Berning TF-10 preamplifier
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.