Why can amps drive so much current?


Many modern amps can drive much more current than what is associated with their rated power into either an 8 ohm or 4 ohm speaker. Why are they designed this way? Is it cheap to over-design the current output of an amp?

By way of example I have an amp rated 150W+150W into an 8 ohm speaker. It is also rated 300W+300W into a 4 ohm speaker. It states it can drive 1200W+1200W into 1 ohm for short periods of time. It also states it can drive 2400W into 2 ohms in bridged mode for short periods of time.

So when driving an 8 ohm speaker it drives 150W, which turns out to be 4.33 amps of current into the 8 ohm speaker at 34.64 volts. (To calculate this I used the following two equations; Power = Voltage x Current, and Voltage = Current x Resistance.) When driving 4 ohms the load is half (4 ohms vs. 8 ohms), therefore the current is double (8.66 amps, while the voltage stays the same at 34.64V (power = 34.64 V x 8.66 A = 300W). When driving 1 ohm the current is 8 times the current of when driving 8 ohms. So the current goes up to 34.4 amps (8 x 4.33 amps) for brief moments of time.

So the amp has it’s internal structures built to be able to drive up to 34.6 amps, but yet probably never drives even half that current. For instance say a speaker is nominally 4 ohms, it most likely never dips below 2 ohms in any circumstance (that’s a pretty big dip for a 4 ohm speaker) and therefore the amp never has an opportunity to put out more than 17.3 amps. And for an 8 ohm speaker the current likely never exceeds 8.6 amps even driving the most demanding of 8 ohm loads (so it is 4 x over-designed for an 8 ohm speaker).

So why do the designers build in so much current overhead? It has to be expensive to do so?


I guess one can compare it to a car. Why do we have/want/need 6, 8, 10 or 12 cylinders, turbo’s and what not. Except for (a few places at) the Autobahn there’s no other place you’ll ever need all that power. Well, the answer is simple: We want that extra power, just in case we need it… Yup, better safe than sorry…

To handle short very high demand dynamic transient loads

For starters, speakers have inductors (coils) and capacitors (crossover networks etc) so, although theyre nominally 8ohms, dependent on frequency can go down to 0.5 ohm. Also power (watts) is actually VI cos(phi) where cos(phi) is the powerefactor and is the angle of the phase change between V & I caused by the complex (inductive/capacitive) load.

First, amps aren’t “designed this way”. Most are not. The amp you are describing is a monster. That is a rare and expensive amp (and heavy too). These monster amps will simply not clip when asked to produce high current into a difficult load. So you can play at higher volume on a speaker with a difficult load… if you don’t play at higher volumes or have a benign speaker, then you don’t need such a monster. BTW, in the old days, such an amp was called an “arc welder”.

I like monster amps simply because they will mate with just about anything and, as a general rule, you have to worry way less about system matching with such an amp. But of course, it costs money.

Why do speakers have difficult loads? Well that is the way the designer built them. The load varies by frequency by the way and that is why Stereophile graphs load to frequency. But when load changes, the current needed changes… why? That is just ohm’s law and the way the universe works. Make a speaker with an easy load and you can either play it louder or get away with a less expensive amp.

I personally feel that if a speaker has a difficult load and is inefficient, the designer did a bad job…regardless of how it sounds… but that is just me.

Bruce in Philly

They don’t have to be heavy or expensive. See M700.

You got me… Class D.

Class D amps were never seriously considered in the past years… PS Audio is a bit of a contrarian given the history of these designs.

Bruce in Philly

Agreed. However, more companies are making them as the better modules have become available. In my situation, I required small, and cool running and it trumped sound quality but I must say I’m very pleased with the M700 and their ability to drive my 87 dB (in)efficient speakers.

Thanks for the input. Yes, the amps are monster amps and I think they just went a bit crazy in over designing them. Oh well, they’ll likely never be stressed and should last forever. Also, they sound outstanding, so I’m happy with the choices the designers made. :slight_smile: