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Old 2nd April 2014, 07:12 AM   #471
waltube is offline waltube  Italy
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Hi,

the use of the formulas is just to have an idea of what can happen.
But ,in real world, different tubes of same type can give you big differences on noise indipendently form Va.
The way to follow for this type of stuff is high gm, low Rp; regarding the mu you can play with trafos ( with certain limits) that must be with a high ratio so you can save a good s/n.
Also the use of regulated power supply is mandatory ( or battery).
Check also a Cag of a tube, because you can find an high Gm tube with a high mu (6C45 p.e.) but with an high value, it will be in parallel with the secondary of the trafo.

W
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Old 2nd April 2014, 08:31 AM   #472
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Quote:
Originally Posted by mm7
if we take mu as a constant, gm~=mu*Ia/Va, then Vfn should depend mostly on Va, not Ia, which is quite counter-intuitive.
As I said, you are combining approximations. The formula I gave for gm gives an estimate; I don't claim that it is accurate or gives the right variation with current or voltage. Formulas for noise often assume that the valve is working somewhere near its 'normal' regime, and are themselves usually only estimates. Hence you can't make the deduction you are making. Experiment is the way forward for valve noise, and you will then find a huge variation between samples and a variation with ageing.

Thermal (white) noise can be accurately calculated. So can temperature-limited shot noise (rare in audio circuits!) and partition noise. All other noise has to be measured, although rough estimates can be calculated. Don't expect to be able to calculate an optimum bias point for noise.
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Old 2nd April 2014, 08:51 AM   #473
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Quote:
Originally Posted by mm7 View Post
may be. but according to the formula Click the image to open in full size. in the article http://protubeaudio.webs.com/tubenoise.htm, the flicker noise (Vfn) is defined by its second part, that depends on Ia and 1/gm. and if we take mu as a constant, gm~=mu*Ia/Va, then Vfn should depend mostly on Va, not Ia, which is quite counter-intuitive.

I see I should simply try it and measure.
I think you made the same mistake twice. When you write gm~=mu*Ia/Va you are assuming that ra = Va/Ia but it is actually dVa/dIa, in other words it is the dynamic resistance of the tube which is the SLOPE of the curve at the point Va, Ia, not the values of Va divided by Ia.

Cheers

Ian
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Old 2nd April 2014, 09:01 AM   #474
DF96 is online now DF96  England
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I fear he has misunderstood the meaning of a formula I suggested.
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Old 2nd April 2014, 11:12 AM   #475
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Quote:
Originally Posted by mm7 View Post
I read about one exotic preamp with a dozen(!) or so 6S45P in parallel.
Review: NVO SPA one Phono Preamplifier | Confessions of a Part-Time Audiophile
nvoaudio
Does it make sense to look in this direction?
Actually the NVO SPA one has only one 6S45P per channel at its input.
The bigger SPA-II has a parallel pair per channel
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Old 2nd April 2014, 01:21 PM   #476
mm7 is offline mm7  Canada
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Quote:
Originally Posted by DF96 View Post
I fear he has misunderstood the meaning of a formula I suggested.
I understand difference between dVa/dIa and Va/Ia.
I just cannot get dVa/dIa in LTSpice simulation. My feeling is, as soon as noise depends on relative values of I or V or both, then it should depend on absolute values too. I understand that K of dependency may (and does) vary depending on where we are on the curve slope.
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Old 2nd April 2014, 01:24 PM   #477
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Hi,

What is the desired S/N ratio that is required for such a mic pre?

Personally I have a MC stage using two twin triode ECC88s per channel. It's dead quiet but it won't beat a solid state design in that department.
The schematic is in the "Analogue Source" section of the forum and the last pages of Salas' Itch phono preamp discuss it as well.

I know of two designers (besides NVO) in the entire who use a raft of triodes (26 to over 50) in a distributed gain arrangement to get the noise floor down enough.

Ciao,
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Old 2nd April 2014, 02:24 PM   #478
waltube is offline waltube  Italy
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Time ago I have built an 8 channel, full balanced, mic tube amp.
I used a trafo in input, 26 dB from Sowter, and one 6922 for each channel.
To reach a very good s/n I used a 12 batteries of 12 Vdc /2 A each one to supply the anodes + 2 batteries of 6 Vdc-12 A, in parallel, for filaments.
The box of power supply weights about 32 kg.
This to have a reasonable time to recording a session of about 3 hrs.
Each tube was selected (Siemens gold pin) for noise and gain and the sockets was the venerable McMurdo with a gummy suspension (for microphonics).
This was the only way to get around -80 dB of s/n with a total gain of about 56-58 dB.

Walter
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Old 2nd April 2014, 02:30 PM   #479
mm7 is offline mm7  Canada
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Quote:
Originally Posted by mm7 View Post
Then flicker noise will be
Vfn = Ia/gm * 2.6 = Ia / (mu * Ia / Va) * 2.6 = Va / mu * 2.6

So it mainly correlates with Va. Is it correct?
Quote:
Originally Posted by ruffrecords View Post
I think you made the same mistake twice. When you write gm~=mu*Ia/Va you are assuming that ra = Va/Ia but it is actually dVa/dIa, in other words it is the dynamic resistance of the tube which is the SLOPE of the curve at the point Va, Ia, not the values of Va divided by Ia.
I just added rough estimation of Gm to your formula. This rough estimation of Gm was advised by DF96,
Quote:
Originally Posted by DF96
No. gm = dIa/dVg, at constant Va. You could get a rough estimate of gm from mu x Ia/Va.
and it should not be reversed as ra = Va/Ia, it should also include mu in this case, to bring it to a proper "slope" on the curve.

I cannot show it in formulae, but I have a feeling that a noise does not depend on dV or dI. It should depend on absolute values of V or I. It is like noise of rain, it depends on an energy of droplets, speed (V) and mass(E) of droplets, and number of droplets per second (I). But it does not much depend (in total, or in average) on some variations of I or V (dV or dI) during gusts of rain.
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Old 2nd April 2014, 02:51 PM   #480
mm7 is offline mm7  Canada
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Quote:
Originally Posted by fdegrove View Post
Hi,
What is the desired S/N ratio that is required for such a mic pre?
Personally I have a MC stage using two twin triode ECC88s per channel. It's dead quiet but it won't beat a solid state design in that department.
Requirement is "as quiet as possible"
It should be quiter than MC preamp.
LOMC generates 300uV signal. Ribbon mic gives 30uV after 1:37 transformer, for 60dB sound. Thanks I'll take a look at your schematic.

To waltube.
My B+ is filtered by 4 RC stages that remove ripple completely. Heater is feed from separate PSU (switch mode). There is no noise from it too. Both PSUs are remote from Preamp. If I unplug PSU from preamp, there is no change in noise level while it runs on energy from the last RC stage.
I also thinking about suspended tube sockets, because microphonics is significant. I hear ringing and I see disturbance around 16kHz area for couple of seconds after I knock a tube.
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