Hafler Transnova 9500 power amplifier

Boy, do we ever get letters. From readers angry that we review too many expensive products. From readers depressed that we review too many affordable products. From readers bemoaning our digital coverage. From readers asking when we're going to get with the 21st century and stop gushing over analog. From readers wanting more coverage of tube products. From readers wanting more coverage of MOSFET amplifiers designed for high voltage gain on the output stage.

Well, okay, we've never received the last type of letter. But if we did, it would have to refer to Hafler's new Transnova 9500 amplifier (or the smaller, 9300 THX that Sam Tellig is currently auditioning). Will coverage of the 9500 confuse or please those readers who want us to review affordable products? Is the top-of-the-line Hafler "affordable"? It's certainly more expensive than past flagship Haflers. But assuming its sound passes muster, it would certainly qualify as high-end for the rest of us.

Our sample of the 9500 arrived in its standard brushed aluminum finish with rack-mount faceplate and handles. It's a surprisingly compact and lightweight amplifier for its rated power—no one would confuse it with a Krell or a Levinson—but it appears to be solidly constructed. The heatsinks, which wrap smoothly around the sides and back of the amplifier, form an integral part of its chassis, saving on sheet metal. The rear panel has your standard RCA unbalanced input jacks and five-way output binding posts, along with a switch which allows for bridged output (a mode I did not audition as I neither had a pair of the amps nor a pair of loudspeakers—or a test load, for that matter—which I really wanted to subject to the bridged configuration's rated 750W/8 ohms output).

I have a slight quibble with the location of the input jacks: because they're internally connected direct to the circuit board, they're quite close to the output jacks, causing some slight cramping when there is actually plenty of real estate available on the rear panel. We understand, however, that a revision will rearrange the rear-panel layout to make better use of the available space. I also disliked the five-way output posts. Their caps are not hexagonal, and could not be tightened down with a nut driver. They're hard to finger-tighten properly, and although there are slots at the end of the caps which seem made for a wide-blade screwdriver or coin, attempting to tighten them this way—while it worked—tended to mangle their soft plastic unless extreme care was taken. Furthermore, two of the four terminals stripped easily and would not tighten securely, forcing me to resort to banana plugs to make the output connections. As if that wasn't enough, one pair of output posts started to work their way loose from the chassis. They never lost their internal connection, but this did not inspire confidence.

Jim Strickland, Hafler's chief engineer, developed the first Transnova amplifier in 1981 for Acoustat, his former company (which was absorbed by Hafler later in the decade). He produced amplifiers for Acoustat using this design from 1982 to 1987. He designed the Transnova topology to counter what he believed to be a misuse of the (then) new late-generation FETs, including power MOSFETs. In his eyes, most designers, accustomed to working with bipolar transistors, were using FETs much as they had bipolar devices—aided by the manufacturers of the devices. These manufacturers were understandably anxious to sell the new FETs without confusing the issue and thus were recommending familiar circuit applications which closely paralleled those of bipolar transistors. Jim felt that because of this "bipolar thinking," the many advantages of FETs, including (but not limited to) high speed, outstanding linearity, low noise, and good thermal stability, were not being fully utilized.

Transnova is actually an acronym which stands for TRANSconductance NOdal Voltage Amplifier. A full discussion of its operation—as might be apparent from the full name alone—is beyond the scope of this discussion (a four-page paper on the topology is available from Hafler). Basically, however, the circuit configuration results in an amplifier which derives most of its voltage gain from the output stage, unlike typical voltage-follower designs (see sidebar). This allows the driver and front-end stages to be low-voltage designs running on ±24V. Because the Transnova design is said to drastically increase the power gain of the output stage, less is demanded of the driver stage, which can be operated with much higher class-A operating current. There are only three gain stages required (input, driver, and power), and the amplifier is said to have inherently high slew rates and wide bandwidth, along with excellent high-frequency stability. Hafler also states that it is inherently more tolerant of capacitive loading—not an unexpected thing in an amplifier circuit originally designed to drive Acoustat's electrostatic loudspeakers.

The 9500 has a beefy power supply, its heavy-duty transformer having separate secondary windings for each channel's high-voltage supply (with a filter capacitance of 20,000µF per rail per channel, footnote 1), and a third set of windings for the regulated low-voltage supply feeding the input and driver stages. The output stage of the 9500 uses eight devices per channel (the smaller 9300 uses six). Because MOSFETs are inherently self-correcting when overheated, no thermal-protection circuitry is needed. There is also no output inductor, implying that the amplifier is unconditionally stable.

Internally, the 9500 appears neat and well-made. The large power transformer takes up at least a quarter of the internal space, and probably makes up a good portion of the weight as well. The wraparound heatsinks give the entire assembly a reassuring solidity.

I began my listening to the Hafler through the Wilson Audio WATT/Puppies. The pairing was successful, though not a revelation. The 9500 turned in a solid performance, but not one which made me want to chuck the Krell KSA-250 which had been in the system immediately prior to the Hafler. The 9500 sounded cooler and leaner than the Krell, with a tighter but less robust extreme bottom end and a brighter, more up-front low treble. Both amplifiers were strong on soundstaging; though the more forward-sounding of the two, the Hafler nonetheless did an impressive job in rendering a sense of space and depth. It lacked some of the Krell's drama, especially in its performance at the bottom end of its range, but kept a very tight grip through it all. While I ultimately came to feel that the Hafler was just a bit too lean and analytical for the WATT/Puppies, it's unlikely that the moderately priced Hafler and the very expensive Wilsons would ever be paired in any case.

Moving on to a more realistic combination, I hooked up the Hafler to the Vandersteen 2Ces. My impressions here could not have been more different. The sound with the Vandersteens was solid and secure, with a powerful and—within the capabilities of the Vandersteens—tight bottom end, an immediate but not pushy midrange, and a clean, open, and detailed treble. Though the extreme highs seemed a shade softened—a quality of the Vandersteens, in my judgment—the Hafler presented an excellent sense of space, with a natural, ambient shell evident around the performers. Sibilance was clean and without exaggeration. The Hafler's slight leanness was evident here, but instead of washing out the warmth of the overall sound it simply translated into an opening-up of the soundspace and a pleasing lack of congestion or muddle.

The longer I listened to the Hafler in this setup, the more I liked it. It was equally at home on small-scale, intimate music or big-boned, rousing works. A new HDCD sampler from Reference Recordings (RR-S3CD) is a superb example of the latter. On Festival Day in Seville, one of the real showpieces on this disc as played by the Dallas Wind Symphony (from Trittico, RR-52CD), the Hafler, like the Energizer bunny (or Frederick Fennell, the conductor here), just kept going and going and going. The bass drum and percussion on this selection will knock you out of your chair, and the Hafler responded without strain or congestion.

Footnote 1: The big 20,000µF electrolytics are bypassed with 4.7µF polypropylene-dielectric types.—John Atkinson
Hafler, A Division of Rockford Corporation
546 South Rockford Drive
Tempe, AZ 85281
(602) 967-3565

Anon2's picture

All these walks down memory lane make me think that, perchance, the state of the art in audio has not advanced too much. I get growing hints with these articles that perhaps I should look on eBay and elsewhere for pennies-on-the-dollar bargains (and someone to install new capacitors), instead of paying thousands, if not tens of thousands, more for successor "new gear."

Capacitors can be replaced if one finds a competent technician--and we have them in large metros. Speaker drivers can be replaced.

Are we not being told that only the Nissan 280Z, or post-1984 Corvettes, were classics that merit refurbishment, for a huge bargain, rather than paying thousands more for products that are "newer," but offer not much else on the performance front?

Worse, perhaps these articles tell us that not much has changed at all? Is the 1980s, 1990s CRT TV but still a CRT TV in new trappings in its audio, 2016 equivalent? Meanwhile, we have phones that outshine their Assembler Language counterparts of the Apollo age. We have 50" LED TVs that on a good day cost a fraction at a big-box retailer, and offer multiples of performance of their CRT 110-pound predecessors of 10-15 years ago.

Keep the articles coming. I keep thinking. Based on a recent article a significant musician, interviewed in this publication, has also thought, and opted for keeping 20-year old gear in favor of "mortgaging a house" on the latest and greatest.

jeffdyer's picture

I am sure that you are correct.

Analogue power electronics achieved almost perfect levels of realism in the 1970s, certainly since the end of the 1980s there has been little change in amplifier technology.

I'm sure you could pick up just about any 1980s quality stereo amplifier and plug it straight into your system. However, I doubt it would satisfy the AC power cable upgrade freaks out there.

Now the sources though, that's where the improvements have been made.

Herb Reichert's picture

and I always wanted one of these

Anon2's picture

I am glad that someone shares my views. I do concur that sources is where the action has occurred. It is also the one rare area of audio with a plausible better-performance-at-lower-prices that we see with TVs, computers, phones, etc. Improvement in sources also tend to re-validate the hi-fi hierarchy of old--we don't hear much of that one anymore either. Optimize the source (where the cost savings and big improvements are). Then go to amplification (again, go used and it's a bonus if you have a good technician in your locale to inspect of refurbish). Then optimize your speakers (an area where performance improvements seem spotty while prices march upward, with some exceptions).