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|A brief detour. For its new Limited Edition 28B SST 1000-watt monos, Bryston of Canada confirms something low-power tube guys have muttered for decades. They probably just didn't expect to hear it now from a maker famous for transistor muscle amps: "A significant part of the design criteria for the 28B SST was to develop a very powerful amplifier that would drive any speaker on the planet but maintain an ideal power curve at 1 watt as well as at 1000 watts and every power level in between. Most amplifiers exhibit a power curve whereby the best noise floor, drive capability and distortion is maintained from about 1/3 power and up. The Bryston 28B-SST Mono amplifier maintains its power curve right from the first watt. This results in a big powerful amplifier that sounds incredibly detailed and musical at very low levels and maintains that same sophistication and drive capability with even the most difficult, inefficient speakers, large or small."
1/3rd power for the Coda CX monos is 150 8-ohm watts. If the Codas were guilty of the general rule, leashing them up to my 101dB 6-ohm Zu Audio Definition Pros should be less than ideal. That's exactly what I did then before working my way up the load challenge curve. Obviously, hi-eff speakers are not the intended targets for such amplifiers. Call it curiosity to explore the first watt credo. And the related one about "power corrupts".
There indeed was corruption, albeit in a very unexpected area. The Codas were tonally a tad cooler than either the Yamamoto A-08S and Fi 2A3 monos, a few tads more so than the 18wpc 6C33C-powered Almarro A318B. Conversely, the Codas asserted far superior control in the upper bass, a region called the power zone since impact, rise times and articulation here convey music's boogie factor. All of this lined up with typical assumptions. The surprise occurred in the treble. Point blank, the Codas killed it. First they overdamped it, then they caused grain. The amount varied from recording to recording but bordered on the truly unacceptable on some.
With the CSX stereo amp on the same Zu speakers above, that grain traded magically for greater sweetness. Power reduction as problem solver? SE-DHT myths being what they are, one might cite massively paralleled output devices. Minuscule offsets might equalize at higher power levels but cause nonlinearities at the micro levels which ultra-efficient speakers consume, all this despite Coda's precision-matching of output transistors. Alas, caution and courtesy warned: Slow down blame assigment. During a dealer dem in Montpellier, Kapra Audio too had encountered the treble kill. Together with the resident tech, measurements were run, then bias current enriched for 'deeper' class A operation. Mystery solved, treble restored.
Added Coda engineer Doug via e-mail from Sacramento: "We typically bias the amps for best THD for a given amount of class A which will account for differences from unit to unit. We have been gain matching the devices into different ranges and that will result in some significant variations in bias. Even when gain matched, the individual devices still have some variation as you have noted. It is widely accepted by those who love class A amps that the sonic benefits gained by more bias outweigh the THD measurements. I am among this group. We have evolved to the point of using the THD as the benchmark because of past experience with reviewers and customers who complain about measurements and heat respectively. The former wants .0000000....% distortion and the latter an amp to run pleasantly cool yet both want full class A. You will discover this as you deal with your clients. The answer is, we can heat these up and they will sound even better but expect to hear about it from the end user."
Point taken. The upshot is, Kapra Audio will adjust the factory bias current on all Coda amps sold by them in conjunction with the owner's speakers and preferences. It's unlikely as ice skating in hell that fanciers of 98dB Lowthers or 110dB horns would be interested in Coda's mightiest monos - but even they can be accommodated because the other base requirement of zero noise is already in place. Never mind the common 60-cycle-plus-harmonics hum. The monos don't even make the faint tweeter surf that requires a Van Gogh right on the driver. They're immaculately mute in that department. However, on the mechanical front of transformer hum that won't parlay through speakers but instead issues forth from amp enclosures, perfection eluded us. Initially. Being a serious firm, Coda solved that as well. The relevant anecdote does them proud.
To wit, two out of four of Coda's initial amp loaners -- two monos, one stereo, one integrated -- proved mechanically dead quiet, one was nearly quiet and one outright hummed. This suggested a quality transformer of insufficiently narrow tolerances. Upon notification, both the importer and Coda cried 'stop' and the Sacramento factory reapproached their original iron vendor as well as competitors for further samples. Here I must add that I've measured up to 260V on my supposedly 240V mains. Such overvoltages fluctuate and certainly aren't kind to transformers especially of the brutish kind. Not one of Coda's 120V-unit sales had netted a single noise complaint. It was overseas 230/240V operation that was still spotty. After my report to the European distributor, they'd taken their demonstrator pair of CXs around and verified that while dead quiet in their own facility, mains scenarios at other dealers could induce mild hum.
|Even with a Variac, Coda couldn't duplicate the hum at headquarters. Still, engineering insisted on 100% customer satisfaction. After a few months, they supplied Europe with two new CX pairs, one with upgraded transformers from the original vendor, the other fitted with competing parts. Kapra Audio tested those units in their system, then forwarded them to Cyprus where CXs suddenly stacked six high. The original vendor's Gen2 transformers not only proved impervious to noise even on my squirrely mains feed, they also sounded superior. Hence they've become stock production outfit. Kudos to Coda and their new European representatives who braved significant ship charges during this process, between the US, Marseilles and Larnaca. While it delayed the review conclusion, an even finer product resulted and it's the consumer who has won.|