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omaha audio

OD-300B SET integrated amplifier

as reviewed by Akhilesh Bajaj

 

 

 

 

AKHILESH BAJAJ'S SYSTEM

LOUDSPEAKERS
1978 Klipschorns with Bob Crite updated crossovers, Vintage Altec VOTT system, modified to use 515B woofers in a large BR box with a 511B horn using 808-8A drivers, with a sub to drive the bottom. Stephens trusonic 80-FRs with Vifa dome tweeters and sub, actively crossed, DCA5.5" driver in PAWO horn, run in pure single driver mode with no passive or active EQ, Vintage University 315 coaxials in largish sealed altec boxes. Subs: Hsu research vtf 2 and vtf 3.

ELECTRONICS
Hand built passive using General Electric single wire rheostats and a Mod Squad Line Drive. 5842/45 SETtube amp (James Burgess built), 2a3 Bez SET amplifer, ASUSA 12ax7/el84 SET mode amplifier, Custom built Dynaco ST35 12ax7/12au7 el84 p/p amplifier, Abraxas Audio Built SET amp 12ax7/12b4 output tube, and a Larry Moore built 2a3 SET amplifier.

SOURCES
Berendsen CDP1 and a Modified Philips CD player with TDA1541 chip in Nonoversampling mode.

 

Omaha Audio, contrary to what its name may indicate, is based in Walnut, California. In conversation with Mr. T.J. Chuang, its president and CEO, I learned that the company designs its amplifiers in the USA, and then has them manufactured in China. I have previously owned a BEZ 2a3 amplifier that was made in China, and, for the money, it performed very well, though not quite in the league of the custom made SET (single ended triode) amplifiers I now own. The Omaha Audio integrated amplifier retails for $1400, and was well spoken of in Enjoy the Music, when it showed at the 2007 Son and Image Festival, in Montreal, Canada. I was intrigued enough by my experiences with the BEZ to want to sample a Chinese SET amplifier that was in a higher price range than the BEZ. About six weeks after a brief conversation with Mr. Chuang, an amplifier showed up at my door in a large brown box (21 x 23 x 16 inches) weighing approximately 45 lbs. This was the OD-300B amplifier. The packaging was excellent. The amplifier was double boxed with no signs of any external damage.

The first thing I looked at was the manual. It was very basic, with several grammatical errors, and in my opinion did not contain much in the way of useful information. Nevertheless, the amplifier itself was very easy to set up, even if someone with no experience were to buy one. Each tube socket was clearly labeled, and the volume control, power button, and input selector switch were very intuitive. The tubes came in a separate well cushioned box, except in one case the smaller tubes were not bound in their containers and as such, collided with each other during the journey. However, no damage was done, and overall I would rate the packing of the amplifier as excellent.

Appearance and Specifications

The OD-300B measures 15.7" wide x 12.2" deep x 7.1" high. It weighs 37 pounds and as per the manual generates 10-watts per channel, with a specified frequency response of 25Hz to 70kHz (+/- 1 dB). Hum is specified as less than 2 mv. Input impedance is 100k ohms. The amplifier comes with a 3 year warranty as specified in the manual, with a 3 month warranty for the tubes.

In terms of appearance, it is a very handsome amplifier with aesthetics that go beyond the usual custom built or small shop tube amplifiers. The external black paintwork is tastefully done and the artwork on the panels is appealing without being lurid. The thick metal faceplate resembles that of much more pricey amplifiers. The speaker outputs are well labeled. The exterior looks clean, simple and elegant. The on/off switch feels expensive, and the volume control knob and input selector knob are a tasteful burnished gray metal. Overall, the OD-300B looks a lot more expensive than it is, and it is very suitable for public viewing, say in a family room, where the acceptance factor amongst family members should be high.

As stated in earlier reviews, I listen primarily to SET amplifiers and own quite a few of them including a 45 tube amplifier (driven with 5842 tubes) built by James Burgess of California, a 2a3 amplifier (driven with 5687 tubes) built by Larry Moore of Ohio, a 12b4a amplifier (with 12ax7 driver) built by Abraxas Audio, as well as a 12ax7/el84 amplifier where the output tube is wired in single-ended mode by ASUSA. My current reference SET is the Larry Moore built 6sn7/46/845 amplifier that I reviewed earlier in Positive Feedback Online.

It's best to clarify up front that my goal with seeking out SET amplifiers is to find components that bring me closer to the music. In that sense, SETs are more about emotion (how does the music make you feel when played through an amplifier?) than reason (how does the amplifier objectively measure?).

The OD-300B uses the 300B output tube, driven by the 6N2 and the 6N3 tubes. These latter tubes are found in a variety of Chinese built audio gear, such as the Shanling T-100 CD player. A search of the Duncan tube website shows that the 6n3p tube is the same as the Soviet 6H3P tube or the 5670 tube. The 6N2 tube (based on information found at http://www.diyaudio.com/forums/showthread/t-22691.html) is like a 12ax7 tube except it uses 6.3 volts across its filaments (a 6ax7 if you will). I am not aware of any other amplifiers on the market that use this combination of tubes. The power and output transformers look proprietary and the OD-300B has 4 and 8 ohm taps on the speaker outputs. The power regulation appears to be solid-state.

Listening

I listened to the OD-300B amplifier for a period of over three months and compared it on and off with my 5842/2a3 SET amplifier that is custom built in the USA by Larry Moore and uses electraprint output transformers. This would have a street value roughly similar to the retail value of the OD-300B. For most sessions, I let the amplifiers warm up for about 10 minutes before listening.

For the first setup, I used a pair of single driver speakers that use a D'Charles Audio 5.5" full range driver in a PAWO horn. A single driver setup like this helps reduce speaker related confounds, as long as it has a reasonable frequency response. For the source, I used the $2200 Berendsen CDP-1 Player. The speaker cable used was 10-gauge cable bought from a large electronics store hand trimmed with no terminators by yours truly to the minimum required length—which was about 5 feet. Interconnects (IC) used were simple Radio Shack gold plated ICs, 1-2 meter, again as short as required. Power filtration was a PS Audio Ultimate Outlet feeding into an Isobar isolator. My main listening room is about 8 feet high by 11 feet long by 15 feet wide, and quite well damped.

The initial material listened to included several female vocalists (Eva Cassidy, Sofia Petersson, Jheena Lodwick etc.). My first impression of the OD-300B was that it is dead quiet, as befits solid-state power regulation. Compared to the Moore 2a3 amplifier, it also seemed just a shade bass shy. When listening to Best Audiophile Voices vol. 2 on the OD-300B versus Moore 2a3, my listening notes indicated that the Moore amp had just a touch more bass impact. The OD-300B was mellower and on the DCA speakers that measure flat from 100Hz-5kHz, the Moore 2a3 seemed to offer a little more depth and insight into the music, though I can imagine that some may prefer the more laidback sound of the OD-300B. Track 5 on this album, "Do that to me one more time" by Jheena Lodwick is somewhat bass heavy and sounded a tad fuller with the Moore amplifier. Track 8 on the same album, "Can't take my eyes off you" by Sofia Petersson has solo piano and voice and the attack and decay (micro dynamics) were a little better with the Moore amplifier. The OD-300B seemed to make instruments sound mellow. However on full range speakers which are peakier (like Fostex or Lowther drivers) the laidback nature of the OD may be a strength.

In order to experiment with that, I took out another set of speakers, consisting of Fostex FE103A drivers that are in a one cubic foot bass reflex box, tuned to about 60Hz. On monitor stands, these speakers behave like bass shy monitors that are peaky. The FE103A Alnico driver has an Fo frequency of only about 90Hz, and also gets noticeably shriller over 3000Hz. Why do I put up with these speakers? The major strength of the Fostex FE103A driver is its huge Alnico magnet and wispy paper cone—a la Lowther—that causes amazing micro dynamics and increases the "you are there" feeling for vocal and acoustic music. A few months ago, I put these speakers on the carpeted floor of my highly damped listening room, and it was one of those magical moments that occur seldom in our audiophile lives. The bass was augmented by the floor to go down to about 50Hz, and the off-axis nature placement reduced the shrillness dramatically. In this position, these are some of the best speakers I have heard for vocal and acoustic music. More information on the Fe103A drivers can be found at http://www.t-linespeakers.org/drivers/tunnelreflex/index.html

It was in the "on the floor" position that I heard these speakers with both the Moore 2A3 and the OD-300B amplifier. On this speaker, the bass difference was less (perhaps because the speaker was less able to resolve any bass differences). The Moore 2A3 amplifier still seemed to offer a more immediate connection with the music, but there was audibly less difference between the two amplifiers.

The next setup was one where most of the listening was done over a period of about two months. This consisted of the same Berendsen source, but the speakers were my 1978 Klipschorns which are setup on the ends of the 15 feet wall of the music room. These have updated capacitors in the crossover, but are otherwise stock. Over the last several years of owning them, I have thrown at them every kind of SET amplification I could come across. They are rated at 104dB efficiency, which means a watt or two is sufficient to drive them to reasonable levels. My current reference 845 SET amplifier synergizes excellently with them. The Klipschorns easily reveal the character of the upstream components, and each SET amplifier I have used with them has sounded quite different.

In order to isolate the qualities of the OD-300B, I listened to the Chesky Ultimate Audio Demonstration Disc, available from Chesky Records. For each quality described below, I'll use a grading scale of poor, fair, good, or excellent. Excellent represents a performance better than comparable SET amplifiers along that dimension, good represents performance approximately on par with comparable gear available from other manufacturers, while fair represents slightly worse. I should clarify that the source (Berendsen CDP-1) and speakers (Klipschorns) were sonically very familiar to me, and I have heard this disc many a time with these components, though with different amplifiers.

The first quality was high resolution, one test of which is that the shaker in track 3 on the Chesky disc should sound different at every shake. The OD-300B rated good. Next, the depth of the soundstage was checked with track 5, where the opening trumpet is 10 feet from the microphone. Again, the OD-300B rated a score of good. The next quality I checked was atmosphere, something SETs are excellent at conveying. Track 7 on the disc should convey a warm, lush and relaxing mood. The OD-300B rated an excellent on this dimension. After atmosphere, the midrange purity was evaluated. The acapella track 9 has finger snaps that should sound fleshly, and the main singer's voice should sound as if it is coming from a whole human body, as opposed to just a microphone. Since the main strength of the 300B tube is midrange purity, not surprisingly the OD-300B rated an excellent on this dimension. On the naturalness dimension, track 11 has rainforest sounds at the beginning, and the cello should feel warm, with no electronic glare. The OD-300B rated an excellent on this dimension as well. The next dimension I checked was transparency, which represents how little the system intercedes between the music and the listener. On track 13, the piano's attack and decay, along with the bass's warmth and fullness and the airiness of cymbals all helped to evaluate this dimension. The OD-300B rated a fair along this dimension, primarily because of slightly weaker bass and a faint sense of masking the immediacy of the music. However, the cymbal highs sounded very nice. Next, the presence dimension can be evaluated on track 15, where one checks the believability of the saxophone playing, and also checks for the virtual saxophone that echoes back from a wall in the studio. I could not hear the virtual saxophone (in all fairness I can only hear it with more expensive amplifiers in this particular sound chain), but the saxophone sounded natural. I would rate the OD-300B a good along this dimension.

The next two qualities were visceral impact (big energy and excitement), and rhythm and pace (foot tapping, does it make you physically respond to the music?). On both these the OD-300B rated a good. On the dimension of focus, where each instrument is delineated along its own space (aka "air" around instruments), the OD-300B rated an excellent. Along the holographic imaging and transients (micro dynamics) dimensions, the OD-300B rated a good. Finally, along the bass resonance dimension I gave it a fair simply because the bass lacked that last bit of fullness compared to other SET amps one can get for similar money.

Conclusion

Like all components, the OD-300B has strengths and weaknesses. Its main weaknesses are a slight lack in the lower bass octaves and a slight lack of immediacy or transparency that ultimately connects you to the music. However, it has significant strengths. It showed excellent air around instruments, midrange purity, nice highs, and naturalness. It also excelled at conveying the mood of the music. It is far better looking than most amplifiers in this price range and extremely easy to operate. It also seems reliably built.

This amplifier would be excellent for folks with slightly bright systems or listening rooms, or with other components that perhaps make the music sound "etched". It is also suitable for systems with monitors and separate subwoofers, as well as single driver speaker systems where the speakers themselves do not go to the lowest octaves. Given its combination of price, aesthetics, quality build, and overall sonics, I strongly recommend the OD-300B. Akhilesh Bajaj

OD-300B amplifier
Retail: $1400

Omaha Audio
web address: www.adaaudio.com

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