What Determines The Amount Of "Current" In An Amp?


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For years I've heard that an amp needs lots of 'current' to drive certain speakers. What in an amps' design will determine current?

Also, how does current differ from watts per channel in the ability to drive a speaker?
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Convert?fit=crop&h=128&rotate=exif&w=128mitch4t
current is one of factor of power(watts per channel). additional current is provided during peaks that is part of discharge of filter capacitors
Per Ohm's law, Current = Voltage/Resistance (I=V/R).

The less resistance in a circuit the more current (power to do work) for a given amount of voltage.

So current is power (and I thought it was knowledge!).
There is confusion around this topic!

Power (wattage) does not exist without voltage.

If on the Voltage rule, loudspeakers are described as being 'Voltage Driven'. This is a sort of shorthand- it means that the amplifier is operating as a voltage source, which in turn means that the amplifier, if driven with a sine wave sweep from 20Hz to 20KHz, will put out the same voltage into the speaker load regardless of the impedance of the speaker.

Essentially this means the amp will be putting out twice as much power into 4 ohms as it does into 8 ohms.

But power still plays a role! and there is still the question of how much current you really need. The problem here is that the industry does not spec the current by itself and so amplifier companies are free to come up with their own expressions, which quite often have nothing to do with the power that the amp makes, and everything to do with how much current is present when your short out the power supplies in the amp!

You might want to read these two articles, which explain some of the intricacies.
http://www.atma-sphere.com/Resources/Common_Amplifier_Myths.php

and

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

The bottom line is how much current your amplifier will need to have will depend on the impedance of your speakers and how efficient they are, such that you will get a satisfying volume in your room.

For example if your speakers are 4 ohms and 100 watts is enough power, your amplifier will need 5 amps, which will allow it to make 100 watts (full power) into 4 ohms.

This is simplified of course, but without further information its a good place to start. If we know the speaker involved and the size of the room, a better calculation can be made.
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300 wpc amp at 8 ohms plays 4 ohms at 450 wpc

300 wpc amp at 8 ohms plays 4 ohms at 600 wpc

Does one of the above amps necessarily have more current than the other? If an amp doubles with each halving of ohms, is that an indicator of its ability to deliver more current?

Is there an identifiable specification in an amp that will tell you how much current it will be able to deliver?

The notorious Apogee Scintilla and Infinity Kappa 9 are two speakers that dip to amp-killing low ohms. What specification in and amp do you look for in its ability to drive a speaker like these. Is it current or wpc you would be looking for in a speaker like these?
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The power supply and output transistors are the primary deteminants of ability to pass current.

The second amp in the example above is able to pass more current, all things being equal.

I don't look for amps that can drive those speakers and never will; there are many good loudspeakers that are high sensitivity. However you will need a high current amp that is stable into low impedances with excellent heat sinking, or a fan, if that is what you want to drive, since the lack of efficiency of these speakers, coupled with the stupid impedance curve and venturesome phase angles, will require continuous high current output as well as high peak current output.
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Thanks Viridian.

In the very big amps of old, they boasted large power supplies. Is it necessary for Class D amps to have a large power supply to deliver continuous high current into low ohms?
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The majority of Class D amps use a Switched Mode Power Supply (SMPS) design. SMPS are also sometimes used with amps in other classes of operation like Class A/B. SMPS designs do not require large transformers as conventional linear PS designs do, since they run much more efficiently. Class D output stages (independent of whether they use an SMPS or linear PS) are also very efficient, so they do not require large heat sinks that other designs do. Class A biased output stages are the most inefficient and therefore require an extremely large amount of heat sink area.
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Yes Bill, but can Class D amps deliver high sustained current into low ohms?
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Ralph, are you saying that the amp is delivering 20 volts to the speakers in the above example? 100=20*(20/4).
Everything is relative, and it depends upon the specific design. One example which uses the Hypex nCore 1200 modules is the Merrill Audio Veritas which is rated at 38 amps max current output, and 400w into 8, 700w into 4, and 1,200w into 2 ohm loads. It isn't clear how long it can provide that amount of current before thermal overload/shutdown.
Where can I purchase a Linear regulated power supply at a decent price?
For my Digital amplifier.? It would be greatly appreciated thank you,Paul.
Some speakers which dip to very low impedances at certain frequencies don't require lots of current, but rather an amp that is stable into a highly-reactive load that presents an almost dead-short operating condition, with, as Viridian mentioned, wild phase angles, a combination that make some amps oscillate in a state of instability. One such speaker is the original Quad, which was sometimes faulted for the sins of the amp driving it.
05-11-15: Mitch4t
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300 wpc amp at 8 ohms plays 4 ohms at 450 wpc

300 wpc amp at 8 ohms plays 4 ohms at 600 wpc

Does one of the above amps necessarily have more current than the other? If an amp doubles with each halving of ohms, is that an indicator of its ability to deliver more current?
yes, the 300W/8Ohms, 600W/4Ohms amp has the ability to supply more current into the load when these 2 amps are compared side-side.
yes, when an amp can double in power for each halving of the load impedance is an indicator that such an amp can supply more current to the load compared to another amp does not double in power for each halving of the load impedance.

Is there an identifiable specification in an amp that will tell you how much current it will be able to deliver?
not really. Atmasphere/Ralph already addressed this point when he said that the industry does not have a standardized method the spec output current & what the manuf do is specify short-circuit current i.e. current delivery capability when the speaker binding posts are short-circuited for a short period of time (such as 10mS). It's no real measure of current since the music signal is faster moving than 10mS.
One hint that an amp can deliver high current to a speaker load is to see the power output specification - if the amp doubles in output power for each halving of the load impedance, it does indicate that such an amp has a large, high-current power transformer & a very robust output stage that can deliver high current. Amplifiers that come to mind are the older Krells, Gryphon Audio, Clayton Audio, Pass Labs, Threshold (to some extent 'cuz they didn't spec their amps below 4 Ohms most of the time).

The notorious Apogee Scintilla and Infinity Kappa 9 are two speakers that dip to amp-killing low ohms. What specification in and amp do you look for in its ability to drive a speaker like these. Is it current or wpc you would be looking for in a speaker like these?
I should be able to answer this one as I own a 1-ohm Apogee Scintilla. ;-) When selecting a suitable amp I looked for both WPC & current delivery capability. I found that looking at only one spec was not sufficient - I needed high current into a 1-Ohm impedance load & I needed voltage to prevent the output stage from slamming into the power rails.
For example: let's take your 300W/8 Ohm amp that you cited in your prev post. It needs a 50VAC secondary transformer to generate its 300W/ch. The manuf could install a 400VA/50VAC dual secondary power transformer. You can do the math - it means 8A of total secondary current. Since it's a stereo amp (I assumed this), it'll be 4A per channel total. That's not a lot of current when driving a Scintilla or a Kappa 9. Further the manuf could advertise that the output has 16 or 32 matched output transistors each rated for 1A. You might be impressed - what a mother load of output transistors; i'll get plenty of output power. Wrong! the current is limited by the 400VA/50VAC power transformer & it doesn't matter that the output stage has 16/32 match transistors. So, when you read the power output spec of such an amp, it'll be like your 1st cited amp: 300W/8, 450W/4 - it won't double in power as the load halves. For that to happen, the power transformer will have to be larger.

Also, how does current differ from watts per channel in the ability to drive a speaker?
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Mitch4t
As you well know, Power = Voltage * Current.
For a high wattage amplifier to deliver high current into a load, the amp needs to have a large power transformer which is rated for high current. Otherwise, that high wattage amplifier has, what I call, 'empty watts' - it's a large number of watts without substantial current delivery support. It's a term coined by me for my personal use. And, as you can tell, 'substantial' is a very fuzzy term. How much is substantial? I do the calculation for each scenario that i'm evaluating & determine if it's substantial or not.

hope this helps....
If you ain't got enough current, I can sell some Amperes for you (i can pour'em to the medicine bottle); trade-ins are OK as well.
05-11-15: Mitch4t
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Thanks Viridian.

In the very big amps of old, they boasted large power supplies. Is it necessary for Class D amps to have a large power supply to deliver continuous high current into low ohms?
In general, yes, SMPS power supplies are capable of supplying large amounts of current into a low impedance. Today one can see several class-D amps capable of 20-30A into a low impedance such as 2 Ohms. Bill_k already cited the Merrill Veritas. There are a few others too - names are eluding me right now....
In a SMPS the amount of power delivered depends on the size of the power inductor & the size of the output power switches (that switch with PWM control, most often). Ultimately, in SMPS power supplies too the amount of output current is limited by the size of the AC power transformer because SMPS too have to take as input the AC from the wall outlet, rectify it to DC & regulate that for the class-D power amp. Whatever current the class-D power amp needs comes from the SMPS power supply & in turn the SMPS power supply takes its current from the wall outlet i.e. AC power transformer.
So, high current is possible thru a SMPS-based class-D power amp - just have to size the AC power transformer correctly (given that the SMPS design is correct for high current).
Many class-D amps are stated to be stable down to 2 Ohms but I see very few capable of stability into 1 Ohm. Part of the reason could be that very few 1 Ohm speakers exist today.