

The Aurora: built by Gary Dahl, current sources by Gary Pimm, metalwork by Josh Stippich,

rosewood chassis by Phil Bruckbauer, and circuit design by Lynn Olson.

The Amity, Raven, and Aurora

by Lynn Olson I've been designing loudspeaker systems since 1975, but I didn't move into electronic design until 1996, when I started the Amity project. I'd been reviewing various amplifiers for Positive Feedback magazine since 1991, and was intrigued by the whole SE-DHT phenomenon - writing the first US review of the Ongaku left an impression as well. I auditioned many SE-DHT amplifiers after that, but in all honesty, none came up to the mark set by the Ongaku, although some came pretty close. I met the designer of the Ongaku a couple of years later at the CES, and Kondo confirmed my impression that the circuit of the Ongaku wasn't anything remarkable. It was the implementation - the all-silver signal path, especially the hand-made silver coupling cap and the all-silver output transformer, that gave the Ongaku its distinctive clarity and insight. Kondo-san said that building one on the cheap would just result in a quite ordinary SE amplifier - the Ongaku could be thought of as the ultimate parts-tweaker amplifier, a design that would sound completely different if all parts weren't exactly as specified. Anyone that listens to SE amplifiers over any length of time is going to become aware just how different they sound from each other. Transistor amplifiers, at least if competently designed, sound more or less alike. Vacuum tube amps that replicate the design philosophy of the Fifties (Class AB PP pentode, quasi-Williamson, 20dB of loop feedback) also have a characteristic "group sound" - and of course different than transistor amps. These two classes of amps are what most audiophiles the world over had heard until the re-introduction of SE-DHT amps in the early Nineties, which created turmoil and dissension in the industry and trade magazines that lasts to this day. Joe Robert's Sound Practices magazine promoted the combination of (very) low power, direct-heated triodes, single-ended circuits, and efficient speakers. By the mid-Nineties, SE-DHT almost became a religious cult, diverging rapidly away from the mainstream universe of hyper-expensive, super-power amps, low-efficiency speakers, with the whole industry driven by reviews in the Big Two magazines. Politics combined with a technological package, in other words. As reviewer of the Ongaku (and designer of the 92dB efficient Ariel) I was drawn into the SE-DHT debate as well - but I never bought the religious angle. I agreed more speaker efficiency was a Good Thing, but didn't care for horn coloration (and still don't). The superiority of direct-heated triodes, by measurement and audition, is clear-cut, but there are design challenges related to filament power (and isolation) and serious demands on the driver stage - as well as coming to terms with the modest power, fragility, and considerable cost of direct-heated triodes. Where I parted company with the SE-DHT crowd was single-ended circuits; for the output stage, at least, there are drawbacks to SE circuits compared to Class A PP (Class AB PP is another matter, and sonically is the worst of all). But a little research disclosed that PP amplifier design had essentially stagnated since the mid-Fifties; in fact, there was a period between the introduction of the Williamson in 1948 and the mid-Fifties when there were essentially no new designs at all! More surprisingly, the mainstream high-end industry was just endlessly recycling trivial variations of Fifties designs over the last thirty years. Almost all products in the mainstream high-end market used the same stale tubes - 12AX7, 12AU7, 12AT7, 6DJ8, EL84, EL34, KT88 - and the same stale circuits - Williamson, Dynaco, Acrosound, or Marantz derivatives (which are all very similar). No wonder SE-DHT was making a stir - it was thirty years overdue! By contrast with the US market, the Japanese had a wild variety of tubes and circuits, and had been playing with them since at least the early Seventies. SE, PP, OTL, hybrid, exotic ultra-rare tubes, specialized radio-transmitter tubes, circuits that ran in positive-grid bias, transformer coupling, anything you could imagine. In addition to the fun articles in MJ magazine, I'd been been reading Norman Crowhurst articles for many years, and I wanted to try a circuit that had the lowest possible intrinsic distortion, especially into typical speaker loads. As a loudspeaker designer, I knew all too well that expecting speakers to be resistive was a vain hope - in fact, the best speakers are typically the most reactive and hardest to drive! I wasn't willing to cherry-pick speakers that were "SE-friendly" - to me, that's another way of saying "speaker-unfriendly." Speakers, by their nature as electromechanical transducers, are not just reactive, but store resonant energy for tens of milliseconds and transmit it back to the amplifier - and all speakers do that, electrostatic, planar, horn, single or multiple-driver. Speaker drivers are intrinsically resonant. Thinking along these lines - an amplifier that remains linear when driving a complex, nonlinear, and poorly-defined load - leads to Class A PP, preferably with direct-heated triodes instead of triode-connected pentodes. (Tests in Vacuum Tube Valley magazine shows DHTs have about one-half to one-third the distortion of triode-connected pentodes, not a small difference.) The reason is clear when you look at the composite curves of Class A PP triodes: the grid-lines are essentially straight and parallel, so when the load-line opens into an ellipse (becomes reactive) there is no change in distortion. BY contrast, all other other circuits and devices ... SE, or PP with Class AB, or pentodes, transistors, MOSFETs, etc. all have grid-lines that are curved. Class AB pentode (or transistor) is the worst, with high curvatures centered around the zero-signal region. In others words, the first watt is the worst watt ... but it only appears with reactive loads! Unfortunately, speakers are reactive: they can't help it, they're bandpass filters, and they also store energy for relatively long periods of time (milliseconds) and reflect it back to the amplifier. With most amplifiers, the reactive and delayed energy from the speaker driver greatly increases the amplifier distortion. This is why amplifiers can sound so different with different speakers. With SE, the overall curvature of the grid-lines is still there, but is quite mild in the zero-signal region; the first watt really is the best watt. This is a critical point; music spends most of the time at low levels, with brief peaks that jump to 10 to 14 dB above the average. On a statistical basis, the greatest percentage of the time is in the 1-watt or lower region, at least if the speaker has any kind of reasonable efficiency. But ... if you want linearity at low and high levels, near-total immunity from reactive loads, only PP triodes operating in very deep Class A can deliver straight and nearly parallel grid lines. The very deep Class A operating point keeps the circuit well away from the undesirable Class AB region; this is only entered when the load becomes a near-short, one that would current-limit and clip a SE amplifier. Another goal is getting driver-stage coloration and distortion out of the picture ... quite different from the distortion-cancellation technique seen in some SE amps. I've never felt good about dissimilar devices maintaining cancellation over a wide range of signal levels and frequencies, and most of all I wanted an amplifier with a stable harmonic signature. Since I worked with cancellation techniques back when I patented the Shadow Vector Quadraphonic Decoder in 1975, I was all too aware that the "gotcha" of any subtractive techniques is the dirty nature of the residual, which calls attention to itself if the "dirt" changes character with different types of music. I am beginning to suspect the ear compensates for amplifier distortion if it is simple in nature and stable (which is another reason for the excellent subjective quality of SE circuits), but does not like distortion that is constantly changing. Unfortunately, conventional phase-splitters have asymmetric output impedances, an asymmetric Miller capacitance, or sensitivity to B+ voltage, so the cancellation of the even-harmonic distortion terms (2nd, 4th, etc.) bounces up and down with the program material. The instability of the distortion characteristic is very likely the source of the usual PP "fog" and vagueness compared to the more direct and immediate SE sound. To solve this problem, we need to go all the way back to classic Thirties-style interstage-transformer coupling. For the lowest distortion, the drivers need to have essentially horizontal load-lines, i.e. a very high impedance load relative to the plate impedance ... preferably more than ten times higher.



The Amity at night

The only way to accomplish this is some kind of dynamic load ... and the extra B+ headroom that requires ... or choke or transformer loading. Since I also wanted highly symmetric phase splitting that was invariant with signal level, I chose the interstage-transformer, and sought out the classic UTC and Sakuma amps for inspiration. My personal esthetic favors wide bandwidth, so the Lundahl transformers presented themselves as high-quality choices. In the PP world, high-frequency balance is more rare than you would expect; many expensive transformers have asymmetries in the capacitive balance, and this has a disastrous effect on HF distortion. It doesn't do any good to have precision-balanced tubes if the transformer itself is out of balance. Another difference between this amplifier and the "classics" is of course much wider bandwidth than the "old days" in the Thirties when these circuits were last used. Back then, no wideband sources were available (with the exception of Major Armstrong's Yankee FM network). Everything else was both narrowband and noisy: intercity AM network radio (carried by Bell System land-lines), shellac 78rpm records, and optical sound-tracks for movies. Modern expectations for 65 to 100dB S/N ratios and 20Hz to 20kHz bandwidth simply didn't exist. Recycling antique parts and antique circuits will, of course, deliver vintage sound. That wasn't my goal. I wanted the unmatched linearity of triodes and transformer coupling, combined with modern bandwidths of 15Hz to 50kHz. This is where modern transformers and carefully selecting low-plate-resistance tubes makes a difference. Transformer bandwidth is improved by having a low impedance on the primary, secondary, or both. One reason that interstage transformers are harder to design than a conventional output transformer is that impedance on both primary and secondary is high, while for the output transformer at least the secondary is low. Since the secondary is ideally unloaded in an interstage application (this gives the lowest distortion for a triode), the only good way to get satisfactory bandwidth from the interstage transformer is select a driver tube that has a low plate impedance. Tubes that fit this application are the 5687/7044/7119 family or the Russian 6H30. (Although the familiar 6DJ8/6922/E88CC has low plate impedances, it also has moderately high third-harmonic distortion and limited output swing, which make it inappropriate for a PP DHT-triode amplifier.) Moving on to the main B+ supply, I first tried a 5R4-GY rectifier (a traditional choice for 300B circuits), but was dismayed with the arc-overs and poor reliability in several examples. Maybe they were old and weak, but this sort of failure should never happen in the first place. By contrast, TV damper diodes, including the New-Old-Stock 6C*3 family and the new Svetlana 6D22S, have more-than-ample peak curves, and derating for continuous use gives more headroom in current and voltage than the traditional tube rectifiers seen in 2A3 or 300B amps. The low voltage drop (15V), huge peak currents (2A), and slow warm-ups (30 seconds) are just additional bonuses. Matt Kamna also demonstrated a technique for zooming in on the waveform on the power-transformer secondary (about 10V/div on the scope screen). The rough appearance around the zero-crossing was very obvious with solid-state diodes. HEXFRED's gave a small improvement, but conventional tube rectifiers looked much smoother, and the TV damper diodes were by far the smoothest of all. So even in low-current preamp applications, TV damper diodes give the least noise. I know from experience in the Tektronix Spectrum Analyzer division that it's much easier to eliminate noise at the source than filter it afterward. If there was an even quieter device, I'd use that, but as far as I know, TV damper diodes are the quietest from the viewpoint of switching noise. Considering that the main B+ supply is switching five hundred volts, this is not a small consideration, since switch-noise is radiated in all directions, into the B+ supply, the interior of the chassis, and back into the power cord.



Another View, showing VV32B's and VR Tubes

A deluxe feature, which was easy to add, is the Voltage Regulator (VR) tube shunt-regulation for the driver. I wanted to drop 270 volts, so why not regulate at the same time? The noise of VR tubes is only 1mV of very smooth broadband noise, while the more common Zener diodes have 3 to 5mV of spectrally nonflat noise, occasional LF bumps and pops from "popcorn" noise, problems with temperature coefficients, and a huge amount of grossly nonlinear capacitance. Zener diodes need a lot of filtering and additional circuitry to isolate the problems. By contrast, VR tubes need no additional circuitry at all - just keep them away from capacitive loads. Not only are old-timey VR tubes quiet, they're nice to look at. The purple glow of the OC3's is an subtle "ON" indicator. When the VR's light up after the lengthy 30 sec warmup from the damper diodes, you know the amp is well and truly running. The original 1997 Amity used the simplest isolation between the VR tube and the main B+ possible; a pair of 2K 20-watt power resistors in series with a 100 uF filter cap between them. This simple RC filter worked well and was essentially unbreakable. The latest version, shown when you click the diagram, features current sources for better isolation between driver and output stage. Either version works well; the choice is yours.

Click for full schematic diagram

Although the Amity is a beautifully simple design, it makes severe demands on the preamp. The preamp has to drive the combined capacitance of the interconnect cable, the input transformer, and the Miller capacitance of the input tubes. This can easily add up to 200 to 600pF of capacitance, depending on how long your cables are and if you choose to use a 1:2 step-up in the input transformer (which quadruples the 60pF Miller capacitance of the input tubes). In all honesty, this is difficult load for most tube preamps, with 12AX7 cathode-follower preamps falling down the worst. (1mA cannot drive 600pF!) Preamps with 12AU7 cathode-followers aren't much better, sounding very "tubey," rolled-off, and old-fashioned. (Passive preamps cannot be used with the Amity since the input transformer must be driven with less than 600 ohms source impedance. A passive preamp using a 10K pot presents 2.5K source impedance to the power amp in the -6dB position, which will cause a significant decrease in transformer bandwidth. No harm to the Amity will result, just degraded sound quality.) I borrowed many different preamps, bought some of them too, sold them at a loss, and just about the only one that sounded decent was a borrowed Jeff Rowland Consonance preamp. The A/B test was simple: I had a very high-quality DAC using the Burr-Brown PCM-63K converters, and I would compare a direct connection to the Amity versus a preamp. A preamp compared to a piece of wire - awkward on many CD's since the only volume setting was full up (no passive volume control allowed, remember). Still, it was obvious that most preamps added odd-sounding electronic colorations, as well as more serious deficits that flattened or removed qualities of air, space, musical textures, and an intangible quality that I call vitality, or immediacy. Most tube preamps, in fact, were downright dull and flat-sounding. The Amity is transparent enough that with most preamps, you could never hear the Amity - just the preamp, mimicking the colorations of a lesser-quality power amp. When the preamp was removed, though, the truth was out - it was the preamp, not the power amp, that was adding the amp-like colorations. A pretty disappointing state of affairs, and I didn't want to mindlessly copy the Jeff Rowland, although I appreciated the elegance (and boldness) of transformer-coupling a high-speed opamp in a preamp, thus keeping the solid-state electronics happy by filtering off ultrasonics from the CD source. So it was time to try my hand at designing a linestage preamp. I dreamed up the original Raven concept some time around 1998 - click the picture for the full schematic.

Click for full schematic diagram