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Old 14th January 2004, 10:21 PM   #11
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Just an additional point: I also find it to be sound degrading to connect any capacitors of any kind in parallel with Black Gate's, they simply work better on their own.

Further parallelling transistors / MOSFET's especially in output stages will always result in loss of resolution in the mid-high region, and add harshness to voices. This is because their VBE / VGS curves do not match exactly. As the current changes, high order distorsion is added to the signal. And no you can not use a feedback loop to remove THD in the amplifier chain, however you can by using feedback with a non linear element in the loop create a modulator. This basicly means you convert THD into TIM.

Better avoid THD alltogether.
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Old 14th January 2004, 10:29 PM   #12
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If you re-read my comment, you'll see that I have made an
allowance for the actual existence of such a thing. I merely
dismiss it as not a real problem.
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Old 14th January 2004, 10:49 PM   #13
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Nelson: Then consider this setup of the two capacitors in parallel, when you view it as the capacitor's practical equivalent circuit.

As you know every capacitor consist of a capacitive part (obviously), an inductive part and a resistive part (last two from wire leads and internal windings of the aluminum foil). There is also a fourth part a parallel resistor, but it has no significance in this discussion.

When you have one capacitor it will act as a series filter with it's lowest impedance at one specific frequency. But when you parallel two caps, they can interact with each other's internal circuit. Especially if the inductance or capacitance is not exactly matched. (Which they can never be in a real-life scenario).
Consider the possibility that C1 might form a parallel resonance with L2 and vise versa. Might occur if the parallel connection resistors (here 1 milliOhm) are low enough. I think that anyone can see that would be a problem in your amplifier power supply.

In the V4P circuit shown above, the problem is eliminated by breaking the circuits apart into separate series filters.
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Old 14th January 2004, 11:12 PM   #14
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Default A picture's worth a thousand whopples..

'I will just emphasize one point, so everyone in here can understand, also Fred"

I wonder how much the benifits of this are from lowering the Q of each of the caps that are in parallel and the reduction of charging current by the additiona series resistance. Lowering the Q of caps in parallel when adding bypass caps is probably a good idea to avoid resonant circuits from the parasitic inductances interacting with the capacitance. For an extremely low ESR cap like the Black gates or Oscons bypassing is not a good idea unless you really understand the capacitor and PCB parasistics. It seems that current sharing effect could be largely acheived by making sure the PCB ESR of traces between cap terminals is less than that for the traces to rectifier and from the caps to the load. Also each of these traces should have have the same resistance to distribute the currents equally. As for telling whoppers.......


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Old 14th January 2004, 11:22 PM   #15
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What about the following solution? The upper one is frequently
recommended as "the proper" way to connect PSU caps in
series to even out the lead impedances. The schematics are
intended to reflect the physical PCB layout.

Now, what about extrapolating from this to the lower figure,
where I intend the bridge connected between a and b and
the load between c and d. I don't think it will even out
differences between the caps (too late to think properly now)
but at least it evens out the lead impedances and you get
rid of the extra resistors. These may be beneficial on the bridge
side, but hardly on the load side, or...?
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Old 14th January 2004, 11:41 PM   #16
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I agree with Lars that two units with resonant frequencies that are different could cause beats - this is common to many situations in engineering sin(a) + sin (b) = Look it up.

Whether that is a problem or not in this case I don't know. We should try it and evaluate.

Petter
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Old 15th January 2004, 12:11 AM   #17
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Quote:
Originally posted by Petter
I agree with Lars that two units with resonant frequencies that are different could cause beats - this is common to many situations in engineering sin(a) + sin (b) = Look it up.

Whether that is a problem or not in this case I don't know. We should try it and evaluate.

Petter
But do you really gain anything from parallel caps instead of
a single bigger one i this case since the resistors add to the ESR?
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Old 15th January 2004, 01:36 AM   #18
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Default Adding 2 cents to the confusion.

If I understand this effect correctly, we're dealing with ever so slight differences in the rise time of the supply capacitors, due to their different capacities, and the different resistances between each capacitor, and whatever charges it. Please correct me if I am wrong.

While paralelling several RCR sections should improve current sharing, at least during most of the charge cycle, it does not entirely eliminate this wobbling, as the (typically) +/- 10% tolerance of the capacitors will cause a difference in rise time that I expect will swamp the resistance. If aiming to eliminate this effect, we would also need to match the capacitance of our cans.

Apart from that, I think the PSU schematic presented by Lars makes for a sensible positive half.

Anyway, such 'charge sloshing' between capacitors will be well covered during the course of this debate, I expect, so I will bring up another point that I, for one, am curious about.

Constructors of CLC supplies and such will be familiar with the idea of placing the smaller cap earlier in the chain to avoid oscillation during changes to the load.

Since most of us tend to invest a tremendous amount of effort in keeping high frequency garbage from our circuits, should we not also examine the potential for high frequency oscillation due to interactions between the load and what essentially is a CLC supply with low values for L?

Lastly, I would point out that I have done a brief experiment on parallelling capacitors for a headphone amp at some earlier time, and there was a slight subjective improvement in reverting to a single cap. I believe I might have been the one who mailed Nelson about this at that time, slightly prior to doing the experiment.

PS! Lars: 330uF polypropylene caps are readily available in Scandinavia. How about trying a similar PSU technique for the Sidewinder, but with PP caps in the RCR bits?
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Old 15th January 2004, 02:28 AM   #19
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I think, when discussing load distrubution and differential currents between different caps, you also might talk about the same effects inside a single large cap as nobody can guarantee, that the electrons are equally distributed on the internal surfaces as well.
You can view a large cap as a combination of many little ones.

But all this is a more theoretically aspect than a practical, as I personally think the effect is cancelled out the more caps you use. Proper connection of those estimated.

I do not think, that desperately making the overall features worse by adding extra resistance to avoid those effects will result in any final benefits.
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Old 15th January 2004, 02:39 AM   #20
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