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In Reply to: RE: LM317 as a CCS , how to? posted by Alex Kitic on October 26, 2014 at 00:04:20
"The cap is not going to be efficient against potential noise from the zener because the zener might introduce noise that is rather high in frequency for the cap to filter."
I don't agree. Zener noise decreases with frequency, whereas the XC of the cap goes down. A single cap can easily reduce Zener noise by a factor of ten, and it's performance does not suffer at greater offsets. If anything, the difficulty lies in attenuating noise close-in (low frequency). I'm very familiar with this as part of my day job.
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Buy Chinese. Bury freedom.
Follow Ups:
While I remain with what I already stated - just like I stated, I have nothing against experiments and real-life trials of hypothesized solutions.
In this case, I guess it is rather easy to connect g2 to B+ via the zener diode, and try it as it is. After that, a cap can be added from g2 to ground, and the result auditioned: if one finds that is sounds better with the added cap, or that there is less noise -- I really cannot object to the solution being implemented.
From my experience, there is no relevant noise to start with - which might have been contributed by the zener diode. Once there is no noise to cancel or filter - the cap becomes redundant, or superfluous, you are free to choose the term...
Whoever wants to try it for himself, should do it, and let us know :)
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"From my experience, there is no relevant noise to start with - which might have been contributed by the zener diode."
Yes, it may very well be that noise contributed by the Zener is insignificant in the larger picture.
"Whoever wants to try it for himself, should do it, and let us know :)"
This is a place for measurement first, listening later. The question is whether Zener noise contributes to system idle noise at the output of the amplifier. Whether a particular owner can hear it with a given system is irrelevant to optimizing the design of the amplifier. A FFT (hardware or software) is required to analyze this issue accurately.
Also, just to address one other point you raised to support the claim that the cap is superflous...
"Adding a 10uF cap from g2 to ground will most probably create an inadequate filtering cell."
The 6BQ5 G2 draws approximately 6mA. That current through the Zener, along with a 21V drop, equates to an effective impedance of 3,500 ohms. Under these conditions, a 10uF cap at the screen will decouple the grid sufficiently to create more than 45 dB attenuation at 1K. Whether this is effective at the output depends on many factors, including the AC impedance of the primary B+ supply. At the very least, I would consider this a good safeguard against degradation (likely in the form of distortion) by a screen that that might not otherwise be sufficiently regulated.
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Buy Chinese. Bury freedom.
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