April 2023

OAD Ultrafidelity Vajra Power Amplifier Review
Flawless sound and million dollars looks.
Review By Greg Borrowman

  Although the Australian-made OAD Ultrafidelity brand is new to the country, its founder, owner and designer, Jon De Sensi, is an old hand in the high-end hi-fi business, having been designing and building high-end audio equipment for several decades, most recently under the MusicLabs Australia brand name. He is also well-known for his willingness to help out other Australian audio designers, such as Redgum's Ian Robinson, for whom he designed the digital-to-analog stages for the Redgum RGCD2 CD Player.

De Sensi is also well-known for "thinking outside the box", which may very well be why he is using slit-foil capacitors in the Vajra's power supply (a first for any power amplifier anywhere in the world, he says) as well as a very unusual transistor topology in the output stage that is rarely encountered in the world of high-power audio amplifiers.

The Equipment
As you can see from the image on the left, the OAD Vajra is the very model of a model power amplifier, in that its front panel has only one real control — the power button. There is no 'main' power button on the rear of the unit, either; that front panel on/off key is all you get. That means there's no standby function — the OAD is either on or off. And because it's a completely linear design — De Sensi is obviously having no truck with either switch-mode power supplies or Class D output stages (and I totally agree with him regarding both these design choices) — it's going to pull quite a few ergs from your 240V mains supply whether it's playing music or not.

If you are wondering about that second front-panel button you spy on the image, it actually has nothing much to do with the operation of the Vajra: it simply increases the sensitivity of the two 'output power' meters by a factor of 10. You need this magnification factor because unless you listen to your music really loudly, the needles on these two meters will be bumping around at the bottom of the scale where they're really hard to read. That legibility is not at all helped by the display's low-light level, which may be tastefully ambient but doesn't throw much light on the meter calibrations.

But even if the meters were brightly lit, the information on them would not really be useful anyway. Like all such meters, they only give truly accurate readings when your loudspeakers present a purely resistive load of exactly eight ohms at every frequency across the audio band — and there isn't a single loudspeaker on the planet that presents such a load. Also, the meters are only truly accurate at a single frequency and when the signal being measured is a sine wave, neither of which really occurs when you're playing music.

To its eternal credit, OAD recognizes this and presents you with this exact information in Vajra's user manual, where you will find the words: "Meter readings are only accurate for a sinusoidal signal into a pure 8-Ohm resistive load." So why would a designer include power output meters at all, knowing full well that they are inaccurate when playing music through loudspeakers?

The obvious answer is that the meters don't have to be particularly accurate to give you an idea of what power levels the amplifier is delivering to your loudspeakers. A less obvious answer is that power meters look great on the front of an amplifier, to the point where it could swing a buying decision in the desired direction for a manufacturer.

Another less obvious reason is that many of the world's most desirable and expensive power amplifiers (here I am thinking specifically of models from McIntosh and Audio Research, though there are literally dozens of other brands I could have mentioned) have power meters on their front panels, and it's always a good idea to do what the famous are doing.

What I would actually have preferred to see on the front of the OAD Ultrafidelity Vajra is a pair of clipping indicators, to indicate when you're delivering so much power to your loudspeakers that you've reached the absolute limit and it might be time to back off the volume a bit. If it were a question of cost, I think I would've installed these indicators rather than the meters, but since cost obviously wasn't a factor here, I'd want them in addition to the meters.

One reason De Sensi may have chosen not to include clipping indicators in the Vajra is because he has included in it a special circuit specifically designed to prevent such clipping in the first place — or to at least prevent hard clipping.

Indeed, when the amplifier is about to go into hard clipping, a 'soft clipping' circuit 'rounds off' the audio signal by limiting the voltage on either side of the signal peak. This circuit type was invented by NAD designer Bjørn Erik Edvardsen for his famous 3020 integrated amplifier, which was so low-powered that it was easy to overdrive and cause output stage clipping, resulting in excessive distortion and a hard sound in the bass.

The problem with such soft-clipping circuits is that when they activate, they introduce distortion into the audio signal. This is a fact that OAD Ultrafidelity makes clear in the manual, which states: "The activated soft clip circuit may distort the output signal significantly, thereby causing a reduction in sonic performance."

As for the distortions introduced by soft-clipping circuits, the perverse fact is that some listeners actually prefer the sound of the particular distortions introduced by hard-clipping over those introduced by soft-clipping circuits — which is one of the reasons why Edvardsen allowed owners to choose between having his soft-clipping circuit switched on or off. Such a switch would have been a nice addition to the Vajra's circuitry.

One reason I suspect De Sensi did not include this switch is that the Vajra's power output is so high (180 watts per channel into eight ohms, and 360 watts per channel into four ohms) that it is very, very unlikely anyone using it will ever use enough power to cause the soft-clipping circuit to activate... even if they play their music extremely loudly and pair the Vajra with extremely inefficient loudspeakers.

However, in the event that you do manage to exceed the maximum capability of the amplifier, even with the soft-clipping circuit engaged, De Sensi has provided a second line of defense against amplifier damage in the form of an over-current protection circuit that trips automatically if the output current exceeds 15 Amperes. This threshold level is so high that you'd be hard-pressed to exceed it under any normal listening situation, so it's not likely to trigger in any circumstance short of a short-circuit (such as if you were to accidentally touch the positive speaker wire to the negative).

Around Back
The rear of the OAD Vajra is almost as sparsely equipped as the front. There is a pair of gold-plated RCA terminals to which you could connect unbalanced (i.e. single-ended) signal wires from your preamplifier, and a pair of gold-plated XLR terminals to which you could connect balanced cables. There's a switch to shift between the two, and OAD warns in the manual that you should connect using only one of the two options at one time, not both at the same time. (Some people tend to connect different front-end components to the different terminals.)

There is also a remote input on the rear panel that, if the Vajra is connected to OAD Ultrafidelity's Padma preamplifier, allows you to switch both amplifier units on and off together via the Padma's remote control.

The speaker terminals on the Vajra are labeled '+' and '–' but come accompanied by a warning label that reads: 'Do not connect any speaker terminal to ground.' I was a bit dismayed that this warning did not appear in the manual and that there was no explanation of why such a warning had been included in the first place.

The usual reason for not connecting the negative terminal to ground is that the amplifier is using a bridged output stage, where two amplifier sections are used for each channel and each of the pairs is tied together. Since there is no mention of this in any of OAD's literature, I asked De Sensi if this was indeed a technique being used in the Vajra.

Complementary Feedback
De Sensi told us that the OAD Vajra uses a Complementary Feedback Pair topology which "makes it different to most — or possibly all — amplifiers". Pressed for a reason for using such an unusual circuit, he said: "The advantage of this type of topology is that it is generally more linear than your more typical Emitter Follower topology. Especially large signal non-linearity is significantly lower than Emitter Follower output, where the non-linearity of a diode junction plays a significant part."

If this all sounds a bit technical, it might help to explain that for non-Class D amplifiers, almost all of them use one of three different topologies. The oldest of the three is the quasi-complementary symmetry output stage, developed to get around the problem that the PNP output transistors available at the time were not good matches for the NPN transistors available back then. Once semiconductor manufacturers developed matched NPN/PNP transistor pairs, most audio amplifier designers started using them in full complementary symmetry output stages using Darlington pairs (the second of the three topologies).

But some designers — an elite group that now includes De Sensi — use the third type of output stage, which is full complementary symmetry using compound pairs. Why? Due to the low distortion, extreme linearity, and superior thermal stability of this topology. As to how large a group of designers this may be, I am unsure, but I reckon there must be some — not least as an amplifier designed by Sir Clive Sinclair that used this circuit delivered such an impressive performance that the topology was likely to be copied... or emulated.

It seemed to make sense at this juncture to ask De Sensi about his use of slit-foil capacitors in the Vajra's power supply. "The continuous foil used in normal electrolytic capacitors allows eddy currents to circulate in the foil," he said. "By using a slit foil electrolytic capacitor, these eddy currents cannot circulate and the result is cleaner, more natural midrange and high frequencies. It is definitely a world-first and is possibly the only power amplifier that uses slit-foil electrolytic power capacitors."

It also seemed like a good time to ask about the unusual name of De Sensi's new amplifier. "Does the name Vajra mean anything?" I asked. "Vajra is Sanskrit for something that has diamond-like attributes," he replied. "In spiritual Sanskrit traditions, the diamond-like properties most usually attributed are those of purity, faithfulness, and constancy."

Listening Sessions
Since a power amplifier requires a preamplifier, OAD loaned me one of its Padma preamplifiers to use to drive the Vajra for my listening sessions. The most obvious advantage of using it with the Vajra was that I was able to switch the Vajra on and off remotely (not possible if the Vajra is paired with something else).

I have already mentioned the Vajra's power meters, so I won't rehash their usefulness or readability, but I should say that although the x10 meter sensitivity button on my review sample worked perfectly, it did seem to be a bit wobbly when I switched it in and out. This was probably an isolated condition, but it's something you might want to check if you think it will bother you (it didn't me).

What was most immediately noticeable from the get-go was the extreme quietness of the Vajra. Most amplifiers make a bit of a noise when you switch them on, and then when there is no audio signal and the volume is high there can sometimes be the faintest of circuit noises. Not with the OAD Vajra: it was as silent as the grave — so silent I wasn't even sure I'd successfully turned the amplifier on in the first place! (I had.)

The only downside I can see of this is that you won't have any audible cues in the event you've accidentally left your Vajra switched on. You can leave it switched on permanently, of course, but given the lack of a standby mode I'd recommend switching it off whenever you're not using it. It doesn't take long to warm up, after all, thanks to it operating in pure Class A mode up to 15 watts before switching to Class A/B.

The real advantage of the Vajra being so quiet is the contribution to dynamics when you're playing music, particularly as the amplifier is so powerful. When an orchestral crescendo ceases, the following silence is total; when there's a rest in the score during a piano sonata, you'll hear nothing until the pianist plays the next note; and between the notes of a melody, there's that beautiful nothingness that only serves to highlight the melody itself.

I found myself entranced like never before by Beethoven's Piano Sonata No. 29 in B major, Op. 106 (Hammerklavier). Although I prefer the interpretation delivered by Solomon back in 1952, the sound quality of that old recording leaves quite a bit to be desired, which meant I was listening to Vladimir Ashkenazy's interpretation (on Decca, with the Waldstein). He extracts the maximum power from the bottom octaves of the Steinway, and the Vajra delivered them with something approaching its maximum power – despite the extremely high volume I was listening at, the Vajra obviously still had some power in reserve. But it wasn't just the power on tap that impressed me: the trills at the end of the first movement Allegro were marvels of clarity — again proving the importance of circuit quietness.

This was also demonstrated in the echoed repeated staccato of the Scherzo, where Ashkenazy's left hand mirrors his right. The musical tension was conveyed perfectly by the Vajra. When Ashkenazy then sustains some of the notes a little longer, the gorgeous resonances of the Steinway filled my listening room with sound. There was also another telling reveal of the Vajra's lack of background noise: before starting in on the Adagio, Ashkenazy takes a longer-than-his-usual pause, and in the silence I could hear his breathing, controlled though it was — something I can't recall having ever heard before. The sonorities of the Adagio were delivered to perfection by the Vajra, as were the micro-variations in Ashkenazy's finger-touch... and yet again, those trills!

The primal scream that kicks off Welcome To My Island, on Caroline Polachek's fabulous album 'Desire, I Want To Turn Into You', was made even more achingly primal by the OAD Vajra, leaping from the silence like a cry for help. Then, when the electronica kicks in, it was syncopated perfectly with Polachek's staccato vocal delivery, while in the background her guitar screamed in concert with her voice.

On Pretty in Possible, I first admired Polachek's wordless vocalization, and then the way the Vajra clearly delineated between the left and right channels, while positioning the vocal exactly where she wanted it to go — sometimes left of center, sometimes right, sometimes centered.

The way too that the Vajra delivered Danny Harle's bass on Bunny Rider was stupendously effective. Listen to the depth of that oh-so-stringy sound. Also admire how completely separate the acoustic of the high-frequency synthesized 'ting' is from the overall acoustic. Then there's the way you can correctly hear the sounds of the two guitars of Marc López Fernández and Samuel Organ on Sunset (incidentally one of my favorites on this superb album, not least because of the quality of Polachek's vocals on it.)

The bagpipes are instruments you don't often hear on an album that identifies as alt or art-pop, so it's a joy when Brìghde Chaimbeul chimes in with hers on Blood and Butter. I would've preferred her to have been given a little more time to shine, though. The studio sound FX on Butterfly Net was delivered ethereally by the Vajra, too, to the point where it really made my listening room sound like Chartres Cathedral!

If you want to hear Arctic Monkey's 'The Car' album better than you've ever heard it before, do yourself a favor and listen to it via the Vajra, connected to a pair of loudspeakers that have plenty of bass and can handle the Vajra's power! I guarantee you'll be impressed... and not only because I think it was the best album to come out of last year. Alex Turner's musical vision is far-seeing, and his musicianship, along with that of the rest of the band, is revealed in all its glory. The lavishness of the layering and the intertwining of multiple musical strands were handled holistically by the Vajra, to the benefit of the songwriting and the performances.

Conclusion
Would you believe I've left one of the best things until the last, and that is that the OAD Ultrafidelity Vajra looks like a million dollars. The finish is extraordinarily good. But of course, it's the sound quality that counts, and the Vajra is flawless in this regard, across all the yardsticks that could be used to measure it — particularly those of purity, faithfulness, and constancy. Yes, please! I want one.

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Specifications
Type: Solid-state stereo amplifier
Frequency Response: 0 to 100kHz (-3dB)
Output Power: 180 Watt @ 8 Ohms (360 Watts @ 4 Ohms)
Gain: 27dB
Input Sensitivity: 2V balanced and 4V unbalanced
Input Impedance: 24kOhm balanced and 12kOhm unbalanced
Damping Factor: >500
SNR: < 100dB
THD: 0.001% at 1kHz (100kHz Bandwidth)
Dimensions: 430 x 160 x 360 mm (WxHxD)
Price: (coming soon)

Company Information
OAD Ultrafidelity
E-mail: oad@oadultrafidelity.com 
Website: OADultrafidelity.com