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Amp Review

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Langston Holland:

My recent upgrade to the Danley TH118 subs has motivated me to look into larger amplifiers than the PL380 that has been perfect for the TH115's. I've also been underpowering the low passband of my KF730 boxes on the order of 3dB. I've justified this mentally because the low passband only covers 90Hz to 200Hz and the fact that I overlap the early part of this spectrum with subs. Now I'm going to drive those hungry 10" drivers to their limits. The KF730's are 16Ω boxes and the series pair of woofers is rated for 700 watts continuous, thus 1,400 watts "program" per box. A daisy chain of (4) to (6) of those boxes demands an amp that effectively didn't exist when the KF730 first came out. That, boys and girls, is not the case anymore.

The net result of the amp upgrade will be approx. 3dB more output from the same loudspeaker arrays from 200Hz downward. Same truck pack, same weight, same size, but more rig for the gigs. Just need to pony up around $10k (assuming sales of my existing amps) and I’m in. Is 3dB under 200Hz worth 10 grand for my (24) KF730’s and (8) TH118’s? Heck yes.

The higher frequencies are powered to their program ratings already with an amp that I've yet to find an equal to sonically. On that note, I’m going to add a fair amount of subjective commentary in this review that I used to shy from. I’m over it. Class D and the variants I’ve heard are not ready for critical HF reproduction yet. There, I said it - and it’s a fact unless you’re deaf, don’t care, think MP3 is good, or are trying to earn a living in this business. :)

Measurement Setup:

Room temperature 20C, voltages at the input and output of the amps were monitored with a Fluke 192C Scopemeter that has fully isolated inputs. Sine waves in various flavors were used for all tests. Long-term tests were done with 500ms on / 500ms off steps with distortion curves monitored via the amazing STEPS module in the ARTA measurement suite. Benchmark DAC1 and ADC1 converters with instrument grade +30dBu I/O capacity were optically connected to my MacBook Pro running XP in Bootcamp. A 21dB pad was used to feed the input of the ADC1 for a 51dBu maximum input capability. At the 4Ω resistive loads I placed on both channels of each amp in the test, this allows a maximum level of 18,904 watts to be measured cleanly. My new SuperBucket™ was used to take the heat. This thing presents (2) 4Ω loads with 14,400 watts long-term dissipation and peaks numerically similar to the national debt. You can also use banana plug jumpers to configure single 8Ω and 2Ω loads at double the wattage, assuming the jumpers survive.


Measurement Theory:

Everybody and their mother have different ideas about how to best measure the performance of an amplifier. Some folks think the old fashioned way of feeding continuous sines is the deal because they apparently use their amps to arc weld during the middle of the week. On the plus side, you’ll get absolutely comparable results from every manufacturer, on the down side those results are not very meaningful for our application. My use of 500ms on / 500ms off sine steps yield effectively identical results as continuous sines in my experience and this is included. Since I use amps for powering loudspeakers with music, I prefer using shorter lengths of sines that approximate musical behavior as much as possible so I can have more meaningful metrics for the intended application. That's where the rub is – just what kind of stimulus comprises the most meaningful metric? For some reason folks keep looking for a _single_ metric to describe amp power, but I think that's a recipe for failure. In our application there is a long-term load as well as much, much higher momentary peaks that need to pass cleanly up to the point where the loudspeaker system begins to sound bad or fails. Those two thresholds are separated by about 6dB of voltage and 20dB of attitude. I've included both long-term and short-term tests in a way that makes the most sense to me. As of today. And it’s my party so I can do what I want. :) I’d love contrasting views as always.

Take a trip around the block and you'll notice that Lab Gruppen and Yamaha/NEXO, to name two, are measuring their amps in ways that attempt to account for the behavior of music as opposed to the behavior of welding machines. Both use sines to allow distortion measurement, which is critical. Lab uses a 33ms on / 66ms off step which seems quite reasonable as a single metric, especially at lower and mid frequencies. It might be a little severe (long) for the higher frequencies, but it keeps things consistent, repeatable and logical. Yamaha/NEXO uses a 20ms on / 500ms off step and only measures the last sine wave in the 20ms burst, and that at 1kHz apparently. This probably yields higher power ratings than reasonable and certainly shouldn’t be the only metric considered.

There's a guy named Don Keele that I've been following since the 80's that has contributed much to the loudspeaker industry, and by extension, the amplifiers that power them. The logic behind his attempts to measure both gizmos in a way that simulates actual usage makes sense to me and correlates with my testing and listening efforts, particularly as applied to limiter settings. As a side point, I think advanced limiter topologies that simultaneously deal with long-term heating and peak mechanical issues of loudspeakers in a sonically pleasing way are the next area of serious improvement in the quest for high SPL and good sound. Driver design will need to go hand-in-hand with this. NEXO is a forerunner here.

So what about Keele's short-term tests? Read Shaped Tone Bursts beginning on pg. 6. The earlier pages are an important intro.

Want his stimuli and have time to wait for 300MB? Click here if you dare. I neutralized a very expensive amplifier that was supposed to go to a little outfit that rhymes with “hair” after I was done with it. I wasn't careful enough with the 20Hz tone burst. Whoops.

Bottom line from Keele? From a post he made last November on the SAC list: "What we need is that 100 Watt continuous power amp that can pass 10 kW peaks for short periods of time."

I've had to chew on this concept for years to appreciate it, but I'm convinced he's right and my tests show more than a few amp manufacturers are convinced as well and are building amps accordingly. According to the needs of music instead of welding, imagine that.

The Players:

Camco Vortex 6

Alluded to earlier, this is simply the best sounding professional amplifier I've ever heard. It's a class H design and somehow manages to reveal the most subtle details in high-end recordings with earphone clarity over (excellent) loudspeakers. To date I've yet to hear a class D variant (including the following amps) that can present the width and depth of a recording anywhere near as well. I have no idea why this is the case - no measurements I've ever made are conclusive. The flatter transfer function certainly doesn't hurt, but it's more than that. Full output at 120v 30 amps works perfectly, no need for the 208/240v model unless it’s more convenient for where you live.


One of the best bang for the buck ratios on the planet. My (9) PL380's have been rock solid for several years. They are also one of the better sounding class D amps if you have to use one in the mid/high region. Like the Camco, full output at 120v 30 amps works perfectly, no need for the 208/240v model.

Powersoft K10

Oh my - this beast is ready for your welding rods and sine waves masquerading as music. One rack space of pure attitude - it's squirrel fans will go into overdrive if necessary, much like the old Crest 9001's, but this amp will just keep belting out continuous power unlike any amp I've ever come across. It also is solid on the Keele short-term tests. The amp is very well designed and feels as solid as it runs. Very logical layout and very efficient with power, will accept any voltage on earth and do sine waves all night long (trance, electronica, whatever). Optional DSP card available with FIR filtering, but not on the order of either the Crown HD or Lab PLM series. Quite good nevertheless - you can download the software for your PC and see if it fits your needs. Sound is much better than average for class D - about equal with the PL380, though a more forward sounding midrange. You obviously want 208/240v with this amp if you're going to use its full capacity.

Crown IT5000HD, IT9000HD and IT12000HD

The processing in these things simply rocks. Very steep and well-designed FIR filters, plenty of all-pass and parametric EQ filters. Nice average and peak limiter facilities. The software interface to the ITech amps is probably the best I've ever seen, Crown works very hard at this and releases updates frequently. I noticed earlier this year that the 2nd order all-pass filters weren't working and Crown had it fixed in less than a week - wow. It also made me wonder why I was the first to notice this since they’ve been out for a while... The software makes for very nice multiple amp management in larger company or install environments. The sound is greatly improved from my previous experience with the non-HD models. My guess is that it's due to the processing upgrade since the amps measure fairly similarly. I prefer the sound of the other amps in this test in the mid/highs when ignoring the processing, but you shouldn’t do that. I designed FIR filtered, flat-phase presets for Troy’s KF730’s to mimic what I’ve done with the Dolby Lake processor and was amazed at how good the net result was. The limiters sounded good too. I noticed a similar result when doing listening tests with the Lab PLM20000Q with internal processing vs. the FP14000 fed by a DLP. Interesting. Reliability reports are good with the HD series. The ITech's have always been good at making power and these amps are no exception, but it’s silly that they increased the numbers in their model numbers – output has not increased as far as I can tell. Though I don't report it here, short-term output into 2Ω’s will surprise you with either the older or newer ITech’s. You think this is a 4Ω only amp? Nope. It shows a similar increase in output into 2Ω’s that you see in typical amps as long as you don't ask it to do so for un-musically long periods of time. Still, voltage is king and is what SPL is made out of and you should stick to a nominal 4Ω’s regardless of amp IMO. The ITech’s play nicely with others due to worldwide voltage and PFC.

If you push these amps to their limits, you're going to want to feed the 9000 and 12000 with 208/240v. At 120v even at 30 amps, both these amps "groan" when under load. That kind of vibration can't be good in the long run. At the 240v feed they got during my measurements, they were nearly silent other than fan noise, which can get up into the Crest 9001 range. The 5000 is happy with 120v 30 amps into anything that will ever happen in practice, though it did want 240v for continuous sine testing.

Lab Gruppen FP14000

I'm buying (4) of these as a result of these tests. Their output to size, weight, sonics, warranty and cost is a winner. These amps are not auto-ranging, you have to choose either 120v or 208/240v - the latter is the only thing that makes sense given the horsepower involved. This amp is all about allowing the peaks of music to get through without clipping as their testing method indicates. It has both hard (lows and subs) and soft (mids and highs) limiting available as well as several cool protection circuits with a light show on the front panel. I made all the pretty lights come on without letting the smoke out. Too bad they make you buy a thousand dollar gizmo to connect your PC to the amp to monitor its behavior - they should nix that thing and its cost and allow you to connect your computer directly to the amp(s). Maybe the smaller Labs sound better in the mid/highs, but this amp showed its class D heritage in my listening tests, which nixed my thought of replacing the Camco's with a Lab on the mid/highs. Still, this amp is probably the best sounding class D amp I've heard.

Long-term Measurements:

Though I expected this to be boring, I accidentally learned some interesting stuff here about design philosophy between the various manufacturers. All have made reasonable compromises in trying to skin the cat of high output without damaging itself or the stuff at the other end of the loudspeaker cable when operated by a bunch of lunatics.

Each of the below show a reference measurement of magnitude and phase at 50 watts and at about 1dB beyond their maximum output capability with both channels driven into 4Ω. Do not freak out about the max power traces, I could have backed off a hair and made them all pretty, but that wouldn’t have been very helpful, would it? Plus I would have missed out on the flames from some of the manufacturers – and what fun would that be? Actually, I learn from those as well and look forward to it. Be sure to read the caption at the bottom of each graphic for details.

Camco Vortex 6 (low level and 1dB beyond max)

QSC PL380 (low level and 1dB beyond max)

Powersoft K10 (low level and max)

Lab Gruppen FP14000 (low level and 1dB beyond max)

Crown ITech HD (low level)

Crown ITech HD (1dB beyond max)

Short-term Measurements:

This is really interesting.

The following scope traces are from the FP14000 to demonstrate how I made these measurements. The scope is used to determine the point at which the shaped tone burst begins to deform and plotted with the input signal scaled to for reference. The red trace is the input signal and the blue trace is the output at the point where I judged it to be the maximum. I lost the 100Hz measurement like a dummy, and I’m too lazy to do the measurement again, but it’s immaterial. One thing you’ll notice is at the higher frequencies, the output trace is late to the party – a simple measurement of phase lag as predicted by the long-term STEPS trace. The scope's phase calculations agreed with STEPS. I think I’m getting the hang of this. :)

FP14000 Shaped Tone Burst Measurements

And now, ladies and gents… These numbers are per channel in stereo mode into 4Ω. They would have been higher at 2Ω, though voltage and the SPL from your loudspeakers would be lower. Chew on that. I hate this whole watts thing – just did it for the peanut gallery – next time it’s volts like God intended. :)

The Data

The Picture

A single post that is more useful than a year of issues from any of the trade rags, Production Partner excepted.

Tom Danley:
Wow, Langston you have been busy!  
Very nice work and it will take a good while before you will have to worry about melting your Abdul-EQ superbucket too.
Again, Nice work!
Tom Danley

Ivan Beaver:
It's great that you took the time and put in the effort to really look at how different products perform-and use those results for products that best meet your needs.

Those types of test can yield very interesting results.  

And give good insight into a lot more about the performance of a product than a simple spec sheet would provide.

And another example of why it is hard to describe the complicated performance of an audio product with a couple of "simple" numbers.

For those looking for more specific details about how a product performs-you often have to look a bit deeper.  And it is always a good idea to perform the exact same tests on different products and chart the results to see "trends".

Good work and an interesting read.

Thanks for the review(s).

One thing I would like to see that you didn't post-was how well the amps did producing a sine wave for an "extended" period of time.  Say 1 or 2 seconds.

Yes regular "music" doesn't have usually have durations that long, but electronic music often does, and at very low freq.

It would be interesting to see how things would change (if they did) with that type of test.

Franz Francis:
+ 1 on the Camco V6. This amplifier seem to have the characteristics of the audiophile types.



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