Current vs Watts

I was visting a high end audio retailer (their are a few lest) and was looking at several amps/intergrades at similiar wattage rating (I'll leave the brand names out for discussions sake) and he was explaining that one of the models had a higher current rating (looked like a larger power supply) and thus had the ability at driving complex loads cleaner.
When I left, it dawned on me I have seeing these various rating for years and just assumed this was true. And I can't say I have really ever thought about this much when auditioning amps- I have always used the wpc rating (at 20-20,000 of course) as a guide, and listening as the ultimate decision maker.
If someone has the time, I would like to really know the watts vs current relationship in amplifier performance.

Watts are voltage (v) x current (I), and also can be calculated with I squared x resistance (R). The more current an amp can produce, the less voltage drop you will get as resistance decreases. That's why some amps can provide 120 watts @ 8 ohms, or 240 watts @ 4 ohms, because they can still put the same voltage across the 4 ohm load. Some amps can't so you don't get twice the wattage at half the impedence. An amp with higher current will probably be better for deep bass notes, or loud dynamic transients.

how the amp will handle complex (i.e., reactive) loads depends on more than its ability to deliver large amounts of current.

As an example... Polk Audio's 1970s era Cobra Cables, which manifest high capacitive reactance, were notorious for driving some marginally unstable amps into ultrasonic oscillation, with catastrophic results.

The traditional problem is poorly implemented (and excessive) negative feedback.

Watts is amps x volts.

OK, I'll explain.

Amplifier A's transformer can produce a max of 3 amps rms. It can produce 40 volts RMS. With an 8 ohm load, 40 v / 8 ohms = 5 amps. Oops! It can't produce 5 amps. It can only produce 3 amps. 8 ohms x 3 amps = 24 volts required. So, at 24 volts and 3 amps into an 8 ohm load, it can produce 24 v x 3 A = 72 watts. But it sure isn't going to produce 40 v x 3 A = 120 watts. Not with 8 ohms. It can only produce 72 watts continuous, but with some good headroom.*

What if we put a 4 ohm load on the amp? In that case 4 ohms x 3 amps = 12 volts. The transformer can only produce 3 amps. That's it. So, 12 v x 3 A = 36 watts. That's all folks. 36 watts with the 4 ohm load.

What if we used another transformer, one that also produces 120 watts, but with a different volt and amp relationship?

OK, let's look at another amplifer, Amp B. This amp use a transformer that will produce 24 volts and 5 amps. 24 v x 5 A = 120 watts, same as Amp A.

Let's try this with an 8 ohm load. 24 v / 8 ohms = 3 amps. Yep, this amp can do that, it can produce 5 A. OK 24 v x 3 A = 72 Watts. Same as Amp A.

Let's see what it does with a 4 ohm load. 24 v / 4 ohms = 6 amps. Oops! Can't do that. OK, work the other way... 4 ohms x 5 A = 20 volts. Yes, the amp will produce 20 volt, in fact, it will make 24 v.

So, 20 v / 4 ohms = 5 A. And 20v x 5A = 100 w with a 4 ohm load.

Both amps have transformers that will produce 120 watts.

Amp A produces 72 watts into an 8 ohm load, and due to excess voltage, has some good headroom.

Amp A will only produce 36 watts into a 4 ohm load.

Amp A has plenty of voltage, but is limited in current, so it does not perform well with with low impedance loads. Yet it has lots of headroom for momentary burst of high power.

Amp B will produce an identical 72 watts into an 8 ohm load, only with no headroom.

Amp B will produce 100 watts into a 4 ohm load, with good headroom.

Amp B is limited by voltage, but has lots of current. It will easily handle low impedance loads.

Note:

* Amp A, 40 v / 16 ohms = 2.5 A. 40 v x 2.5 A = 100 watts. 100 watts may be impressive, but who has 16 ohm speakers these days?

So, what does all this mean? For an amp to be "high current" look for its 4 ohm power CONTINUOUS rating to be equal to or greater than its 8 ohm power CONTINUOUS rating.

hypertone said:

Watts are voltage (v) x current (I), and also can be calculated with I squared x resistance (R). The more current an amp can produce, the less voltage drop you will get as resistance decreases. That's why some amps can provide 120 watts @ 8 ohms, or 240 watts @ 4 ohms, because they can still put the same voltage across the 4 ohm load. Some amps can't so you don't get twice the wattage at half the impedance. An amp with higher current will probably be better for deep bass notes, or loud dynamic transients.

Click to expand...

In AC power can most easily be expressed V*I*Cos Phi (I can't remember the abbreviation for the angle, it is pronounced FEE0.

That takes into account the reactivity of the load. V times I is the maximum it can be. The phase angle cosine starts at one and becomes zero at ninety degrees.

But take note: Most speakers are a difference impedance at every frequency.

An audio amplifier is (or tries to be) a voltage source. What that means is for any given volume setting it puts out a certain voltage and tries to maintain that voltage at it's output. Watts are then determined by the amount of current developed when this voltage is applied to the load (speaker impedance).

For example, let's use a 100 watt amplifer...

A 100w amplifer (driven to rated output) will have an output voltage of about 28 volts and produce about 3.5 amps of current into an 8 ohm load.

Because of the voltage source characteristics, that amplifier will try to maintain our 28 volt example even as the load increases/impedance drops.

Theoretically, that 100w amplifer should produce 200w at 4 ohms because our 28 volts applied to a 4 ohm load results in twice as much current as it did at 8 ohms, or about 7 amps. Some amps with large power supplies and or regulated power supplies will do this and "double down" as it's often referred to. However, most won't because of the current limitations of the power supply.

So, what happens to most amps is that they reach a point with increasing load where the power supply cannot produce enough current to maintain the output voltage. When this point is reached, the output voltage begins to sag, and as a result the "double down" effect is not observed.

Going back to our example, the amp could produce 3.5 amps of current with the 28v output @ 8 ohms resulting in about 100w. But, for sake of discussion, let's say it's power supply couldn't produce the full 7 amps of current necessary to maintain 28V and double down at 4 ohms. However, it's power supply could manage 6 amps of current at 4 ohms. This would result in about 24V and an output of approximately 144 watts - which is not at all unusual for most amps to have 30-50% more power at 4 ohms compared to 8 ohms. Of course, some may be more, some may be less.

Usually, at 8 ohms and higher, the output limitation is voltage related based on the power supply's rail voltages. As the impedance drops/load increases, the output limitations become the inability of the power supply to keep increasing current to maintain the rail voltage.

Keep in mind there is a lot of fluff when it comes to talking about "high current" amplifers. Some mfg, such as H/K, cite high instantaneous current specifications. Unfortunately, these don't mean much in the real world because the impedances used for these measurements are extremely low; short circuits, essentially.


Prepare for sidetrack...





For example, the H/K 3385 stereo receiver (rated 85wpc @ 8 ohms) claims 42A of peak current. I don't doubt it's ability to do that, but it won't do that at any normal (or even most abnormal) speaker load. And, you can't force more current to the speaker than the voltage and impedance dictate.

That receiver will develop about 26V and 3.25A to produce it's 85w rated power at 8 Ohms. People see the spec saying 42A of peak current and never really look into the math and assume it's going to do that with THEIR speakers. Let's say your speaker is rated 8 ohms, but drops to 4 ohms at certain frequencies; not unusual. Well, 42A @ 8 ohms produces 14,114 watts and requires 336 Volts to get there, if the impedance drops to 4 ohms, it requires 168V to get that 42A and results in 7,056 watts. I think we can see that isn't going to happen with a 85wpc amp or receiver that has a rail voltage of probably around 40V, give or take.

Since the receiver's power is spec'ed at 85w I'll give the benefit of 3dB headroom resulting in 170w peak (at 8 ohms). On this assumption, 42A of current could be developed if the speaker impedance dropped to around 0.1 Ohm.

I don't mean to disparage H/K gear, it's fine stuff. They just happen to provide the numbers for a good example.

Wow! Great, all your responses were very helpful. It's much appreciated!:thmbsp: