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Reviewer: Srajan Ebaen
Source: Zanden Audio Model 2000P/5000S; Ancient Audio Lektor Prime; AMR CD-77 [on loan]; Weiss Engineering Jason + Medea [on review]
Preamp/Integrated: Supratek Cabernet Dual; Wyetech Labs Jade; ModWright SWL 9.0SE; Bel Canto PRe3; Melody HiFi I2A3; ModWright LS 36.5 [on review]; Trafomatic Audio Experience One [on review]; Raysonic C200 [on review]

Amp: 2 x Audiosector Patek SE; First Watt F3 and F4 [on review]; Yamamoto A-08S; Fi 2A3 monos w. JJ 2A3-40s; Coda CX, CSX and CSi [on review]; Raysonic M100s [on review]
Speakers: Zu Definition Pro with WLM Sys V active crossover; Mark & Daniel Ruby or Maximus Monitor with OmniHarmonizer; DeVore Fidelity Nines; Rethm Saadhana; Eryk S Concept Ketsu S [on review]

Cables: Crystal Cable Ultra loom; Zanden Audio proprietary I²S cable; Crystal Cable Reference power cords; double cryo'd Acrolink with Furutech UK plug between wall and transformer
Stands: 2 x Grand Prix Audio Monaco Modular 4-tier
Powerline conditioning: 2 x Walker Audio Velocitor S fed from custom AudioSector 1.5KV Plitron step-down transformer with balanced power output option; Furutech RTP-6
Sundry accessories: GPA Formula Carbon/Kevlar shelf for transport; GPA Apex footers underneath stand, DAC and amp; Walker Audio Extreme SST on all connections; Walker Audio Vivid CD cleaner; Walker Audio Reference HDLs; Furutech RD-2 CD demagnetizer; Nanotech Nespa Pro; Acoustic System resonators for room tuning
Room size: 16' w x 21' d x 9' h in short-wall setup, with openly adjoining 15' x 35' living room

Review Component Retail: £4,399 | $8,500

Everything you always wanted to know about sex - single-ended class X...
Isn't it embarrassing how the sex word triggers hard-wired reflexes to get our instant attention? Mention integrated amplifiers to the average audiophile male and you'll get a yawn instead of a bulge. Abbingdon Music Research believes they've rediscovered the sex in this category with their new 75 lbs/34kg AM-77. It combines a 5687/6900 NOS valve voltage gain stage with a class A bipolar power buffer. There's patent-pending non NFB error correction. 48,000uF of per-channel capacitance. And a 1200VA custom dual C-core power transformer. These ingredients combine for 180wpc into 8/4 ohms (220/320 watts into 8/2+4 ohms mono strapped). Output impedance is 0.5 ohms and S/N a colossal 100dB or better. The touch-sensitive, auto-illuminating remote shifts volume in 1dB increments over a 71dB range. It also selects from 5 RCA inputs (one is shared with the balanced connectors, one with a 3.5mm iPod link).

For sexy mating -- who wants mismatches -- there's stereo, mono/mono or high-power mono mode. Unlimited sync'ing with multiple units. Integrated or power amp mode. A provision to adapt the XLR input to transformer-coupled sources by linking pins 2 and 3. The above-mentioned USB port for an iPod-ish portable source with 5V DC power mounted into the right side edge of the fascia. Paralleled 5-way and SpeakOn speaker outputs. Home theater direct conversion to transform one input into a pass-thru bypassing the volume control. Input labeling from a stored list. Input level offsets for volume matching. Display brightness adjustments. Touch-sensitive chassis controls. And a 9-pin RS232 uplink port for future upgrades. Phew, quite the run-on gymnastics on the interface front. Conspicuously lacking is a variable output for biamping with non-AMR amps that possess the necessary input trim control or, driving an active subwoofer line level: "It is our experience," explained AMR, "that not only is it necessary to have matched gain with biamping but that using amplifiers of notably different sonic signatures leads to problems, meaning the use of identical amplifiers is de facto mandatory." Certain users will beg to differ but here the AM-77 plays prude and vetoes variable outs as taboo.

The same generally applies to subwoofer feeds. We have never been able to get a truly seamless subwoofer integration using line level feeds. Again, the subwoofer will not pick up the sonic flavor of the main amplifier and speaker interaction and one ends up with the subwoofer playing one tune and the main speaker another. This has nothing to do with frequency response per se but more with harmonics, compression etc. A speaker level feed invariably integrates better, hence that is what we recommend. During the design of the AM-77, we considered adding extra outputs, however any extra output drawn after the attenuator would have to be additionally buffered in order to not load the attenuator, significantly reducing sound quality. There is no buffer that does not interact in some way with the source. We could have introduced a simple attenuated feed directly from the output stage but this makes little real sense."

For cosmic sex aka cosmetix, there are the champagne or Titanium finish options. For serious locker room envy, there are dimensions of 17.9" x 6.2" x 18.3" WxHxD (46cm x 16cm x 47cm). That's mondo big and heavy. For more boasts, there are IHF dynamic (i.e. short-term peak) ratings of 270 stereo watts, 380/480 mono watts into 8 and 4/2-ohm impedances. Flex that muscle.

For tech sex -- an oxymoron to most -- you're referred to AMR's website for the long session. In short, unusuals are to be found in the volume control implementation (inspired by Mr. Loesch's prior work on transformer volume controls); and the regulator-free power supply with critical voltage stabilization regardless of incoming AC values (if utility power falls outside 90-135V or 190-260V, the adaptive circuitry won't allow the main circuits to go on-line). There's 'crow bar' speaker protection not in series with the outputs but outside the signal path, activated only by equipment failure or overdrive conditions. "OptiPower refers to positions where very large currents are required at low voltages and with low noise, such as the bipolar output stage of the AM-77 and the heater circuit of the CD-77's analogue stage. Here AMR employs a circuit called Virtual Battery Power Supply. It combines a battery of capacitors after a low-noise rectifier followed by an electronic circuit. This utilizes a large-value capacitor as a reference source which is 'amplified' by a high current-gain bipolar transistor. The amplification factor of the bipolar transistor is several thousand, producing a behavior identical to a capacitor several farad in size, which is constantly being recharged. The result is a very stable voltage with a low source impedance and very low noise, extremely similar in behavior to the use of a large battery."

Technical director Thorsten Loesch views the AM-77 project as an attempt to wrest zero feedback low-power triode sonics from average 4-ohm 85dB speakers. The tech sex highlight thus is the 'X-rated' topology. Truly. CEO Vince Luke sums it up as "most amplifiers are class A, AB or D. The AM-77 is genuinely different which we refer to as class X. Among other things, it explains why its power output is the same for both 8 and 4 ohm despite having a transistor buffer stage. The AM-77 has much in common with a valve-based amplifier, meaning it operates 'power-matched' just like a valve circuit. Hence the same power into 4 and 8 ohms (and a little more into 6 ohms). All this has mostly to do with the fact that our output stage is in effect a valve circuit realized with solid-state devices but no output transformer." [Below are block diagrams to show the feed-forward error correction of the X circuit.]

The Traditional Approach and Customary Circuit
In order to interface signals, often some form of gain is needed. Audio equipment requires both voltage gain and power buffering - voltage gain where levels need to be increased and power buffering where loads like speakers need to be driven. In power amplifiers, a common approach is to design the amplifier circuit to have very high gain (which requires a large number of amplification stages and invariably adds noise and distortion for each added stage) and a large amount of negative feedback to set circuit gain. Often several such multi-stage circuits with their attendant negative feedback circuits are connected in series. This is depicted in the Customary Circuit diagram."

Class X
In the AM-77, two stages are used for the OptiGain®circuit. First, the signal is amplified to the desired voltage using an NOS 5687 electron valve with linearity and drive without the need for a negative feedback loop. This solution is much more costly and difficult when compared with more ubiquitous and cheaper operational amplifiers (op-amps). In our view, this represents a very significant sonic improvement over a circuit with negative feedback.

"The second half of the OptiGain®circuit is the power buffer stage where solid-state technology is combined with a thermionic electron valve phase-separation circuit. The power buffer stage uses AMR's "Class X: Jikoda circuit" with bipolar output devices, which in real-time, simultaneously detects and cancels unwanted artifacts in the audio signal. None pass to effect the playback of music, exceeding the ability of even traditional well-regarded class A circuitry.

"In addition, making the power buffer stage balanced and by actively driving both terminals of the speaker (as opposed to connecting the negative terminal of the speaker to some ill-defined 'ground') will ensure that the amplifier remains in full and direct control of the attached speaker and that any noise or error signals impressed upon the power supply are suppressed.

"Of course, this requires in effect two complete power buffer stages for one channel of amplification instead of the more common use of just one power buffer stage. It is thus twice as costly but the extra cost is well justified by the superior sonic performance. Finally, having a balanced power stage allows the application of a very specific form of feed-forward error correction, a unique method of reducing nonlinearities that is far superior to the common use of so-called negative feedback in amplification devices.

"In the AMR circuit, the error between input and output signal in one of the two balanced phases is precisely extracted and applied in reverse polarity to the opposite phase section. As a result, the output is only left with two signals which are in opposite polarity to each other. With all the errors removed, the result is a perfect replica of the input signal driving the speaker's output. This feed-forward error correction in effect completely removes the 'solid-state' sonic signature of the power buffer stage. This leaves an amplifier that behaves sonically like an optimal, extremely high power (>160W) and very linear (0.09% THD @ 160W) single-ended triode amplifier with only two stages, which also has a constant high damping factor (over 15 from 1Hz to 100kHz)."
The AM-77 thus is a hybrid combining a single-ended valve voltage gain stage with a dual-differential transistor power buffer wherein one half of each 5687 provides gain, the other phase splitting. The output stage is non-traditional in a number of ways, hence normal classification isn't applicable. "One might call it class A insofar as there is no switching off the 'other half' of the circuit when signal levels are high, yet it is not class A insofar as it does not operate with a bias current that is equal to the maximum output curent. So it is not traditional class A but also not traditional class A/B. We call it class X."

As the review of their companion CD-77 player proved, the AMR team gives more than lip service to esoteric hifi sensibilities. Valve-powered, zero oversampled or fully upsampled (and everything in-between while spinning their own transport section), the massive AMR digital source proved Zanden-esque in spirit and execution. It suggested that the SET ambitions for the integrated amp would be likewise rooted in demonstrability and not mere propaganda.

The CD-77's lengthy introduction explained how AMR employs split teams in the UK and the Far East to fabricate luxury fi at sub $10K stickers. The kit is delivered in alu flight cases hidden in double cardboard cartons. A dispatch of the AM-77 includes a custom power cord and interconnect; a system test disk (with mono pink noise, out-of-phase mono pink noise, burn-in signal and demagnetization signal); a Telarc Jazz Piano sampler (Chestnut, Peterson, Loussier, Tyner, Brubeck, Camilo, Shearing et al); comprehensive bound documentation with a quick-start insert; a velcro-sealed cloth cover for added transit protection; and the custom remote with spare batteries. This stuff has been tweaked intensively by a euro team and then is on-site checked by company principal Pat in AMR's 4000sft plant. The AMR organization is a tightly knit transcontinental affair.

That's not to say these products launched quite on schedule or without twists. Despite strong and positive reactions at its first trade shows, AMR was slow to push its pieces forward until sufficient international distributor feedback signed off. Early tester units for example wouldn't power up where wall voltages fell outside the preset window to trigger protection. Learning what permutations around the world to expect, that window has been widened in production. Such are common birthing signs. AMR apparently has exited this tunnel now. I expect this to mean that marketing will soon commence in earnest, with global reviews pursued and the big drum banged.

As the innards show, parts density is high, assembly impressive and copper lining generous.