Current limit onset definition?


My Spectral DMA 180 indicates in specs that Current Limit Onset is 40 amps. It also says peak current 60 amps. Anyone know what this means?
ptss
The 60 amp figure is the maximum amount of current the amplifier can supply into a very low impedance (probably a short circuit, i.e. zero ohms) for a very short period of time (probably a few milliseconds). It is not representative of anything the amplifier would be able to do under realistic operating conditions, as 60 amps into 2 ohms is 7200 watts; 60 amps into 4 ohms is 14400 watts; 60 amps into 8 ohms is 28800 watts.

The 40 amp figure is the point at which one of the self-protection mechanisms in the amplifier starts to kick in. I suspect that the reason that figure is less than the 60 amp figure is that the response time of the protection mechanism is a bit slower than the amount of time the 60 amp figure is based on. In other words, the amplifier could provide more than 40 amps into a short circuit for a few milliseconds, but a few milliseconds later the protection mechanism would kick in and either limit that current or shut down the output altogether.

I have no specific knowledge of the design of Spectral amplifiers, but that would be my interpretation of those specs.

Regards,
-- Al
Almarg... since the Spectral DMA 180 is 200 Wpc amp into 8 ohms and can deliver 683 Wpc into 2-ohms, wouldn't it look like a melted puddle of sludge at any of the wattage figures you presented? I'm assuming this is wattage pulled out of the wall (not delivered to the speakers) we're speaking of... but really!!!)???
Hi Plato,

I'm not sure that you read my comment accurately.

The point to my mentioning those ridiculous wattage numbers was to illustrate that, as I said, "the 60 amp figure ... is **NOT** [emphasis added] representative of anything the amplifier would be able to do under realistic operating conditions."

As I indicated, the 60 amp figure most likely represents how much current the amplifier would be able to output into a dead short across its output terminals, for a few thousandths of a second. Since 60 amps into a realistic load impedance (2 ohms, 4 ohms, 8 ohms, etc.) would correspond to an absurd amount of power, the amplifier is obviously not capable of putting an amount of current remotely close to 60 amps into those impedances.

Some further comments:

A peak current spec such as that is therefore essentially meaningless. In saying so, btw, I'm not singling out Spectral, as a lot of other amp manufacturers provide similar specs. And adding to the meaninglessness of that kind of spec is the fact that the amount of time the specified peak current can be sustained for never seems to be indicated.

A more meaningful indication of the current capability and general robustness of a solid state amplifier would be whether or not its maximum rated continuous power capability doubles into 4 ohms, relative to 8 ohms (or if not, how closely the 4 ohm rating approaches being double the 8 ohm rating), and doubles again into 2 ohms, relative to 4 ohms (or if not, and if the amp is rated for 2 ohms, how closely the 2 ohm rating approaches being double the 4 ohm rating or four times the 8 ohm rating).

Regards,
-- Al
Peak current is akin to saying a circuit breaker may let through 150A of *instantaneous* current before it trips (in fact, in a short circuit that many instantaneous amps aren't unusual before the breaker trips or wires start to melt); and continuous current is akin to saying the breaker may trip when you reach that many continuous amps flowing through.

Instantaneous current will only be available in low impedances, and as a "spec" it's mostly meaningless, other that to probably somehow try to infer high slew rates (though they are spec'd in Volts, so there, this is why it's really meaningless to me)...
I'm still not clear how Spectral is defining amps
if in fact it outputs only 683 watts into 2 ohms.
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
The "6" is indeed an "8" - 8A. see this clear picture of the back panel at http://hifi.myinsales.ru/collection/usiliteli/product/spectral-dma-180

Newer amps have 5A fuses.
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
Clearly the 40A figure has nothing to do with power draw...
01-05-15: Ack
The "6" is indeed an "8" - 8A.... Newer amps have 5A fuses.
Thanks for the info, Ack. It seems that the specs at the Spectral site for fuse ratings have not been done very carefully. In addition to the discrepancy regarding the DMA-180, the ratings listed in the specs for all four of the currently produced amplifiers (2.5 amps for three models, 5 amps for the other, for 120 volt operation) would in every case not allow as much power into the amplifier as it is rated to output continuously into a 2 ohm load. And likewise for a 4 ohm load for two of the four amplifiers, with zero provision for internal power consumption or margin in one of the other two cases. (In saying this I'm assuming that the output power ratings for the stereo amplifiers are per channel, although that is not explicitly stated). Also, the top of the line DMA-400 monoblock is described as having two 5 amp AC line fuses, when the picture clearly shows only one fuse being provided (as can be expected).
01-06-15: Atmasphere
The current rating is the amount of current that flows when the power supply is shorted out for 10 milliseconds.... But we are not talking about anything to do with the output; simply the power supply itself.
Ralph (Atmasphere), thanks for your comments also. As a point of information, all of the peak current specs Spectral provides at their site for the recent models as well as for the DMA-180 are described as "peak **output** current" [emphasis added]. So I would have to assume that **if** their wording is accurate those specs are based on the short we have referred to being applied to the output of the amplifier, rather than to the output of the power supply. The numbers, btw, are 60 amps in each of those cases, except for the DMA-400 monoblock which is 90 amps.

Best regards,
-- Al
Ack, confused, there are no newer DMA180's that I'm aware of.
What do you mean?
Atma, I'm even more confused, sorry. What does the 40 amp have to do with?
Atma, did you mean 40? or 60?
I appreciate you have genuine knowledge on these things.
01-07-15: Ptss
Ack, confused, there are no newer DMA180's that I'm aware of. What do you mean?
He means newer models having different model numbers (DMA-200S, DMA-260, DMA-300, DMA-400). Although as I indicated in my previous post the first three of those models are specified at the Spectral site (perhaps incorrectly) as having 2.5 amp fuses for each channel, for 120 volt operation.

Regards,
-- Al
Ralph (Atmasphere), thanks for your comments also. As a point of information, all of the peak current specs Spectral provides at their site for the recent models as well as for the DMA-180 are described as "peak **output** current" [emphasis added]. So I would have to assume that **if** their wording is accurate those specs are based on the short we have referred to being applied to the output of the amplifier, rather than to the output of the power supply. The numbers, btw, are 60 amps in each of those cases, except for the DMA-400 monoblock which is 90 amps.

Giving it the benefit of the doubt (using the power formula), 90 amps squared is 8100 watts into one ohm (into 2 ohms it would be 16,200 watts...). Looking at the specs we see that the 2 ohm power is no-where near that. So the current cannot be anything to do with the output section! It is how much current occurs when the power supply is subjected to a dead short for 10 milliseconds. It is a measure of the power supply capacity and really not anything else.

By the same measure our MA-2 amplifier, which is an OTL, makes about 60 amps.
Hi Ralph,

Yes, but it is also true that 90 amps into 0 ohms is 0 watts (or, more precisely, some relatively small amount of power since no conductor can be exactly 0 ohms, assuming the temperature is not near absolute zero). Isn't it possible **in the case of a solid state amplifier** that the dead short is placed across the output of the amp, and the current through that short which results from application of a 10 ms signal to the input of the amp is measured?

Best regards,
-- Al
No, I don't think so. 90 amps through a 600 watt output section would toast the output devices in a heartbeat. Or less.

The 90 amp figure simply has nothing to do with the output section. It has to do with the power supply.
Ralph,(Atmasphere I think). Why square the 90?
I see nothing about squaring in the power formula.
Why do you say 600 watt output section?
Ptss, the amount of power delivered into a resistive load is:

P = (I squared) x R

where P = power (in watts)
I = current (in amperes)
R = resistance (in ohms)

An alternative formula which produces the same result is:

P = (E squared)/R

where E = voltage (in volts)

Both of those formulas can be mathematically derived from the definition of power (E x I for a resistive load) in combination with Ohm's Law (I = E/R).

Also, the reason Ralph referred to a 600 watt output section is that most of the amplifiers which have been discussed are rated as being able to continuously supply around 600 to 700 watts or so into a 2 ohm load.

Regards,
-- Al
Thx Al. I'll look into this.
Thanks Al.