Pass Laboratories Aleph 0 monoblock power amplifier Measurements

Sidebar 2: Measurements

A full set of measurements of the Pass Aleph 0 was made in the unbalanced mode—the mode used by DO for most of his listening. Selected measurements were also made in the balanced mode. Unless otherwise noted, the measurements shown are for the unbalanced configuration.

Following the 1/3-power, one-hour preconditioning test, the Aleph 0's heatsinks were hot, though not too hot to touch comfortably. The Aleph 0 is non-inverting in its unbalanced mode; in the balanced, pin 2 is configured as the positive leg, pin 3 the negative.

The Aleph 0's input impedance measured just under 6.5k ohms (unbalanced) and just over 7k ohms (balanced). These are low figures. While they should cause no difficulty with preamplifiers having a low output impedance (preferably 600 ohms or below), the Aleph 0 may be less than a perfect match to those few preamplifiers having output impedances over 1k ohm (some tube preamplifiers fall into this category, as do most passive "preamps"). [The subjective result of such a mismatch will be to roll off the lows prematurely, the result being too lean a tonal balance. Some preamplifiers will also clip prematurely into such a load.—Ed.]

The output impedance of the Aleph 0 was under 0.02 ohms at either 1kHz or 20Hz, increasing to a maximum of 0.025 ohms at 20kHz. Voltage gain into an 8 ohm load measured 25.8dB, balanced or unbalanced. DC offset was a rather high 180mV two minutes after turn-on, but decreased gradually to 60mV after 20 minutes, 38mV after 1 hour. The signal/noise ratio (unweighted, 22Hz–22kHz, ref. 1W into 8 ohms) measured 111dB unbalanced, 109dB balanced.

Fig.1 shows the frequency response of the Aleph 0 at 1W into 8 ohms, unbalanced. The balanced response, as well as the response at 2W into 4 ohms, is virtually identical, and isn't shown. The small blip at about 180Hz is a measurement artifact, and in any case is no more than 0.1dB. The Aleph 0's 10kHz squarewave response (fig.2) indicates an excellent risetime and only the slightest rounding of the leading edge. The 1kHz squarewave was close to perfect, and is not shown.

395PAS0FIG1

Fig.1 Pass Aleph 0, frequency response into 8 ohms (0.5dB/vertical div.).

395PAS0fig2

Fig.2 Pass Aleph 0, 10kHz squarewave.

The THD+noise vs frequency curves are shown in fig.3. The result is astonishingly low for a single-ended amplifier, and any increase in THD into lower impedance loads is minimal. The result for a balanced input, not shown, was marginally better—about 0.0001% lower at high frequencies. The 1kHz distortion waveform at 10W output into 4 ohms (fig.4) indicates primarily second-harmonic content overlaid with noise. The waveforms into 8 ohms and 2 ohms are virtually identical (second harmonic plus noise), and are not shown.

395PAS0FIG3

Fig.3 Pass Aleph 0, THD+noise (%) vs frequency at (from bottom to top at 20kHz): 1W into 8 ohms, 2W into 4 ohms, 4W into 2 ohms.

395Pas0fig4

Fig.4 Pass Aleph 0, 1kHz waveform at 10W into 4 ohms (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).

The spectrum of the Aleph 0's output reproducing 50Hz at a 107W output level into 4 ohms (2/3 the rated power of 160W) is shown in fig.5. All of the distortion artifacts are extremely low here. Only the second harmonic—at –68.6dB, or just under 0.04%—is greater than –90dB (0.003%). Fig.6 shows a similar spectrum, with the amplifier reproducing an equal combination of 19kHz and 20kHz sinewaves at 80W into 4 ohms (visible clipping was evident with this signal just above this power level). The spectrum shows the products resulting from intermodulation between these two frequencies. The largest artifacts here are at 18kHz and 21kHz (at approximately –56dB, or 0.15%), with the next largest at 1kHz (–65dB, or 0.06%). The spectrum at 43W into 8 ohms was very similar, and isn't shown.

395PAS0FIG5

Fig.5 Pass Aleph 0, spectrum of 50Hz sinewave, DC–1kHz, at 107W into 4 ohms (linear frequency scale). Note that the second harmonic at 100Hz is the highest in level at –68.6dB (0.04%).

395PAS0FIG6

Fig.6 Pass Aleph 0, HF intermodulation spectrum, DC–22kHz, 19+20kHz at 80W into 4 ohms (linear frequency scale).

The 1kHz, THD+N vs output power curves (at 1kHz) are shown in fig.7. The distortion characteristics are typical of a solid-state amplifier, remaining very low up to the knee of the curves, and increasing rapidly above that; there was nothing unexpected due to the single-ended design. The apparent drop in measured THD up to the 4W level is due to the true distortion lying below the amplifier's noise floor below that power level. The discrete clipping levels (at 1% THD+N) were 95W into 8 ohms (19.6dBW), 167W into 4 ohms (19.2dBW), and 279W into 2 ohms (18.5dBW), all of which are well above specification. (The wall voltage was 117V AC at these powers into 8 and 4 ohms, 116V AC into 2 ohms.)

395PAS0FIG7

Fig.7 Pass Aleph 0, distortion (%) vs output power into (from bottom to top at 10W): 2 ohms, 4 ohms, and 8 ohms.

Based on my previous experience with single-ended amplifiers (all of them tube designs), I expected mundane test-bench results at best from the Aleph 0. This was certainly not the case. Its measured performance was excellent across the board.—Thomas J. Norton

COMPANY INFO
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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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