The Volksamp Aleph 30 SE Power Amplifier
Review by Dick Olsher
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To fully understand amplifier evolution over the past 50 years, it is essential to examine the decade of the
70's, a time when tube amplification came close to extinction. Although the solid-state era was launched in the 60s, its pull in the high-end market place did not become significant until the mid
70's. The 60's were defined by the M & M tube gear: Marantz and McIntosh. The Marantz 8B, 9A and the McIntosh MC-60 were long-standing Class A recommendations by the old Stereophile Magazine. While the lower price points was clearly locked up by Dyna Company's Dynaco Stereo 70 and Mk. III models.
In December 1964, J. Gordon Holt offered this prognostication: "Transistors, of course, are with us more than ever, and as designers gradually lick their original problems - hiss, easy overload and inherent vice of the output stage, tube adherents are being left with less and less to claim in the way of superiority." The general thinking at the time was that progress was inevitable and that it was only a matter of time before solid-state designs relegated tubes to the trash bin. Sure enough, about four years later, JGH went gaga over the Crown DC-300, declaring it to be "…the best power amplifier that is currently available, regardless of cost. We will even go so far as to guess that it will not be possible to build a better one, although we have sometimes been proven wrong on some similarly rash statements in the past."
The transistor's choke hold intensified with Jim Bongiorno's Dynaco Stereo 400 and the splash made by his Ampzilla kit project while at
SAE. Bongiorno's burst of creative energy with his own "The Great American Sound Company" (GAS) set the design template for decades to come. Design features such as servo-controls, fully complementary differential balanced topologies, and sliding class A bias, transformed solid-state into a technological powerhouse. By the late 70s everyone, it seemed, rushed to compete. Even Audio Research blinked, as it felt the pressure, and began to offer transistorized fare.
Audiophiles were mesmerized by solid-state's macho image of lots of horsepower under the hood and sufficient current drive to vaporize speaker voice coils. Marketing hype at this time was intensely specification based, emphasizing the vanishing harmonic distortion levels of solid-state designs. Tubes were dismissed on the weight of "objective" measurements as obsolete and/or inferior. This is not the time nor place to debate why amplifiers that measure so well, can sound so poorly. However, as with wine tasting, the whole high-end experience is predicated on perception. This concept was well known and respected in the old days. The late Harry Olson used to say that in all things audio, the ear is the final arbiter. Unfortunately, the scope had by now replaced listening tests for may designers. An illusion of live music was (and still is) what I was after. For the record, I was never seduced by any solid-state amplifiers during this period. It was the Dynaco Stereo 70 that proved to be the great destroyer. I couldn't find anything transistor that came close to the musicality of this vintage design. No, I didn't believe even then that it was the most detailed or extended amp on the market, but it allowed me to transcend beyond a mere hi-fi experience to savor the music's drama and passion. It was precisely solid-state's emotional sterility that sealed my life-long bond with tubes. To their credit, many audiophiles and music lovers eventually became dissatisfied with the status quo: a mindless quest for ever increasing detail, bass extension, and smoothness. The push to restore the musical balance with a healthy dose of passion and fire laid the foundation for the tube renaissance of the 80s.
So why is Toob Man reviewing the Aleph 30 - a solid-state design? In a nutshell, it is the most musical solid-state I have yet to audition over the past forty years. It effectively bridges the gap between solid-state and tube sound, blending tube and transistor virtues into a musically satisfying whole.
Back in the 80s, Bob Carver issued an amplifier challenge to Stereophile Magazine. The gist of it was that Bob claimed that he could transform the sound of one his solid-state amps (I can't recall the exact model) to mimic that of an expensive tube amp of the Magazine's own choosing. So here we were huddled in JGH's listening room in Santa Fe, with Bob on his hands and knees tinkering with his circuitry so as to match the sonics of a very expensive Conrad-Johnson power amp. I've got to hand it to Carver - that's some bravado and one hell of a publicity stunt. Nice try, Bob, but no cigar. At the time, Larry Archibald and JGH seemed to think that Carver had in fact succeeded. But to my ears, Carver had merely managed to reduce the damping factor of his amp; in essence making it sound as loose and uncontrolled in the bass range as the Conrad-Johnson. There was still a large residual difference in the midrange. Some 20 years later, I can report that Nelson Pass has succeeded where Bob Carver has failed: the Aleph 30 enjoys a healthy dose of tube magic in the midrange.
The Volksamp Aleph 30 represents an upgraded version of the much praised Aleph 3, and is the result of Nelson's vision of single-ended Class A topology and commitment to simple circuitry having the most natural response characteristic. Only two gain stages are used, and there are no adjustments of any kind in the circuitry. There are absolutely no potentiometers to adjust, as the operating parameters of bias current and DC offset are defined by physical constants, and will not go out of adjustment.
Let me turn the podium over to Nelson Pass for a discussion of his current design philosophy and specific design aspects of the Aleph 30. This is not in a general our editorial policy, but Nelson has so much to good stuff to say that we'll permit this exception: "When I started designing amplifiers 25 years ago, solid state amplifiers had just achieved a firm grasp on the market. Power and harmonic distortion numbers were king, and the largest audio magazine said that amplifiers with the same specs sounded the same.
We have heard Triodes, Pentodes, Bipolar, VFET, Mosfet, TFET valves, IGBT, Hybrids, THD distortion, IM distortion, TIM distortion, phase distortion, quantization, feedback, nested feedback, no feedback, feed forward, Stasis, harmonic time alignment, high slew, Class AB, Class A, Pure Class A, Class AA, Class A/AB, Class D, Class H, Constant bias, dynamic bias, optical bias, Real Life Bias, Sustained Plateau Bias, big supplies, smart supplies, regulated
supplies, separate supplies, switching supplies, dynamic headroom, high current, balanced inputs and balanced outputs. Apart from digitally recorded source material, things have not changed very much in twenty five years. Solid state amplifiers still dominate the market, the largest audio magazine still doesn't hear the difference, and many audiophiles are still hanging on to their tubes. Leaving
aside the examples of marketing hype, we have a large number of attempts to improve the sound of amplifiers, each attempting to address a hypothesized flaw in the performance. Audiophiles have voted on the various designs with their pocketbooks, and products go down in history as classics or are forgotten. The used market speaks eloquently: Marantz 9's command a high price, while Dyna 120's are largely unwanted. . .
I authored the first patent on the dynamically biased Class A amplifier in 1974, however I have not used the technique for the last 15 years. The reason is that I found the quality of sound associated with an efficient Class A operating mode inferior in depth and less liquid at high frequencies, simply because it operates at reduced bias at low levels. Given the plethora of cool running "Class A" amplifiers on the market, you might say I opened a Pandora's box.
A very important consideration in attempting to create an amplifier with a natural characteristic is the selection of the gain devices. A single ended Class A topology is appropriate, and we want a characteristic where the positive amplitude is very, very slightly greater than the negative. For a current gain device, that would mean gain that smoothly increases with current, and for a tube or field effect device a transconductance that smoothly increases with current. Triodes and Mosfets share a useful characteristic: their transconductance tends to increase with current. Bipolar power devices have a slight gain increase until they hit about an amp or so, and then they decline at higher currents. In general the use of bipolar in a single ended Class A circuit is a poor fit. Another performance advantage shared by Tubes and Fets is the high performance they deliver in simple Class A circuits. Bipolar designs on the market have between five and seven gain stages associated with the signal path, but with tubes and Mosfets good objective specifications are achievable with only 2 or 3 gain devices in the signal path. Readers of The Audio Amateur Magazine will be familiar with my "Zen" design, which uses a Mosfet in a power amplifier which has only one gain stage, and only one gain transistor. Research continues at Pass Labs in improving the performance of very simple gain circuits.
Yet a third advantage tubes and Mosfets have over bipolar devices is their greater reliability at higher temperatures. As noted, single ended power amplifiers dissipate comparatively high wattages and run hot. Bipolar devices are much more prone to failure at high temperatures. In a decision between Triodes and Mosfets, the Mosfet's advantage is in naturally operating at the voltages and currents we want to deliver to a loudspeaker. Efforts to create a direct coupled single ended triode power amplifier have been severely limited by the high voltages and low plate currents that are the province of tubes. The commercial offerings have not exceeded 8 watts or so, in spite of hundreds of dissipated watts. Transformer coupled single ended triode amplifiers are the alternative, using very large gapped-core transformers to avoid core saturation from the high DC current, but they suffer the characteristic of such a loosely coupled transformer as well. The promise of the transconductance characteristic in power amplifiers in providing the most realistic amplified representation of music is best fulfilled in Mosfet single ended Class A circuitry where it can be used very simply and biased very high.
The Pass Aleph 30 uses International Rectifier Hexfet Power Mosfets exclusively for all gain stages. These Mosfets were chosen because they have the most ideal transfer curve for an asymmetric Class A design. Made in the United States, they have the highest quality of power Mosfets we have tested to date. We match output devices to within 2%. The input devices are matched in circuit for lowest noise and distortion. The smallest of these, the input devices, are capable of peak currents of 5 amps. The largest are capable of peaks of 25 amps each, and are run in parallel pairs. The power Mosfets in the Pass Aleph 30 have chip temperatures ratings to 150 degrees Centigrade, and we operate them at small fractions, typically 20% of their ratings. For extended life, we do not allow chip temperatures to exceed 85 degrees C. Regardless of the type of gain device, in systems where the utmost in natural reproduction is the goal, simple single ended Class A circuits are the topologies of choice.
It is a very simple topology, which is a key part of the sound quality. Other solid state amplifier designs have five to seven gain stages in the signal path in order to get enough gain to use feedback to provide adequate performance. In this amplifier, we get greater linearity by providing much more bias through two gain stages: a differential input stage, and the output transistors.
Mosfets provide the widest bandwidth of solid state power devices, however they were not chosen for this reason. The design of the Aleph 30 does not seek to maximize the amplifier bandwidth as such. The capacitances of the Mosfets provide a natural
roll-off in conjunction with the resistive impedances found in the circuit, and the simplicity of the circuit allows for what is largely a single pole
roll-off characteristic. The slew rate of the amplifier is about 40 Volts/µS load, which is about 10 times faster than the fastest signal you will ever see, and about 100 times faster than what you will be listening to. In and of itself, the slew rate is an unimportant factor when evaluating tube and simple Mosfet designs. It becomes more important with complex circuit topologies where there is heavy dependence on feedback correction, but even then its importance has been overstated.
The amplifier is powered by a toroidal transformer which charges 120,000 µF capacitance. This unregulated supply feeds the output transistors only with a full power ripple of about .3 volt. The power draw of this system is constant regardless of the music playing through the amplifier. As such, it does not depend on a high quality AC outlet or special power cords, since the dynamic performance does not create a variation in AC line draw. If the AC line is running low, the output stage will bias to a higher current level by way of compensation.
The amplifier is stable into any load impedance or reactance including a direct short. The Aleph 30 is impervious to electrostatic shock at the input and dead shorts at the output. You can safely plug and unplug inputs and outputs while the amplifier is running. (Do not try this with other
products). The Aleph 30 is protected from overheating by a 75 degree C. thermostatic switch, and from internal failure by a slow-blow fuse."
Virtually all of the original circuit design and choice of parts remains intact. However, the enhancements are significant. For starters, the number of output power Mosfet devices has been increased from four to six per channel. This lowers the distortion and improves the damping factor by about 50%. It also makes it possible to run the bias up a bit, giving higher heat sink temperatures but with the same on-chip temperatures. Second, the power supply has been beefed up with half-again more power supply capacitors with twice the voltage ratings, giving lower ESR figures. Also the power supply incorporates much improved line noise filtering in both differential and common mode. The input impedance on the single-ended input, has been increased to 47
Kohm, from the original Aleph 3's 23 Kohm. The current output Mosfets are said to not only be a bit better than those in the Aleph 3, more importantly are better matched. The end result is a reduced noise floor and about a 6 dB greater dynamic range relative to the Aleph 3. Finally, a balanced XLR input has been provided in addition to the single-ended RCA input of the Aleph 3.
The Aleph 30 runs hot, with the heat sink temperature rising to 120 to 130 degree F. Good ventilation is essential when
sitting the amplifier. At least six inches of clearance are recommended on the sides and top. According to Pass Labs it takes about an hour of warm up time to get the best performance out of the amplifier. This is based on actual distortion and noise measurements. Apparently, many customers have reported that an even longer warm up period yields more improvement.
There are cosmetic differences, having to do with the type of chassis and heat sink used. The Aleph 3 was machined entirely from aluminum extrusions stock, which proved to be very expensive and not particularly functional or ergonomic. There was limited access space to the input and output terminals on the rear, and the old layout required the power switch to situated in the rear underneath the wiring. The Aleph 30 features a conventional chassis with side-mounted heat sinks, and a front-panel mounted power switch. The amp is now more convenient to use, sounds better, and costs quite a bit less as the production cost savings have been passed directly to the consumer (the original Aleph 3 cost $2,500!)
Note that the Aleph 30 is produced under license from Nelson Pass and Pass Laboratories Inc., and is distributed and serviced by Pass Labs Domestic Sales Inc. and Pass Labs Foreign Sales Inc. The warranty period is 3 years. When can you expect the first sign of trouble? As Nelson explains: "In fifteen years the electrolytic power supply capacitors will get old. Depending on usage, you will begin to have semiconductor and other failures between 10 and 50 years after date of manufacture. Later, the sun will cool to a white dwarf, and after that the universe will experience heat death."
The BassZilla loudspeaker (both the Fostex and Lowther DX4 versions) was used almost exclusively for all of the listening tests. The accompanying preamps were the Air Tight ATC-2 and the DIVA M-7. The digital front end was the exceptional BAT VK-D5SE CD player.
Let me start with traditional solid-state virtues; namely, bass drive, resolution, and
macrodynamics. There was absolutely no hesitation or stall while revving up its output stage to negotiate loud passages. This sensation of instantaneous power, a hallmark of upper echelon solid-state amplifiers, was present in spades. In fact, the change in "RPM", from low to high level was linear with no perception of any compression. Take, for example, the following live recording of the Milt Jackson Quintet with Ray Brown (That's the Way it is: MCA/Impulse! MCAD-33112). I'm always amused by the cautionary note that accompanies the AAD designation on the back cover: "This compact disc contains programs transferred from analog tape and therefore may contain some tape hiss and other anomalies that exist with analog recording." I suppose that by 'anomalies' they might be referring to a couple of spots where Jackson's vibes overload the tape recorder. When this happens, the startle factor with the Aleph 30 is very high as the transient hits the proverbial brick wall.
There is no place where resolution becomes a bigger issue than with reproduction of a piano. The challenge is that piano sound is quite detailed or crisp due to its percussive nature, yet its tone is often capable of great lyricism. These two opposing aspects - the beauty and the beast - must be kept in balance during both the recording process as well as the playback. Otherwise, the nature of the performance is adulterated. Solid-state typically errs toward a crisper picture of the real thing, while tubes often soften (liquefy) and dull the full transient impact of piano sound. The Aleph 30 manages to walk a fine line, in that it is able to retrieve transient nuances without sounding bright or etched in the process. This is one smooth cat!
Bass impact and definition were superlative. My impression was that the Aleph was pushing bass lines to the limit allowed by the loudspeaker. This was a complete package. The bass range was tightly integrated with exceptional body and pitch definition. The last thing I look forward to in an amplifier is an anemic upper bass or lower midrange. For someone like me who's enamored with the majesty of a double bass or cello, the fundamental and first harmonic must be served up correctly - or there's hell to pay.
One of the Aleph's most remarkable attributes is its vintage tube-like tonal balance. The power range of the orchestra is full bodied with a rich harmonic palette. In fact, when its cold, the Aleph sounds a bit too dark and recessed. However, as it warms up, the highs come into almost perfect balance with the midrange. So if you're tweaking your system to suit the Aleph, be sure to allow it sufficient time before taking any action. For example, my first impression when I first switch it on is that I should change over to a brighter sounding preamp. Allow the Aleph to cook for a couple of hours, and life is good again.
The exceptional retrieval of microdynamics or low-level pitch and volume modulations is what gives single-ended triode (SET) amplifiers a significant advantage in recreating a musical tapestry rich in emotional intensity. The music's drama resides in such detail. The subtle tremolo or vibrato of a singer's voice holds the key to its expressiveness. Again, the Aleph 30 held its own in the company of SET amps, although it was outdone in this regard by the Art Audio PX25 ($5,000).
Space - the final frontier. This has always been the domain of tube magic. The rendition of the recording acoustic as a believable three-dimensional space populated by palpable image outlines has been better facilitated by tubes since the beginning of time. Transistors have had a hard time in painting anything remotely approaching a 3-D space. Even some of the highly touted gear of the past (e.g., PS Audio) was shockingly deficient in this area. No, I'm not going to tell you that the Aleph is indistinguishable in spatial matters from a SET amplifier, because it isn't. It lacks the spatial incisiveness and focus of a good 2A3 based SET amplifier. But it represents a breath of fresh air relative to expensive Krell or Mark Levinson amplification. The Aleph even competes effectively with highly touted push-pull tube amplifiers. It's just that it lacks the holographic focus and mastery of the spatial perspective afforded by the finest SET amplifiers. It slightly constricts the depth perspective and diffuses image outlines a bit. Nothing very serious, to be sure, unless soundstaging is very high on your list of sonic priorities.
The Aleph 30 has performed superbly in the context of my current reference system. It consistently pushed my buttons like only a handful of amps have done - irrespective of cost. Forget the ideology of tubes versus transistors, and don't let its "plain Jane" looks fool you: this is one amazing little amplifier. It spins out a magical musical carpet better than any solid-state amp I've auditioned to date. AND GET A HOLD OF THIS: it dishes out ample tube magic to satisfy even Toob Man. Let me be even more blunt: it's imperative that you audition this amp before you make any purchase decision - at any price point. Its performance/price ratio is simply astounding. The Aleph 30 is truly a case of simpler is better.