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Product Review - Bruce Moore Dual 70 Tube Power Amplifier - July, 2000


Jason Serinus

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Bruce Moore Dual 70 Tube Power Amplifier

Power Output: 70 watts per channel ultralinear mode, 37 watts triode mode

THD: 0.1%, 20 Hz to 20 kHz @ 1 watt; 1%,  20 Hz to 20 kHz @ full power

Frequency Response: +0,-1 dB @ 1 Hz to 200 kHz (1 watt)

Input Impedance: 412 kOhm or 100 kOhm, depending upon date of manufacture.

Input Sensitivity: 1.3V for full output

Outputs: 4 & 8 Ohm  Taps

Damping factor: 15

Absolute Phase: Non-inverting

Tube Complement: Four 6922, Four 6550/KT88

Features: Triode/ultralinear switch, independent biasing for each output tube

Dimensions:   7.5" (191mm) H x 15" (381mm) W x 15" (381mm) D

Shipping Weight: 70 Pounds

MSRP: $4,300

 

Bruce Moore Audio Design, Distributed by R.B. Electronics (Bob Bergner), 5492 Linden Street, Dublin, California 94568; Phone 925-875-1055; E-Mail bbergner@cvc.com or bergner@pacbel.net

Preface

For readers coming to my reviews for the first time, I currently review two channel equipment and classical recordings for Secrets. Before long, I also expect to serve as its Music Editor. 

Weaned on classical music, and specifically on old opera ‘78s, I rediscovered opera and symphonic music when I was 11, and have been hooked every since. While in my teenage years, I listened to Donovan, Buddy Holly, and Little Richard just as frequently as Caruso (Little Richard was played at high volume to drive my mother out of the house), classical music increasingly became my first love. I turned many a head in my twenties whenever I moved, because boxes of classical LPs stood out among the few possessions I lugged from place to place. Both my performing career (see http://www.planeteria.net/home/whistler) and equipment CD/performance reviewing have evolved naturally as music has assumed greater and greater importance to my emotional and spiritual well-being.

You can be assured that, when I review equipment, I am most concerned with how accurately it conveys ultimate musicality. My reference, except in the case of synthesized and normally amplified music, is always the sound of live music heard in a good, un-amplified acoustic setting. Thanks to the San Francisco Symphony, the San Francisco Opera, the rich concert series of Cal Performances and San Francisco Performances (two of the best performance series in the U.S.), and my current work as an East Bay performance reviewer, I have frequent opportunities to hear great music in fine acoustic settings. I am even able, upon occasion, to sit everywhere from first row center to the sides of the rear balcony. All this plus a fine sound system have afforded me ample references for evaluating various equipment configurations.  

Introduction

This review continues my exploration of the Bruce Moore line of tube amplifiers and preamplifiers. Background information on designer Moore, who lives in Silicon Valley, may be found in my review of the Bruce Moore Companion II-C tube preamp, located in the Secrets of Home Theater and High Fidelity archives: http://www.hometheaterhifi.com/volume_7_2/bruce-moore-companion-ii-c-tube-preamplifier-5-2000.html.

While the Dual 70 Stereo Power amplifier is designed to work with a plethora of systems, it was specifically engineered to complement the Bruce Moore Companion III preamp. Since that preamp (review forthcoming) has become a treasured mainstay of my reference system, I especially looked forward to hearing Bruce’s amp paired with it in my system.  

The amp can be preset to work in either 70 watt ultralinear mode or 37 watt triode mode. Most clients prefer the 70-watt setting, due to its greater power and dynamic range. Since my Chameleon III speakers are not particularly sensitive, the amp was preset to operate in 70 watt ultralinear mode.

The Dual 70 offers both 8 Ohm and 4 Ohm speaker connections. The binding posts are easily accessible on the rear of the amp (far more comfortably spaced than those on my Pass Aleph 5, I must add). While most 8 ohm speakers prefer the 8 Ohm connection, there are occasional rebels that prefer the 4 ohm setting. My Chameleons were quite content with the 8 Ohm connections.

Bias settings for each of the amp’s four output tubes are conveniently located on the lower front of the amp. All you need is a simple screwdriver to adjust bias. You simply turn the knob shown on the bottom left of the amp to choose which tube you’re biasing. After noting where the needle points on the front bias adjustment window (the round window that dominates the front of the amp), you turn the appropriate bias adjustment screw until the needle falls in the right place. Easy as pie. Bob Bergner, who distributes the Moore products, suggests that bias be accomplished late at night, when electricity is most uniform. Those who plug the amp into a PS Audio P600 (or above) Power Plant can probably ignore this suggestion.

Most of the weight of the amp falls in front, where the power supply is located. Please be aware of this when moving and situating this component. Otherwise, you may discover it and you rocking one direction or another. Given the options of broken tubes or a broken back, I lifted consciously and chose neither.

Testing

I spent a significant amount of time with the Dual mono 70, auditioning it not only with the Companion III preamp but with the Wright and Parasound preamps previously reviewed for this site. For my last test, I listened to it via Passive Attenuators, eliminating an active preamp entirely. My reference amp throughout the audition process was my 60W Pass Aleph 5, considered one of the simplest, most neutral, low distortion, tube-like solid state amps of the recent past. Unfortunately, the Pass’ input impedance is a very low 10K Ohm, which does not make it an ideal match for a tube preamp such as the Companion III. While it sounds so good with my Companion III that I retain it as my reference, it must be acknowledged that the much higher input impedance of the Bruce Moore amp makes for a better match.

One major difference between tube amps and solid state amps is that the output of tube amps remains relatively constant over the entire impedance response of the speaker, whereas the output of solid state amps varies inversely to the impedance of the speaker. This is why tube amps whose wattage equals that of solid state amps frequently sound both more powerful and more dynamic.

This was certainly the case chez Serinus. What I heard in dynamic difference between Pass and Bruce Moore far exceeded the difference between 60W and 70W. The Dual mono 70 is as dynamic as all get out, with the Moore amp/preamp combo definitely pushing the limits of my 14.5’ x 17.5’ x 9’ listening area. This amp definitely deserves room to breathe.

The CDs employed for assessment were my usual standbys. Not all are the last word in sonic excellence, but I have either used them on many occasions or found their music so rewarding that hearing it over and over again does not become a chore. These review discs, in no particular order, include the Beethoven “Archduke Trio,” a 20 to 16 bit recording played by the Chung Trio; Susan Graham’s exquisite “La Belle Epoque” Reynaldo Hahn song recital, recorded in 24 to 16 bits; the Bach Brandenburg Concerti played with extraordinary verve and color by the Akademie fur Alte Music, Berlin; the Blues No More track of Terry Evans’ JVC XRCD “Puttin’ It Down;” the reverse polarity beginning of Mahler’s Symphony No. 6, conducted by Pierre Boulez (I know, there are musically superior versions); and, based on extraordinary sonic merit, the 24 to 16 bit Domenic Argento “Valentino Dances” on Reference Recordings. Because I had recently reviewed it, and was thus quite familiar with its sound, I also took a listen to Antonin Dvorak’s Symphony No. 9 conducted by Claudio Abbado.

In the course of many weeks of intense listening, I accomplished several switches back and forth between the Pass and the Moore. I also invited my “partner in high-end crime,” my friend Joey who began building his own high-end system after hearing mine for the first time, to come take a listen and tell me if he thought my perceptions were on the mark. As a result, I feel that my conclusions are highly accurate.

All my conclusions result from tests conducted with the active preamplifiers mentioned above. The passive attenuators, when connected to either the Pass Aleph 5 or Moore Dual Mono 70, were fine on highs, but failed to convey midrange and bass fullness. A significant part of the tonal picture was lost, and what remained was not enough to facilitate clear comparison. Despite frequent claims that the simplicity of passive preamps and passive attenuators makes them ideal for many systems, I have so far tried using either one or the other with three very different solid state amps, and have never achieved success. Amps such as Pass, Moore and Krell seem to need an active preamp in order to manifest their magic.

Furthermore, plans to isolate the sound of my various interconnects by using the passive attenuators and switching interconnects back and forth – a test that would make me even more aware of what was responsible for what I was hearing when auditioning equipment -- had to be scrapped because a faulty locking termination on one of my Taras would not open far enough to allow me to attach it to a passive attenuator. The Tara has since been repaired under lifetime warranty.

What I heard

The Bruce Moore amp excels in the following areas:

1.      Depth. Whether depth signals are actually embedded in a recording, or the product of electronic hocus pocus, they help compensate for the reduction in listening area size from concert hall dimensions to one’s living room. The Moore amp definitely conveys depth, more depth than the Pass.

2.      Sound of the hall. I don’t know if it’s because of the Pass’s less than perfect input impedance match with my tubed preamp, but the Pass has never fully convinced me that I am hearing the “sound of the hall” touted in many audio reviews. With the Moore, I know that such claims are not thin air. Having a visceral sense of sound bouncing off walls, as opposed to a generalized sense of reverb, made listening quite compelling.

3.      Lower midrange and bass, bass, bass. The Dual Mono 70 has them both, so rich, controlled and powerful that I had to turn down my subwoofer amps (connected to the preamps’ tape outputs) in order to balance the subs with my main Chameleon speakers. The Dual Mono 70’s bass is also the tightest and fastest I have heard in my system. It is mighty impressive, natural sounding, and totally satisfying – the best I have heard in my system.

4.      Highs. The highs on the Dual 70 are quite vivid without being overly bright. I found them quite pleasing.

5.      Power and ease. Even on the most complex and demanding orchestral passages, there was never a sense that this amp was being pushed beyond its limits. Rather, the problem was that the volume increments between detents on the Companion III preamp were too great to allow proper volume adjustment in my size room for an amp this dynamic. In fact, even the very first preamp volume setting resulted in volume too loud for late night apartment listening. If you use a preamp with detented volume controls, it may need to be readjusted to supply the ideal volume settings called for by such a powerful amp.

6.      Pace. The pace of this amp was superb. Everything sounded solid, crisp and perfectly timed. Never did a drum stick sound like a spatula (something I have heard on an inferior preamp). Anything you have heard about a tube preamp sounding fuzzy or less focused than a solid state amp does not apply.

7.      Noise floor, vividness of presentation and transparency. There is a sense of reality that I experienced with this amp that exceeded that heard with my Aleph 5/ Companion III combo. Part of this I ascribe to a very low noise floor and  resultant transparency. Another reason may simply be this amps extra power, superior design, and perfect match with the preamp. All I know is that, on one level, everything sounded more involving, vivid and real.

Yes but

After reading all this, you may wonder why I did not pass on my Pass amp and replace it with the “perfect complement” to my Companion III preamp, the Bruce Moore Dual Mono 70. Funny you should ask . . . .

Before I owned the Pass, I owned a Krell KSA 50-S. I once heard a manufacturer of superb cables call my 50W Krell the best amp that Dan d’Agostino ever designed. Perhaps if I had paired the Krell with my current Companion III preamp, I would have been able to comment on this assertion with a semblance of surety. All I know is that, when paired with my Classe 6 preamp (now used only for phono), the Krell

(a)   created an amazing, huge, three-dimensional, space age soundstage that seemed a bit more like a psychedelic experience than one related to the sonic reality of unaltered consciousness;

(b)    had enough slam to make my downstairs neighbor bang her broomstick on the ceiling to get me to turn the music down; and

(c)     had a combination of a recessed midrange and across the board sweetness that eventually drove me nuts.

With the Krell, everything sounded sweet sweet sweet, from sweet-voiced sopranos to cacophonous 20th century orchestral fare. Music that should have chilled my soul or reminded me of the horrors of war instead sounded sweet. Drum thwacks had a sweet edge, brass had a sweet edge, even sour notes had a sweet edge. Gaaah! I grew to hate it. Is there any wonder that I went from the Krell to the neutral sounding Pass?

The Bruce Moore is nothing like the Krell. Its soundstage seems real and convincing, and its slam achieves perfect integration with the rest of the sonic spectrum. It also has a glorious midrange, one that deeply impresses. But I do not find its timbre entirely true. Everything is just a little bit shinier and sweeter than I find it in real life.

The analogy I come up with is of looking through a window. If we posit, rightly or wrongly, that the ultra-simple Pass offers a view through a clear window, then I’d say that the Moore’s window has a bit of a shine to it. It is a beautiful shine, very inviting and seductive. It certainly does not obscure the view, or alter one’s feeling about what rests on the other side of the window. But it’s one of those shines that makes everything on the other side seem to have a little bit of mother of pearl sheen.

Compared to the Moore, the Pass sounds most unglamorous. Just as many will find that a person’s natural beauty is enhanced with even a touch of make-up, many will prefer the slightly glamorous shine of the Moore. After all, it only serves to further drive home how clean, rich, full, deep, solid and impressive this amp is. Alas, this extra little bit of sweetness, which the Moore shares with many other tube products, is not what I seek. I want to hear music just as it is, heaven and its hell, warts and all.

For reasons I hope to be able to pinpoint clearly as I test more speaker cables, interconnects and power cords in my system, my current equipment configuration conveys the reverb around instruments and in acoustic spaces as occasionally sounding a tinge gray and powdery. This is especially true on some (not all) chamber music recordings. “Space” does not always sound the same as it does in real life. The Moore does not do this. Both reverberation around instruments, and the sound of the hall, are quite convincing through the Moore; instruments and voices stand out in vivid color. But the Moore’s slight extra shine, glamour and sweetness are not real. As much as I love many of the Moore’s qualities, I prefer a more neutral sound. While, with the Pass connected to the rest of my current equipment/cable configuration, a cello may sound less brilliant and rich than in real life, what it does sound like is as close to the sound of a cello as I have yet heard reproduced in my 14.5 x 17.5 x 9 listening room.

I wish to make one more point. Bruce Moore products are designed by one man, who in turn has one major distributor who in turn serves both other distributors and his own customers. All the people I have met or communicated with in the Bruce Moore circle are fine men, men of the highest integrity who love what they do. But they are not always easily accessible when it comes to service. As many readers know, this is true for many small operations. There is an advantage to working with a large company that really is there to answer the phone during the “normal business hours” of this anything but normal industry. The advantage is moot when one large company wanted to charge hundreds of dollars for replacement stenciling of front panel lettering on a $3,200 preamp whose inexpensive stenciled lettering had worn off, or when another company promised immediate upgrading for a piece of equipment, but ended up holding it for weeks until the parts arrived. But when a company combines regular accessibility with integrity, there is a distinct advantage. The Moore operation would benefit from a second in command who can help expedite service issues.

Conclusion

I review CDs for two newspapers that insist that I state all my opinions as though they are absolute fact. To write “I feel” in such a review is, according to the editors of those publications, to draw undue attention away from the music and towards myself.

Tell me, what is the life of music without the listener? Just what is the sound of one hand clapping?

I do not pretend that my reviews are more than a reflection of how I choose to experience reality. My hope, however, is that I provide you with enough clues about myself and my values to enable you to sense if your viewpoint may prove in synch with my own.

In many respects, the Bruce Moore Dual 70 is the finest amp I have yet auditioned in my system. Matched with the right preamp and cables, I have absolute certainty that it will bring great pleasure to many. I also am equally certain that, while my criticism may speak to some, others will find my reservations an indication that this amp is right for them.

Bruce Moore is a superb designer, and his value-laden products deserve your attention. This amp can stand comparison with many name brand amps that consistently receive praise in the audiophile press. I encourage you to take a listen.

REVIEW SYSTEM:

Michael Green Chameleon III tunable speakers (modified with Nirvana hook-up wire and Scan Speak 2905/9700 tweeters);

Hsu HRSW12V powered subwoofers (stereo pair)

PASS Aleph 5 60W pure Class A power amplifier

Bruce Moore Companion III tube preamp

Theta Gen. 5A single-ended DAC

Genesis Digital Lens with BNC in and out

Audio Alchemy DDS-Pro transport

PS Audio P300 Power Plant

AQ Dragon II speaker cable to the Chameleons

AQ Clear II speaker cable to the Hsus

Nirvana interconnect between amp and preamp

AQ Diamond II co-ax interconnect from Hsu amps to preamp

Tara Decade interconnect between preamp and DAC

Nirvana digital (BNC) interconnects from Theta to Lens and Lens to transport

Power cords by MIT, Synergistic, Harmonic Technology and XLO

Michael Green Deluxe Ultrarack and Basic Racks; MG Audiopoints and room treatment; Black Diamond Racing Cones; inner tubes, maple cutting boards and bags of sand, homemade bass traps; Shakti stone and many Shakti On-Lines; Bedini Ultraclarifier, Audioprism Stoplight and Blacklight

- Jason Serinus -

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