onece you roll out of bed each
morning.
Otherwise it won't happen, and we have to waste all our time
telling you yet again and again the same old answers to the same old
questions,
because you are lazy.

Is this OK? My amps' bandwidths check out to specs
and the numbers are excellent.


I have no idea, because there is not enough data upon which to make an
assement.


What is an advantage to a line stage over a
passive stage?


In my designs of preamps I often have a deletable gain stage.
Often this a single µ-follower using 12AU7/6CG7 with open loop gain
reduced from a maximum of about 18
down to about 5, using a shunt NFB loop.
This reduces the gain block Rout also to less than 1k.

See my website for details.

The gain stage feeds gain pot or attenuator of at least 50k, and the
wiper
goes to a cathode follower grid, and output is from the cathode.
What is the impedance change effeced by having a CF after the gain
attenuator?
Why is it put there?



Stop picking fly **** out of pepper and don't put me to the
ringer to ask perfect questions.


I'm not picking flu **** out of peper, and insist i have the right
to say what is thought provoking on a discussion group.

I will NEVER give up the right to ask dozens of questions if I can so
that
you and all the others have to actually use your god given brain to
work out the answers or remain dumb, just fed answers by me acting like
a damn parrot.

I am happy to give you just enough of a guide to work out answers on
your own.


If you do not understand me, then move on
to another thread. Thank you.


You are not alone in not understanding the interactions between a given
preamp,
and a given power amp, neither of which was PERHAPS designed to run
optimally
with each other.
The question you have asked has been asked before in news groups.

Rather than give a short generic one statement fits all answer, except
its never enough,
I will give you the full story,
depending on where I think you are according to skill, abilities as I
see them,
and sometimes that means you have to go and take all the covers off your
amps, trace all the circuits,
do it all over to make sure there is no mistake, measure responses
involved, and then calculate out what you should be getting and then
work out a fully detailed
cure to problems, then carry out the mods and check everything.

Don't just post a winge about poor bandwidth on a NG concerning your own
gear
about which we assume you are responsible for correct working.

I am not there to look over your shoulder to guide your soldering or to
be your tutor
with the most very basic frequency response issues with R&C networks.

And you have not offered to pay me for my professional services.

The weather here today was a glorious autumn day of about 21C. I rode by
bicycle about 85km, not sure how far;
I was out for about 4.5 hours.
It seemed to go a bit better after cleaning off a pound of oily grunge
on the chain and gears
before i left.
Afterwards the pool beckoned, and a short swim of 300M got the arms and
back re-aliged after the ride.
I sometimes ask, Why do they have the swim as the first item in an
triathlon?
It'd make more sense to make it the last thing, so that when contestents
got to the
podium finish, they'd be clean and refreshed, but probably embarrassed
appearing in a swimsuit,
or pair of budgy smugglers as we call them here.
Men are such fickle creatures.
I'm too old and decrepit to run 26 miles after a swim of 3 miles and
ride of 100 miles.
The run is the real killer; the other two can almost be done by age
challenged personel
such as meself, who I know ain't as good as I was.

I'm happy doing 80kM thse days by bike, only 50 miles, and not very
fast,
I like to see the scenery, and not worry about competion, or what anyone
else might think about
what i am doing, or not doing.


Being Easter Sunday, not many others were going very fast and I passed a
large number on my way around the town.

One guy I know cycled nearly every day until he was about 84, I think a
bad hip
eventually crumbled.
But my sporting hero is Hubert Opperman, see
http://canberrabicyclemuseum.com.au/...he_cyclist.htm

This man makes me look like a fool.

I bet he asked a lot of questions of those around him, and of himself.

Patrick Turner.











west

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Ian Iveson Ian Iveson is offline
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Eeyore wrote

You're using a 100k 'pot' effectively to drive a cable.. With as
little as 1 nF
of cable capacitance that forms a low pass filter at 6kHz !


Little? 1nF is over 10 metres of ordinary audio coax.

The transistor amps will have a lower input impedance that loads
this which will
push the turnover frequency out of the audible area (just).



Equally, reducing cable length to less than 1m would be just about OK.

You should never drive a cable from a pot or attenuator of high
value.


High is entirely relative.

Try using a 10k stepped attenuator instead. The whole point of
cathode followers
is to drive lower impedances.



Yes, assuming all sources have output resistances of less than 1k, but
doesn't your explanation equally lead to the conclusion that the
problem could be solved by reducing the input resistance of the valve
amps? That would be a much cheaper solution, but not a good one IMO.

All things in proportion.

cheers, Ian




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west wrote:

I built a typical phono amp with 3 12AX7s, the last tube a cathode follower
(low impedance). It has plenty of gain (70db) and can easily drive any amp
directly. I like using a 100k step resistor volume control instead of a line
amp.
My problem is that when I use any of my tube amps, the sound has limited
bandwidth. A SS amp does not exhibit this problem, if fact the SS sound is
rather impressive to me (for silicone).
With my limited know-how, I'm swagging that an impedance mismatch is the
culprit.
If my contention is correct, is there a simple cure besides using a line
amp? Perhaps some kind of buffer is called for. All comments welcomed and
thank you.

Cordially,
west


It's not an impedance problem, West.

You've built a filter consisting of the 100K pot and the following
cable's capacitance.

Reduce the pot to 10k, shorten the cable to no more than a meter (3ft
3in) or preferably 18 inches, and your problem should go away. Or you
can move the pot to before the cathode follower to break the link with
the cable capacitance.

Don't worry about the wretched kibbitzers who will now wield their
little bit of knowledge on you like a club. This is an error that is
often made by professional designers and sometimes even gets into
production.

As a matter of taste rather than electronics, you should also lose the
12AX7, which are suitable only for guitar amps, and build your preamp
with octals, preferably 6SL7 and 6SN7. The CF in particular should be
a 6SN7. If you don't want to reengineer the chassis for the bigger
sockets, at least try the 6CG7.

Another taste tip: A 12AT7 for practical purposes has as much gain as
the 12AX7 and sounds more precise. You can also, if you have the gain
in another stage, engineer in more warmth with the 12AU7.

HTH.

Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
"wonderfully well written and reasoned information
for the tube audio constructor"
John Broskie TubeCAD & GlassWare
"an unbelievably comprehensive web site
containing vital gems of wisdom"
Stuart Perry Hi-Fi News & Record Review

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Ian Iveson wrote:

Eeyore wrote

You're using a 100k 'pot' effectively to drive a cable.. With as
little as 1 nF of cable capacitance that forms a low pass filter at 6kHz !


Little? 1nF is over 10 metres of ordinary audio coax.


That depends what you call ordinary. What do you use and what's its capacitance
?

Graham

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Andre Jute wrote:

It's not an impedance problem, West.

You've built a filter consisting of the 100K pot and the following
cable's capacitance.

Reduce the pot to 10k,


Good Lord.

You followed Poopie's advice !

Graham

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Andre Jute wrote:

This is an error that is often made by professional designers and sometimes even
gets into production.


No it isn't. Certainly not any designer I know.

Graham

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Eeyore wrote

You're using a 100k 'pot' effectively to drive a cable.. With as
little as 1 nF of cable capacitance that forms a low pass filter
at 6kHz !


Little? 1nF is over 10 metres of ordinary audio coax.


That depends what you call ordinary. What do you use and what's its
capacitance
?



Erm...it's pearlescent and thick, so it may be this:

http://www.maplin.co.uk/module.aspx?...2&doy=8m4#spec

but around 90pF/m seems common.

I am using a 100k pot too...but with only 1/2m of cable, until I have
finished my remote controlled pre. 10k will be a better load for my
sources.

cheers, Ian




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Eeyore wrote:

Andre Jute wrote:

This is an error that is often made by professional designers and sometimes even
gets into production.


No it isn't. Certainly not any designer I know.



In this particular case I think there are too many one liner replies
which are less than very informative.

The original poster complained of poor bandwidth and probably it is a
case of
the R component being too high to get decent BW with respect to whatever
C is present.

But as I pointed out before, the R with an attenuator varies.

And a 100k pot will, when hooked to a low Rout CF have
maximum Ro at the -6dB position and Rout = 25kpot.
But at -20dB, its 10k in parallel to 90k = 9k, and at -26dB, its
much lower again, and as you reduce the Rout of the attenuator, HF
banwidth gets higher
since the same C causes a rising pole with lower shunt R.
My previous long post was about the whole issue, not just parts of it,
and made strong, even obnoxious hints that anyone with such response
problems need to analyse carefully
before applying any solution, and teach themselves some basic R&C
network theory in the process,
and they'd then be well equipped for fixing the next curly problem
that arrises, and about which they might post.

Patrick Turner.




Graham

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Eeyore wrote:
Andre Jute wrote:

It's not an impedance problem, West.

You've built a filter consisting of the 100K pot and the following
cable's capacitance.

Reduce the pot to 10k,


Good Lord.

You followed Poopie's advice !

Graham


Who is Poopie? What a disgusting name!

Andre Jute
Our legislators managed to criminalize fox-hunting and smoking; when
they will get off their collective fat backside and criminalize
negative feedback? It is clearly consumed only by thickoes like Graham
Stevenson.

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Andre Jute wrote:

Our legislators managed to criminalize fox-hunting and smoking; when
they will get off their collective fat backside and criminalize
negative feedback?


You wouldn't understand the value of well-applied negative feedback if it stood
up and smacked in you in the face.

Graham

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Patrick Turner wrote:
Eeyore wrote:

Andre Jute wrote:

This is an error that is often made by professional designers and sometimes even
gets into production.


No it isn't. Certainly not any designer I know.



In this particular case I think there are too many one liner replies
which are less than very informative.


You aren't so innocent, Patrick, that you imagine Poopie is capable of
an informative answer, are you? Poor bugger is still trying to master
joined-up writing, never mind joined-up thinking!

Just in case you want to send him an example of a complete answer,
with subsidiary thoughts. here is what I sent again:

********
Andre Jute wrote:

It's not an impedance problem, West.


You've built a filter consisting of the 100K pot and the following
cable's capacitance.


Reduce the pot to 10k, shorten the cable to no more than a meter (3ft
3in) or preferably 18 inches, and your problem should go away. Or you
can move the pot to before the cathode follower to break the link
with
the cable capacitance.


Don't worry about the wretched kibbitzers who will now wield their
little bit of knowledge on you like a club. This is an error that is
often made by professional designers and sometimes even gets into
production.


As a matter of taste rather than electronics, you should also lose
the
12AX7, which are suitable only for guitar amps, and build your preamp
with octals, preferably 6SL7 and 6SN7. The CF in particular should be
a 6SN7. If you don't want to reengineer the chassis for the bigger
sockets, at least try the 6CG7.


Another taste tip: A 12AT7 for practical purposes has as much gain as
the 12AX7 and sounds more precise. You can also, if you have the gain
in another stage, engineer in more warmth with the 12AU7.


HTH.


Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
"wonderfully well written and reasoned information
for the tube audio constructor"
John Broskie TubeCAD & GlassWare
"an unbelievably comprehensive web site
containing vital gems of wisdom"
Stuart Perry Hi-Fi News & Record Review

********

On second thoughts, better not to confuse poor Poopie. He might ask
you what "thought" is, and you will never finish explaining to him...

Andre Jute
A little, a very little thought will suffice -- Lord Keynes


The original poster complained of poor bandwidth and probably it is a
case of
the R component being too high to get decent BW with respect to whatever
C is present.

But as I pointed out before, the R with an attenuator varies.

And a 100k pot will, when hooked to a low Rout CF have
maximum Ro at the -6dB position and Rout = 25kpot.
But at -20dB, its 10k in parallel to 90k = 9k, and at -26dB, its
much lower again, and as you reduce the Rout of the attenuator, HF
banwidth gets higher
since the same C causes a rising pole with lower shunt R.
My previous long post was about the whole issue, not just parts of it,
and made strong, even obnoxious hints that anyone with such response
problems need to analyse carefully
before applying any solution, and teach themselves some basic R&C
network theory in the process,
and they'd then be well equipped for fixing the next curly problem
that arrises, and about which they might post.

Patrick Turner.




Graham


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"Eeyore" wrote in message
...


west wrote:

I built a typical phono amp with 3 12AX7s, the last tube a cathode

follower
(low impedance). It has plenty of gain (70db) and can easily drive any

amp
directly. I like using a 100k step resistor volume control instead of a

line
amp.
My problem is that when I use any of my tube amps, the sound has limited
bandwidth. A SS amp does not exhibit this problem, if fact the SS sound

is
rather impressive to me (for silicone).
With my limited know-how, I'm swagging that an impedance mismatch is the
culprit.
If my contention is correct, is there a simple cure besides using a line
amp? Perhaps some kind of buffer is called for. All comments welcomed

and
thank you.


You're using a 100k 'pot' effectively to drive a cable.. With as little as

1 nF
of cable capacitance that forms a low pass filter at 6kHz !

The transistor amps will have a lower input impedance that loads this

which will
push the turnover frequency out of the audible area (just).

You should never drive a cable from a pot or attenuator of high value.

Try using a 10k stepped attenuator instead. The whole point of cathode

followers
is to drive lower impedances.

Graham


Your answer is what I suspected but not having much experience, I asked this
NG. I was looking for a Tube Basic 101 generic answer that I did not find in
my reference books. I don't understand where Patrick is coming from. I have
never asked for specific help. That's the most enjoyable aspect of this
hobby (for me), is rolling your own, having some problem, and ultimately
solving it. Thanks again Graham.

west





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Andre Jute wrote:

Poor bugger is still trying to master joined-up writing, never mind joined-up thinking!


Projecting again I see !

Graham

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PS

You just helped me solve a serious problem. On inspection, I found I
had one of my cables pointing the wrong way. Now I have turned it
round the sound is...er...hmm...perhaps it wasn't so serious after
all.

Apart from the arrows, the cable says "Shark wire - 80 * 0.1 - OFC -
Aud/Vid" regularly, three times per metre. It seems quite proud of
itself, even though it is too thick for Neutrik plugs unless you
dispense with the collets.

cheers, Ian


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west wrote:
I built a typical phono amp with 3 12AX7s... CF... 100k step resistor volume control
,,,My problem is that when I use any of my tube amps, the sound has limited
bandwidth. A SS amp does not exhibit this problem


You're using a 100k 'pot' effectively to drive a cable...
With as little as 1 nF of cable capacitance that forms a low pass filter at 6kHz !


Your answer is what I suspected but not having much experience, I asked this
NG. I was looking for a Tube Basic 101 generic answer that I did not find in
my reference books. I don't understand where Patrick is coming from....


(1) the limited bandwidth problem being only there is presumably due
to the high output impedance (100k step resistors not the same as 100k
potentiometer, which is not the same as a 100k output impedance though -
you might not even need 100pF to get a serious high frequency cut). The
test is to measure (on a scope or whatever, with a low capacitance
probe) directly at the output socket, with no cable connected, what is
going on with the frequency response. The trouble with this test is that
you said it is a phono preamp, so you have to rig up a reverse RIAA
circuit for your signal generator's output. Another, easier, test would
be to shove a 22k resistor or so across the output and then see if the
valve amps are then happy. Presumably the solid state amp's low input
impedance is reducing the effects of the RC filter you make with the
interconnect cable. Or the valve amps have lots of capacitance/Miller
effect at their input and want a lower input impedance (not so likely to
the a significant factor I guess). If the oscilloscope test shows a
high frequency cut is still going on, maybe the RIAA values are wrong
and maybe the SS amp has some slight high frequency lift (some nfb
circuits do this) that is compensating - two wrongs making a right??
(pretty unlikely).

(2) I get the feeling Patrick sometimes growls about the questioners
as a sign of friendship.

Mark A
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Mark Aitchison wrote:

100k step resistors not the same as 100k potentiometer


Would you care to explain your philosphy behind that statement ?

Graham

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Ian Iveson wrote:

Eeyore wrote

You're using a 100k 'pot' effectively to drive a cable.. With as
little as 1 nF of cable capacitance that forms a low pass filter
at 6kHz !

Little? 1nF is over 10 metres of ordinary audio coax.


That depends what you call ordinary. What do you use and what's its
capacitance
?


Erm...it's pearlescent and thick, so it may be this:

http://www.maplin.co.uk/module.aspx?...2&doy=8m4#spec

but around 90pF/m seems common.

I am using a 100k pot too...but with only 1/2m of cable, until I have
finished my remote controlled pre. 10k will be a better load for my
sources.

cheers, Ian


With a 100k log pot set at -20dB, R source = 9k.

1.2M of cable at 90pF/M = 108pF.

The pole caused by pot and cable = 163kHz.

The worst HF losses are where pot is set for -6dB, R source = 25k, so
the pole is at 59kHz.

So a 100k pot would offer no audible losses over a 10k pot.

The 10k pot would load a previous CF with too low a load if tubed.
Just because we have a triode set up as a CF which gives low Rout 1k,
it doesn't mean we are permitted to use loads lower that the common
cathode gain stage.
Ideally a 47k load on a 1/2 6SN7 should be used for eithe ranode of
cathode load.
Where the THD may have been 0.1% at 1V out with anode loading, it
becomes 0.0066%
when the CF is used, but if the load = 10k, perhaps THD becomes 0.02% at
a volt output.
So thus the gains brought by the CF are eroded if the load is reduced.
The bandwidth won't change at the CF; its Rout = 1/gm in parallel with
RL, or about 500 ohms
and whether RL =10k or 100k does not make any great difference.

Patrick Turner.


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Eeyore wrote:

Andre Jute wrote:

Our legislators managed to criminalize fox-hunting and smoking; when
they will get off their collective fat backside and criminalize
negative feedback?


You wouldn't understand the value of well-applied negative feedback if it stood
up and smacked in you in the face.

Graham


Dear Graham,

Andre comes at you with the mildest of proposterous wit, and you talk of
smackings.

Get thee thither, find a sense of humourator before 'tis two late.

Patrick Turner.
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Mark Aitchison wrote:

west wrote:
I built a typical phono amp with 3 12AX7s... CF... 100k step resistor volume control
,,,My problem is that when I use any of my tube amps, the sound has limited
bandwidth. A SS amp does not exhibit this problem


You're using a 100k 'pot' effectively to drive a cable...
With as little as 1 nF of cable capacitance that forms a low pass filter at 6kHz !


Your answer is what I suspected but not having much experience, I asked this
NG. I was looking for a Tube Basic 101 generic answer that I did not find in
my reference books. I don't understand where Patrick is coming from....


(1) the limited bandwidth problem being only there is presumably due
to the high output impedance (100k step resistors not the same as 100k
potentiometer, which is not the same as a 100k output impedance though -
you might not even need 100pF to get a serious high frequency cut). The
test is to measure (on a scope or whatever, with a low capacitance
probe) directly at the output socket, with no cable connected, what is
going on with the frequency response. The trouble with this test is that
you said it is a phono preamp, so you have to rig up a reverse RIAA
circuit for your signal generator's output. Another, easier, test would
be to shove a 22k resistor or so across the output and then see if the
valve amps are then happy. Presumably the solid state amp's low input
impedance is reducing the effects of the RC filter you make with the
interconnect cable. Or the valve amps have lots of capacitance/Miller
effect at their input and want a lower input impedance (not so likely to
the a significant factor I guess). If the oscilloscope test shows a
high frequency cut is still going on, maybe the RIAA values are wrong
and maybe the SS amp has some slight high frequency lift (some nfb
circuits do this) that is compensating - two wrongs making a right??
(pretty unlikely).

(2) I get the feeling Patrick sometimes growls about the questioners
as a sign of friendship.


Indeed, so GGGRRRRRRR!

If only petioners would apply what they were supposed to have learnt....

I'd like people to not stagger around in the dark.

Patrick Turner.


Mark A

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west wrote:
I built a typical phono amp with 3 12AX7s... CF... 100k step resistor volume control
...My problem is that when I use any of my tube amps, the sound has limited
bandwidth. A SS amp does not exhibit this problem


[re-editing my previous post that contained errers]

My understanding of "step resistor" is the resistance between volume
control positions* - but does "100k step resistor volume control" mean
a volume control made from 100k step resistors (perhaps around 2M
total?!) or "a 100k volume control that contains step resistors"? I
jumped to the conclusion it was the first, which isn't a sensible
conclusion, thinking about it.


You're using a 100k 'pot' effectively to drive a cable...
With as little as 1 nF of cable capacitance that forms a low pass filter at 6kHz !


Your answer is what I suspected but not having much experience, I asked this
NG. I was looking for a Tube Basic 101 generic answer that I did not find in
my reference books. I don't understand where Patrick is coming from....


(1) the limited bandwidth problem being only there _when driving a
valve amp_ is presumably due to the high output impedance (not the same
as a 100k output impedance though - depends on volume setting). The
test is to measure (on a scope or whatever, with a low capacitance
probe) directly at the output socket, with no cable connected, what is
going on with the frequency response. The trouble with this test is that
you have to rig up a reverse RIAA circuit for your signal generator's
output. Another, easier, test would be to shove a 22k resistor or so
across the output and then see if the valve amps are then happier
(albeit with a low volume). If the test shows a high frequency cut is
still going on, maybe the RIAA values are wrong and maybe the SS amp has
some slight high frequency lift (some nfb circuits do this) that is
compensating - two wrongs making a right?? (pretty unlikely).

How much "limited bandwidth" are we talking about, by the way?


*defined at http://ww1.microchip.com/downloads/e...es/01080a.pdf:
The Step resistance. This is the change in resistance that occurs
between two adjacent wiper register values. It is also the RAB
resistance divided by the number of RS resistors (resolution) in the
Resistor Ladder.


Mark A
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Patrick Turner said:


So a 100k pot would offer no audible losses over a 10k pot.



Indeed.


The 10k pot would load a previous CF with too low a load if tubed.
Just because we have a triode set up as a CF which gives low Rout 1k,
it doesn't mean we are permitted to use loads lower that the common
cathode gain stage.



That's why I suggested to put the pot (switched attenuator) in front
of the CF, wit a selector switch if one so desires.

My initial post seems to have been lost in the battle of wits that
followed.
I don't have the stamina to write hundreds of lines of text about
something that can be done with in a single paragraph..........

--

- Maggies are an addiction for life. -
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On Apr 8, 10:16 am, "Andrew Jute McCoy" distilled:

... a bunch of good stuff cribbed from the rest of the thread.


As always.

When are you going to have an original thought or be the 'first' with
good advice? Typically, a snippet here and a snippet there... Ah,
well. Your little Timmee will lap it up and thank you for it. Patrick
gets in in one and is criticized for detail. You say nothing in
highlights...

Let's see if West actually gets it right, and if so, based on which
set of suggestions.

Peter Wieck
Wyncote, PA



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Sander deWaal wrote

That's why I suggested to put the pot (switched attenuator) in front
of the CF, wit a selector switch if one so desires.

My initial post seems to have been lost in the battle of wits that
followed.
I don't have the stamina to write hundreds of lines of text about
something that can be done with in a single paragraph..........


No battle of wits I can see; just the odd foul mouth.

West said he wants to learn, so a discussion around the subject seems
more appropriate than a definitive answer.

The problem with your perfectly correct post for West, perhaps, is
that the phono amp is a special purpose machine, and the "passive pre"
is general purpose, with provision for other inputs. Your solution
would require that the two are entirely combined.

I haven't suggested that West uses a 10k pot. The cathode follower
would prefer the 100k. I use only SS sources and they sound like they
prefer 10k. Sometimes I wonder about this, considering I know sod all
about what the typical SS source is *supposed* to prefer. Should I
stick with the 100k?

The conundrum for me is why West said he prefers an attenuator over a
pre with gain. Clearly he doesn't, considering the problems he is
having. However, that is what he said he wanted, so I saw no point in
offering a solution that negated his first premise.

cheers, Ian


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Sander deWaal wrote:

Patrick Turner said:

So a 100k pot would offer no audible losses over a 10k pot.


Indeed.


In the specified circumstances.


The 10k pot would load a previous CF with too low a load if tubed.
Just because we have a triode set up as a CF which gives low Rout 1k,
it doesn't mean we are permitted to use loads lower that the common
cathode gain stage.


That's why I suggested to put the pot (switched attenuator) in front
of the CF, wit a selector switch if one so desires.


That is in fact the far better solution. I'll be backing that horse too.


My initial post seems to have been lost in the battle of wits that
followed.
I don't have the stamina to write hundreds of lines of text about
something that can be done with in a single paragraph..........


I agree.

Graham

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Peter Wieck wrote:

"Andrew Jute McCoy" distilled:

... a bunch of good stuff cribbed from the rest of the thread.


As always.

When are you going to have an original thought or be the 'first' with
good advice?


Android McCoy hasn't had an original thought in the entire time I've been
posting here.

Graham

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Ian Iveson wrote:

I haven't suggested that West uses a 10k pot. The cathode follower
would prefer the 100k.


On the basis of what principle do you say that ?


I use only SS sources and they sound like they prefer 10k.


Solid state sources will be largely agnostic about the matter until you get down
the to few k ohms or even hundreds of ohms.


Sometimes I wonder about this, considering I know sod all
about what the typical SS source is *supposed* to prefer.


Please specifiy what source and internal style of circuitry if known.


Should I stick with the 100k?


As I posted earlier. 100k is not a great choice for driving cables. It can also
increase the background noise level.

Graham

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Eeyore wrote

I haven't suggested that West uses a 10k pot. The cathode follower
would prefer the 100k.


On the basis of what principle do you say that ?


It's a 12AX7. Look up the curves and draw yourself a 10k loadline.

As I posted earlier. 100k is not a great choice for driving cables.
It can also
increase the background noise level.


It's OK in my case, thank you. We have already explained why.

The idea of a "passive pre" is problematic for valve sources in my
view.

Ian




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Ian Iveson wrote:

Eeyore wrote

I haven't suggested that West uses a 10k pot. The cathode follower
would prefer the 100k.


On the basis of what principle do you say that ?


It's a 12AX7. Look up the curves and draw yourself a 10k loadline.


What good would that be for a cathode follower ?

Graham

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On Apr 7, 5:57 pm, "west" wrote:
I built a typical phono amp with 3 12AX7s, the last tube a cathode follower
(low impedance). It has plenty of gain (70db) and can easily drive any amp
directly. I like using a 100k step resistor volume control instead of a line
amp.
My problem is that when I use any of my tube amps, the sound has limited
bandwidth. A SS amp does not exhibit this problem, if fact the SS sound is
rather impressive to me (for silicone).
With my limited know-how, I'm swagging that an impedance mismatch is the
culprit.
If my contention is correct, is there a simple cure besides using a line
amp? Perhaps some kind of buffer is called for. All comments welcomed and
thank you.


Not exactly an impedance mismatch.

Say an amp (doesn't matter if it's tube or solid state) plus
interconnect cable has an input capacitance of 100pF. This is on the
high end of reasonable but it doesn't take many feet of coax to get
there. And many power amps (especially tube ones) will have a input
circuit that looks like a few hundred k ohms in series with a
capacitor of 50 pF or so.

Then at 20kHz, it will have a reactance of 1/2*pi*f*C, which is 80k.

So if your preamp has an output impedance of 100K, you are a little
more than 3db down at 20kHz.

No need for spice modeling. No need for anything other than pencil and
paper.

If you have a cathode follower as the last stage of your preamp, you
could probably be using a 10K attenuator at the output, but I don't
know what else you might doing to spoil an already low output
impedance stage.

Tim.

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Eeyore wrote:

I haven't suggested that West uses a 10k pot. The cathode
follower
would prefer the 100k.

On the basis of what principle do you say that ?


It's a 12AX7. Look up the curves and draw yourself a 10k loadline.


What good would that be for a cathode follower ?



The characteristic curves don't care about circumstance.

You would have to make a guess at West's operating point, since he
appears to be keeping it a secret.

You might consider, having drawn your line, whether a 12AX7 is
appropriate for driving a long cable in any case.

My opinion is that a CF is a clumsy instrument, and the more the gain,
and the less the current, the clumsier it becomes.

Ian


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"Ian Iveson" wrote in
message k
Eeyore wrote:

I haven't suggested that West uses a 10k pot. The
cathode follower
would prefer the 100k.

On the basis of what principle do you say that ?


It's a 12AX7. Look up the curves and draw yourself a
10k loadline.


What good would that be for a cathode follower ?


The characteristic curves don't care about circumstance.

You would have to make a guess at West's operating point,
since he appears to be keeping it a secret.


You might consider, having drawn your line, whether a
12AX7 is appropriate for driving a long cable in any case.


My opinion is that a CF is a clumsy instrument, and the
more the gain, and the less the current, the clumsier it
becomes.


When does a CF have any gain at all?

A CF built on a 12AX7 is a bit of a straw man. Why use a hi-mu triode for a
stage that *always* has less than unity gain?

Looking at good quality circuits of sucessful commercial designs, a 12AU7
seems to be more to the point.


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"Eeyore" wrote in
message
Peter Wieck wrote:

"Andrew Jute McCoy" distilled:

... a bunch of good stuff cribbed from the rest of the
thread.


As always.

When are you going to have an original thought or be the
'first' with good advice?


Android McCoy hasn't had an original thought in the
entire time I've been posting here.


The style of writing, if you call what McCoy does *style*, seems origional
enough. ;-)


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