Need understanding on amperage

To further clarify.

When you look at a "current rating" spec you need to know at least two things:

(1) At what voltage is the current being specified.

(2) Is the rating CONTINUOUS or transient.

The ratings on wall power are usually continuous maximums (in root mean square or RMS) and are at your line voltage of 120 V AC.

The 60 amp rating for a power amp like the Parasound are TRANSIENT, good for only a very short time and at a voltage much lower than your line power levels.

Make no mistake, if that A21 tried to output 60 amps for more than a few milliseconds, fuses would blow.

In the old days, manufacturers were required to avoid transient power/voltage/current specs because they can be so misleading for consumers. Only RMS continuous specs were allowed to be published.

From the A21 schematic published by Parasound, each power buss to the output stage is fused to 8 amps. No amplifier fused for 8 amps will run 60 amps very long before vaporizing the fuse link.

From the Littlefuse data sheets for an 8 amp fast blow fuse, 60 amps blows the fuse in about 10 milliseconds. Each channel has 50,000 mfds of capacitance, and left and right channels have separate power supplies (so the full 100,000 mfds is not applied to a single 60 amp transient, only 50,000 mfds is, and those are further divided to a V+ cap at 25,000 and a V- cap at 25,000 mfd).

If you calculate the drop in voltage of the 25,000 mfd capacitor, I=C*dV/dt, so I=60 amps, C=25,000 mfd, and dt = 10 milliseconds (the time to blow the fuse), we find dV = 24 volts, or about 1/3 of the 80V supply voltage.

So if you shorted this amp, it would deliver 60 amps for the 10 milliseconds needed to burn the fuse, and have plenty of power supply voltage left in the capacitor.

It could potentially drive a 1 ohm load to 60V (or 3600 W) for 10 milliseconds. That's it, and it's limited by the fusing.

Atmasphere/Tonywise

Yes, the remaining voltage would be 56 volts, and the average power delivered would be between 3600 and 3136 Wts. And it may be somewhat less because we are not accounting for the voltage drop across the output stage transistors (it won't be zero, and will be dependant whether the transistors are being driven into saturation or not).

Guys, it was just a ballpark analysis. It also did not include the re-charge of the capacitor by the transformer or that perhaps both the V+ and V- capacitors might be discharged partially.

My point being is that the delivery of 60 amps to the load will be limited by the fuse characterisitcs primarily. Whether it is 10 milliseconds or 15 milliseconds is not that important.

BTW, I would hope that the fuse was sized to prevent the bipolar output transistors from going into secondary breakdown. If this happens, the power transistors fail and the entire output stage is toast before the fuse can protect them.

Yes, I have often wondered whether this claim for 60 amps by Parasound was an actual measured performance spec or strictly hypothetical "potential" capability. I guess only John Curl can answer this question.

If the current limiters kick in long before the 60 amps, then its hypothetical. Something I did not consider in my analysis.

Tony,

It is possible that the entire output stage could fail if the safe operating area of those transistors is not broad enough to allow the fuse to burn before the transistors fail. This is the potential weakness of having a plurality of power transistors in parallel in the output stage. The weakest one fails first, then the others have to carry the remaining current and they start failing in a cascade phenomena.

Atmasphere:
You are correct re the re-charge. 1/2 sine at 60 Hz is about 8 milliseconds, so it would depend on where in time the transient occured. Nevertheless, I doubt of the power transformer could supply anything near 60 amps. Bombay also seems to assume the 8 amp fuse is in the AC input side but it is actually on the DC power busses supplying the output stages.

Tonyw:

Murphy's law should have nothing to do with it. But the amp designer should by specing the right power transistors. Safe operating area data are provided by all the power transistor manufacturers.