I'm sure a volume pot is not ever going to be an "ideal" resistor. Do people here have a favourite pot? (Less noisy or more reliable as far as not getting scratchy) Or does the stepped attenuator have less noise and rule the day?
Also pretty much every guitar amp on the planet places it's volume pot after the first valve. I would assume to reduce the noise introduced from said pot which Popilin showed so nicely in his first post.
Also pretty much every guitar amp on the planet places it's volume pot after the first valve. I would assume to reduce the noise introduced from said pot which Popilin showed so nicely in his first post.
Hi MelB
Nice to see you around here !
No resistive compound is exempt from thermal noise, given by the equation of post#1, only depends on the resistance value, not its composition.
I love my Alps Black Beauty, is made of a resistive polymer which minimizes other types of noise.
What makes a good pot so expensive, is its precision, which is harder if it is a dual device.
For the rest, there are no major differences with a stepped attenuator.
I prefer the pot, have that feeling that makes it so enjoyable, and without breaks.
Among the specs of my Alps, appears the momentum necessary for operation !
Thanks for your feedback and kind words, encourages me to continue the thread.
Nice to see you around here !
No resistive compound is exempt from thermal noise, given by the equation of post#1, only depends on the resistance value, not its composition.
I love my Alps Black Beauty, is made of a resistive polymer which minimizes other types of noise.
What makes a good pot so expensive, is its precision, which is harder if it is a dual device.
For the rest, there are no major differences with a stepped attenuator.
I prefer the pot, have that feeling that makes it so enjoyable, and without breaks.
Among the specs of my Alps, appears the momentum necessary for operation !
Thanks for your feedback and kind words, encourages me to continue the thread.
OK, if you prefer, you can think it's just marketing.
As we have seen in the simulations and other estimates, the volume pot and ECC82, contributing almost 92% of the total noise, although it is a noisy valve, its contribution is of the same order as the pot.
Using high transconductance valves, and the final scheme of connection (3), valve noise is negligible compared to the pot noise, being valid the result for the last simulation of post#1.
Do not ask me to do the math on a weekend, my religion forbids me.
If it bothers you is the title of the thread, no problem, you can propound another and we ask the moderators to change it.
Most guitars have a volume pot built in, which itself is a source of noise, so that's not why. Think again...
In fact neither statement is entirely true. I have checked the thread carefully and can only find references to calculating the thermal noise of the resistor and the old 2.5/gm chestnut for the tube noise. It is not clear how you determined this is 92% of the total noise. However:
1. In the case with the pot between stages you have ignored the source impedance feeding the first tube. There would be no point using a 100k pot if this source was not of the order of 20k in which case the noise is no better than with the pot at the front. If the source is a lot less than 20k you don't need a 100k pot.
2. Tube noise is much worse than you think. 2.5/gm only applies to shot noise and only at rf. At audio frequencies flicker noise is at least as important and gets less as Ia is reduced, the exact opposite of shot noise. Minimising tube noise is not therefore simply a matter of choosing a high gm tube and winding up the idle current.
3. I have yet to measure a real cathode follower driven by a non zero source impedance that has less than 80uV output noise in a 20KHz bandwidth.
Cheers
Ian
Back in post 18 I said:ruffrecords said:
It surprises me just how common is this myth that high gm guarantees low audio noise. I can't remember where I saw it, but someone did some measurements comparing 12AY7 (gm around 2mA/V?) and 6DJ8 (around 12mA/V?) and found very similar audio noise, with the 12AY7 being slightly better.DF96 said:
The funny thing is, 2.5/gm for triodes (which is only a rough approximation anyway) is most useful in the very region (IF/HF) where almost everyone uses pentodes - so partition noise dominates. Below that flicker noise dominates. Above that grid noise dominates.
As I said before, the analyzes are only conceptual, and a reasonable approximation can be used in most cases.
Doing again the third simulation, of post#1 for a 20KHz bandwidth, we obtain
pot noise ≈ 35μV RMS, other sources noise ≈ 5μV RMS
Using
N = √[∑(Ni)^2]
And the wrong estimate of valve noise of post#16, we obtain
N ≈ 94μV RMS
So, noise is dominated by valve noise and pot noise, about 95%
I'll tell for a third time, for simplicity, we assumed a zero source impedance, in which case all simulations were made.
If it is necessary to place a source of about 20K, such as a phono stage, things can only get worse.
Leaving aside the fact that not everyone can afford to have a separate phono stage, and I say this for the second time, a 100K pot prevents us using an additional CF before pot, in approach (1).
The latter approach (3), is always the most favored, and I say it for the second time, that between the two stages where is located the pot there are high voltages, which require the use of coupling capacitors, again, a 100K pot avoid us using huge capacitors, regardless of the source impedance for the first stage.
If you put a 10K pot and 30μF capacitors, is fine, but it doesn't seems good engineering.
An implicit detail that is quite obvious, in this configuration is necessary to reduce the gain of the first stage, in order to reduce distortion, so a 20K source has less impact on noise.
Also tell for the fourth time, that the only decent pot I own, is a 100K Alps Black Beauty, which is the cause that motivated me to notice the problem and start this thread.
My Tarzan-English does not help much, but neither need a cryptographer.
This has been already clarified by DF96 in post#18, Morgan Jones also makes clear in his "Valve Amplifiers" third edition, at page 534 sic "Unfortunately, there is no way of predicting flicker noise for a valve as it is highly sample dependent, although its level is directly related to the level of thermal noise for a given valve".
Curiously, on page 533, he uses the same old 2.5/gm chestnut for triode noise.
Assuming that electrons are fermions, using the Fermi-Dirac distribution, along with the addition theorem of spherical harmonics...no, just kidding.
The only reference to noise, I could find is
The Sound of Silence: Lowest-Noise Riaa Phono-Amps: Designer's Guide - Burkhard Vogel - Google Libros
Which gives us for the ECC82, on a 20KHz bandwith, something like Vn ≈ 6.4μV RMS, well below the estimate of post#16.
Redoing calculations
N ≈ 37μV RMS
Now pot noise predominates, about 95%.
This result is consistent with some known line stages based on the ECC82.
I just own a multimeter and an old 15MHz oscilloscope, so I can not measure noise.
However a CF asisted by a CCS could help, or even better, Allen Wright SLCF.
This is enlightening, but also I have a good alibi.
I am gullible as a kid, maybe I should ask Morgan Jones to return my money.
Vogel's treatment of noise is quite good.
FWIW, Mullard's measurements of EF86 at 1.25mA anode current are about 2uV over 25Hz-10kHz (equivalent to 2.8uV over 25-20k). That's quite quiet for a pentode! As MJ points out, this is still somewhat higher than the RF-based equations predict, but still the same order of magnitude. If I remember correctly, Vogel ended up with a triode ENR equation which was roughly double that of the RF equation, i.e., something like 5/gm.
As a practical matter, I don't ever use phono stages with 20k output impedances, and I've never found the noise from a 50-100k pot in a tube line amp to be an issue. If you're concerned about it, use a transformer volume control.
Yes, right? I love it, unfortunately I can't read the whole thing, it's just a sample.
BTW. Morgan Jones third edition is one of my favorite books.
I have read that section several times, and predictions are astonishing.
I have seen something like 3/gm.
Taking as a reference to Allen Wright, and your His Master's Noise, I would never do that, just said that to support my point, but keep it quiet.
I don't get along great with transformers, is also out of my budget.
<snip>
I don't get along great with transformers, is also out of my budget.
As big fan of good transformers (my system typically has 4 in the path, and never less than 3 regardless of source) I do have to say a mediocre transformer will be worse than no transformer at all..
As big fan of good transformers (my system typically has 4 in the path, and never less than 3 regardless of source) I do have to say a mediocre transformer will be worse than no transformer at all..
I agree with every word, and of memories to me left my last five transformers, more than 80 windings, handmade, during three months.
I hate transformers!
For a full treatment of noise in tubes, including flicker noise, I would recommend chapter 8 of 'Amplifying Devices and Low Pass Amplifier Design' by E.M. Cherry and D.E. Hooper. Copies are expensive and hard to come by so I would recommend getting your local library to obtain a copy for you.
Cheers
Ian
Cheers
Ian
Hi Ian
The scope of our actions is often unexpected, maybe you never imagined that your posts and knowledge you helped me on the other side of the world, always with a clear and friendly style.
It is for this reason that you are among the superheroes of the forum, along with Wavebourn, SY, DF96, Salas and some other, who knows excuse my forgetfulness.
As well I have noted, noise is a rather complex issue, thank you very much for your advice.
Thank you for the kind words. It is really nice to know that your efforts are appreciated.
Unfortunately there is very little information on tube noise in the audio band and even less in data sheets. One of the few tube data sheets I have found that does have this information is the E83F. It's Req at RF is 1000 ohms max but in the audio band up to 10KHz it is 30K. OK it is a pentode so partition noise is a big contributor but the difference is 15dB. It's data sheet is here:
E83F @ The National Valve Museum
Cheers
Ian
Rise time is inversely proportional to bandwidth, so a phono cartridge which can go up to 70 KHz must be pretty fast. IMHO.
With a properly designed EQ adding the missing 3.18 µs time constant, you can go easily to 10 Hz - 50 KHz (± 0.1 dB), so I don't see the problem.
RIAA curves place the sensitivity of a flat cartridge at least 40db down at 20 khz from 20hz.
You are correct that in having a mechanical structure that can resolve 50khz is a very good thing.
When youdid the volume pot noise tests, Did you leave the amplifier input impedance at normal value or short the amplifier input to ground?
You are correct that in having a mechanical structure that can resolve 50khz is a very good thing.
When youdid the volume pot noise tests, Did you leave the amplifier input impedance at normal value or short the amplifier input to ground?
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