|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
63.246.183.223
In Reply to: RE: LM317 as a CCS , how to? posted by Alex Kitic on October 24, 2014 at 23:20:40
"Remove the cap and the choke from the circuit, leaving just the zener from B+ to g2."
Is there a reason you wouldn't want to install a cap to ground at the grid, in order to reduce any potential noise and improve short-term regulation? I'm thinking something on the order of 10uF might be beneficial.
--------------------------
Buy Chinese. Bury freedom.
Follow Ups:
Yes, there are several reasons, that can all be summarized as "superfluous".
The purpose of the cap is to stabilize the voltage and provide an AC path to ground. None of those functions is necessary in this case.
With the CCS under the cathode, current draw is pretty much fixed, and in an SE amplifier this means further guarantee for little or no change in B+.
With a stable B+, and stable/fixed current draw (where the change in current draw for g2 is mere statistics from the zener's point of view), the g2 voltage needs no further stabilization.
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. Caps are usually placed in parallel with zeners in order to try filtering the noise (something you shouldn't do with VR tubes due to the possibility of oscillation), not in series with zeners towards ground.
Last but not least, the cap and the resistor (in a usual arrangement, resistor to g2 and from g2 cap bypass to ground) present an RC filter cell: the higher the resistor value, the lower the cap can be without cutting too much low frequency content. Adding a 10uF cap from g2 to ground will most probably create an inadequate filtering cell.
More often than not, my explanations are misunderstood or not-understood. Most people find it difficult to think outside the envelope, or to think ahead when they want to do something. Zeners are not resistors, and they are solid state technology thus seldom found in old tube technology textbooks. Furthermore, they are usually used as shunt and not series devices. I guess that is the reason why it is difficult to convey those ideas to people with some previous knowledge and experience. But it is easy to check: install the zener, as advised - and check it out. OK, now you can experiment by adding the cap. Are you going to like the result? That's up to you to decide: I guess you are not going to like the result, but it seems that unless you do it, you are going to be left with doubts on your mind.
******
http://rh-amps.blogspot.com/
"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.
--------------------------
Buy Chinese. Bury freedom.
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 :)
******
http://rh-amps.blogspot.com/
"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.
--------------------------
Buy Chinese. Bury freedom.
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: