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In Reply to: RE: Linear Power Supply or Battery on Mac Mini - pics posted by audioengr on December 23, 2014 at 10:42:09
I have compared several LI battery supplies with ultracaps on the output to the linear I use. No contest. The fast linear wins every time.
I'm not sure how much help the ultracaps should be with higher frequency transients. Much smaller value caps in parallel might actually do better. I'm thinking of the parasitic inductance of the larger caps having a cancelling effect on it's effectiveness at higher frequencies. As you should know, a bypass capacitor becomes more inductive at higher freqs where even lead lengths, as short as they may be, become a factor.
Do you know of an extremely 'fast' 12V linear power supply that can source upwards of 6 to 8 Amps on peaks? I know that the Belleson local regulators are good for only 2-Amps or so, so they don't qualify for my application.
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What I don't get is why the concern about ultra fast responding power supplies being good despite the inductance of the cabling vs. well bypassed stiff batteries being bad because of the inductance of the cabling.
It makes no sense as described. There has to be something else going on.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
Nothing else going on. The speed makes a BIG difference. Once you hear it, you are a believer.
It comes down to the undesired inductive reactance in the capacitor design, capacitor leads, and the power supply DC cables having a cancelling effect on the desired capacitive reactance as the frequency goes up.
In a bypass capacitor, you want a low resistance (reactance actually because we're talking AC) path to ground for the high frequency noise.
In other words, a perfect capacitor will have capacitive reactance only, but that is not the case in the real world. The capacitor is more correctly represented as having series resistance and series inductance along with it's capacitance.
Capacitive reactance is inversely proportional to frequency so as frequency goes up, the reactance goes down, making for an easier path for bypassing high frequency noise (to ground).... but, inductive reactance goes up with frequency cancelling the effectiveness of the bypass capacitor at higher frequencies.
How do you accommodate for that? Inductance in parallel goes down in value so to reduce the effects of inductive reactance in the capacitors, you can wire them in parallel. That's why you often see smaller value capacitors wired in parallel with larger capacitors.... and depending on the circuit needs, you might see several very small value capacitors wired in parallel rather than using just one very large capacitor. As you wire the capacitors in parallel, their capacitance goes up AND the total series inductance goes down. The effects of inductive reactance is reduced.
It's also worth noting that when someone speaks of impedance, impedance is comprised of resistance, capacitive reactance, and inductive reactance.
Google Electrical Reactance and separately Google Impedance.
While it might be fun to 'look at' the output of various power supplies and compare their ripple, wide band noise, and transient response, but beyond being sufficient to power the computer, does it matter much?
Personally, I would be more concerned with the power supply in the DAC and more so for the DAC's analog section.
Since my computer is now set up to accept 12VDC, it will be pretty easy to try various 12V power sources on it and listen. I have a couple linear power supplies, a couple switchers, and some batteries.
Quite right. Real Capacitors are far from the ideal text-book models. Only the most expensive Teflon and paper-oils even come close, and these are usually too large to be practical for power decoupling.
Abe - putting small value decoupling caps at the power supply output does not have much effect, because of the inductance of the cabling to the load. I have some in there, but only for noise filtering of the power supply itself, not for load regulation. The ultracaps do help with medium frequency transients. They have very low ESR.
The best fast linear supply is the paulhynesdesign.com SR5-12 or SR7-12, 5 amps and 7 amps respectively.
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