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发表于 2012-2-13 17:03:28
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The First Watt Model B1 Buffer Preamplifier
by Dick Olsher
Life at 55 MPH
First Watt’s latest DIY offering, the B1 buffer stage, is described by Nelson Pass as “a no-feedback JFET buffer that offers ultra low distortion and noise with ultra wide bandwidth.” Nelson’s article may be found here: www.firstwatt.com. If you are unfamiliar with First Watt’s product spectrum and with the underlying philosophy of its progenitor – Nelson Pass – I strongly suggest that you visit the First Watt website for an orientation. In a nutshell, Nelson believes that simple elegant designs are the most worthy of his time and your money.
The speed limit metaphor refers to the fact that the buffer has no voltage gain and thus the system’s overall gain is limited to the gain of the front-end source component plus that of the power amp. Well, that brings up the question of just how much gain is needed? Obviously, a CD player with a nominal 2V line signal presents no problems at all for the buffer preamp. Most power amps feature an input sensitivity between 1 and 2 V, which mean that a typical CD player needs no additional gain in order to redline most power amps. But take a look at the line inputs on most line stages and preamplifiers. The signal is forced to undergo 12 to 18 dB or more of amplification. That’s very likely a throwback to the days when line inputs were nominally 0.2 V (tuners and tape decks, for example), which require a gain of 1.5/0.2 (assuming a power amp sensitivity of 1.5 V) or a factor of 7.5. Now, that’s about 18 dB of gain. I am certain that no CD player, whatever its pedigree, would sound its best when fed into a high-gain line-level input. Imagine the sonic indignity of first throwing away most of the signal at the volume control and then forcing it through an active gain stage to build it up again – hardly an optimal situation.
Things may be less clear with respect to a phono front end, so let’s take a look at my current setup. My Grado Reference MM cartridge is specified at 5 mV nominal output. Again, resorting to a hypothetical power amp with a sensitivity of 1.5 V means that the phono stage requires a gain of 1,500/5 or 300 to drive the amp off the cliff. That’s equivalent to a gain of about 50 dB, not a problem for my Air Tight ATE-2 phono stage or most any other phono stage. Again, no gain boost is needed from a line stage. All of this should lead to the realization that the ideal line stage may well be a buffer stage with provisions for input selection and volume control. Nelson asks the following rhetorical question: “Do I feel like the pedal’s to the metal and I’m only doing 55?” He speaks for me as well when he responds with “No - I’m listening as loud as I want and I sleep soundly at night, knowing that I’m not throwing away signal with my volume control.”
A Few Technical Details
The so-called “passive preamp,” an attempt to achieve the ideal line stage without a buffer stage, has in my experience been a sonic failure. Lifeless and comatose are two descriptors that immediately come to mind. Simply inserting a volume control between the source component and power amp ignores two important interface issues. First, there’s the issue of impedance matching. Ideally, input impedance should be high and output impedance should be low. That’s difficult to achieve without the use of an active buffer or at least a transformer. My sample of the B1 was built by Nelson - as shown in the construction article schematic. The input impedance is a function of the setting of the 25 kOhm potentiometers and ranges up to a maximum of 25 kOhm. Nelson admits that the pots on the B1 are perfectly ordinary, but that they appear to benefit from the very high impedance of the active circuit. And while he has not perceived any sonic issues with them, he points out that you are perfectly free to use your favorite volume control in this application. I would have preferred, for example, a 50 kOhm pot, or even better for my birthday, a motorized remote-controlled attenuator. If you’re into stepped attenuators, note that his favorite for a long time has been the Elna with type RN55D resistors. The actual output impedance is dominated by a 1 kOhm resistor (R104 and R204 in the schematic). That’s fairly low, but Nelson says that this value can be arbitrarily lowered down to say 200 Ohm, though he cautions that could cause problems with wideband equipment. The second interface issue has to do with the ability of a stage to drive long and/or reactive interconnects. And that’s an area that an active buffer stage excels in, though I can’t say that I specifically challenged the B1 in this respect, having simply dropped it in to the an existing system which uses fairly short FMS Nexus interconnects.
Despite the Spartan appearance of my B1, it actually offers a capability lacking in much more expensive high-end preamp designs; namely, channel balance control. The dual volume controls allow for independent tweaking of each channel and to my mind this approach represents the preferred method of implementing a balance control. It’s surprising that so few modern preamps bother with such a control. In fact, the great J. Gordon Holt once opined that lack of a balance control is nothing short of a fatal design flaw. I find it extremely useful in compensating for inter-channel gain differences and for loudspeaker sensitivity deviations. For example, rarely are loudspeaker drivers matched to better than 1 to 2 dB, and with the B1, a simple tweak of either the left or right pot can re-center the soundstage.
My unit was outfitted with an Ault +18V (70W) Model MW116 switching power supply specified for medical applications. No complaints so far, but I’m very curious to substitute a linear DC power supply such as the B+K 1671A in the near future.
The Sound of One Hand Clapping
The B1 took its place in a reference system headed by the PrimaLuna Eight CD player and an analog system that included the Kuzma Reference turntable, outfitted with the Graham Engineering model 2.5 tonearm and Grado Reference MM cartridge. The associated phono stage was the all-tube Air Tight ATE-2. The rest of the system consisted of the TEAC Esoteric MG-20 loudspeakers driven by either the Vincent Audio SP-T800 monoblocks or Esoteric’s new A-100 tube amplifier.
For starters, the B1 displaced the Vincent Audio SA-T8 line preamplifier, which in concert with the SP-T800 monoblocks had already impressed me favorably in terms of detail resolution and rhythmic drive. Well, the B1 did even better in these respects. Lots of kinetic energy. But more than anything else, rising from below the threshold of conscious perception, a single word began to form: wow! A sensation of extreme clarity and transparency took hold of the soundstage. The sonic presentation was spectacularly open and spacious. It was as if my sonic window received the Windex treatment. Transients unfolded with speed and control. Image outlines were nicely focused. Massed voices were readily resolvable, thus enhancing the spatial impression of large choral works.
This darn gadget made familiar recordings sound more immediate and less electronic. Harmonic textures were squeaky clean, with commendable purity of timbre. And forget about listening fatigue. This is a low-distortion device, well-behaved over the entire audible bandwidth, and retains its textural suaveness from soft to loud levels. Bass lines were thunderous and well defined, as you would generally expect from well-designed solid-state gear.
It became clear fairly quickly that the B1 took its sonic cues from the associated gear. It didn’t impose a particular tonal character over the presentation but rather allowed each power amp to dictate matters. Tonal differences between the Esoteric and Vincent Audio amps were readily discernible. The A-100 evinced greater lower midrange weight, as it should, being a transformer-coupled tube amplifier. Differences in treble character and bandwidth were also evident. In my opinion, neutrality in a preamplifier is usually a good thing. But if, like me, you’re craving a touch of tube sweetness and 3-D image palpability then you’ll have to get your tube fix at the power amp. The key is to carefully select either a matching tube amp or a hybrid design such as the SP-T8000, which really clicked sonically with the B1.
At this point, I was really curious to find out just how competitive the B1 was with the best line preamplifier currently known to me – the Concert Fidelity CF-080. Note that the CF-080 does employ a tube based gain stage – a single 12AU7 per channel. And I do use the best sounding 12AU7 out there for this application – the Mullard CV4003 with box anode construction. Now, some of you may object to the notion of comparing a modest DIY design to an $18,000 ultra high-end design. It sounds crazy, but I needed to know. Well, there were differences. The CF-080 possessed more palpable image outlines and rendered harmonic textures a bit more richly. The B1, on the other hand, showed a tad more immediacy and better defined bass lines. In the end, I felt that most of these differences were simply explainable on the basis of tube versus transistor sound.
Conclusion
Don’t let its modest appearance fool you: this is one fantastically effective buffer stage. And it’s not about to leave my reference system any time soon. I’m still laughing out loud over this one – a DIY product that succeeds wildly in bringing the listener a significant step closer to the source. In hindsight, that should not be surprising as the B1 buffer stage eliminates unnecessary gain stages between the source and power amp. A powerful demonstration of the “simpler is better” principle, or more accurately, less gain – less sonic pain. My advice to preamplifier manufacturers is this: be sure to dedicate at least one line-level input and output to a buffer stage. It really is better to operate at the speed limit, especially in the context of CD players.
[ 本帖最后由 davidxtb 于 2012-2-13 17:04 编辑 ] |
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