Berning TF-10 preamplifier
By J. Gordon Holt
Several issues back, we reviewed rather enthusiastically a pre-production prototype of this preamp. The original was an unprepossessing-looking device on two chassis, interconnected by a 3' umbilical, with a squat little preamp box and an even squatter power supply with humongous cans sticking out the top. We averred that it sounded nice. The production model is so nicely styled and functionally smooth that we wondered if it might not be another Japanese product. 'T'ain't.
The Berning TF-10 is what may justifiably be described as a basic preamp with frills. It has no tone controls, but outside of that, it is one of the more flexible preamps around. It has three magnetic phono input pairs--all identical, and selectable by a front-panel switch. There are two sets of Tape Outs and Tape Ins, with switch selection of 1-to-2 or 2-to-1 dubbing. There are separate switches for input selection and for monitoring, so that it is possible to listen to one program source while recording another. There are 5 switch-selectable high-level input pairs (including 2 from Tape), and offtape monitoring provisions for either tape machine. And there's another, quite unusual feature: A remote muting switch that connects to the preamp via 20' of thin cable, and allows its outputs to be shutdown when desired.
AC convenience outlets are minimal and marginal. There is only one of each. (switched and unswitched) and the switched one has a current rating of a mere 3 amperes, which is too low to operate high-powered amplifiers. In most cases, the power amplifier will have to be switched on separately, or be controlled by a separate power switcher such as the one from Audio Research. (A solid-state power amplifier should be separately switched anyway as a matter of routine practice be cause most preamplifiers, including this one, produce some kind of extraneous noise during warm-up, and some generate enough of a pop to damage loudspeakers if the power amp is on when the pop occurs. The Berning preamp has a delay relay to keep the worst warmup noises from the outputs, but it does produce a mild DC pulse or two when the relay closes and when the unit is turned off; and the pulses are strong enough to put some power amplifiers into DC-offset shutdown.)
The thing that makes the TF-10 unique is its hybrid amplifying circuitry. Each amplifying stage consists of a triode tube and an FET in what David Berning describes rather evasively as a "patented special complementary arrangement." The basic idea behind this is that the tandem hookup of a tube and an FET having complementary distortion characteristics will tend to cancel out the distortion from each device. The entire TF-10 preamplifier uses minimal negative feedback, and this is local feedback only. No loop feedback is used--at all--thus eliminating any possibility of instability or of any of those kinds of distortion (such as TIM and SID) that relate to feedback. The phono equalization, too, is entirely passive, to minimize any interaction between the equalization circuitry and the electrical characteristics of the cartridge.
Measurements showed that the frequency response of the high-level section was flat from 10Hz to beyond 200kHz (within 0.1dB) and that the RIAA equalization was accurate to within 0.2dB from 30Hz to 15kHz (which is the limit of the range specified by the RIAA standard). This is the highest RIAA-equalization accuracy we have ever encountered in a preamp, but when one considers the fact that designer David Berning works for the US Bureau of Standards, it seems somehow less surprising that this should be so. As a result, our bypass tests (using tape as a source) revealed no audible difference whatsoever between the signal going directly to the power amplifier and that passing through the entire preamp.
It should be noted though that inverse-RIAA bypass testing tells only half the story about a preamp. Tape is bandwidth-limiting (although the new digital recorders approach DC at the low end) and thus does not feed a preamp the ultrasonic garbage that imperfect groove-tracking and surface noise elicits front a cartridge. Many preamplifiers we have tested, which sounded more than acceptable on the bypass test, have failed miserably with a phono source through inability to cope with those ultrasonics. The Berning didn't.
The way a preamplifier handles spurious ultrasonics determines to a great extent the "texture" of its sound. One which copes very well but not perfectly will tend to have a clean but slightly dry or "chalky" sound. Diminishing ability to cope therewith adds a progressively wiry edge to the high end and an increasingly coarse "grain" to the sound, reflecting how far the ultrasonic crud is being splattered downward into the audible range. Increased audibility of surface noise is another unwelcome bonus resulting from a preamp's inability to handle the steep wavefronts of ticks and pops. The TF-10 sets a new standard for freedom from both.
Let it first be said that this is unquestionably the most listenable wideband preamplifier that we have auditioned to date. It seems to be entirely free from any sonic texturing, producing the most liquidly transparent sound we have ever heard from discs. Surface noise, too, is so subdued that one's first reaction is that the preamp's high end must be rolled off. It isn't. Hard transients--those producing the fastest rise times--are reproduced with incredible sharpness, yet there is no trace of that irritating edginess at the top which has characterized every other preamplifier having comparable rise time. Some such preamps sound "faster" than they really are because of that high-end edge, but have been found unacceptable by musically-oriented listeners because of the way they distort overtone structure, turning brass cymbals and nylon strings into steel. The TF-10 is an unprecedented amalgam of quickness and accuracy with musicality.
The high end is open, delicate and downright exquisite, sounding as if it has no upper limit. Instrumental timbres are reproduced flawlessly, without a trace of hardness or edginess, and depth, detail and inner definition are for all intents and purposes indistinguishable from that Of the original program source. The low end is full and rich, yet as taut and detailed as the best we have heard from all-solid-state preamplifiers, and the entire middle range has that textureless liquidity we have only heard previously from tubed preamplifiers that fell short in various other ways. In other words, this is now the preamplifier by which others must be judged.
As usual, it is necessary to add that many listeners aren't going to like this. Apart from those audiophiles who don't know what live music sounds like anyway, many will find that the TF-10 doesn't do as good a job of complementing other aberrations in their systems as well as does the Blotzblotz Mk-XVII-k preamp. But audiophiles who are seeking the most accurate components they can get--and particularly those who admire the quickness of transistors but prefer the musicality of tubes--will find the TF-10 to be the answer to their prayers. Like the Infinity HCA, this seems to represent the best of all possible worlds.
The Berning's stiffest competition is of course the similarly priced Audio Research SP-6A -- generally acknowledged thus far to be the best preamplifier money can buy. We borrowed an SP-6A from a subscriber for a brief time, and can report that the difference, while not dramatic, was unmistakable. The ARC was quite a bit brighter, a little warmer in the lower-middle range, not quite as tight at the bottom, and not quite as open at the extreme top. It had somewhat more high-end texture (very subtly rough by comparison) than did the Berning, and tended to reproduce surface noise a little more conspicuously. In short, we would say that, whereas the SP-6 had all of the attributes of the very best tubed components, the Berning seemed to have all the attributes of the best tubed and solid-state components with none of the shortcomings of either.
Can the TF-10 be improved upon? Well, it could have lower phono-preamp noise. Our sample measured 58dB of S/N relative to 5 millivolts input -- enough to be audible at high volume-control settings when no disc was playing. Perhaps another design will better the TF-10 in detail or inner definition, but there is so little room for improvement there that we can't conceive of this being "obsoleted" within the foreseeable future.
What about tube life, though? How long will a TF-10 continue to deliver this level of performance? Tubes, after all, are known to have a finite life, and can be expected to deliver progressively impaired performance for many months prior to their outright failure. They also continue to become harder to get and higher in price, making routine periodic replacement an increasing problem. The TF-10's designer, David Berning, expressed his conviction that all tubes in the TF-10 should be "good for at "least 20,000 hours." These are industrial-quality tubes, rated at 10,000 hours under normal-use conditions.
And they are not being operated "normally." Heater voltage in the TF-10 is lower than the rated 12 volts (virtually guaranteeing double the heater life), and normal operating currents are far below the manufacturer's rated values. The tubes don't even get hot; just warm to the touch. In fact, after four hours of operation on a hot day, there is less heat coming from the vents at the top of the unit than comes from most transistor-only preamplifiers (and far less than from class-A transistor designs). We have no way of verifying Berning's optimistic estimate of tube life, except to say that the tubes in the 1710 appear to be "coasting," which has always contributed to extended tube life. Even assuming that the tubes only meet the manufacturer's minimum-life expectancy, 10,000 hours works out to over 13 years of life, assuming 2 hours of use per day. That's longer than most transistors can be expected to last. And of that 13 years, at least eight years represent the peak-performance life of the tubes. That kind of overkill should be reassurance enough to anyone considering the purchase of a device using "old-fashioned tubes."
The vacuum-tube, incidentally, does not seem destined for imminent demise. The old, familiar 12AX7 may well be on its way out, but industrial-grade tubes with lower noise and distortion are in widespread use throughout the world in military electronic devices which would cost far more to replace than to keep retubing. As long as that situation continues, those tubes will continue to be readily available to manufacturers, if not to your local RadioShack. (The Soviets, for example, have used more powerful rocket engines than we had, and thus never had the need to miniaturize as much of their onboard electronics for reduced weight.)
This, then, has to be considered the state of the art preamplifier as of now. Now, how about a cheaper version of it that most audiophiles can afford?
Sidebar 1: Specifications
Description: Hybrid (tube/transistor) basic preamplifier; 3 magnetic phono inputs, 5 high-level inputs including 2 Monitor, 2 Tape outputs, 2 Main outputs, tape cross-copy switch (1-to-2, 2-to-1), separate Input Source and Monitor switches, stepped volume control with vernier, remote Muting switch, 1 switched and I unswitched AC convenience outlet.
Dimensions: 19" W by 12" D by 3?" H, overall.
Manufacturer: Precedent Audio Products, Inc., 306 E. Oliver Street, Baltimore, MD 21202 (1979). David Berning Co., 12430 McCrossin Lane, Potomac, MD 20854. Tel: (301) 926-3371 (2002). Web: www.davidberning.com
Sidebar 2: Manufacturer's Comment
What can I say but "Thanks" for the fine review. I would also like to express my gratitude to Precedent Audio's Murray Zeligman, whose many hours of critical listening to pre-production prototypes contributed to the performance of the final production version of the TF-10.
Since initial production -- one of which Stereophile had for testing -- we made a minor circuit change which effects a very slight improvement in the reproduction of detail. That improved version is the one we have been delivering to our dealers. --David Berning