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Multi-Way Conventional loudspeakers with crossovers

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Old 17th January 2006, 05:18 PM   #21
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I read that back when you posted it; the reread was a good idea, as I had already forgotten about the IM.

Also, it's important to remember that even in applications like passive crossovers where there is no DC bias applied, instrument waveforms are often asymmetrical; obviously they integrate to zero over the long term, (depending on the LF characteristics of the chain) but can be quite asymmetrical over the short term.

And with respect to point 5, I believe those were all of the same type; that does not cover the issue of a fancy small cap bypassing more mundane larger capacitors. (Which I have not had good luck with. )
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Old 17th January 2006, 06:14 PM   #22
hermanv is offline hermanv  United States
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Default Capacitor bypassing

Although I have little personal experience with bypassing large caps with high quality smaller caps, the examples I've seen posted are of fairly tiny and probably questionable ratios.

I see electrolytic power supply capacitors running 50,000 uF bypassed with a 0.1uF or two. A ratio of 1/2 million to one or to put it another way 0.0002%.

Looking at the problem from a current flow standpoint, I'd guess that a ratio of 1% to 5% made more sense. So in the example above a bypass of 500uF would be needed for 1% of the capacitance value to be a high grade low ESR path. Clearly a very costly answer.

Does anyone have experience with actual improvement in sound, and if so at what ratio?
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Old 17th January 2006, 06:27 PM   #23
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"I see electrolytic power supply capacitors running 50,000 uF bypassed with a 0.1uF or two."

IMO it's pointless to "bypass" PS caps, whose job is to supply pure DC.

IOW, there's no AC signal to bypass.
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Old 17th January 2006, 07:32 PM   #24
hermanv is offline hermanv  United States
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Quote:
Originally posted by noah katz
IMO it's pointless to "bypass" PS caps, whose job is to supply pure DC.

IOW, there's no AC signal to bypass.
An "ideal" power supply presents a zero Ohm source impedance to all loads AC or DC. Power amplifiers have substantial AC load currents and these currents flow into and out of power supply bypass capacitors. (Actually if you follow the load current all the way around the loop, the power supply capacitors are usually in series with the load)

Having a very low AC impedance prevents a signal voltage from being developed across the power supply output and therefore prevents any intermodulation of other stages.

Needing a low AC impedance means that electrolytic capacitors might not be the best choice, bypassing them is one possible solution. Other solutions include regulating the supply rails (usually results in a lower Z out) or capacitor multipliers where the gain of an active device helps to lower the power supply output impedance (but not its actual stored energy).
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Old 17th January 2006, 07:40 PM   #25
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"Power amplifiers have substantial AC load currents and these currents flow into and out of power supply bypass capacitors."

If you count pulsating DC as AC, then I guess that's true.

Still not sure it applies for PS caps; what's needed is low internal impedance so that when current is delivered the voltage doesn't fall.
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Old 17th January 2006, 07:46 PM   #26
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Anything that pulses has an AC component, or so I read Ohm's law.
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Old 17th January 2006, 08:13 PM   #27
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As HermanV says, the capacitors are in series with the load. It's easier to see if you draw the PS as two batteries, with the center point grounded (or not, as is the case in some full bridge configurations), and the load returned to the center point.

Looking at the cap's job as minimizing AC rail voltage is actually another way of looking at the same thing, and is certainly valid.

Unfortunately really high quality high voltage capacitors in the 100 uF (less than 0.2 Ohms Z at 1 kHz) range are not inexpensive.
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Old 17th January 2006, 08:19 PM   #28
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OK, I see what you guys are saying now.

But I'm still having trouble equating what (I think) bypass caps are supposed to do, which is transmit AC faithfully, to what a PS is supposed to do, which is supply DC.
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Old 17th January 2006, 08:23 PM   #29
poobah is offline poobah  United States
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Quote:
IMO it's pointless to "bypass" PS caps, whose job is to supply pure DC.
There are 2 substantial AC quantities to bypass: 1, the current from the power supply; 2, the current required by the load.

The capacitors "supply pure DC" by storing and releasing charge... i.e. AC current. Pure DC current flows past the capacitors; the AC currents flow through the capacitors.

Electrolytic capacitors really behave their best at low frequencies. At higher frequencies, the effects ESL (equivalent series inductance) and ESR (blah blah resistance) begin to dominate. In other words, electrolytics do not offer a "low impedance" at high frequencies. Paralleling caps of different technology is usefull because it can provide reduce impedance at higher frequencies. There are, however, no golden ratios. Proper selection is based on the frequency spectrum in need of low impedance. Capacitor roulette with the aid of a scope is a popular and sound method.
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Old 17th January 2006, 08:30 PM   #30
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OK, it makes sense now. Thanks for the explanation.
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