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Re: black gates in parallel

Posted by Robert Karl Stonjek on December 21, 2001 at 23:52:45

In Reply to: Re: black gates in parallel posted by Steve Eddy on December 21, 2001 at 22:46:00:

Thanks Steve, most informative. A couple of questions (since you’ve clearly studied this area)
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 whatsoever to the ripple current rating of the caps.

The transformer in question would need a VA rating of 4,175VA (167 amps at 25 volts). At least that’s how the nuvotem catalogue explains it. But where does all that current go?

Let me think about my own amplifiers and follow it through. 1,000VA transformer steps down from 240V to 110 centre tapped (2*55V). The current is given by the manufacturer as 9ARMS, that’s 9 amps ‘all the way around’ (from one phase to the other) so 9*55*2=990W which translates to the 1,000VA at the 240V side.

The capacitors have a ripple rating of 30 amps @ 20kHz @ 70 Deg, and 15.8 amps @ 100Hz @ 85 Deg. There are four of these in circuit, two per rail in parallel. I take it the ripple current adds in parallel so at 100Hz its 32.6 amps per rail.

Well the transformer is only going to let them have 9. Now, my question is ‘where does the current go’. You said:-
Anyway, point being that peak ripple current remains rather constant regardless of what the amplifier's drawing at any given time.

So we are talking idling current (amplifier doing nothing). I rise from my seat and test this theory.

Ok, input current is 380-400mA (average) but has sharp peaks at 50Hz (the ripple). The peak to peak reading was 2.4ARMS (double checking using the meter function). Using the meter function I just get 400mA.

Now the low voltage side. This is a surprise - average reading is 760mA, peak to peak is still 2.5ARMS.

Wait, penny drops. That’s a half wave current!! For a whole wave it would be twice as much, ie 5 amps or about double the primary current. So the wattage is the same on both sides of the transformer.

Ok, the capacitors are near new and the amplifier hasn’t been used very much so I wouldn’t expect much deterioration (in 6 months) so what gives?

Is the peak ripple to narrow/quick to measure? I’m using a Fluke scopemeter. Should cope with the peaks, shouldn’t it? The probe is rated at 100kHz.

Be interested in your comments.

Kind Regards,
Robert Karl Stonjek

PS by "where does it go" I mean "why doesn't the amplifier blow the house fuses when it should be drawing 18 amps (two amplifiers at 9 amps each)". Note that this is a question and not an accusation or such.

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Kimber Kable

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