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23rd September 2003, 01:41 PM  #21  
diyAudio Moderator Emeritus
Join Date: Apr 2002
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Quote:
Thanks, Bas
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23rd September 2003, 01:41 PM  #22 
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Join Date: Sep 2003
Location: Texas

Yes, you are right. When the circuit leaves class A mode many things change. Even so, the class A analysis can be helpful.
For example, the AC resistive behavior of the regulator will more or less apply to situations where current demands are asymmectrical. In a screen regulator for AB mode, etc. where the PS has to supply current pulses, the resistance of the PS still applies (assuming that the regulator is fast enough). And so you can know partly what it will do even in nonclass A modes. I'll try to investigate this some (but things are starting to pile up ). One other thing to be sure to say is that the resistor/capacitor models in the simulator are perfect, whereas real resistors and capacitors (especially electrolytics) are not perfect and do have voltage/frequency nonlinearities. If anyone can supply me with a good electrolytic capacitor model, I'd really like to have one. 
23rd September 2003, 01:48 PM  #23 
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Join Date: Sep 2003
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Bas, theory may simply be for weird folks. Which, for most of us is also "other people."
I do, however, build stuff too. At the moment I'm building a pair of circlotron monoblocks and and preamp to go with them. But I figured that if I posted the schematics for these as my first real post, I might not survive it. 
23rd September 2003, 01:52 PM  #24  
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Quote:
Cheers, Bas
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If you can see the speaker cone move...it's not hifi. 

23rd September 2003, 01:57 PM  #25  
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Join Date: Jun 2002
Location: U.K.

Quote:
Do expect to be quizzed on the why, where and how... Cheers, 

24th September 2003, 02:14 AM  #26 
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Join Date: Sep 2003
Location: Texas

Reactive Effects
OK, so here is part two. Do the reactive components of the PS/Regulator affect the circuit? I’ll use the same 6dj8 grounded cathode amp. But first, we have to talk about phase relationships, because this is what the reactive effects are all about. I'm going to do this in two parts. Everyone on the forum knows what phase is. It is actually very important to audio design, but often left out of consideration. Why is it important? Here is an example. Everyone also knows that a square wave can be represented by an infinite series of sine waves (Fourier analysis). The basic idea is this: Square Wave = A_{1} * sin ( a_{1} * f) + A_{2} * sin (a_{2} * f) + . . . A_{n} * (a_{n} * f) Where n typically goes to infinity, A is an amplitude value, a is a multiple of pi, and f is the fundamental frequency of the square wave. Now there are three ways that a circuit can mess up this square wave trying to amplify it:[list=1][*]Altering the value of any of the As (harmonic distortion)[*]Altering the phase relationship among the different frequency components (phase distortion)[*]Generating new frequencies (intermodulation distortion, which basically leads back to #1)[/list=1] So, if the amplifier causes frequency dependent phase shifts, then the square wave will not be amplified perfectly because the sine waves won’t add up right. You’ll get rounded corners and ringing. This is one reason why good amps must have a near flat response and minimal phase distortion beyond the audio range, so that the transients can be amplified faithfully (or close to it anyway). OK, so do tube amplifiers have frequency dependent phase? You bet they do. And depending on the number of stages, lots of it. Generally, the more stages, the more the phase shift. Not only that, but at as the frequency gets higher, the phase relationships that hold in the audio spectrum (in good designs) can be completely reversed above (below) the audio spectrum. No big deal you say. Maybe, maybe not. But if you’re applying global NFB, very big deal. Because if you’re feeding a signal back to the input that is supposed to be 180 degrees out of phase, but at 100KHz it gets to be 360 degrees in phase, then you have a positive feedback amplifier, or oscillation. I know that you guys all know this, but I figured it was worth the trip. Besides, you guys are better at this than I am . 
24th September 2003, 02:25 AM  #27 
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Reactive Effects Cont'd
OK, so the main effect of the reactive components of the PS (capacitors and chokes) will be in altering the phase behavior of the amplifier. Chokes and capacitors store energy in magnetic fields or in electrostatic charges and this stored energy is returned back to the circuit in a frequency dependent way causing phase shifts in V and I. So, we start here with our basic amp but with a perfect PS. Like this: Looking at the voltage phase at the point marked “Phase” (the output of the amp) we see this: Since this amplifier inverts, the primary phase relationship is 180 degrees. However, at low frequencies the phase is slightly less than 180 and at higher frequencies it is slightly more. What this means is that the high frequencies effectively get to the output “faster” than the low frequencies. Practically speaking, however, these phase shifts are probably audibly unnoticeable. But, I don’t have data to back up this claim. But, no matter what we do with the regulator we can’t get any better than this, unless we can actually improve the phase shifts somehow. Starting as before, if we insert our basic RC regulator and replot the phase shifts caused by the amp we get this: In other words, the RC regulator has absolutely no effect on the phase behavior of the amplifier. So, how about the series regulator? Taking the “raw” regulator with the 470n damping cap we get: Absolutely no effect. In fact, there is no discernable effect for any of the regulators that were used previously. So, for this very simple analysis, we can see that, from a phase point of view, all of these regulators are the same. Meaning that the reactive behavior of the regulator on the circuit is irrelevant. This is primarily due to the fact that the resistive component is also irrelevant and, hence, the phase of the deltaV just doesn’t show up. I should also add that the phase of the current in all of these examples is identical to the voltage phase. And, I should give the caveats that this is simulation, these are perfect Rs and Cs, the circuit is pure class A, and the line side of the PS is not being taken into account. You may ask, is there every any change to the phase relationship? There can be, depending on the exact time constants of the PS and the circuit. For example, here is a result for the RC regulator where the cap is set to 1u instead of 330u. The voltage phase shift is now 180 degrees advanced and the difference is almost 15 degrees at 20Hz. In the case where C=330u, the phase shift at 100Hz is almost exactly 180d. I’ve also plotted the current phase shift in red. Even so, these phase differences are unlikely to be audibly noticeable. Now, when chokes and big caps are involved in high current situations (for example, OTL output PS), YMMV. Maybe we can look at this later. In the meantime, I think I've figured out how to account for most of the preregulator PS. If this works, I'll be back with more. But, please let me know how we're doing. 
24th September 2003, 02:52 AM  #28 
diyAudio Member
Join Date: May 2003
Location: Midwest

first RC network
Runeight, sorry I am late in seeing your first post.
The first analysis is way off at higher frequencies. The reason being that a typical electrolytic capacitor of that size will series resonate at some point, probably between, say, 7khz and 30khz. Then the cap will become an inductor (simplification) as the frequency rises. So the impedance will start rising above the series resonant point. The same with the cathode bypass electrolytic capacitor. The frequency response will also change at both low and high frequencies as the reactance of the cap rises. Just a thought. ps. As you study the subject at hand, you will see other implications that need addressing. 
24th September 2003, 03:32 AM  #29 
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Join Date: Sep 2003
Location: Texas

Thanks for the information. I do know that the electrolytic caps have this behavior. I just don't have a good model for them. If I can find one, I'd like to try this again.
What other implications are you thinking of? Other than the line side, something other than class A, chokes, etc. 
24th September 2003, 04:00 AM  #30 
On Hiatus

It's not too tough to bypass electrolytics and make them look more ideal.
The results of your simulations are not surprising. I assume you'll add some series resistance to the reactive component on the next goround?
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