|
Prodotti, prezzi, dimensioni, possono subire variazioni senza preavviso, chiedere la conferma prima dell'ordine. Prices, products, dimension, could be changed, Ask before order. Prezzi e prodotti aggiornati sul nuovo sito Audiokit e-Shop cliccando qui ! Update prices & products on new e-Shop Audiokit website click Here
|
The Ariel By Lynn Olson
There seems to be a misunderstanding out there about transmission lines and
efficiency. Keep in mind that driver efficiency is almost purely a function of
cone mass, radiating area, and magnetic strength in the voice coil gap, and very
little else. Believe it or not, for a given driver, the box design has no
effect on the driver efficiency. (Read the Neville Theile and Richard Small
articles in the 1972/73 AES Journal if you don't believe me.) If the crossover is really greedy, it can attenuate a lot of upper-band
energy, but very few designers try to remove energy in the 100-500 Hz region,
where any correctly operating driver is naturally flat. So how did TL's get a reputation for being inefficient? Well, if you
mass-load the driver by putting damping material too close to the cone (3 inches
or less), that will indeed remove a couple of dB ... as well as altering the
frequency response. Also, the heyday of TL's was in the mid-Seventies, when
driver efficiencies were at an all-time historic low of 81 to 84 dB/meter. Actually, TL's are well-suited for very efficient drivers that would
otherwise have high cutoff frequencies if you used them in a conventional vented
or closed-box Thiel-Small alignment. For example, the Vifa P13-WH-00 drivers
used in this system typically provide an F3 of 83 Hz in a conventional vented
QB3 alignment, while the Ariel transmission line system is still strong at 55
Hz. As you might imagine, there's a pretty big difference between 55Hz and 83Hz
of bass response! In addition, the transmission line rolls off more gently than a 4th-order
vented system ... indeed, if the TL has the appropriate rolloff frequency, it
can actually match the low frequency room lift and not seem to roll off at all.
(To do this, though, the Ariels would have to roll off at 25 to 30 Hz. That's
another project altogether.) The Vifa midbass drivers in the Ariel are operated in parallel and produce an
efficiency around 92 dB/meter (calculating directly from the Theile/Small
parameters). The Scan-Speak D2905/9500 tweeter produces a measured efficiency
around 90 dB/meter. These numbers are consistent with the target slope of an
overall 2 dB tilt from 100 Hz to 10 kHz. By measuring efficiency in the power center of the musical spectrum, which is
a band between 200-800 Hz, the Ariels provide an efficiency of 92 dB/meter ...
more efficient than most audiophile-class loudspeakers, and much more efficient
than planar speakers and mini-monitors. In practice, a good 8 watt SE triode
will fill the room, and a 20-30 watt amplifier will have all the power you could
ask for. Remember that amps that clip smoothly (absence of feedback and good
design helps here) can easily sound 2 to 3 times more powerful than conventional
amps. Tube watts really are bigger watts.
The perennial question ... which amplifier? For most readers, it's simple; just get a classic old EL84/6BQ5 push-pull amplifier, and there you go. There are plenty of old Fifties-vintage Eico's, Dyna's, and Scott 20-watt amplifiers still kicking around, and restoration can be done any competent guitar-amp tech. The newer Single-Ended 2A3 and 300B amplifiers are a mixed bag; some are designed by newcomers who think that tube amps should sound like old AC/DC table radios ... warm and muffled. Anyone who's heard a properly restored Fifties amp will be quickly disabused of this notion ... they sound fast in comparison to many contemporary amplifiers. When a highly-reviewed "audiophile" amplifier is outperformed by a Dyna Stereo 70, it isn't even really hi-fi, much less "audiophile." So for all practical purposes, a Stereo 70 is entry-level hi-fi. At $119 in 1958 dollars, it was entry-level back in 1958, and remains so today. That you can spend upwards of $3000 on a highly-reviewed audiophile amplifier, and get a less musical and realistic sound than a Stereo-70, is a unspoken comment on "progress" in the hi-fi industry. In a way, the good old Dyna Stereo 70 makes our lives easier; it makes a convenient go-nogo benchmark for assessing amplifiers, and a good fallback for those times when we get ear-burn from the current crop of electronics. All right, now that we've established the minimum standard, where do we go from here? Good question! There are hundreds of tube amps out there ... triode, pentode, single-ended, push-pull, ultra-linear, pentode, etc etc. Some sound terrific, and some are so-so. Keep in mind that 90% of the commercial designs are variants of the Williamson, Dynaco, or Acro circuits of the late 1950's, with the most of the sonic differences coming down to parts selection. (If you look at amplifiers with KT88, 6550, EL34, or EL84 tubes, the percentages rise to about 95 to 98%) By contrast, amplifiers that use direct-heated triodes like the 2A3, 300B, SV572, 211, or 845 have a startling variety of circuit designs, and not surprisingly, sound pretty different from each other. It's a cliche, but you have to listen for yourself, and remember that power amps sound strikingly different on speakers with different efficiency ratings. The amp that sounds best on an audiophile minimonitor almost certainly will sound different, and worse, on an efficient speaker ... and vice versa. There are some very isolated folks who just can't get their hands on tube gear, or are afraid to make the leap to what to them is an alien technology. Unfortunately, good transistor amps are truly rare; they measure nicely all right, but not very many are lifelike in the sense of delivering that "you-are-there" quality to music ... indeed, I would guess that many audiophiles have never heard that effect, even once, due to the degraded condition of modern electronics. Twenty years ago, transistor amps were brittle-sounding, grainy, and very two-dimensional sounding. Nowadays, transistor-amp designers have tweaked the "audiophile" breed of transistor amps to emulate the sound of mediocre tube amps ... sweet and rolled-off sounding, but with little realism or sense of depth. The grain and edge are gone, but so is much of the sense of life and sparkle in the music. I know of exactly three transistor amplifiers that avoid the trap of grain-n-grit on one hand and audiophile flatness on the other ... in short, they sound just like a really good triode amp, but with more power! Unfortunately, all three are really obscure. so don't expect to see them at hi-fi shows or read any reviews any time soon. They are: the R.E. Designs LNPA-150, the latest (yet unnamed) transistor amps from Stan Warren's Supermods, and a Japanese manufacturer called Silicon Arts Design. All three, although apparently mundane Class AB push-pull transistor designers, have the immediacy, the clarity, and the musical rightness of direct-heated triode amps ... but are completely different technologically. How is it done? Don't ask me ... transistor electronics are not my specialty. But it is possible. Moving to the more specific question of how tubes and transistors sound with the Ariels, I've noticed that the midrange is more prominent and immediate-sounding with tube amps (clearer, more prominent vocals), while transistor amps seem to have a distant and recessed midrange, with a stronger emphasis on deep bass and extreme treble. Many transistor amps (especially audiophile models) just don't work with the Ariels, giving a dull and undynamic sound. Switch to a tube amp, and the entire system opens up and really sings. It's not a matter of damping factor; adding a 1-ohm resistor to a transistor amp just makes it sound worse, moving even further away from the clarity and naturalness of the vacuum-tube amps. My only guess for what's going on is the transistor amplifiers are not at their best at the low power levels required by the Ariel, which much of the time is in the milliwatt region. Although theory indicates that Class A transistor amps would sound best with an efficient speaker, in practice I have found the reverse to be true, with the SE transistor amps being the worst type of amplifier, and bearing no sonic resemblance to SE triode amps at all. Looking a liitle deeper, though, and it turns out that SE transistor amps frequently use generous amounts of feedback to stabilize the DC parameters of the circuit, something that is not required by transformer-coupled triode circuits. In short, avoid SE transistor or OTL tube amps with the Ariel. SE combined with feedback seems to magnify the defects of transistor sound, and OTL's (which frequently have high levels of feedback) can't deliver the current that a 4-ohm speaker requires. Not that there's anything wrong with OTL tube amps; they sound great with (some) 8 and (most) 16-ohm speakers. (Note to the reader: no, the Ariel cannot be converted to 16-ohm operation. Designing a good 16-ohm speaker is not as easy as it looks.) Perhaps more surprisingly, the highly regarded audiophile favorites don't always come off very well with the Ariels. In particular, the Ariels are allergic to Krell, Cary, Pass, and Audio Research products; I'm sure they sound great with different speakers, but I can say from experience that these amps just don't work with the Ariels. To sum up, 3 watts isn't enough, 8 to 60 watts is just fine, and 100 watts or more is too much. The big tube amps with smoldering banks of EL34's or 6550's, or big transistor amps with heat sinks on all sides, are not a good match for medium to high efficiency speakers like the Ariel. Well, that certainly sounds gloomy, doesn't it? Which amps sound good on the Ariels, if the ones that most folks know about aren't a good match? Well, I can say from experience the Audio Note Ongaku is a stunner, but then, at $85,000 or whatever it costs these days, it ought to be! The Ariels also sound wonderful with various different SE triode amps, such as the Reichert Silver 300B, the parallel-feed confections from Electronic Tonalities, and various others I can't remember right now. I was knocked out by the WAVAC 833 at the 1998 Winter CES, and I must say it is the most hard-core exotic design I have heard or seen, with rare and costly NOS WE473A inputs, NOS Genelex KT88's as driver tubes, and a spooky-looking 833A transmitter tube pumping out a solid 100 watts of single-ended power. This is one amp where the $35,000 price tag doesn't seem out of line, considering the extreme rarity of the parts, made-to-order Tango transformers, and the NC-milled 60mm-thick aluminum chassis. Oh yeah, it sounded good too, good enough to make anyone forget about audiophile amps forever. Having had (more than one) glimpse of heaven, Karna and I were frustrated by the mundane sound of the amps that came trooping through for review by Positive Feedback magazine. (I don't do that anymore, so don't send review samples to me, send them to the magazine!) Don't go looking through all the back issues of PF trying to find the dogs; it's standard policy at PF not to print bad or even "ho-hum" reviews, and just send the product back to where it came from without comment. The products Karna and I did like were far, far out of our price range, even at deep discounts ... I'm in no position to spend more than the price of a new car on an amplifier! So ... off we went on the adventure of designing our own electronics. Read about the Amity, Raven, and Aurora. If you want additional information, visit the tail end of the Aloha Audio page, and you'll see links to people all over the world. Unlike the Ariels, I can tell you about our experiences, but I can't hold your hand, or offer any additional information, when it comes to vacuum-tube electronics. This stuff is dangerous! Not only that, things like good grounding technique aren't something you can learn over the Internet. Do yourself a favor, find an experienced tube-amp builder, guitar-amp technician, or ham-radio old-timer, and have them train you in the art of vacuum-tube electronics. Good engineering practice is not something you learn over the Internet, any more than you can learn to shift a manual-transmission car, or fly a plane. Some things require hands-on instruction from an experienced professional. Building a vacuum-tube amp is one of them. Good luck!
Prezzi e prodotti aggiornati sul nuovo sito Audiokit e-Shop cliccando qui ! Update prices & products on new e-Shop Audiokit website click Here
mercoledì 02 luglio 2014 |
|
Copyright © 1996 - 2009 Audiokit All Right reserved, name and brands are of respective owners. |