Review Summary Sound "Gloriously transparent," with a "seemingly limitless spatial presentation" and "remarkable bass performance"; as monoblocks, "you get texture and hear levels of detail easily"; bass "solidity and power are awesome" as well. "At the end of each listening session with the Belles 150A Reference amp, either in stereo or mono, I was energized and scintillated." Features "The dual-mono 150A Reference…features an input stage with matched dual J-FET/bipolar transistors in a cascaded differential pair, a voltage-gain stage using bipolar transistors, and a final stage using a source follower with eight power MOSFETs biased in class B." Delivers 125Wpc into 8 ohms and 250Wpc into 4 ohms as a stereo amp and four times the power as a monoblock. Use "I had no expectation that the monoblock setup would be significantly different or better in any way than a single stereo amp, so I was completely surprised by how far the mono setup surpassed the performance of a single amp." Value "Prior to spending time with these amps, I thought there would never be a product that would justify a double Reviewers’ Choice nod from me. Well, here it is, the Belles 150A Reference -- extraordinary and possibly incomparable."

Dave Belles is one of those audio designers with a long track record, in his case more than 25 years in the business of designing and manufacturing audio equipment. When you buy a Belles product today, it is designed and usually built by the man himself. The cosmetics, and price tag, are kept reasonable. Belles amps have almost always been solid-state affairs, sometimes running in class A but more often class AB. Belles preamps have been passive, solid-state and tube. Dave Belles has often made his stereo amps easily convertible to mono. That’s the case with the dual-mono 150A Reference. As a stereo amp it's rated at 125Wpc into 8 ohms and 250Wpc into 4 ohms, but when you use a balanced interconnect and connect the loudspeakers to the R+ and L+ speaker terminals, the 150A Reference becomes a monoblock capable of 500 watts into 8 ohms and in the vicinity of 1000 watts into 4 ohms.

The dual-mono 150A Reference ($2995 USD) features an input stage with matched dual J-FET/bipolar transistors in a cascaded differential pair, a voltage-gain stage using bipolar transistors, and a final stage using a source follower with eight power MOSFETs biased in class B. Both channels of the 150A Reference share a common 1000VA toroidal transformer. According to Belles, a single transformer, rather than two smaller ones as is typical with dual-mono designs, makes for "stronger and cleaner bass." Each amplifier channel is an integrated assembly, and the circuit components are mounted directly to the heatsinks, which eliminates additional wiring. Some negative feedback is used to stabilize the gain, reduce distortion and noise, and lower the output impedance.

The 150A Reference's front panel is unadorned. It contains only a power switch and LED. The rear panel has a pair of RCA inputs and the aforementioned single XLR input. The speaker binding posts are spaced so that dual bananas can connect any pair, including the two central posts, which are the ones you use when running the amp in mono. An IEC power-cord connector allows the use of an aftermarket power cord. Heatsinks are exposed and line the left and right sides of the amp. The review amplifier was supplied with an impressive 1/2"-thick silver faceplate, but black is also an option.

Review system

I used a modified Pioneer DV-525 DVD player as a CD transport. Digital audio signals were processed by an Assemblage D2D-1, which upsamples and interpolates 16 bits/44.1kHz to 24 bits/96kHz, and a modified MSB Link DAC powered by MSB’s P1000 outboard power supply. The preamp was a Belles 21A. Speakers were Vandersteen 3A Signatures. Digital cables were Cardas Lightning and VansEvers Pandora. Balanced interconnects were Nirvana S-L, while single-ended interconnects were Audience Au24, Nirvana S-L, Nordost Quattro Fil and Magnan Signature. Speaker cables were Monster Sigma Retro Gold or Magnan Signature. Listening to vinyl required a preamp change to an Audible Illusions Modulus 3A with Gold phono boards fed by a Roksan Xerxes 'table, SME V 'arm and Cardas Heart MC cartridge. Nordost Moon Glo was connected directly to the cartridge's pins and preamp’s phono input. The amps were given 150 hours of burn-in before initial evaluation. When amps were changed for comparison, another 36 hours of re-burn-in was done before evaluations resumed.

A dedicated 30-amp power line provided AC power to the system. One or two Exact Power EP15A power conditioners stabilized the AC line voltage and frequency while removing noise and distortion. Source components were powered by Exact Power’s SP15 balanced power unit or Equi=Tech’s Son of Q balanced power conditioner -- both were plugged into the EP15A to benefit from the clean, stabilized AC. Power cords were JPS Labs Power AC+, Audience PowerChord, Magnan Signature, and Audio Prism SuperNatural 9.5. With all of the power-conditioning equipment, it's probably no surprise that this system has a remarkably low noise floor and is capable of producing a huge, immersive soundstage.

The specially constructed listening room is treated with 14 Room Tunes Pressure Zone Controllers mounted on the walls. Argent Room Lenses are used in four positions to further tune the acoustics of the room and assist with producing the large, enveloping soundstage. A VansEvers Spatial Lens and Window system further opens up the soundstage.

In stereo -- and a few comparisons

The sound of the 150A Reference was a surprise. Compared to the less expensive Belles 150A Hot Rod amp, the 150A Reference is considerably more transparent and detailed. I have long thought of the 150A ($1200) and 150A Hot Rod ($1700) as being classic amps -- they make everything sound good. They are the sort of amps you can put in your system and forget about. But they never struck me as having state-of-the-art transparency -- nor would I expect them to at their modest prices.

The 150A Reference changes that for Belles designs. The sound is gloriously transparent, but the great musical character of the earlier 150A amps remains. I immediately liked the seemingly limitless spatial presentation of the 150A Reference. Sometimes amps that reviewers describe as "neutral" also sound boring. Not the 150A Reference, which rekindled in me the impossible-to-achieve desire to listen to every recording in the house. It was, in fact, difficult to review the 150A Reference because I would get swept away by the music and forget to listen to the equipment. That’s a clear indicator in my book that an amp is something special.

Like the Belles 350A amp ($3495) that is my current reference, the 150A Reference has remarkable bass performance. It presents a tremendous amount of texture and detail in the bass compared to other amps. Dave Belles attributes this to the method he uses to produce the extremely high damping factor of "over 1000." Damping factor is one indicator of how resistant the amplifier is to EMF coming back to the amp from the drivers in the speaker. Tube amps can have damping factors of less than 10 up to maybe 60, indicating they have little ability to ignore the EMF. Most solid-state amps will have a damping factor between 100 and 600 and sometimes as high as 800. Higher damping factors indicate the amp can control unruly woofers better and thereby produce sound with less distortion.

There are amps with high damping factors that sound over-damped, dry and musically unimpressive in the bass. Not the 150A Reference, whose bass was a constant source of a-music-ment. It’s hard to describe how the sound made by woofers changes when driven by this amp. You will realize that what you had thought was pretty good bass was bass loaded with audible distortion and non-linearities. Frankly, the 150A Reference makes the Vandersteen 3A Signatures sound like they have had a serious woofer upgrade. The Chieftains' Long Black Veil [BMG 09026627022] has a number of tracks useful for evaluating bass performance. One of my favorites is "The Foggy Dew," with its powerful drum whacks coming out of silence. The 150A Reference produces these whacks with superb tightness and a very strong pressure-wave launch. I'm not talking about more prominent bass, but more extended bass -- lower low frequencies than I’ve heard before.

The Monster Power MPA-2230 amp, for example, has an excellent pedigree. Most audiophiles would consider it a good-sounding amp. But its bass, in comparison to that of the 150A Reference, is ill-defined, not as extended or detailed, and lacking in texture and definition. How does Dave Belles do it? He is keeping his secrets to himself. The low-end performance of the 150A Reference certainly isn’t some frequency-response trick. I verified there’s no measurable difference in output levels using a test CD, a Fluke digital voltmeter, and two other amplifiers.

But bass isn't everything. There is a bit more clarity and detail in the 150A Reference's midrange than that of the 350A, and as big as the 350A’s soundstage is, the 150A Reference's is even bigger -- or at least more immersive given the right room and recordings. Less-well-recorded CDs sounded consistently interesting and musical on the 150A Reference, but better-recorded LPs and CDs were almost freak-out huge in terms of the size of the images they could cast in the room.

The 150A Reference's treble is nicely extended and very clean, clear and low in distortion. There’s no noticeable roll-off or emphasis. There’s no grit, edge, grayness or dullness. The highs float in the room nicely when the recording puts them there. There’s no extra warmth or liquidity, no dryness or brightness. The 150A Reference's top end is the answer for those who have grown weary of editorialized highs. You can sit back and hear just how things were recorded without loss or embellishment. Daboa's musical bazaar From the Gekko [Triple Earth TRECD115] (formerly impossible to find, but now listed on Amazon.com and one of my favorite CDs) has a lot of interesting synthesized high-frequency sounds that are reproduced with such precision by the 150A Reference that you can almost visualize the waveform of each sound in your head. Sometimes these sounds mimic crystal bells or steel drums; other times they are unique. Whatever Frank Harris of Daboa does comes out sounding interesting and captivating. Other amps tend to flatten and dull these sounds to the point that they just aren’t as lovely or electrifying as when reproduced by the 150A Reference.

Spatial finesse is another strong point of the 150A Reference. When present on the recording (there's lots of depth on the Daboa CD; for example, try "It’s Not Easy Being Green"), the 150A makes an amazing presentation -- seemingly larger than the listening room itself. This amp produced the best spatial performance I’ve ever heard from an amp near its price. In fact, the 150A Reference has to be put up against some very expensive amps before you’ll find one that does space as well.

In mono

As I've mentioned, the rated power of the 150A Reference increases by four times when the amp is used as a monoblock. The voltage slew rate doubles as well. Slew rate is one indicator of the ability of the amp to generate large voltages very quickly. The 150A Reference is also fully balanced in mono, one benefit of which is that noise common to both the positive and negative halves of the audio signal is canceled out when the halves are combined at the output of the amplifier. This means that power-supply noise and other types of noise present in both signals are essentially canceled out of existence. This has the benefit of lowering the noise floor of the amplifier, so delicate detail emanates from near-total silence.

My first thought when listening to a pair of 150A Reference amps in mono was that the music seemed way loud. Switching from single-ended mode to mono increases the input sensitivity of the amps and makes for a 6dB boost for the same volume setting. This makes the speakers crank. Serious listening could begin only after I had backed off the volume control to match the monoblocks' playback level to that of the stereo amp.

With the level matched, the first difference I noticed was in texture. With most amps you get texture. With the mono 150A Reference amps, you get texture and hear levels of detail easily. With other amps, the detail may not have been completely missing, but it wasn’t as vital and vibrant as reproduced by the 150A Reference amps in mono. There is clarity and "zing" to sounds that other amps don’t fully reveal. A perfect showcase for this is XTC's Apple Venus Vol. 1 [TVT Records TVT3250-2]. It is loaded with texture (and is potentially edgy-sounding on some systems). With the 150A Reference monos, the texture and dynamics were thrilling, and the all-important silences were velvety black, allowing me to discern subtle pitch and harmonic nuance in every instrument along with greater decay and room echo. There was no annoyance with this recording's edginess. It was observable, but not bothersome.

The bass performance of the 150A Reference monoblocks is beyond reproach. The solidity and power are awesome. The bass character is remarkably similar to that of the stereo 150A Reference, but it’s backed up with the power of the gods. These amps make ordinary woofers move serious amounts of air with an impressive absence of overhang and boominess. These amps are the first to make the woofers of my Vandersteen 3A Signature speakers rock my listening room to nearly the same degree as a powerful subwoofer. The 150A Reference in stereo has the extension and texture, but the monoblocks put pile-driver power behind these. No matter what the musical selection, everything about the bass was the best I’ve ever experienced. It was flat-out exciting to listen to bass lines reproduced with such clarity and richness while they were so thoroughly intertwined with the other instruments.

The highs are almost indescribable -- so fast, so clean, so spacious, so detailed, so not grainy. There is no sterility in the highs, no mechanical feel, no electronic character and no grit or harshness -- ever. I simply could not believe how good the high frequencies of ordinary CDs could sound. Case in point is Brian Wilson’s self-titled album from 1989 [Warner Archives/Rhino R279660]. The remastering is typical Rhino sound, which is to say there’s a bit of emphasis in the treble or thereabouts that lets you know the recording is a little bright. Most amplifiers will play this back as an annoying brightness. The 150A Reference monoblocks put it all out there without the annoyance. Pick any track and marvel at the wealth of high-frequency info on this recording, and also at how lovely the highs sound. "Melt Away" does the ballad thing with floaty, romantic highs, while "Little Children" uses the treble almost like a bass line to drive this march-like tune forward. Even in the more aggressive style of "Little Children," the highs never become fatiguing or annoying.

Next up is the effortless speed of these monoblock amps. For the first time I have heard from my audio system music that sounds live. For me, live music has always had scary-real dynamics that just couldn’t quite be captured at home. Music at home always seemed slower and lacked that live edge. Not with the 150A Reference monos. There is effortless speed to all sounds that is missing from the performance of most other amps, even some of the five-figure numbers. The 150A Reference monoblocks’ transient speed is certainly one of the reasons the treble octaves are so exciting, so right. If I’m sounding giddy about this, there’s a reason. For 30 years I’ve longed to hear this sort of live character from a high-end system, and it finally happened when these amps were pulling the train.

A particularly enjoyable moment: "Storms in Africa II," track 12 on Enya’s Watermark [Geffen 9242332], was transformed from an enjoyable tune into a musical tour de force, with powerful dynamics, driving bass, floating highs and endless joy radiating effortlessly from trans-dimensional pressure-wave transducers (loudspeakers). Can’t find track 12 on your copy of Watermark? Sorry, you bought yours too soon. Sometime after Enya’s astounding success with this album, someone, probably Enya, had track 12 added to the CD. Even with the volume level at well above 90dB average, the system seemed like it had an endless well of watts. I’ve never heard the 3A Signatures play so loud without the audible hint of being at the end of the line SPL-wise.

Conclusions

At the end of each listening session with the Belles 150A Reference amp, either in stereo or mono, I was energized and scintillated -- the same sort of feeling you get after an especially invigorating shower. I don’t remember ever having this strong a reaction to listening to music on any other system. I thought the feeling might pass given increasing familiarity with these amps, but it never did -- and never tapered off even during the last week of listening. Over time, I usually grow accustomed to and complacent about the sound of most audio components. They may be incredible, but a couple of months later the sound is just what it is without any of the appreciation I had for it initially. The 150A Reference is the first amp I have used in my system that I stayed excited about over the entire review period -- and beyond. I had no expectation that the monoblock setup would be significantly different or better in any way than a single stereo amp, so I was completely surprised by how far the mono setup surpassed the performance of a single amp.

The last question is regarding Reviewers’ Choice. Prior to spending time with these amps, I thought there would never be a product that would justify a double Reviewers’ Choice nod from me. Well, here it is, the Belles 150A Reference -- extraordinary and possibly incomparable.

...Doug Blackburn
db@soundstage.com

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