Reply
 
Thread Tools Display Modes
  #1   Report Post  
Posted to rec.audio.pro
[email protected][_2_] seanbroderick20003@yahoo.com[_2_] is offline
external usenet poster
 
Posts: 26
Default Simple Test Circuit for Tube Noise

Can anyone suggest a simple circuit to measure vacuum tube noise? How
about negative (emission) grid current? Also, what are some of the
quieter tubes out there? Thanks!

Sean Broderick
  #2   Report Post  
Posted to rec.audio.pro
Scott Dorsey Scott Dorsey is offline
external usenet poster
 
Posts: 16,853
Default Simple Test Circuit for Tube Noise

wrote:
Can anyone suggest a simple circuit to measure vacuum tube noise?


There are some details in the Radiotron Handbook. You might also find
some information in a VERY early version of Motchenbacher's book on low
noise electronics.

However, you're better off testing in-circuit, or in a jig that emulates
the circuit that the tubes will be used in, because the different noise
sources are of different importance in different applications.

How
about negative (emission) grid current?


If you ever have to worry about grid current in an audio circuit, something
has gone horribly wrong. From the standpoint of the input, the tube is a
big capacitor. The only time the tube ever draws current is when the Miller
capacitance is so high that it's acting like a capacitive load on the input.
You might see that in a big power tube occasionally, but that's the last place
to worry about noise.

Also, what are some of the
quieter tubes out there? Thanks!


For what application? Part of the problem is that the input and output loads
have a lot to do with noise performance. Part of the problem is that some
applications have big issues with microphonics and others don't. And if you
are worried about RF noise, all the rules change.

In general, I'll point out that for low level signals with minimal drive,
nuvistors are hard to beat. And there are some really impressive compactrons
out there; people ignore them because they are viewed as TV tubes, but that
means the prices are low and the numbers are good. And in a pinch there is
always the EF86, the 6DJ8, and other frame grid types.

A lot of it has to do with quality of construction and cleanliness of
assembly too. There are very quiet 6AU6 tubes out there... but there are
also lots of very, very noisy ones.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #3   Report Post  
Posted to rec.audio.pro
Mark Mark is offline
external usenet poster
 
Posts: 966
Default Simple Test Circuit for Tube Noise

On Sep 10, 5:02*pm, (Scott Dorsey) wrote:
wrote:
Can anyone suggest a simple circuit to measure vacuum tube noise? *



if you want low noise, why are you using tubes...

just asking...


Mark


  #4   Report Post  
Posted to rec.audio.pro
PStamler PStamler is offline
external usenet poster
 
Posts: 882
Default Simple Test Circuit for Tube Noise

On Sep 10, 4:16*pm, Mark wrote:
On Sep 10, 5:02*pm, (Scott Dorsey) wrote:

wrote:
Can anyone suggest a simple circuit to measure vacuum tube noise? *


if you want low noise, why are you using tubes...


It's entirely possible to get low noise from tubes, IF you do the
design right and if the source impedance is appropriate (high).

As a rule, the quietest performance will be found in tubes with high
gm (mutual conductance); of course, since gm varies with operating
conditions, you need to be cognizant of them. There are typically four
noise sources in a triode circuit: the source resistance at the input,
the cathode resistor (if unbypassed), the plate resistor, and the tube
itself, which has an equivalent noise resistance of 2.5 / gm, where gm
is measured in mhos -- er, Siemenses. The contribution of the plate
resistance is Rp divided by the square of the tube's gain. Add that,
the tube's equivalent input noise resistance, and the cathode
resistance, again if unbypassed. If the total is 1/4 or less of the
source resistance, the circuit will add 1dB or less to the inherent
noise of the source resistance.

In theory. In practice, there are other issues, some of which Scott
has discussed above. But, if the source resistance is appropriate
(either inherently high, or made so with a transformer), tube circuits
can be as quiet as, say, a 5534 opamp.

Peace,
Paul
  #5   Report Post  
Posted to rec.audio.pro
Scott Dorsey Scott Dorsey is offline
external usenet poster
 
Posts: 16,853
Default Simple Test Circuit for Tube Noise

Mark wrote:
On Sep 10, 5:02=A0pm, (Scott Dorsey) wrote:
wrote:
Can anyone suggest a simple circuit to measure vacuum tube noise? =A0



if you want low noise, why are you using tubes...

just asking...


Freedom from overload?
--scott

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


  #6   Report Post  
Posted to rec.audio.pro
Scott Dorsey Scott Dorsey is offline
external usenet poster
 
Posts: 16,853
Default Simple Test Circuit for Tube Noise

PStamler wrote:

In theory. In practice, there are other issues, some of which Scott
has discussed above. But, if the source resistance is appropriate
(either inherently high, or made so with a transformer), tube circuits
can be as quiet as, say, a 5534 opamp.


Unfortunately, the thermal noise in the transformers is sometimes a
bigger worry than the noise of the tubes themselves.... and certainly
the interwinding capacitance of the transformer is always a bigger
destroyer of bandwidth than the tube stage itself.

I'm a big fan of tubes, it's the transformers I don't like...
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #7   Report Post  
Posted to rec.audio.pro
PStamler PStamler is offline
external usenet poster
 
Posts: 882
Default Simple Test Circuit for Tube Noise

On Sep 10, 8:56*pm, (Scott Dorsey) wrote:
PStamler wrote:

In theory. In practice, there are other issues, some of which Scott
has discussed above. But, if the source resistance is appropriate
(either inherently high, or made so with a transformer), tube circuits
can be as quiet as, say, a 5534 opamp.


Unfortunately, the thermal noise in the transformers is sometimes a
bigger worry than the noise of the tubes themselves.... *and certainly
the interwinding capacitance of the transformer is always a bigger
destroyer of bandwidth than the tube stage itself.

I'm a big fan of tubes, it's the transformers I don't like...


Use the right transformer, and it works fine. Try the Jensen JT-13k7;
the thermal noise only adds 1 dB, and the bandwidth is good. It's the
tube used in Dan Kennedy's original preamp design, which is very clean
and neutral. It turns out the same transformer does a fine job with a
vacuum tube.

Peace,
Paul
  #8   Report Post  
Posted to rec.audio.pro
Scott Dorsey Scott Dorsey is offline
external usenet poster
 
Posts: 16,853
Default Simple Test Circuit for Tube Noise

PStamler wrote:
Use the right transformer, and it works fine. Try the Jensen JT-13k7;
the thermal noise only adds 1 dB, and the bandwidth is good. It's the
tube used in Dan Kennedy's original preamp design, which is very clean
and neutral. It turns out the same transformer does a fine job with a
vacuum tube.


That's about as good as you can get, yeah!
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #9   Report Post  
Posted to rec.audio.pro
Mark Mark is offline
external usenet poster
 
Posts: 966
Default Simple Test Circuit for Tube Noise

On Sep 10, 5:45*pm, PStamler wrote:
On Sep 10, 4:16*pm, Mark wrote:

On Sep 10, 5:02*pm, (Scott Dorsey) wrote:


wrote:
Can anyone suggest a simple circuit to measure vacuum tube noise? *


if you want low noise, why are you using tubes...


It's entirely possible to get low noise from tubes, IF you do the
design right and if the source impedance is appropriate (high).



true enough,

but if you put an equal amount of effort into the design and impedance
using a low noise FET, it will beat the tube hands down...

Mark

  #10   Report Post  
Posted to rec.audio.pro
Scott Dorsey Scott Dorsey is offline
external usenet poster
 
Posts: 16,853
Default Simple Test Circuit for Tube Noise

Mark wrote:

true enough,

but if you put an equal amount of effort into the design and impedance
using a low noise FET, it will beat the tube hands down...


Not really. There's a really nice paper from Marshall Leach on the subject,
too. It's probably referenced on his website.

Tubes and FETs come out about neck and neck, with similar transformers.
Transformerless bipolar input stage comes out hands down, though, assuming
600 ohm source impedance.

That said, Fred Forssell makes a transformerless tube preamp that really
is shockingly quiet. It's not as quiet as the Great River or the Millennia
Media, but it's way more quiet than it has any right to be.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."


  #11   Report Post  
Posted to rec.audio.pro
PStamler PStamler is offline
external usenet poster
 
Posts: 882
Default Simple Test Circuit for Tube Noise

On Sep 11, 8:54*pm, (Scott Dorsey) wrote:
Mark wrote:

true enough,


but if you put an equal amount of effort into the design and impedance
using a low noise FET, it will beat the tube hands down...


Not really. *There's a really nice paper from Marshall Leach on the subject,
too. *It's probably referenced on his website.

Tubes and FETs come out about neck and neck, with similar transformers.
Transformerless bipolar input stage comes out hands down, though, assuming
600 ohm source impedance.


When you're looking at attainable noise figures of 2 dB over the
inherent noise of the microphone, which you can do with a tube or a
FET and the right transformer, there's not a lot of hands-down
improvement possible. In fact, there's only 2dB of improvement
possible, max, unless you dip the mic into liquid helium.
Realistically, more like 1dB, which is barely noticeable.

Modern designs, with any of the three technologies, are scraping
pretty close to the theoretical limits.

Peace,
Paul
  #12   Report Post  
Posted to rec.audio.pro
Mark Mark is offline
external usenet poster
 
Posts: 966
Default Simple Test Circuit for Tube Noise

On Sep 12, 2:30*am, PStamler wrote:
On Sep 11, 8:54*pm, (Scott Dorsey) wrote:

Mark wrote:


true enough,


but if you put an equal amount of effort into the design and impedance
using a low noise FET, it will beat the tube hands down...


Not really. *There's a really nice paper from Marshall Leach on the subject,
too. *It's probably referenced on his website.


Tubes and FETs come out about neck and neck, with similar transformers.
Transformerless bipolar input stage comes out hands down, though, assuming
600 ohm source impedance.


When you're looking at attainable noise figures of 2 dB over the
inherent noise of the microphone, which you can do with a tube or a
FET and the right transformer, there's not a lot of hands-down
improvement possible. In fact, there's only 2dB of improvement
possible, max, unless you dip the mic into liquid helium.
Realistically, more like 1dB, which is barely noticeable.

Modern designs, with any of the three technologies, are scraping
pretty close to the theoretical limits.

Peace,
Paul


I agree that an FET can achieve a 2 dB noise figure and that means an
FET can be 2 dB away from as good as it gets at room temperature...


Can you achieve a 2 dB noise figure with a 12AX7? Or are you talking
about some special tube?



Mark


  #13   Report Post  
Posted to rec.audio.pro
PStamler PStamler is offline
external usenet poster
 
Posts: 882
Default Simple Test Circuit for Tube Noise

On Sep 12, 5:27*pm, Mark wrote:
On Sep 12, 2:30*am, PStamler wrote:



On Sep 11, 8:54*pm, (Scott Dorsey) wrote:


Mark wrote:


true enough,


but if you put an equal amount of effort into the design and impedance
using a low noise FET, it will beat the tube hands down...


Not really. *There's a really nice paper from Marshall Leach on the subject,
too. *It's probably referenced on his website.


Tubes and FETs come out about neck and neck, with similar transformers.
Transformerless bipolar input stage comes out hands down, though, assuming
600 ohm source impedance.


When you're looking at attainable noise figures of 2 dB over the
inherent noise of the microphone, which you can do with a tube or a
FET and the right transformer, there's not a lot of hands-down
improvement possible. In fact, there's only 2dB of improvement
possible, max, unless you dip the mic into liquid helium.
Realistically, more like 1dB, which is barely noticeable.


Modern designs, with any of the three technologies, are scraping
pretty close to the theoretical limits.


Peace,
Paul


I agree that an FET can achieve a 2 dB noise figure and that means an
FET can be 2 dB away from as good as it gets at room temperature...

Can you achieve a 2 dB noise figure with a 12AX7? *Or are you talking
about some special tube?


It's a little hard with a 12AX7, but it can be done with a 12AU7 or a
6SN7, neither of which is exotic. The main thing is to use a
transformer with low noise, like the Jensen 13k7 I mentioned earlier.
Oh, and run the tube at a highish current to maximize gm (lower noise)
and minimize the value of the cathode resistor.

Peace,
Paul
  #14   Report Post  
Posted to rec.audio.pro
Don Pearce[_3_] Don Pearce[_3_] is offline
external usenet poster
 
Posts: 2,417
Default Simple Test Circuit for Tube Noise

On 13 Sep 2010 16:36:26 -0400, (Scott Dorsey) wrote:

wrote:

The 33 Meg resistor is being bootstrapped to at least 330 Meg. The
bootstrap is direct coupled. The output Z of the capsules I'm using
is ~50 Meg at 40 Hz.


How did you get that number? What is the capacitance of the capsule when
idle?

I'm getting plenty of low end, and no obvious
distortion. The problem with a higher value grid resistor is the bias
becomes drifty. The U47 used a 60 Meg grid leak, which I think is too
low, but there was no bootstrapping in that case.


As long as the leak resistance is low enough to permit all the spurious
charge appearing on the grid to be grounded out, you're fine. The amount
of spurious charge that appears there is dependant on the kind of tube you
use and it's one of the more important things to select when you pick a
tube for microphone applications.

Are 1 or 2 Gigohm thick film resistors quiet enough in this
application? I can't find metal film above about 50 Meg.


Read the previous message. The capsule is bypassing them, it doesn't
matter how noisy they are.


Oh it does. The same RC pair that is lowpass to the bias and its
associated noise is highpass to the wanted signal. In the region of
crossover (ok cutoff for both) any resistor noise will be coming
through loud and clear.

d
  #15   Report Post  
Posted to rec.audio.pro
[email protected][_2_] seanbroderick20003@yahoo.com[_2_] is offline
external usenet poster
 
Posts: 26
Default Simple Test Circuit for Tube Noise

On Sep 13, 3:36*pm, (Scott Dorsey) wrote:
wrote:

The 33 Meg resistor is being bootstrapped to at least 330 Meg. *The
bootstrap is direct coupled. *The output Z of the capsules I'm using
is ~50 Meg at 40 Hz.


How did you get that number? *What is the capacitance of the capsule when
idle?


Capsule capacitance is about 80 pF. So its output Z it just its
capacitive reactance, right?

As long as the leak resistance is low enough to permit all the spurious
charge appearing on the grid to be grounded out, you're fine. *The amount
of spurious charge that appears there is dependant on the kind of tube you
use and it's one of the more important things to select when you pick a
tube for microphone applications.


What do you mean by "spurious charge"? Do you mean input current?


Sean B


  #16   Report Post  
Posted to rec.audio.pro
Scott Dorsey Scott Dorsey is offline
external usenet poster
 
Posts: 16,853
Default Simple Test Circuit for Tube Noise

wrote:
On Sep 13, 3:36=A0pm, (Scott Dorsey) wrote:
wrote:

The 33 Meg resistor is being bootstrapped to at least 330 Meg. =A0The
bootstrap is direct coupled. =A0The output Z of the capsules I'm using
is ~50 Meg at 40 Hz.


How did you get that number? =A0What is the capacitance of the capsule wh=

en
idle?


Capsule capacitance is about 80 pF. So its output Z it just its
capacitive reactance, right?


I'm not sure if you can assume that, but it would make sense. 80 pF is
a pretty high capacitance capsule and you can probably get away with a higher
Z than most with it.

As long as the leak resistance is low enough to permit all the spurious
charge appearing on the grid to be grounded out, you're fine. =A0The amou=

nt
of spurious charge that appears there is dependant on the kind of tube yo=

u
use and it's one of the more important things to select when you pick a
tube for microphone applications.


What do you mean by "spurious charge"? Do you mean input current?


No, the whole point of the leak resistor is to drain off the charge that
appears spuriously on the grid..... as electron flow occurs inside the tube,
the grid slowly picks up a charge. As long as there is a path to ground
for that charge, everything is fine... but if the grid is completely
disconnected it will gradually charge up toward the plate value.

The value of the leak resistor required is determined by the degree to
which this happens. There is a good discussion in the Radiotron Handbook.

Don't think about input current... it's really not a useful concept here
anyway.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #17   Report Post  
Posted to rec.audio.pro
[email protected][_2_] seanbroderick20003@yahoo.com[_2_] is offline
external usenet poster
 
Posts: 26
Default Simple Test Circuit for Tube Noise

On Sep 14, 8:48*am, (Scott Dorsey) wrote:
wrote:
On Sep 13, 3:36=A0pm, (Scott Dorsey) wrote:
wrote:


The 33 Meg resistor is being bootstrapped to at least 330 Meg. =A0The
bootstrap is direct coupled. =A0The output Z of the capsules I'm using
is ~50 Meg at 40 Hz.


How did you get that number? =A0What is the capacitance of the capsule wh=

en
idle?


Capsule capacitance is about 80 pF. *So its output Z it just its
capacitive reactance, right?


I'm not sure if you can assume that, but it would make sense. *80 pF is
a pretty high capacitance capsule and you can probably get away with a higher
Z than most with it.

As long as the leak resistance is low enough to permit all the spurious
charge appearing on the grid to be grounded out, you're fine. =A0The amou=

nt
of spurious charge that appears there is dependant on the kind of tube yo=

u
use and it's one of the more important things to select when you pick a
tube for microphone applications.


What do you mean by "spurious charge"? *Do you mean input current?


No, the whole point of the leak resistor is to drain off the charge that
appears spuriously on the grid..... as electron flow occurs inside the tube,
the grid slowly picks up a charge. *As long as there is a path to ground
for that charge, everything is fine... but if the grid is completely
disconnected it will gradually charge up toward the plate value.

The value of the leak resistor required is determined by the degree to
which this happens. *There is a good discussion in the Radiotron Handbook.

Don't think about input current... it's really not a useful concept here
anyway.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."



Apparently you're right about enlarging the grid resistor, but for a
different reason. A bigger grid/charge resistor results in LOWER
noise. How's that for counter-intuitive? My beef with my prototype
is noise, so hopefully this is the answer. I'll let y'all know.
Check out the paper below, page 2:

http://www.etymotic.com/publications/erl-0053-1967.pdf


Sean B
  #19   Report Post  
Posted to rec.audio.pro
[email protected][_2_] seanbroderick20003@yahoo.com[_2_] is offline
external usenet poster
 
Posts: 26
Default Simple Test Circuit for Tube Noise

On Sep 14, 1:08*pm, (Scott Dorsey) wrote:
wrote:

Apparently you're right about enlarging the grid resistor, but for a
different reason. *A bigger grid/charge resistor results in LOWER
noise. *


It's not lower noise, it's more signal.


I'm surprised by this. I thought once the input Z is about 4 times
the source Z, there is little to be gained in level by raising the
input Z further.

Sean B
Reply
Thread Tools
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Simple Audio Test Chamber Jerry Steiger Pro Audio 31 July 30th 08 06:24 AM
Power Circuit Noise? Dan Tech 9 May 15th 05 03:55 AM
Ambient Noise - Volume Adjustment Intercom Circuit zipzit Tech 5 October 5th 04 04:34 AM
Simple tube compressor schematic Patrick Turner Vacuum Tubes 0 April 1st 04 01:39 AM
Oscilloscope for Tube circuit analysis? Revolvr Vacuum Tubes 18 October 12th 03 04:23 AM


All times are GMT +1. The time now is 08:26 PM.

Powered by: vBulletin
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 AudioBanter.com.
The comments are property of their posters.
 

About Us

"It's about Audio and hi-fi"