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Old 14th September 2007, 05:33 PM   #41
Magura is offline Magura  Denmark
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Quote:
Originally posted by rdf


H Conrad. Take this one for a spin. Consider a cap with, let's say, two bypasses. A general rule of thumb I work with is any bypass much smaller than a 1/10th the main cap risks resonating against the main's parasitic. Now preceed each of the three caps with an inductor sized on the same order as the following cap's inductance. Take a look at the total network's effect on PS filtering and the effective source impedance looking back from the load.

As an example, a 100uF with 30nH primary inductance bypassed with 10uf/20nH and 1uF/15nH. Adjust nH to taste to account for traces, etc.. So the power supply filtering (after the rectification cap) becomes:

30nH > 100uF > 20nH > 10uF > 15nH > 1uF > load

Quote:
Originally posted by rdf


Guilty as charged. I'll take that as a complement. Naturally as such I didn't think about large currents but it should work well for preamps, low power amps, etc. If the target is lowering HF PS impedance for reasons of stability no reason a small 'RF' bypass won't work with a large main cap jumps out. I wouldn't build a glass circuit without using this technique, to my ....perception it always brings clear benefits.
This approach works fine for more current hungry applications as welol. The only detail I can add is that when crossing the 500uF mark, bypass ratios up to 1:200 is just fine.

I have recently made a few experiments with a 100.000uF > 680uF > 50uF (film from this point and down) > 10uF >1uF > 0.22uF.

This layout showed no ill effects. I started out with a 10.000uF cap in between the 100.000uF and the 680uF, removing it made no difference.

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Old 14th September 2007, 06:02 PM   #42
rdf is offline rdf  Canada
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If you play around with the technique in Spice it quickly becomes obvious significantly larger values for the inductors work better. The example numbers were min values, Andrew's right they're in the range of isolated (non-groundplane) traces, as you'ld expect since the inductance of modern film caps is in the range of their physical length.
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Old 14th September 2007, 09:07 PM   #43
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Though I'm a big LTSpice user, I'm very wary of what it tells me regarding caps when using a frequency scan. As far as I can tell, it models losses with a simple fixed resistor, and that's only good at a single frequency. Though I've had a couple suggestions on how to deal with this, I haven't really grasped how to do it. BTW, there was a circuit presented for comparing caps in audio equipment, I think by Jung. IMO, bridge type circuits have the same problem- when you null the DF between two caps it's with a fixed resistor, good at a single frequency, but insufficient for a listening test over the audio range, depending on the characteristics of the caps in question. So we have two levels of insufficiency, simulation and actual comparison! I can do a series of measurements that should be illuminating, but tying it all together is another story.
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Old 14th September 2007, 09:18 PM   #44
Magura is offline Magura  Denmark
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Quote:
Originally posted by Conrad Hoffman
I can do a series of measurements that should be illuminating, but tying it all together is another story.

That would be much appreciated! Tying it all together should be possible as well if you have made enough measurements of different caps in different configurations...no?

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Old 14th September 2007, 09:39 PM   #45
WRWSTD is offline WRWSTD  United States
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I have been reading through this and other threads about power supplies, caps, wire, etc.
The way I see it is that an amplifier is really nothing more than a modulated power supply. This "power supply" is supposed to be Impedance-matched to the load (speakers) the best as possibly can be within a passband of frequencies. All the impedances between the stages of the amplifier (modulated power supply) all must follow this same idea.

With all of that in mind, it does not matter much what type of parts are used if everything is impedance matched within a particular passband.
It is a balancing act.
The hard part is gathering data to figure out all those impedances.
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Old 14th September 2007, 10:07 PM   #46
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Impedance matching is important for RF, where you have reflections to deal with, and can create matches with baluns and ununs. For audio, it's neither desirable or a useful concept. What we generally want is a "stiff" supply, impervious to sudden transient loads. To me, that means an energy storage capacitor that holds its properties over a wide range is more desirable than one which doesn't.

Eva- just to clarify your beliefs, you don't think large electrolytics should generally be bypassed at the cap (I agree to a large extent, but still want to create a perfect cap), but surely you believe local bypassing, say for a high bandwidth op-amp, using small low ESR caps is desirable? Or is a big electrolytic enough?

FWIW, I recently changed the supply and bypassing for my phono preamp to consist of 47uF OS-CONs, but no smaller value bypasses. I'm not very happy with it now, and intend to try a variety of other schemes. Given the PSRR and all, I don't even know why it matters, but it seems to.
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Old 14th September 2007, 10:21 PM   #47
rdf is offline rdf  Canada
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Hi Conrad, no doubt about LTSpice's limitations but unless I'm way off it really doesn't matter. In general the aggregate topology doesn't appear to be that sensitive to wide variances in the cap's self-inductance if the preceeding inductor is large enough. LT should do a good enough job modelling small air core inductors.
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Old 15th September 2007, 04:22 AM   #48
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Can't do the tests I want to do right now, but here's something I was able to throw together with what's on the bench right now. This may not be important, but it's another way of looking at things that might be useful. Let's say at high frequencies a capacitor's phase shift is closer to zero than -90 degrees. That means it's closer to a resistor, albeit a low value one. Last time I checked, a resistor isn't a very good energy storage device. So let's say I want two things. I want to use enough capacitance so the reactance (X) stays under some arbitrary value, like 1 ohm. Let's say I also dictate that the phase shift (loss tangent) shall stay between -80 and -90 degrees. It turns out that this is pretty easy below about 3khz. It's also pretty easy above 10 or 20khz, where you can use reasonably obtainable film caps. The middle region is surprisingly difficult. Take a look at the following Excel spreadsheet. Check my math, of course, as I've been known to make pretty big mistakes! Most important, tell me why the phase shift, loss tangent, DF, or however you want to describe it, doesn't matter. Also, I'm now thinking the poly motor capacitors might be a great answer, but the one I have is too small for the frequency range where I need it.

Cap spreadsheet
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Old 15th September 2007, 08:21 AM   #49
mzzj is offline mzzj  Finland
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Quote:
Originally posted by Conrad Hoffman
Can't do the tests I want to do right now, but here's something I was able to throw together with what's on the bench right now. This may not be important, but it's another way of looking at things that might be useful. Let's say at high frequencies a capacitor's phase shift is closer to zero than -90 degrees. That means it's closer to a resistor, albeit a low value one. Last time I checked, a resistor isn't a very good energy storage device. So let's say I want two things. I want to use enough capacitance so the reactance (X) stays under some arbitrary value, like 1 ohm. Let's say I also dictate that the phase shift (loss tangent) shall stay between -80 and -90 degrees. It turns out that this is pretty easy below about 3khz. It's also pretty easy above 10 or 20khz, where you can use reasonably obtainable film caps. The middle region is surprisingly difficult. Take a look at the following Excel spreadsheet. Check my math, of course, as I've been known to make pretty big mistakes! Most important, tell me why the phase shift, loss tangent, DF, or however you want to describe it, doesn't matter. Also, I'm now thinking the poly motor capacitors might be a great answer, but the one I have is too small for the frequency range where I need it.

Cap spreadsheet
Doesnt matter if you dont have considerable voltage swing accross your caps. And we dont want that if its about power supply caps.

Lets take another look at large 1000uF 50v electrolytic capacitor not able give and take energy at 50khz swithing frequency. Lets say that supply voltage is 5.0025v and 0.025ohms ESR. we charge our DUT with 500mA for 10us and discharce 500mA for 10us.

1000uF cap voltage rises 5mV during charge perioid and ESR drop is 12.5mV. Energy input to cap: 0.5*0.001*5.005^2 0.5*0.001*5.0v^2 =12.525mJ-12.5mJ =25uJ
Energy loss in ESR: 0.5A*0.025*10us=62.5nJ
So our elko stores 99.7% of the energy.
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Old 15th September 2007, 11:53 AM   #50
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mzzj- As usual, looking at things from an energy standpoint clears them up quickly! I'm probably better off ditching the AC model and just looking at the thing as a battery and a small resistor- you're right, there shouldn't be a significant AC voltage across the cap anyway. Still, any power amp I've ever looked at (with unregulated supplies) does have easily visible "music" signal on the supply rails. It would be interesting to see if a spectral analysis of this "music" showed LF content due to insufficient capacitance, and MF content where it's hardest to get low DF, that 3-10khz range.
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