| For many years, I have been fascinated with single-ended class A amplifier
design. To me, this design simply makes musical instruments sound more correct and
natural. I do not understand why more designers are not into this topology. Maybe it is
because, like vacuum tubes, they do not measure as good as they sound. In a sense,
single-ended amplifiers can "push" current into the load but not actively
"pull" current from it, so they must be biased high enough to encompass the full
power waveform. Single-ended, pure class A with a total of two gain stages lay the
groundwork for this remarkable amplifier, the Pass Aleph 2. According to Pass, most power
amplifiers have five to seven gain stages in the signal path in order to obtain enough
gain to use feedback to provide adequate performance. When the design is right from the
start, all the band aid circuits are not necessary. I am also a big fan of the less is
more design philosophy, and that is what the Pass Aleph 2 amplifier is all about.
Features Nelson Pass has been designing amplifiers for 25 years, and most recently he
has come up with a line of monoblock and stereo amplifiers that are both physically and
sonically unique. The Aleph 2 is a monoblock 100 W, single-ended Class A amplifier that is
biased so high that it idles at 300 W! This high idle current is very desirable as a means
of maximizing linearity. All of the Aleph models are of moderate power (30 W to 200 W) and
run not warm, but hot!
The Aleph 2 takes up about a cubic foot of space and is all heatsink except for the top
and bottom covers. This amp is beautifully made and has both RCA unbalanced and XLR
balanced inputs. On the output, two sets of gold-plated five-way binding posts allow for
bi-wiring. On the front is a power switch, an AC line connector and a nice blue LED
between, the heatsink fins - nothing you do not need. When powering up, the amp's turn-on
peak is roughly 50 amps with a 120 V system. Once the amp is on, its current from the wall
is constant and is rather impervious to power line variations. The amplifier is said to be
stable into any load impedance or reactance including a direct short, and will deliver
clean audio into 4 ohms at 300 W peak.
Mr. Pass uses a characteristic of air as a model for designing his amplifiers, in that
air is single ended in nature. He says, "We can push on air and raise the pressure an
arbitrary amount, but we cannot pull on it. We can only let it relax and fill the space as
it will, and the pressure will never go below 0. As we push on air the increase in
pressure is greater than the corresponding decrease when we allow air to expand. This
means that for a given motion of diaphragm acting on air, the positive pressure
perturbations will be slightly greater than the negative. From this, we see air is phase
sensitive."
Mr. Pass goes on to say, "Air's distortion is monotonic, which is to say its
distortion products decrease smoothly as the acoustic level decreases. I believe this is
an important element that has often been overlooked in audio design and is reflected in
the poor quality of early solid state amplifiers and D/A and A/D converters. They are not
monotonic: the distortion increases as the level decreases," (definitely not a good
thing). The end result is that the distortion products this amplifier does have are
directly proportional to the output power.
I had to pop the hood to check out what was inside of Pass Aleph. How does this thing
work with so few parts, I asked? The answer: very well. The signal path in this design has
a MOSFET on the input and six MOSFETs in parallel on the output and that is it. It seems
as though ever since op-amps arrived on the scene a few years ago audio quality took a
giant step backwards. Designers use op-amps freely because they are cheap, easy, measure
pretty well and can potentially disguise a poor design. I believe we are able to hear many
things we cannot measure, and some distortions we can measure are not necessarily
objectionable to listen to. Since I wrote this review before Ed Foster did his bench
tests, I can only guess that it sounds better than it measures. (Other than THD at 4 ohms,
it measures pretty good. -Ed.)
There has been much interest recently in single-ended triode tube designs, which are
limited by high voltages and low plate currents and result in low maximum power outputs.
Incredibly efficient speakers are necessary to make this type of system work, although the
results can be rewarding. Many of the sonic advantages of the tube design can be had,
without all the limitations, with this Pass approach.
In use When UPS delivered the review pair of amps, I was so pumped about getting them
connected, I threw my back out carrying the amps from the garage to the studio. They weigh
about 60 pounds each and have no handles (that is no excuse, I just lifted wrong). As I
lay in pain on the floor, my son Paul (audiophile in training) connected the Pass
amplifiers to my Westlake Lc 8.1 monitor speakers. After about an hour of warm up the amps
were almost too hot to touch, so I figured they were ready for auditioning.
The first thing I listened to was the new Duke Ellington CD recording that DMP just
released. I do not think any type of music is more demanding on a monitor system than a
big band brass section. The Aleph 2 sounded smooth and natural without the harshness
associated with so many solid state amplifiers. The control over the bass was amazing -
deep without getting woolly.
One of the nicest sonic signatures of this amplifier was its sense of space; there is a
feeling of depth and air around the instruments. As I remembered from my experiences with
single-ended class A microphone preamps, the Pass amplifier has the same kind of harmonic
accuracy that I do not hear from bi-polar designs. (In talking with Nelson Pass, I hope I
planted the seed for a Pass microphone preamp someday; at least he did not say no).
I ran the Aleph 2 amplifiers for several weeks without so much as a hiccup. The only
thing negative (pun intended) I can say about these amps is the heat. At certain times of
the year this may be welcome, but with the amps on the floor beneath my speakers, this
would be a problem in the summer. If I had five Aleph 2 amplifiers for my surround setup I
would have to take the summer off and go sailing!
Summary
With its less-is-more design, this single-ended Class A amplifier is second to none. It
has all the virtues of the best vacuum tube designs but with control over the low end that
tube amps seldom achieve - plus MOSFETs do not go bad like tubes. If you are looking for
truly top-of-the-line amplifier and can deal with the heat, you should definitely check
out the Pass Aleph 2 amplifier.
Tom Jung is technical consultant for Pro Audio Review and is the owner and
mastering/recording engineer of Digital Music Products (DMP). |
