What Power Amplifier Should I Buy?

Sorry, not a JBL guy. "Smooth" is not the word that comes to mind when I think of JBL. They have their fans, I’m just not one of them.

I prefer modern horns if possible ... see the Australian AH425 Azurahorn, which was designed by Bjorn Kolbrek using BEM computer simulation. The AH425 simulation became part of his Doctoral Thesis in Norway, and the physical horns were built in Perth, Australia by Martin Seddon.

Azurahorns

AH425 Frequency Response

They were part of a project to design a modern successor to the Altec Model 19, which I have set aside, but the AH425 was a nice spin-off from the project. The other half of the project was an Altec/Great Plains Audio 416 woofer in a 4 cubic foot closed box with a crossover in the 640~800 Hz range.

Going back to the OP, I still think, given his budget constraints, he should try a much more powerful amplifier (not just another 30 watts) and see if dynamics improve. I doubt it will, but I’m not the OP, and the experiment should be tried. In my experience, speakers usually limit the dynamic range of a system, not the amp. The true Theile/Small efficiency of the woofer typically sets overall speaker efficiency, and that in turn sets the overall system headroom. To a first approximation, 1% conversion efficiency in the T/S sense results in 92 dB/meter/watt efficiency for the bass driver, which is almost never attenuated in the crossover.

From previous conversations, the T/S efficiency of the compact Revels appears to be in the 0.2% to 0.3% range. This would imply only 10 watts/channel are needed to reach 95 dB at a one meter distance. Allow another 2X for a 2 meter listening distance, and maybe we need 20 watts/channel.

Anything more is just there for additional headroom, and to allow for the occasional transient peak. The fact that a 100 watt amplifier is falling short points to marginal amplifier design, or the speaker itself running out of steam. More likely it’s the speaker, if a 6.5" woofer is all we have to work with.

Speaker manufacturers like to brag about "long excursion", but an old rule of thumb in the loudspeaker biz is: "If you can see the cone moving, it’s distorting". Still true today.

One of the jokes we used to tell back when I did this for living was: "If the amp is bigger than the speaker, something ain't right".

@lynn_olson 

I will probably buy the Buckeye Hypex NC502MP 2-Channel amp. Spec is 500W in to 4 ohms at 1% THD. I'm not happy with that way of specifying, but the power should be far more than the Bryston and should allow me to discover what more headroom will get me

I notice that in your last post, you seem to be implying that the amp may not be the problem. I've come to that conclusion. So the upcoming test will probably highlight any deficiencies in the Revels.

I am quite comfortable using REW. Is there a measurement in REW that will highlight the speaker non-linearities? Distortion perhaps?

You’ve heard the expression, "A Quart in a Pint Pot"? Yeah, like that. A little woofer sounds little. Maybe a 500-watt amplifier will make the 6.5" woofer sound bigger. Maybe. But I am dubious.

Which is where subs come in. Two of them have twice the radiating area, twice the amplifier power, and twice the first efficiency of a single sub. That’s a 6 dB gain in headroom, which is definitely audible. But ... restricted to under 40~80 Hz, depending how your crossovers are set.

And you have a $5000 (or less) spending constraint. I’ll be honest, if you are not into DIY builds or hacking old Klipsch loudspeakers, your options are limited. Commercial speakers with 15" woofers are usually aimed at the party speaker market, not hi-fi. Studio monitors with 15" woofers are serious money unless you buy used, and you have to be very careful buying anything that’s seen professional use (more likely abuse).

A big problem are the high profit margins built into high-end audio products. 40% for the dealer, and another 5~10% for the regional distributor. If imported from Europe or the UK, another 10~15% for the USA agent. Audio companies need a minimum of a 3 to 4 times ratio between total assembly cost and the retail price to turn a (minimal) profit. This means building at least the wood cabinet in China is a good idea, and maybe the drivers too.

Looked at as a strict cost exercise, a small 2-way sitting directly on top of a subwoofer with a 12" driver, and an electronic crossover between the two of them, is the smart and economic choice. The little guy gets an electronic highpass filter between 80 and 120 Hz to stop the 6.5" woofer moving, which makes it a midrange driver. That also takes the power burden from the mid/high amplifier, so anything beyond 30 watts is plenty. You might even go Class A and enjoy the clarity and lack of fatigue of Class A sound. I am a big fan of Class A operation, whether solid-state of vacuum tube.

Vera-Fi Audio makes a $400 subwoofer with a 12" driver and 500-watt Class D amplifier called the Caldera. From what I can tell, it’s a solid product. I bought one for my son as a Christmas present.

Slap one under each Revel, and get a good electronic crossover between the Revel amplifier and the plate amp of the subs. With the subs directly underneath the Revels, or any other compact loudspeaker, you can set the crossover as high as desired, and decrease the IM distortion many times. With a setup like that, I would ditch the traditional Class AB amplifier and try something else.