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Old 16th May 2007, 01:37 AM   #1
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Default PI-filter (or CLC vs CRC vs C)

I've been reading various items on this, including the following threads from this esteemed forum: PI-Filter, PS question - CLC vs C filtering, First Pass project, need Aleph-M PSU advice please and proper design of pi-filter for Aleph 30.

There appear to be varying ideas on the usefulness of such filters for the power supply, especially for a class A, balanced amplifier with regulation on the power supply. Nelson Pass' post in the second thread especially seems to indicate this.

And one other thing I've been wondering is, how much of the effect of the filter is caused by the series resistance of the inductor used in the filter, and how much is due to the inductance? Most of the inductors of the 2mH variety would run between 0.2 and 0.5 ohms, which is almost exactly the value of the resistor that you would use if your were going to use a CRC type filter.

So the question, what's more useful in general, CLC, CRC or just plain C?

In my case, looking at a ZV6 with a regulated power supply (like the one described in ZV5), I probably won't bother with the extra space/cost vs performance that the four inductors would give me. And, yes, I know I'm doing a lot of talking without much action, but I'm still buying parts (and I'm a real slow-starter at the best of times - just ask the missus...).
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Old 16th May 2007, 03:10 AM   #2
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The ripple voltage is of saw teeth shape after the caps.
When the size of PSU caps is getting bigger, the ripple is getting
smaller. And, the additional L or R reduces the ripple further down.

If the L and R have the same ac resistance, they give the
ripple reduction at the same rate. The only difference is that the L
changes the ripple shape to the sine wave while thr R keeps it to
the saw teeth shape. But, I do not know how much these
different shape of ripple voltage will affect the sound, assuming
that the ripple voltage is very low anyhow . . .


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Old 16th May 2007, 05:36 AM   #3
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Quote:
Originally posted by Babowana
The only difference is that the L changes the ripple shape to the sine wave while thr R keeps it to the saw teeth shape.
. . .




The penny has dropped!

So how do you find these wonderful inductors to do this for you? I guess there is always "roll your own" (now where is that inductor calculator?). As big air-cored inductors seem to cost about $50 each (for a 2mH) in Australia. Maybe Mundorf has a iron-cored one available here for less. And I can't afford to get them sent from the US any more as it's too expensive now .

Thanks, Babowana!
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Old 16th May 2007, 09:39 AM   #4
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Hi,

you can look at the effect of L and R filters with PSU Designer2 (search).

A CLC filter has a lot more ripple reduction than a CRC one without losing too much voltage and generating too much heat.

Up to 3A of bias you can use a siple 1.8mH, 1.4mm air choke (less than 15€)

For my Aleph-X (7-8A bias) I´m using Torobar coils wich work fine.

William
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Old 16th May 2007, 04:58 PM   #5
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For such a simple subject, this generates far more than its fair share of angst. Never have understood why. Just me being silly, I suppose.
Here's the hierarchy from most best to least best:
1) REGULATED
Pro--excellent isolation from line problems, absolute control over rail voltage
Con--heat, complexity, potential for transient problems (ringing) if not executed properly, potential for dynamic restriction, potential for noise
2) CLC
Pro--good isolation from line problems, very quiet
Con--lack of control over rail voltage, bulky, heavy
3) CRC
Pro--good isolation from line problems, very quiet
Con--lack of control over rail voltage, not as good at eliminating high frequency aberrations as CLC
4) C and Capacitance multiplier
Pro--decent isolation from line problems
Con--lack of control over rail voltage, not as good at eliminating high frequency aberrations as CRC

The list is far from exhaustive. For instance you could have something like a CRCLC-Capacitance multiplier-C. Where would that fit in? Flip a coin. Also, my inclusion of Capacitance multipliers with straight C filters is rather arbitrary. I did so because a capacitance multiplier acts like an arbitrarily large capacitor; in principle, you could duplicate the effect with a really, really big cap. Given that capacitance multipliers are generally used in combination with other things, it gets complicated. If someone wants to put it higher up in the hierarchy, I won't put up much of a fight.
But so-and-so used this filter and what's-his-face used the other filter and they both say their way is the best and I just don't know what to do...
This is one of those human psychology things. If they used it and it worked out okay, to them it's the best thing in the world. Just ask anyone who's ever built a speaker. They always love it, even if it sounds like fingernails on a blackboard. (Beranek's Law)
One general rule of thumb applies: Make it big.
Or, as a fellow I know used to say: If some is good, then more is better, and too much is just enough.
As for inductors vs. resistors, this is covered ad nauseum in every electronics text that's ever been written. Basically, resistors are neutral as far as frequency goes. DC (aka 0Hz) gets attenuated exactly the same as 120Hz, which gets attenuated exactly the same as RF. Inductors attenuate high frequencies more than low frequencies. RF is attenuated more than DC. If a resistor is .5 Ohms and an inductor has a DC resistance of .5 Ohms, then in that limited sense they will behave the same...at DC. But that's not why you use an inductor. If you already had perfect DC, you wouldn't need the extra filtration.
Quit griping and build something. You can always add an L or an R later if you think you need it. Power supplies are easy.
Quote: Nothing is hard, but thinking makes it so. (Meaning if you think it's hard, it will be. Clear your mind and the problem will solve itself.)

Grey

DISCLAIMER: The young 'uns are being particularly demanding. I caught a few typos and oddities, but can't claim that I got them all.
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Old 16th May 2007, 07:55 PM   #6
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Quote:
Originally posted by GRollins
For such a simple subject....
Grey

Quite true…

Never the less a CLC will render quite good regulation and a very clean rail something that a CRC will never deliver. I have compare both on the same amp and there is no contest soundwise.
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Old 17th May 2007, 12:39 AM   #7
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Either a CLC or CRC is still a lot better than C. As an example,
I have here a supply with 30,000 uF after the rectifier, then
.12 ohms, then another 30,000 uF. The difference on either side
of the resistor is about 10 dB worth of ripple, and that's a lot.

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Old 17th May 2007, 02:11 AM   #8
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Thanks for your replies everyone. Especially Grey, very clear and erudite. I'll take your advice "Quit griping and build something. You can always add an L or an R later if you think you need it." when the bits arrive. You guys are blessed over there, big heatsinks are rare as hen's teeth over here.
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Old 17th May 2007, 02:13 AM   #9
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Pretend the filter is in a speaker instead of an amp. Suddenly that CLC snaps into focus as an 18dB/oct lowpass filter as opposed to a C filter being--obviously--a 6dB/oct lowpass filter.
Wanna take bets as to which one does a better job of filtering?

Grey
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Old 17th May 2007, 02:16 AM   #10
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Quote:
Originally posted by Cloth Ears


You guys are blessed over there, big heatsinks are rare as hen's teeth over here.


Wanna bet? Why do do think I "invented" water cooling for audio?
No heatsinks here in the hinterlands.

Grey
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