LC Audio story on current woppling?

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Member
Joined 2002
Paid Member
LC Audio describes "the theory" behind their capacitor connections in their Class A amplifier and they describe something they call "current woppling". Would this be an issue for one of our DIY projects where we connect a number of smaller capacitors in parallel to create the necessary PS capacitance? To explain it in further detail. If my Aleph-X requires four 22,000uf caps in its PS I was under the impression that I could parallel up four 5600uF caps to form each 22,000uF cap. Am I off track here?

Our novelty Virtual 4 pole Capacitor bank is one example. In this bank 10 high speed low impedance power electrolytic capacitors are connected together to form 2 big caps for the amplifier rails. None of these caps are connected in parallel, as this would pose problems with current transfer woppling. This is when several low impedance caps are connected directly in parallel, they tend to compete for the power. The first cap gets current first, then the next tries to take some of the charge from the first, and so on. In terms of sound performance, you get a loss of precision in the high frequencies and a cold midrange at larger power outputs - where the woppling is more severe. One alternative is to use only large can capacitors. This to some extent also will solve the two problems described, however almost any large can capacitor will work very slowly, and play with a slow and out-of-beat bass. The V4P setup however solves both problems! Bass is fast and in sync, while precision in mid and high ranges are intact at all levels. Due to the special V4P network, power is charged and discharged to each cap at exactly the same time and rate. At the output of the V4P network no electrolytic caps are present, so here we found a perfect opportunity to place a huge Polyprop (22uF) which this way will be the only capacitor the amplifier can 'see' above 1 kHz.

Regards,
Dan
 
diyAudio Retiree
Joined 2002
woppling

One entry found for whopper.
Main Entry: whop·per
Pronunciation: 'hwä-p&r, 'wä-
Function: noun
Etymology: 1whop
Date: circa 1785
1 : something unusually large or otherwise extreme of its kind
2 : an extravagant or monstrous lie

Maybe it is the verb for telling a whooper......
 
I say that's mostly crap.

I'm sorry Nelson, we didn't invent this phenomena, it was explained at a passive seminar by the norther European representative of Chemicon (Former Sprague) Capacitors factory.

Now it was back in 1991, so i'm sure the word woppling is not the right word he said, but the phenomena is real enough.

If you have say 5 capacitors in parallel, in a dual rail connection, where the rectifier is in one end, and the amplifier is in the other, and we decide the ESR of each capacitor is 10 milliOhms, and the connection resistance between each capacitor is 2 milliOhms.
Do you (Nelson Pass) dispute that the capacitor nearest to the rectifier is charged with the largest current (since it has only 10 mOhm ESR) , and the one furthest away is charged with only about half the current (since it has 20 mOhm as seen from the rectifier). Then when the charging spike is over, the charge rolls forward from the first capacitor to the last and attempts to equalize the charge over the entire bank. So each capacitor ends up having the same voltage. This is what i have (maybe wrong word) called current woppling.
 
Just wanted to add the schematic to our solution discussed. So easier to follow.

Sorry the text is in Dansih language, but the schematic should speak for itself.

The resistors ensure that the charging spike is equally distributed over the entire capacitor bank, and also equally discharged from all 4 capacitors to the amplifier.

I will just emphasize one point, so everyone in here can understand, also Fred: :D Only the positive side of the power supply is shown here on this schematic. There need also be a corresponding negative side of this power supply circuit to make it work properly.
 

Attachments

  • v4p.gif
    v4p.gif
    7.8 KB · Views: 2,343
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.
 
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.
 

Attachments

  • 2caps.gif
    2caps.gif
    8.1 KB · Views: 1,947
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.......


Truth is the most valuable thing we have. Let us economize it.

Familiarity breeds contempt. How accurate that is. The reason we hold truth in such respect is because we have so little opportunity to get familiar with it.

Mark Twain
 
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...?
 

Attachments

  • multicap.gif
    multicap.gif
    1.9 KB · Views: 1,885
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
 
Petter said:
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?
 
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? ;)
 
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.
 
The Original And Best Whopper ???..........

"The Original WHOPPPER® Sandwich has been doing its thing since 1957. It's a fire-grilled classic and everything you would expect from a great-tasting burger – 1/4 pound of beef, red ripe tomatoes, crisp lettuce, creamy mayonnaise, ketchup, crunchy pickles and onions all on a freshly baked bun. Add extra ketchup and HAVE IT YOUR WAY®."
 

Attachments

  • whopper.jpeg
    whopper.jpeg
    13.5 KB · Views: 1,111
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.