Current limit onset definition?

Briefly. I understand a 15 amp circuit at 120 voltd is 1800 watts available to the amp. How does this factor in?

The 683 watt spec (for a 2 ohm load), like most amplifier output power ratings, is a rating of how much power it can supply CONTINUOUSLY. Note in the specs for the DMA-180 that just above that number and the numbers for 4 ohm and 8 ohm loads, it says "power output (continuous)."

Amplifier continuous output power specs are also usually based on the assumption that the load impedance is purely resistive, although that is a very rough approximation at best for most speakers. For a resistive load the relation between power, current, and resistance is as follows (note that in this context "amps" is short for "amperes"):

P = (I squared) x R

where P = power in watts.
I = current in amps.
R = resistance in ohms.

Based on that equation it can be calculated that 683 watts into 2 ohms corresponds to 18.48 amps into the 2 ohms.

For the specified ratings into other load impedances:

400 watts into 4 ohms corresponds to 10 amps.
200 watts into 8 ohms corresponds to 5 amps.

I'm somewhat skeptical, btw, that the amplifier can really supply 683 watts per channel into 2 ohms on a continuous basis. The amplifier is specified as having two 4 amp AC line fuses for 120 volt operation, one fuse for each channel. 4 amps x 120 volts = 480 watts. Since the amplifier can't continuously put out more power than it takes in, it would therefore blow the fuse if it were required to put out 683 watts for a sustained period of time.

In any event, though, the 60 ampere current rating has nothing to do with any of that. As I indicated in my earlier posts that rating can be presumed to represent what the amplifier can put out into a short circuit (0 ohms) for a few thousandths of a second. A few thousandths of a second later the amplifier's self-protection mechanisms would kick in, to protect it from the damage it would otherwise suffer it it were to supply such an excessive amount of current for a significant amount of time.

I understand a 15 amp circuit at 120 volts is 1800 watts available to the amp. How does this factor in?

The amplifier is specified as consuming a maximum of 1600 watts, and 250 watts at idle. A 1600 watt draw by the amp would presumably only occur for brief amounts of time, or else the fuses in the amp would blow, as I indicated. Also, for brief amounts of time (say fractions of a second) the AC outlet and its associated circuit breaker and wiring can supply much more than 15 amps, without the breaker tripping. That kind of brief surge of large amounts of current commonly occurs when an appliance such as a refrigerator or an air conditioner compressor switches on, which is why lights are sometimes seen to dim briefly at those times. The large amount of current times the resistance of the house wiring results in a substantial voltage drop in the house wiring, for the brief duration of the current surge.

Regards,
-- Al

P.S. to my previous post: I found some rear panel photos of the DMA-180, which although difficult to read I believe indicate that the AC line fuses for 120 volt operation are 6 amp fuses, not 4 amp fuses as indicated at the Spectral website. That seems more consistent with the 683 watt 2 ohm continuous output power rating (6 amps x 120 volts = 720 watts AC input), although still marginal at best, as I would feel safe in assuming that the amplifier is not close to being 683/720 = 95% efficient.

Although in the photos it's hard to tell for sure if the 6 is really a 6, and not an 8. 8 amps x 120 volts = 960 watts. 683/960 = 71% efficiency, which roughly speaking would seem to be in the ballpark of being reasonable.

Regards,
-- Al

Thanks very much Al. On my DMA180 the fuses are specified as 8 amp for 100 and 120 volts. 4 for 220.
I bought my Spectral and MIT gear based on discussions with MICHEAL PERCY, who sells components via his website. I am reminded that he was especially knowledgeable and helpful. From my experience one can be very confident in dealing with Michael. I would not hesitate to send a bank draft or money order-with my order. He is a reliable gentleman.

The current rating is the amount of current that flows when the power supply is shorted out for 10 milliseconds. As Al pointed out, it has nothing to do with the power or the current that the output section can make.

The rating is really only there to show how the power supply is built up, and it is without question confusing! For example, you might be surprised normally to find that a tube amp (and for that matter an OTL) has the same rating. But we are not talking about anything to do with the output; simply the power supply itself. more:

http://www.atma-sphere.com/Resources/Common_Amplifier_Myths.php