Those who've followed our SE's writings for a while know a few things about him. He's got a long nose and tiny ears. This automatically undermines any serious aural abilities. It necessitates ultra-sensitive hornspeakers to guarantee capture via the under-sized receptors and penetrate the cramped ear canals. He fancies single-ended valve amps true to his initials, but solely those of the rare VV32B, PX25 and KT88 persuasions. No 300B or 2A3 classics for him, hence more audiophile mud slinging from diverse camps of entrenched disbelievers. He personally met Eduardo de Lima at three consecutive CES shows. He chronicled this Brazilian designer's continuous emergence and now owns 200 discontinued cruzeiros worth of stock in his company. When publication of his introductory write up of a pre-production pair killed legitimate reasons to hold on to it any longer, he properly acquired a pair of the firm's AUDIOPAX Model 88 monoblocks from the first production run. He's used it ever since.
How many bloody disqualifications does this add up to, about ability, political correctness, conceptual leanings, believability and heroic non-bias, in one brief paragraph leading to a presumably objective and iron-clad review of these amplifiers? If your tally comes up uncomfortably high, adios. I bought 'em, I love 'em, I'm guilty as charged. Now let's get on with the task at hand.
|Their arrival vividly brought to mind the seafaring past of the Phoenicians and Portuguese. Why be assaulted by stern and haughty nautical brass, for not carefully crating and safely stowing away officer's canteen delights like finely aged Osborne Oloroso 10 RF Sherry from Jerez de la Frontera? As diligently crated, foamed and packed as the images show, Eduardo's amps would have survived far worse than tumultuous waves, irate captains or modern-day UPS. Considering their immaculate automotive lacquer finish -- mine from the German Audi palette to match the Avantgardes' Cactus Grün Mica -- the non-scratch draw-string baggie they came enveloped in was a classy and appreciated practical touch.
These amps have worked flawlessly and without hiccups since their delivery. They've powered the resident DUOs, Triangle Ventis 222 and nOrh SM6.9 as well as Coincident Victory and DeVore Fidelity Gibbon 8 review loaners. For a design sans feedback to control power supply noise, the 88s are shockingly quiet - no surf sounds on the 103dB midrange horns whatsoever, only some very mild tweeter hiss at extreme close-up range, the equal of any solid-state amp that's been through. From a reliability perspective, they're also blatantly over-engineered. Their modest but exceptionally bright maker's torture test to sign off on output transformers? He ran prototypes for two days nonstop without a load attached. Make no mistake - such abuse would melt conventional transformer-coupled amps in a matter of hours.
I nearly suffered a self-induced heart attack after discovering the reason one speaker wasn't playing one day - not for some failed tubes but a slipped spade. The amps had been pounding away at illegal boogie levels for hours, to break in some other piece of gear while I was running errands 'round town to protect my hearing. Certain to have violated one of tubeman's 10 commandments -- thou shalt not run signal without a load -- I was resigned to chalk up one humdinger of a faux-pas to carelessness.
When I reseated the connection before recrating the amp just in case ? Be still my heart - glorious stereo again. After I duly confessed to Eduardo how I had nearly fried his creation, he chuckled and shared his own above war story. (Naturally, shorting outputs goes from careless to criminal - although de Lima has tried that too, deliberately and by mistake, and without catastrophic failure. But that he won't guarantee.) All this by way of suggesting that the spotty reputation of certain finicky and sparky High-End valve amplifiers should not include the 88s.
|The amps' nomenclature refers to the uncommon usage of twin KT-88s. Each independent zero feedback single-ended KT-88 circuit uses its own pi-filter choke and output transformer. The latter are series-connected at the output terminals. Dual-windings filament and power transformers are shared between the two halves. A quasi precursor to de Lima's ununusal tube wiring scheme appeared in the circuit architecture of the original push-pull Quad II. It employed a tertiary winding for partial cathode coupling to reduce plate resistance, output impedance and distortion levels while maintaining pentode power efficiency. However, a joint article by Quad-founder Walker and independent thinker/designer Williamson argued against the ultralinear screen tap championed by Hafler and Keroes who, chasing fashionably low THD figures, advocated against Peter Walker's cathode coupling themselves. Thus these camps split into twain.
In the Model 88, an advanced variant of de Lima's original LM3 Low Mu Triode with Higher Raw Efficiency Emulator circuit appears. It's a successor to the so-called super-ultralinear mode never commercially exploited. It employs Quad's tertiary winding, Hafler's screen tap and de Lima's unbypassed resistor in the cathode leg to control the exact amount of coupling and thus harmonic distortion levels, spectra and output impedance.
This makes the Model 88s into refined technology demonstrators for how to model the complex harmonic distortion spectra of micro-power triodes while guaranteeing sufficient drive and output power for conventional loudspeakers. Diligent mathematical studies of low-output triode behavior, and how it varies with frequency and amplitude, convinced de Lima of a radical potential. A clever marriage of unconventional pentode hookup with the combined response of two slightly dissimilar but series-connected output transformers would allow the "cloning" of two ordinary KT-88. They could be "tricked" to timbrally behave like the best-sounding of the micro-power triodes -- the famed 45 and 2A3 -- while generating 30 watts of conservatively derived go-juice.
|The first graph to the right depicts perfectly parallel THD versus power and frequency curves essentially flat to 1kHz. Below that a first gently, then acceleratingly tapered rise occurs in the bass. De Lima experimented with widely differing response curves. Based on extensive listening tests with various speakers, he identified this deliberate target profile as the realistic (audibly superior versus conformist perfect-on-paper) ideal.
The lower graph depicts intermodulation behavior. It shows a very unusual and complete absence of I.M. artifacts in most of the band except for the 19kHz+ and lowest bass region. Further standard fitness measurements that every AUDIOPAX amp prior to shipping must undergo can be found on Eduardo's Avantgarde-USA pages. This includes a nearly perfect squarewave test. According to de Lima, to arrive at such square wave performance with a zero feedback tube amp usually limits the designer to the Russian 6C33C. Its low plate resistance uncomplicates transformer requirements. His series-connected twin (and dissimilar) output transformers make things significantly more challenging instead. Without giving away secrets, he would like music lovers to know this: Designing a SET amp with low output impedance and minimal harmonic distortion is far easier than knowing how to deliberately design these variables such that they give best sonic results after having interfaced with the THD and impedance patterns of connected loudspeakers.
By virtue of discrete separation between each monoblock's twin halves, de Lima discovered an additional feature. It would optimize the harmonic distortion interface between amp and speaker, to tailor what's heard at the listener's seat. After all, measured test bench distortion into simulated loads via resistors doesn't account for behavior of real-world speakers. Under transient, wildly fluctuating music signal, speaker impedance, phase angle and harmonic distortion all vary with frequency (the latter with amplitude as well). De Lima's modeling and subsequent implementation in the Model 88 accounts for these normal phenomena in a complex manner of which these and other published graphs are mere conceptual indicators.
Granted, the impact of basic bias current adjustments on amplifier sonics -- tube and solid-state -- have long since been understood and incorporated. Class 'A', anybody? De Lima's novel angle is the ability for the end user to not only alter these values in lockstep parallel. You can dial in precise yet variable offsets or asymmetry between the dual output bottles preceded by one 12AT7 driver each. Because of its powerful effects, de Lima dubbed this feature Timbrelock™.
Use is child's play, too. Forget voltmeter probes and jeweler's screwdrivers. Without signal, flick the toggles to illuminate whatever positions in the 10-LED rings surrounding the front-edge trim pots come up. 7 o'clock corresponds to the lowest, 5:30 to the highest possible value. Lighting up two neighboring diodes at once locks in an intermediary value. Set both amps the same. Then flick the switches back into the bypass forward position to extinguish the diodes and subtract their associated circuitry from the signal path. You're back to the music. Evaluate the effects, readjust until you obtain the most realistic combination of definition and harmonic density. While each speaker will react differently -- validating the extreme useful of this invention -- higher bias values saturate color while progressively blurring tonal separation. Lower values increase resolution but lean out harmonics. Start off changing the values in parallel first to get a feel for the sonic effects. Then experiment with offset. In my experience, a discrepancy of two blank diodes seems a commonly successful match.
|These brief technical and usage descriptions barely skim the surface of what makes the Model 88 a truly novel valved amplifier. For those so inclined, I shall at review's end refer you to my prior articles and interviews for blow-by-blow accounts on the elaborate philosophy and its designer. Even more can be found in Eduardo's White Papers on the Avantgarde-USA site. Today's evaluation considers these technical aspects a closed case. I've already and extensively covered them. Instead for 6moons, I'll concentrate on the even more exciting sonic observations.