It does my heart good to witness the continued popularity of vacuum tube electronics. Despite the technical "arrogance" of solid-state designs, flaunting as they do "superior" specifications, tubes continue to elicit the most musically passionate reproduction and enchant a new generation of music lovers. And so it is, with these two headphone amplifiers at both extremes at the price spectrum. Ray Samuels’ Stealth (US$2,495) is a two-chassis affair with an external power supply. It features high-quality parts throughout, including a DACT stepped-attenuator volume control, Halco H2 film resistors, and Hovland Musicaps. The affordable (US$700) Antique Sound Lab MG-Head OTL 32 DT (ASL) comes outfitted with a tube cage, which I hide in the closet, and a handy remote control that provides both input source selection and volume adjustment. Both units share some design similarities: no global feedback, transformerless coupling of the output stage, and use of a Series-Regulated Push-Pull (SRPP) output stage topology.
Yes, I know that both of these devices are being promoted as dual purpose - preamplifier-headphone amplifier – but I prefer to focus on what they most likely do best, and that is headphone amplification. Let me explain. Both designs use two stages of amplification. In both cases, the preamplifier output is taken immediately after the first stage. The Stealth’s first stage is the simplest of all possible tube circuits – a grounded cathode circuit with a bypassed cathode resistor, while the ASL uses an SRPP input stage. I estimate the output impedance of the first stage to be about 6,000 ohm for the Stealth, while that of the ASL is given as 10,000 ohm. In my book, a line preamplifier should ideally feature a low output impedance of less than 1,000 ohm. Otherwise, its ability to drive long interconnect cables and interface with a variety of power amplifiers is compromised. As, neither of the devices under test met this criterion, my strategy was to concentrate on the complete circuit which is intended for headphone amplification.
Technical Details: Headphone Amplifier
The ASL is big on the SRPP topology, as both the input and output stages use it. A 12AU7 dual triode makes up the input stage, while a pair of 6BQ5/EL84 pentodes are used per channel in the output stage. Oh, I should mention that these are connected as triodes with the screen grid tied to the plate. Low and High Z output impedance settings are provided and may be selected via a toggle switch for matching headphone impedance ratings. In the high Z position, a 1k Ohm resistor shunts the output of the amplifier, while in the Low Z position a 33 Ohm resistor is selected. When the output impedance matches that of the load, maximum power transfer takes place, though the damping factor may suffer. For example, in the High Z position, output power into a 300 Ohm load is rated at 160 milliwatt (mW) and only 15 mW into a 33 Ohm load. The power supply is dominated by a heavy transformer and is tube rectified (5AR4), which gets my seal of approval. In my opinion, this is the only path toward true vintage tube sound.
Conveniently for my tube rolling experiments (see below), the Stealth also uses a 12AU7-based input stage, though only half of the tube is used per channel. There is some confusion over choice of input stage tube as the official web page states that: "This design decision permits the circuit to operate many low, medium, and high gain tubes that are available today from current manufacturers. NOS tubes are also available and will provide the user with even more choices. You can use 12AU7, 12AT7, 12AX7, and many more." Although it is certainly possible to press 12AT7 and 12AX7 9-pin miniatures into service, I believe, based on the circuit’s operating point, that a 12AU7 is optimum in this application. As with the ASL, the output stage also uses an SRPP circuit. However, the tube of choice here is the 6SN7GT dual triode, one of my favorite driver tubes. The output is shunted by a 10 kOhm resistor which results in a rather high output impedance estimated at around 3,000 ohm.
The ASL comes outfitted with Russian-made Electro-Harmonix 12AU7 and Sovtek EL84 tubes. The Stealth’s stock tube complement is a bit more difficult to pin down. My review sample was "juiced" with a Mullard 12AU7 and GE brand 6SN7s. Ray Samuels was also kind enough to include a smooth-plate Telefunken 12AU7 for me to experiment with. The website, on the other hand, states that "for drive tubes, the choice was made to use a 6SN7 from Electro-Harmonix." Since I have yet to develop much affection for the sound of Russian 6SN7s, this could be a problem. I personally resent paying for stock tubes that will simply be tossed away, and believe me, the Russian 6SN7 is firmly in this category.
Setting The Table
Listening sessions were conducted with both the well-known Sennheiser HD 600 and Grado SR80 cans. First of all, I wanted to be sure to obtain impressions at both load impedance extremes. The $95 SR80 is rated at 33 Ohms and qualifies as a low-impedance headphone, while the HD 600 at 300 Ohms easily rates as a high-impedance device. And second, I happen to own both which made the process very convenient. This strategy paid dividends, as you will see shortly, uncovering significant load-amplifier interactions. To say that an amplifier is best on the basis of a single load is a dangerous and misleading proposition and highlights the danger of extrapolating sonic observations outside of the actual review context.
Standard stereo recordings (read: non binaural) fail to form a realistic soundstage when listened to through headphones. The soundstage degenerates to one or more pools of sound localized within the head or at the ears. This is due to the lack of spatial clues that are generated by the head and the external part of the ears (pinna). When sound is leaked directly into the ear, bypassing the head, it is devoid of the intensity, phase and spectral clues normally used by the auditory system to map out a soundfield external to the head. There are devices that try to restore spatial clues to headphone listening by processing and modifying the signal to add a head-related transfer function. Such devices were not used during these tests. And since I did not use binaural recordings during the evaluation, there was no meaningful soundstage experience (e.g., depth) to describe and report upon. Image focus was, however, a valid observable. I could easily discern differences in image palpability with changes in amplification and recordings.
Out of the gate, the ASL quickly affirmed its all-tube signal path by fleshing out a full-bodied tonal balance. The lower midrange was portrayed with the sort of authority and weight that is typically associated with vintage tube gear. I decided to experiment early on with the High/Low impedance settings to establish the preferred setting for subsequent listening sessions. With the HD 600, High Z gave slightly better dynamics, while Low Z gave the best bass control. In the case of the Grado SR80, which entered the scene last, Low Z was my preferred setting.
The stock Electro-Harmonix 12AU7 is said to be built to match the performance of the original military-grade GE 6189. However, in the context of the already smooth and mellow HD 600 cans, this tube sounded distinctly veiled, that is to say, lacking convincing immediacy and clarity. Harmonic color contrast was diminished, and its biggest liability - a rough treble range - detracted from the overall suaveness of the presentation.
The Electron-Harmonix had to go. I happened to have on hand NOS samples of the Radiotechnique French military grade 12AU7, vintage 1956 and 1965. These are offered for sale by Upscale Audio’s Kevin Deal and turned out to sound fabulous in the context of the ASL. Harmonic colors glowed vibrantly, blossoming with life-like contrasts. Image focus improved, as did the impression of transient speed. Treble finesse was now in line with rest of range. The 1956 vintage Radiotechnique was similar in character to 1965 production lot, but slightly darker in color. Hence, at least in this application, I would give the nod to the 1965 vintage. It seemed like a good idea to also check out the US Philips brand, which is commonly available (read: inexpensive). OK, it was nicely balanced, but there was nothing else noteworthy to write home about, as it lacked the focus, immediacy, and harmonic bloom of the Radiotechnique tubes.
At this point in the proceedings, and based strictly on my auditions with the HD 600, I had reached several conclusions. My notes read as follows: Nice fat lower midrange. Plenty of harmonic integrity. Easy to listen to but lacking in clarity and detail resolution. Slight loss of transient speed blunts impact of piano sound. In other words, there was no way I was going to reach a sonic orgasm with this amplifier using the HD 600 cans. Enter the Grado SR80. The Grado is certainly rougher around the edges relative to the HD 600, but offers a brighter, livelier balance, and strong bass drive. The result in a word was synergy, defined here to mean total system sound that is greater than the sum of its individual parts. The impression of clarity and focus was strong. Bass lines were delivered with plenty of punch, rhythmic drive, and an open non-boxy quality. And most critically, it was impossible to ignore the music’s emotional content. The system sound was no longer pretty and relaxing (read: decaffeinated), but rather laced with a shot of caffeine. In case you did not know this, caffeine is the most widely used psychoactive substance in the world; the listening experience was now emotionally charged, involving the listener in the music’s passion and drama.
Overall Rating: 4 out of 5 Blue Notes when used with the Radiotechnique 12AU7 and the Grado SR80 or comparable low-Z headphones. My guess is that the entire Grado headphone line is compatible with the ASL amplifier.
Sonic Impressions: Ray Samuels Audio Stealth
First impressions were obtained with the HD 600 cans. When outfitted with the Mullard 12AU7 and the GE 6SN7GT tubes, the Stealth quickly revealed its sonic calling cards: harmonic purity, clarity in spades, and tight image focus. The sense of immediacy was much greater than with the ASL. Reverberant decay was more clearly enunciated, as layers of veiling were stripped away. At least with the HD 600, the Stealth made the ASL sound like a low-resolution device. The ebb and flow of microdynamic nuances was communicated with greater conviction, recovering more of the music’s dramatic impact. The tonal balance was closer to neutral, lacking the rich lower midrange of the ASL. On the minus side, and this represents a significant fly in the ointment, bass definition could be better. The midbass was under damped, detracting, for example, from the tightness of double bass fundamentals.
A round of tube rolling followed, all based on results with the HD 600. I noted that the internal construction of the French Radiotechnique 12AU7 was similar to that of the British Mullard. However, I did find that as good as the Mullard was, its sound was eclipsed by that of the Radiotechnique tube, being smother overall, with better highs. The smooth-plate Telefunken proved to be even sweeter sounding and more luxuriant than the Radiotechnique, offering a slightly superior midrange and improved transient speed. Often described as mellow and dark sounding, that was superficially the case in this application as well. But to its credit, it offered a winning combination of immediacy with low-level resolution, and proved to be the most compelling musically, bringing me closest to the emotional content of the music. Next was a Philips brand, made in Great Britain, which appears to be identical internally to Mullard. Not surprisingly, it sounded very close to Mullard, taking into account differences in production runs. I thought I would give the Electro- Harmonix another chance. And again, it failed to distinguish itself. Problems were noted with diffuse image outlines, some veiling, and harmonically the sound was on the dry side of the real thing. Finally, the US Philips 12AU7A, was pressed into service. It sounded slightly out of focus, pleasant and bland; not sufficiently incisive.
In an effort to improve bass definition, I decided to roll in my favorite 6SN7GT type, the RCA VT-231, while keeping the Telefunken 12AU7 in the mix. That turned out to be a move in the right direction, nudging the purity of harmonic textures closer to heaven, while at the same time firming up bass lines. And although the music’s rhythm and pace improved, bass lines still lacked the sort of punch and pitch definition I had come to expect from a high-end system. I was left with the conclusion that the bass issue was a consequence of the unit’s high output impedance and resultant poor damping factor.
So far, it was clear that if the HD 600 were the only headphones on the planet, that the Stealth would make for a spectacular coupling, bass damping issues aside. It was time to throw something completely different at the Stealth: a low Z headphone. I expected issues with dynamic headroom due to the impedance mismatch, but it was in fact shocking to find out just how poorly the Stealth performed with the SR80 cans. The Grado was completely unrecognizable, as if it had a close encounter with a blood-sucking vampire. Its life energy was completely sucked dry. It sounded bland, polite, and devoid of dynamic nuances. And anyone who has listened to the SR80 will recognize just how sick that is. The moral of this story is that high output impedance devices are very load sensitive and require a careful audition with prospective headphones to establish compatibility.
Overall Rating: 3.75 out of 5 Blue Notes when used with Sennheiser HD 600 cans or similar high-Z models and tube recommendations given above.
Antique Sound Labs MG OTL 32 DT
Type: Tube preamplifier and headphone amplifier
Ray Samuels Emmeline The Stealth
Type: headphone preamplifier
Antique Sound Lab
Emmeline By Ray Samuels Audio