Stupid/Basic Questions about Power and Current for Amps

[tmtomh]

Continuing with my "better to look stupid and learn something than keep quiet and remain ignorant" theme, I have a couple of questions about amp current capacity.

Let's say you have an amp - like the Purifi 400 module - that's rated for max current capacity of 25A. And let's say you're driving a low-impedance, 2-ohm speaker (I know impedance varies with frequency - let's say that's the average for the sake of discussion).

My understanding is that 25 amps into 2 ohms gives 50 volts and 1250 watts, yes? But the Purifi 400 module is rated at 450w into 2 ohms, which works out to 15 amps. For 8 ohms it's even more absurd: the module is rated for 225 watts into 8 ohms, but 25 amps of current into 8 ohms is 5000 watts.

So it would appear the current capacity of the amp far exceeds its maximum wattage. This would suggest that the amp can drive almost any load without ever getting anywhere near its maximum current capacity - or, to put it differently, that if it's driven into clipping or protection it will not be because too much current is being drawn. But that doesn't make any sense (does it)? So what obvious thing am I missing?

Finally, a perhaps even dumber question: most home audio setups are going to be plugged into 20-amp circuits. How can one make full use of high-current amps rated at 25-40A if they're plugged into 20A electrical circuits?

Thanks in advance for setting me straight!

 

[DVDdoug]

I'm not sure what the amplifier's "current rating" means... But if it has excess current capability that would mean the output power is limited by the voltage.

A well regulated power supply with adequate current capability means that the voltage "holds up" under load and the voltage, along with the minimum rated impedance is what limits the power..

Finally, a perhaps even dumber question: most home audio setups are going to be plugged into 20-amp circuits. How can one make full use of high-current amps rated at 25-40A if they're plugged into 20A electrical circuits?

The transformer in the power supply can convert high-voltage low-current into lower-voltage higher-current.

The "odd thing" is, some amplifiers are rated for more power than you can get out of a standard outlet! An amplifier is always less than 100% efficient and there's no way you can get 5000W from a standard power outlet so you can't get 5000W continuously from an amplifier unless you have a "special" power source. I've never seen amplifier specs that call for a high-current power outlet... Capacitors store power that can be delivered in short bursts but you can't get more continuous power out, or more average power out, than is coming from the power outlet. Music or normal audio doesn't require continuous full-power but the specs are fishy.

 

[Mnyb]

Amps can voltage clip or current clip depending on impedance it’s usually volts that is the problem it’s asked to reproduce a waveform bigger than power supply rating say 50 volts .

A modern amp is basically a voltage source ( simplified) the current drawn is merely a consequence of the impedance of the circuit driven ( your speakers ).

U=I*R ( or U=I*Z )

In your ballpark calculations you probably assumed only resistive impedance a more real complex impedance can make everything draw more current , so called 8 ohm speakers are rarely that in all their range .

And then it’s a lot of marketing fud even in good products, context matters one must consider how the spec is measured ?

 

[Mnyb]

I'm not sure what the amplifier's "current rating" means... But if it has excess current capability that would mean the output power is limited by the voltage.

A well regulated power supply with adequate current capability means that the voltage "holds up" under load and the voltage, along with the minimum rated impedance is what limits the power..


The transformer in the power supply can convert high-voltage low-current into lower-voltage higher-current.

The "odd thing" is, some amplifiers are rated for more power than you can get out of a standard outlet! An amplifier is always less than 100% efficient and there's no way you can get 5000W from a standard power outlet so you can't get 5000W continuously from an amplifier unless you have a "special" power source. I've never seen amplifier specs that call for a high-current power outlet... Capacitors store power that can be delivered in short bursts but you can't get more continuous power out, or more average power out, than is coming from the power outlet. Music or normal audio doesn't require continuous full-power but the specs are fishy.

That’s is what’s saves your fuses music is not continuous at all so the stored energy in you power supply can deliver peaks that far exceeds the average power you migth get from your outlet.
In normal listening you need good instruments to se a difference between playing music and idle consumption in many cases

 

[KSTR]

My understanding is that 25 amps into 2 ohms gives 50 volts and 1250 watts, yes? But the Purifi 400 module is rated at 450w into 2 ohms, which works out to 15 amps. For 8 ohms it's even more absurd: the module is rated for 225 watts into 8 ohms, but 25 amps of current into 8 ohms is 5000 watts.

So it would appear the current capacity of the amp far exceeds its maximum wattage. This would suggest that the amp can drive almost any load without ever getting anywhere near its maximum current capacity - or, to put it differently, that if it's driven into clipping or protection it will not be because too much current is being drawn. But that doesn't make any sense (does it)? So what obvious thing am I missing?

Power ratings are for sinusodial signal and resistive load and restricted by maximum output voltage that can be sustained for the required current. That maximum can either be reached by voltage clipping, or by current clipping, or by thermal overload. Note that the voltage and currents you derive from that are RMS values, that is, the peak values are 1.42x of that.

Because loudspeakers are reactive loads, in extreme cases momentary peak currents can be 2x...3x higher than the peak current that would be induced by a resistive load of nominal value.
In case of the Purifi the 2-Ohms rating is already on the edge because the resistive peak current is 21.3A, there is not much headroom left until the current limit at >= 25A kicks in. For 4 Ohms, OTOH, the headroom is now almost 2x and thus the amp will most likely never run into current overload with 99.9% of music signals.

So to answer your question, yes, the amp will not be limited by current overload unless you a) go below 2Ohms purely resistive load or b) you have a very nasty speaker impedance paired with totally compressed/clipped music signal so that peak current limit is reached.

 

[Blumlein 88]

What you want to see in an amp.

A little less capable.

Less capable, but able to do something into 2 ohm loads.

Also often amps note peak amperage, which is going to be at peak voltage, not the RMS voltage. So you can reduce that amperage rating by .707. Or it might refer to a peak amperage at peak voltage of the power supply which might be well more than peak voltage at lower distortion levels of the amp. So I've found peak amperage ratings to not have all that much meaning in terms of use. The test like Amir has done with recent amps under review will tell the story.