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Re: black gates in parallel

Posted by Steve Eddy on December 21, 2001 at 22:46:00

In Reply to: Re: black gates in parallel posted by Robert Karl Stonjek on December 21, 2001 at 21:17:11:

I'm in a 50Hz country. I have seen ESR (Equivalent Series Resistance) quoted at 50Hz but know not why.

I've seen it specified even at 20kHz (Cornell-Dubilier does this, but that's in addition to a 120 Hz spec). There are other applications beside audio so I'm sure that knowing ESR at frequencies other than twice the power line frequency must useful for others.

I would have thought that temperature is less of a problem with power supply caps in power amplifiers as the power supply of a good amplifier has a rating capable of handling the amps quoted maximum output. The max output is made up of at least 6dB headroom - not much for audio output but a huge difference for the power supply. So a few transients barely trouble the main caps.

No, I'm not talking about the current drawn by the amplifier itself. I'm talking about ripple current. That's the current drawn from the AC side of the power supply by the capacitors during the refresh cycle when the caps are being topped up. And in a full wave rectified power supply, this occurs every 120th of a second. Or every 100th of a second depending on power line frequency.

When the rectifier diodes turn on during the refresh cycle, the secondary of the power transformer sees the impedance of the reservoir caps at either 100 or 120 Hz. That impedance is usually quite low. Just as a convenient example, consider a 10,000 uF, 25 volt cap from United Chemicon (KMH25VN103M25X35). At 120 Hz, its impedance is about 0.15 ohms.

With 25 volts across such an impedance, the cap would ideally want to draw about 167 amps. And if the transformer's VA rating were sufficiently high to deliver that kind of current (and of course if your AC line didn't load down too much), that cap would quickly burn itself up. That particular cap has a rated ripple current (at 105 degrees C) of only 2.45 amps RMS. Which is why I wince whenever I see someone thoughtlessly slapping some monstrously huge power transformer into an amplifier without giving any consideration whatsover to the ripple current rating of the caps.

Anyway, point being that peak ripple current remains rather constant regardless of what the amplifier's drawing at any given time.

Not all designs are so generous. I ‘refreshed’ a Marantz pre and power amp for a friend including placing new ‘grain of wheat’ globes around the big meters. Looked great but the lights dimmed in time with the music when the amp was pushed. I think the moderate transformer may have had something to do with this :)

Well you can somewhat make up for a weedy power transformer by increasing reservoir capacitance. Just be sure to consider ripple current when making up for a weedy power transformer by replacing it with a higher VA transformer.

I note that extra headroom would not be of much help for class A amplifiers or tube amps (heater supply circuit). I've replaced a few bridge rectifiers on heater circuits. I can’t remember if I did the caps as well - I must have? Class A amps are hot all over - heatsink, power supply, case, etc.

Well "headroom" is lragely marketing smoke and mirrors. It's basically just another way of stating the load regulation of the power supply. In other words its power output at some average level versus its power output at peak level. The greater the "headroom" figure, the weedier the power supply. Of course class A amplifiers keep the power supply loaded down more constantly versus class B or low-bias AB so its power output remains more consistent and therefore less "headroom."

As for the shunt cap extending life, I read it again just a day or so ago but can't find where.

Well if you find it again, let me know. I'd like to see the argument behind it. I still can't think of anything off the top of my head that would account for it but I may be overlooking something.

se

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