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