David Lamkins picked up his first guitar a long time ago. As best he can recall the year was 1967: the year of the Summer of Love. Four decades later David has conjured up an amalgam of folk, rock and jazz solo guitar music for the occasional intimate Portland audience.
location: Portland, OR USA

Facets: Atomic, FBT, QSC, amplifiers, comparison, evaluation, gatherings, power, review, sound reinforcement, @musings info

Powered Speaker Comparison

I recently spent an afternoon with friends listening to an Axe-FX though four different pairs of powered speakers:

This is the first time I had heard any of these speakers except for my K10s. What follows are my purely subjective opinions and observations. As always, if you don't rely on your own ears when making a purchase decision, you get what you deserve...

The FR is the largest of the group. Viewed from the front, the FR has the appearance of a guitar cabinet with a traditional grille cloth and tolex covering. Only the rear view tells you that you're dealing with something different, with the sealed back and the diminutive chassis hosting a just a few switches and knobs and a pair of XLR jacks. I'm told that the amp is built around a pair of 6L6s. The chassis looks way too small for that, but then I'm used to thinking in terms of traditional guitar amps.

The 12ma is the next in size: a bit shorter and perhaps a bit deeper. The 12ma, like the all of the powered monitors except for the FR, is shaped to allow placement either upright as a main speaker or tilted back as a monitor. The cabinet is plywood, coated (IIRC) with a spatter-coat finish but without protective metal corners. The grille is perforated metal.

The K10 has roughly the same form factor as the FBTs, but its cabinet is plastic (again, without protective metal corners). The grille is perforated metal. The K10 is smaller and lighter than the 12ma by about the amount you'd expect given the difference in the driver size.

The 8ma, although the driver is only two inches smaller, seems disproportionately small compared to the K10. I think this is because the 8ma's tweeter is coaxial with the LF driver. (The 8ma's size is not a bad thing at all, especially if you have a need for a small rig.)

These are all two-way speakers; in addition to the LF driver, all have a tweeter. The FR has a variable pad to control tweeter level. The 12ma has bass and treble tone controls. The K10 has two switches to alter EQ: one for a LF boost; the other for a presence boost.

The FR, despite being the only tube-powered amp, is designed to be tonally neutral. The FR delivers 50 watts of power to its speakers. Atomic does not provide a maximum SPL rating for the FR. Suffice to say that it'll play plenty loud for its intended use.

The rest of the speakers are internally bi-amped, with separate power amps driving the LF and HF drivers. The 8ma is rated at 150W to its LF driver and 50W to its HF driver, for a 117 dB maximum SPL. The 12ma is rated at 300W/100W for a maximum 123 dB SPL. The K10 is rated at 500W/500W (yes, all those zeroes are supposed to be there) for a maximum 129 dB SPL.

Before going any further, I need to discuss power ratings. The only "honest" rating in the bunch is the FR's 50 watts. I say that not because I believe that FBT and QSC are fudging the numbers, but rather because I'm pretty sure that the FR doesn't have any limiter or speaker-protection circuitry to throttle the amplifier power. Like any tube amp, pushing the FR harder will add distortion. The FR's designer no doubt picked speakers capable of handling the full rated power plus a safety margin.

Other powered speakers (not just the FBT and QSC, but essentially all powered speakers) are different in that they include limiter circuitry to keep the power amps from clipping (because clipping a SS power amp sounds really bad and can cause damage to HF drivers) and to protect the drivers from melting down or bottoming out. What this means is that you should take those power numbers with a grain of salt. When you push the amps too hard, they'll be throttled back to protect the speakers.

Not all limiters are created equal. If you intend to push your powered speakers hard, listen to how their behavior changes at high volume levels. It's better to find this out in-store than to be unpleasantly surprised after the purchase.

That said, I have nothing but good things to say about KSC's use of unusually high-powered amplifiers. This design choice seems to endow the K-series with the ability to handle transient peaks at high volumes without "flattening out" like comparably-sized units from other vendors.

Keep in mind that small multiplicative factors in power ratings don't mean a whole lot in terms of perceived loudness. All other things being equal, a doubling of power will cause a just noticeable difference in loudness. To achieve the perception of "twice as loud" requires ten times as much power!

Also remember that the power doesn't get delivered directly to your ears. Speakers convert the electrical energy into sound. The efficiency of the speakers determines how much of the amplifier's electrical energy gets converted into sound. A more efficient speaker will deliver more sound pressure for a given amount of power delivered to the speaker.

And then there are your ears, which (audiophiles' assertions notwithstanding) are among the most inaccurate, subjective, unstable "measuring devices" one can possibly imagine. A measuring tape made of taffy is about as accurate as our ears. Why? Because like the measuring tape of taffy, our ears "stretch" to accomodate the measurement. The louder a particular sound, the more our ears adapt to convince us that the sound isn't really all that loud. If that's not bad enough, our ears are not uniformly sensitive to different frequencies. An emphasis in a frequency range to which our ears are more sensitive gives the perception of increased loudness without any further investment in power or efficiency. Don't think for a moment that speaker manufacturers aren't fully aware of this phenomenon.

We started the listening test with music in order to better judge the full-range response of the speakers. All of the speakers sounded good. I wouldn't have been at all disappointed to have any of these speakers at the core of a good stereo system. However, I was a bit annoyed with the choice of the 12ma as the tonal reference. This is a brighter speaker than I like. Despite the extra crispness, I find the extra energy in the "presence" frequencies to be fatiguing over an extended listening session. Furthermore, because of the ear's adaptation mechanism, the K10s - which are my personal tonal reference - sounded "dull" by comparison.

Indeed, adaptation is an insidious thing. You expect to hear what you're used to hearing. After an afternoon of listening to these overly-bright tones, my ears had become accustomed to accepting that high-frequency emphasis as normal, so much so that I set up an EQ curve with a similar emphasis for my own music listening. It took me a while before I was willing to listen to music without that emphasis; even then it took over an hour for my ears to adapt. I mention all of this not to denigrate the FBT sound, but rather to illustrate how there are no absolutes in tonality. You're always going to gravitate toward the tonality you're accustomed to hearing.

Most of the distinctions in tonality amongst the four systems (and they are different - don't think for a moment that the "flat" in FRFR is anything but a relative term) can be explained by differences in frequency response in different ranges. I spent some time watching a 1/3-octave RTA on my iPhone just to get a feel for the relative differences among the four systems. Both of the FBT models seem to have an emphasis at around 8 KHz and a distinct rolloff below 100 Hz or so. The FR seemed to have an emphasis at around 200 to 300Hz and 2 KHz and a slight rolloff above 8 KHz. The K10 seemed to have more energy below 100 Hz than the other systems; a slight dip at around 4 to 8 KHz was corrected by engaging the presence switch.

Keep in mind that, despite the use of the RTA, these observations are extremely subjective. I was not measuring a test signal; I was forming an impression based upon observing the real-time graph for the same piece of music (which was, of course, presenting a constantly changing reading). Also keep in mind that neither the iPhone's microphone nor the RTA itself is calibrated in any manner, so I can only report relative observations.

With the Axe-FX, we used two patches. The first was a "clean" patch based on a Trainwreck amp (with the gain turned down) through a Bogner cab. I didn't have a particular preference for any of the speakers when used with this patch. They all sounded good. The K10 sounded a little more rounded. The FR had a little more thump on the lower notes. The FBTs sounded a bit crisper and a little bit thinner on the low end.

The second patch was a Marshall patch. This brought out some very obvious preferences among the attendees. The 8ma sounded "small"; it simply couldn't move enough air to convey the full impact of a cranked stack. BTW, we listened at sane levels throughout all of the tests. I measured approximately 85 dBA in the third row from the stage. The FR was very convincing. The K10 was the clear winner, though, for this particular sound.

It has become clear to me, as a direct result of this listening test, that higher-gain sounds bring out more of the differences among different speakers. This should come as no surprise. Distortion increases the density of frequencies present in your guitar tone, which makes it easier to distinguish small differences in frequency response. Anyone who has every searched for that "perfect" distortion tone knows how frustrating it can be to satisfy one's expectations.

These four powered speakers are all fine products, suitable for many FRFR applications. However, I wouldn't recommend one over another without consideration of both the application and your own personal preferences.

January 15 2010 04:16:01 GMT