Question about high current amps versus "not high current amps"

The topic of "high current" wouldn’t matter if speakers worked like plain resistors - e.g. acting as the same constant 8 ohms value (or 4 ohms, or 2 ohms) at every frequency from 20Hz - 20kHz. But speakers don’t do this; not even close. Hop onto Stereophile’s online archives and look through the measurements for various speaker models - they often include a graph of impedance (ohms) versus frequency. The curves you’ll see are very, VERY far from the straight flat line you’d get from a plain resistor. Of course it’s more complicated than just that, so they also include a phase angle curve.

So what happens when an amplifiers tries to drive one of these "variable impedance" curves? Well, if it’s a:

• "High Current" amplifiers will produce more power into the "dips" of the curve (lower impedance), thus relatively boosting the SPL at those frequencies. The gold standard of these is amps which claim to "double down", which is the theoretical limit for load invariant amplifiers (e.g. produces twice the power into a 4 ohms load verus an 8 ohm load). These amps are also called "voltage sources", becasue their voltage output remains constant (does not dip) while driving loads of increasingly smaller ohms.
• "Low Current" amplifiers (though nobody calls them that) will NOT produce more power into the dips - though in practice they are affected, and will produce some amount that is more or less depending on numerous factors. But they typically will NOT come close to "doubling down". These amps are also called "power sources".

So which is better? It totally depends on how the speaker was designed! The designer has to (somehow) balance these disparete drivers, crossover, and (likely) amplifier behaviors off of each other to produce a result that sounds natural. If they designed with "only" high-current amplifiers, the speaker will surely sound different driven from a tube amp, and vice versa! What’s perhaps surprising is how many speaker models sound good - albeit different - from both high-current SS amps and tube amps.

Most ported speakers will show a HUGE bump in their impedance in the bass region, right around where their actual bass response starts rolling off. That’s due to the port tuning. You’d think this would mean a high current amplifier should sound "anemic" at these frequiencies, from the high impedance (where they produce much less power), but the reality is this bump represents a "resonance" where the speaker system is actually particularly efficient at generating those frequencies. So yeah...a lot of complexity for a speaker designer to juggle...

@thecarpathian  Maybe an easier analogy is legs vs. arms. 

You need both to be able to dead lift.

That’s it, I’m out of aspirin...

Plus I use a leg press machine, better on your back...

Therefore, my conclusion is slew rate is important to SQ, though not as much impedance matching, To the physicists and electrical engineers, is my conclusion valid?

@jsalerno277 No. Its a lot more complicated than that. Slew rate is only one of 40 or 50 variables that make up sound quality. There are some excellent sounding amps that have very low slew rates.

"Low Current" amplifiers (though nobody calls them that) will NOT produce more power into the dips - though in practice they are affected, and will produce some amount that is more or less depending on numerous factors. But they typically will NOT come close to "doubling down". These amps are also called "power sources".

@mulveling This statement is incorrect. You could be referring to a tube amplifier in this. Tube amps, if imbued with enough feedback, can behave as a Voltage source yet won’t double power when presented with a load of half the impedance. Instead, they will cut power in half when presented with a load of double the impedance. If the tube amp has no feedback, then it will behave more like a power source although it will be only roughly so. If the amp has both current and Voltage feedback and the two are balanced to the same amount then the amp can behave as a true power source. If you did this with a solid state amp it would behave that way too.

At any rate, a tube amp with enough feedback can drive a variable impedance test load with minimal variation- around 1/10dB, yet no tube amp is considered ’high current’.

A better model for a ’high current’ amp is one that can double power from 4 Ohms to 2 Ohms. Whether that’s of any help with most speakers is highly dubious; any power amplifier will sound better (have lower distortion) if its not having to work hard for a living!

@thecarpathian 

The capacitance reduces power supply ripple and attempts to feed the amplifier constant DC  voltages. 

Theoretically, the more muscle he has (capacitance), the more weight (low impedance speaker) he can lift with minimal strain.

But I thought when someone was really muscular, they had more ripples?