Pass Laboratories Aleph 0 monoblock power amplifier

The Aleph Null, or 0, represents Nelson Pass's maiden product under the Pass Laboratories banner. When he left Threshold several years ago, Pass had the luxury of starting over with a clean slate, and decided immediately that he wanted to design a single-ended MOSFET amp. The result is aptly named after Georg Cantor's first transfinite number: Aleph Null, the gateway to higher-order infinities. Just as Cantor's transfinite mathematics stretched minds with its novel conceptual view of the infinite, the Pass Aleph 0 tantalizes the imagination with a new dimension in the future of solid-state amplification: a single-ended output stage.

The sound of one transistor clapping
If only objective measurements are used to assess the performance of a power amp, then complex circuit topologies will be seen to excel compared with much simpler designs. The problem with measurements, of course, is that they describe performance not in the perceptual but in the physical domain. What something sounds like does not always correlate with how it measures. I'm not advocating or excusing poor measurements—decent measurements serve to validate a given design's veracity and, by implication, the designer's competence. Rarely will an amp that has gross test-bench measurements sound good. Conversely, I've heard many test-bench wonders sound downright ugly. There's a point beyond which measurements fail to convey the musical potential of a device. If it were true—as some mainline reviewers and magazines maintain—that the story begins and ends with measurements, then a Japanese receiver ought to sound superior to almost every expensive high-end tube design.

As Nelson Pass is fond of saying, audiophiles have voted with their pocketbooks: on the used-equipment market, a Marantz 9 is highly coveted, while the early solid-state Dynaco 120 is largely unwanted. Looking at the last 50 years of audio history, I've found that simple designs (tube and transistor) have come closest to capturing the textural purity and intimacy of live music.

SE designs aren't exactly new to Pass, who published a 20W single-ended DIY design in 1977 in the pages of Audio magazine. To paraphrase his words, the Aleph's design rests upon the philosophical tripod of simple circuitry, transconductance gain devices, and maximally linear operation.

The Aleph uses only three gain stages. The differential input stage accepts both balanced (XLR) and unbalanced (RCA) inputs, the former tied directly into the feedback loop without additional active circuitry. A cascoded voltage gain stage drives an n-channel MOSFET output stage, which is biased at 2.5 amps quiescent current. Out to about 40W into 8 ohms—that is, almost up to 2.5 amps output current—this stage operates single-ended: both the negative and the positive signal swings are accommodated by paralleled n-channel devices. Beyond this point, p-channel power MOSFETs kick in to handle the positive signal swings. The amplifier therefore operates in class-A push-pull out to its rated power-delivery limit. Pass believes that defaulting to class-A push-pull is sonically superior to simply clipping the SE stage at a lower power level.

The MOSFET, Pass's current gain device of choice, is a transconductance device, like a vacuum tube. However, it doesn't require transformer-coupling to the low-impedance load—the Achilles' heel of tubed SE amplifiers. Also, like a triode tube, a MOSFET's transconductance, or gain, tends to increase with current. Pass believes that the bad rap many MOSFET-based designs have received is attributable to two factors: most designers simply dropped MOSFETs into the same (read: complex) topologies optimized for bipolar transistors, thus failing to capitalize on the MOSFET's full potential. And MOSFETs perform poorly at low bias currents, exhibiting low-level non-linearities in traditional class-AB designs. Nelson's bottom line is that a MOSFET must be operated in class-A—where maximum intrinsic linearity is achievable—to fully realize its benefits.

US-made, International Rectifier, HexFET power MOSFETs are used exclusively for all of the Aleph 0's gain stages. Input devices are matched to within 0.2%, output devices to within 2%. Each output MOSFET is capable of handling peaks of 25 amps; two banks of eight MOSFETs are run in parallel. Since no current-limiting circuitry of any sort is used, peak currents of 50 amps can be delivered before the fuses blow. The circuit's inherent linearity means that only some 20dB of global negative feedback need be used to tailor the overall voltage gain and increase the damping factor to about 800—equivalent to an output impedance of 0.01 ohms.

Is that you, Mother?
So runs the punch line of the joke that begins: What does the baby porcupine say after he backs into a cactus? The Aleph 0, with heatsink fins on all four sides, reminds me of a porcupine. So it doesn't look very French or German—an American original, indeed! The machined-aluminum chassis is grained and anodized for a cool metallic look.

In 1974, Nelson Pass ushered in a new era of "cool-running" class-A amps with his patented, dynamically biased "Stasis" output stage. Pass now admits that he hasn't used this technique for the past 15 years, because he's found that operation at reduced bias levels degrades sound quality—primarily soundstage depth and textural liquidity.

So, no surprise, the Aleph 0 runs hot, its heatsinks warming up to 120–130°F after one hour of operation. When the Aleph's internal temperatures exceed 160°F, a thermal protection system in the unit shuts the amp off until the thermal sensor has cooled. With adequate ventilation (at least 6" of clearance is recommended on the sides and top), and ambient temperatures of up to even 90°F, chassis temperature is supposed to remain below the trigger level of the protection circuit. Of course, the chassis' rate of warmup is also a function of the type of signal used, test signals being harder on the amp than music.

So when one channel went mute one hot summer evening, it took me a while to realize what had happened. I checked all cable connections and scratched my head; and when the amp came back to life some ten minutes later, it dawned on me: thermal protection! This prophylaxis is aimed at protecting the output transistors, but I found it inconvenient at the time. A few minutes after the first channel came back to life, the other channel played dead. The final solution was to shut the amps off for a while. Although it was a hot New Mexico day and the amps had been running for several hours, they were well-ventilated, and ambient conditions were within the envelope specified in the owner's manual. The thermal-protection system needs fine-tuning.

The Aleph 0 is DC-coupled from input to output—there are no capacitors in the signal path. It takes an hour for the chassis temperature to stabilize, during which offset levels are a bit higher than 50mV. After the first hour, DC offset at the Aleph's output drops to around 50mV. Such DC offsets do not interfere with loudspeaker performance. DC offset levels could, however, be exacerbated by the preamp—when the preamp's own output stage is DC-coupled. Another demand the Aleph places on the partnering preamp is its ability to drive a rather low input impedance, which means that, in general, the preamp's output impedance should not exceed several hundred ohms.

COMPANY INFO
Pass Laboratories Inc.
13395 New Airport Road
Suite G, Auburn, CA 95602
(530) 878-5350
ARTICLE CONTENTS

COMMENTS
Bogolu Haranath's picture

Aleph 0 has a damping factor of 400 into 8 Ohms and 200 into 4 Ohms ....... It has a tight fist control over partnering loudspeakers :-) .......

Ortofan's picture

... Adcom GFA-565 and Threshold T-200 - both from the 1990s, as well.

Explain, if you would, exactly how a power amplifier might exert "tight fist control" over a loudspeaker?

Bogolu Haranath's picture

Both those Adcom and Threshold amps were designed by Nelson Pass ....... Mark Levinson and Krell amps of the 90's also had very low output impedance and very high damping factors .....

Martin Colloms wrote articles about high damping factors and tight control over loudspeaker drivers for Stereophile :-) ........

Bogolu Haranath's picture

Martin Colloms article 'The unseen variable' is one of the good articles to read :-) .........

Ortofan's picture

... mention, let alone describe or explain, the term "tight fist control" that you used?

Bogolu Haranath's picture

Ok ..... That is my description for the dramatic effect ...... High damping factor means tight control of the loudspeaker drivers ....... Little variation in frequency response of the loudspeaker :-) ........

Bogolu Haranath's picture

Pass also makes First Watt amps, reviewed by Stereophile, which have low damping factors and high harmonic distortions ........ HR loved the sound quality of those First Watt amps :-) .......

dworkman's picture

Bogulu

This has been asked before but you never answered. Why…do…you…insist…on…using…ellipses…at…end…of…every…sentence…?

It is entirely incorrect grammatically and IT’S VERY ANNOYING. WOULD YOU LIKE IT IF EVERYONE TYPED ALL CAPS, ALL THE TIME?

Or perhaps ifwedidntliveanyspacesbetweenwords?Thatsprettyannoyingtoo,isn’t it?

Bogolu Haranath's picture

I just want to get attention ....... I got your attention, didn't I :-) ........

Jack L's picture

....... 200 into 4 Ohms" quoted B. Haranath.

Too high damping factor is NOT always a good thing as it so often over-control the loudspeaker units diaphram movements, particularly the bass driver or woofer. You will find bass notes often too tight, too clinical, not as musical & enjoyable for power amps with lower damping factor.

Jack L

Bogolu Haranath's picture

Tube amps usually have low damping factors ....... The First Watt amps which I mentioned above, also have low damping factors ......

What is the output impedance of your tube amp? :-) .........

Jack L's picture

Hi

Not all tube amps get low damping factor.

There are quite a few tube amps get very high damping factor, e.g.
Berning ZH-240, ZH-270 OTL tube amps.

Those Zero Hysteresis OTL TUBE amps get DF up to 300 !!!!!!!

Jack L

Bogolu Haranath's picture

Do you know what the output impedances are for those Berning model amps? ........ At what loudspeaker load impedance, that 300 damping factor is? ........ What are the damping factors for 8 Ohm and 4 Ohm speaker load impedances? :-) ........

Bogolu Haranath's picture

If you want almost no damping factors into 8 and 4 Ohms, look into BAT and PrimaLuna tube amps reviewed by Stereophile :-) .......

AaronGarrett's picture

Thanks for posting this JA! Dick Olsher describes my listening experience very well. I had the Pass 0s set up with a Pass Aleph P and an Ono phono stage and it sounded great. Just too damned hot for older non-air conditioned apartments.

Bogolu Haranath's picture

Not only the rent is too damn high, those amps run too damn hot :-) .......

Jack L's picture

.......transconductance device, like a vacuum tube." quoted Dick Olsher.

Yes but NO.

MOS-FET, like any bipolar junction solid state devices, gets a set of signal transfer curves with a kink or 'knee' on all, which make it NON-linear. It limits signal swings before going into non-linear zone. This explains why solid state amp usually sound hash & unpleasant when driven into overload.

That said, not ALL vacuum tubes are linear as well. Pentodes & tetrodes all get signal transfer curves with kinks or 'knees' like a FET.

Only TRIODES get a set of linear signal transfer curves withOUT kinks & 'knees', providing adequate headroom for large music signal swing. That's why triode amps sound so accurate & yet musical even when driven into overload.

As a die-hard triodes lover, I only love triode music, not even pentodes & tetrodes, period as my critical ears do detect the noticeable difference.

All vacuum tubes need to be heated up their filaments to start conducting current like any light bulbs.

There are two way of heating up the filaments. One is direct heating so that the heated up filament emits electrons to the plate at very high positive operating voltage. Generally only power tubes are direct heated.

Another way is indirect heating. The filament after heated up, in turn heats up the cathode which envolopes the filament. So the heated up cathode emits electroncs to the plate inside the tube instead of the filament. All small signal driver tubes & most most power tubes are indirect heating.

Sonically, direct-filament-heating power triodes sound the very BEST. e.g. 2A3, 300B etc. NO, repeat, zip solid state devices, not even indirect heated power triode can match their accurate, dynamic & yet musical sound, period.

That's why my ALL design/built phonstages, linestages & power amps used only triodes, period.

Listening is believing

Jack L

Ortofan's picture

... a datasheet for a triode tube that includes plots for the transfer characteristic "curves" which do not exhibit any "kinks & knees".

Bogolu Haranath's picture

Triodes don't exhibit any 'kinkiness' ...... according to Jack :-) ......

Jack L's picture

Hi

Google the best sounding direct-filament-heating power triodes: 300B & 2A3 spec. You will see both tubes exhibit pretty straight transfer curves without any kinks or 'knees'.

I particularly love the sound of 300B - so elegant, dynamic, & musically engaging.

The USD125,000 Audio Note Japan 'Kegon' single-ended class A 17W+17W power amp used 2x300B each channel. It sounded so enjoying!!!!

Listening is believing

Jack L

Ortofan's picture

... the datasheet for the current iteration of the Western Electric 300B.
Are you suggesting that the characteristic graphs shown therein are devoid of any "any kinks or 'knees'?"

https://www.westernelectric.com/products/images/300b/300b-technical-specifications.pdf

Herb Reichert's picture

a better set of plate curves than a 300B or 2A3? Or a more linear transfer function?

Or any solid state device capable of being used with zero feedback?

(asking for a friend)

hr

Bogolu Haranath's picture

CH Precision amps say 0% feedback, reviewed by Stereophile ...... First Watt amps have very low, may be 0% feedback, reviewed by Stereophile ....... However, none of those SS amps have as high output impedance as some tube amps :-) ........

Jack L's picture

.......zero feedback?" quoted H Reichert.

Negative feedback is a musical signal disaster !

Since day one decades back, manufacturers promoted straight-line flat frequency responses & almost zero total harmonic distortion in their audio amplifiers by applying huge global feedback in their bench test using simple sinewave test signals.

It is a 'white wash' to cover up the inadequate original design to fool the consumer at large.

In fact, negative feedback is never ever designed to handle realtime music signals built of complex high orders of harmonics DYNAMICALLY
riding on the fundamental signal.

So it is apple to orange situation. The specs published shows only how
an audio amp handles pure static sinewave test signals instead of realtime dynamic complex music signals.

Overdosed global negative feedbacks only drive the amp into momentarily unstable state when handling large realime dynamic complex signals.

Such split-second instability sounds hash & unpleasant to the ears.

This happens in most, if not all, audio amp using integrated circuit (IC) chips which are built in with tons of global feedback as standard feature.

Therefore NO NO for me with preamps & power amps with negative feedback & built with IC chips as my ears do detect the sonic problems.

All my design/built phonostages, linesatages & power amps used only triodes with zero same-stage local feedbacks & global feedbaks. Absolutely zip IC chips.

Listening is believing

Jack L

Bogolu Haranath's picture

Give yourself a gift of an 8K TV for this Father's Day, Jack :-) ........

Jack L's picture

Hi

Excellent idea! Surely 8K UHD TV is better than the 50" 4K UHD wifi TV I am now using. But its price still too high that I want to wait a couple more years to allow the hot price cooling down. There is not many 8K movies available anyway.

Jack L

PS I just brought for myself a Father's Day gift: a mouth mask from a local tailor shop with military like flamatage pattern on it. I like such fashionable design.

Ortofan's picture

... a "large realime dynamic complex" test signal that, in your opinion, could be used to satisfactorily gauge the performance of an amplifier?

Bogolu Haranath's picture

If we play 'realime' music through an amplifier designed by Jack, we get 'lime juice' on the other side :-) .......

Jack L's picture

Hi.

Very funny. I like 'ginger ale' better.

Did I tell you here before the 'Little David' 5W+5W all triode single-ended class A power amp which I design/built 4 years back, rocks my 700 sq.ft basement audio den when playing the firecrack music: Tchaikovsky
1812 Overture ???

Listening is believing

Jack L

Jack L's picture

Hi

Yes, this is music signal. Not pure sinewave test signals used in bench test & measurement.

Unless I miss a lot, I don't think any amp manufacturers todate have managed to test their products qualitatively using real music signals as there is no such testers yet available.

So get the right design to start with: use active devices with LINEAR transfer function only. Zero negative feedback throughout. No IC chips.

Don't forget the function of an audio amp is to amplify MUSIC signals, not pure sinvewave test signals always used in bench tests.

Since no such realtime music signal testers are yet available todate, use our ears to test the amps! It is that basic & simple.

Listening, not measuring, is believing

Jack L

Ortofan's picture

... that "realtime music" signal look like?

Suppose that one were to develop a "realtime music signal tester".
What sort of test signal (that could be produced by a signal generator) would fit your definition of a "realtime music" test signal?

Jack L's picture

Hi

Google "music signal" to know more about how a "realtime music signal" looks like.

You will see the waveforms are totally unlike the pure sinewave test signals used in all bench tests. Much much more complex to put the amp under test working much harder.

Jack L

Bogolu Haranath's picture

Have you tested that music signal with your custom made amp? ...... If so, what test equipment have you used? ..... Have you tested that music signal with other amps? ...... If so, what other amps did you test them with? :-) .......

How do you know the Class-A rated amps in Stereophile list do or don't accurately reproduce that music signal? :-) ........

Jack L's picture

Hi

First off, you got to know a music signal is built up of a number of sinewaves of different orders of harmonics riding on its basic frequency, say Middle C music note: 261.63Hz.

So now you understand music note wave shape is so different from pure sinewave signals used in bench tests. So WHY we still bank on such tests which bear nothing to do with music signals our ears perceive ??????

While such music note testers are not available in the foreseeable future, simply test it with your own ears playing good music recordings available to you.

"How do you know the Class-A rated amps in Stereophile list do or don't accurately reproduce that music signal?"

I never said the above! My friend, it is your wrong interpretation.

To answer your question, any currently available testing methodology can't measure how "accurately reproduces" music signal by an amp, period.

we are comparing apple (sinewave test signals) to orange (music note signals). Irrelevant !!

Class A is not a cure-all, pal. For solidstate amp, class A topology is to eliminate the cross-over distortion caused by push-pull interaction between the opposite polarity output power transistors conducting/non-conducting during one complete signal cycle.

The downside is the output power transistors generate huge heat on clsss A operation. So to prevent the output transistors melt down due to overheat, the output power needed to be reduced bigtime to be workable without failure.

Class A output topology comes in either 'push-pull' output (for all transistor amps & some tube amps) for high power & 'single-ended' non-push-pull (for tube amps only) for low output power.

Single-ended non-push-pull output power topology sounds much better than
high ouput Class A push-pull designs.

Jack L

Ortofan's picture

... "much much more complex" test signal look like?

How "much much more complex" does a test signal need to be?

If not "pure sinewave test signals" then what sort of test signal would you want to use in order to make an amp under test work "much harder?"

Herb Reichert's picture

recommends testing with a drum track and a piano track.......But I recommend examining bird songs with a real time analyzer. Bird song in bird song out - all distortions will be completely obvious.

just sayin'

hr

Ortofan's picture

... a stately raven flew into my listening room.
Permission was requested to make a recording of his call for use as an audio system test signal.
The raven agreed - but on one condition.
That the recording be used only once, then nevermore.

Bogolu Haranath's picture

Get a rooster ...... Every morning you can record his song :-) ........

Ortofan's picture

... tell us more about this feedback phobia that is being experienced?
When and how did it start?
Has any attempt been made to confront it or treat it?
Is there anything in particular that helps to ameliorate the symptoms?

Bogolu Haranath's picture

Feedback phobia causes night time sweating and nightmares ....... The cure is 300B tube amp with no feedback .... and a nice beverage or two, to go with it :-) .......

Jack L's picture

YES !

Bogolu Haranath's picture

Cary Audio CAD 805-RS mono blocks reviewed by Stereophile, uses 300B driver tube :-) .......

Bogolu Haranath's picture

BorderPatrol EXD amp uses 4 300B tubes ...... Reviewed by Stereophile :-) ........

Jack L's picture

Hi

Linear transfer function is not the only factor to deliver best sound.
It is the direct filament heating that makes the substantial sound improvement.
Question: Why 300B sounds much better than 2A3 (i.e. much more ELEGANT, airy & dynamic), given both direct filament heating, both exhibit very closely similar linear transfer curves?

300B gets its proprietary design in internal electrodes size/profile & layout that beats 2A3 sonically.

Listening is believing

Jack L

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