March 2009

conrad-johnson ET250S Hybrid Stereo Power Amplifier
The best of both worlds.
Review By Tom Lyle
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  I have been a conrad-johnson fan ever since I purchased a tubed PV-10A preamplifier somewhere around 1990. OK, since I've never owned a PV-5 some might consider me a Johnny-come-lately, but still, ever since then I have had more than a few positive experiences with a number of their solid-state and tubed preamplifiers and power amplifiers. In general, what I've discovered is that C-J products have a midrange to die for (well, or at least within risk of doing so, since no audio product is worth that kind of sacrifice), and a very lifelike and musical character. But in their products from the early 1990s I sensed a hint of a burnished euphonic "golden glow" throughout their frequency range, especially in their more affordable components. In more recent times that slight patina has evaporated, and has been replaced by an ultra-transparent sound that manages to embrace the best of solid-state and tube worlds, regardless if they were designed with glass bottles or transistors. Even though there are sonic nuances between C-J's solid-state and tube units, they all maintain a lifelike, musical personality that always serves at the pleasure of the source. This is especially true when one considers their more ambitious products such as their ACT2 series of preamplifiers, components that should be regarded by all who have ears as state-of-the-art. Even their current line-up's less expensive solid-state "integrated", the CA-200 control amplifier has a sound that approaches a near ideal mix of the best of the solid-state and tubed attributes (see Todd Warnke's March 2007 review). In view of that, it is surprising that conrad-johnson proclaims in their advertising campaign that the ET250S hybrid power amplifier's tubed input stage supplies "harmonic character" whilst it's solid-state output stage "produces the muscle". If only other things in life were so simple. Are conrad-johnson's solid-state amps deficient in harmonic character? Do conrad-johnson's tube amps lack muscle? In both cases, absolutely not.

Now Familiar
The ET in the amp's moniker denotes Enhanced Triode, in reference to the two 6992 tubes that provide voltage gain in its input section. These tubes are located front and center in the now familiar style of the latest conrad-johnson product's curved recess of its front panel. There are three flat, clear, semicircular plexiglass plates — two with openings for the tubes and one covering the top of the tubes. The top plate is removable via a provided tool, and as a nice touch, four pink rubberized tube damper rings are supplied. The amp's high level of fit and finish is evident in its 0.25-inch gold colored aluminum faceplate and its black cabinet with 1-inch heat sinks lining the side panel. On the rear of the cabinet are a pair of heavy-duty gold-plated speaker posts that accepted my Audioquest binding post wrench, a pair of gold-plated unbalanced RCA inputs, an IEC AC jack, a switchable12V input trigger, and a pair of round fuse holders. Besides the partially naked input tubes on the recessed front panel is the one inch in diameter power switch. The circumference of the switch lights when in the on position. Peeking inside the cabinet reveals excellent electronic craftsmanship, sort of like an electronic work of art for those inclined to think those types of things.

The ET250S is built with a zero feedback design that conrad-johnson says is judiciously employed to allow for, among other benefits, finer the gradations of microdynamics and to accurately reproduce the decay on musical transients. They go on to say that while increasing negative feedback decreases distortion, their careful design of the circuits, high parts quality, and their power supply design make zero feedback design achievable. The solid-state output stage of the ET250S uses what conrad-johnson claims are high-current bi-polar devices that have been selected for their extremely low output impedance and their ability to handle a great deal of current. C-J also claims that the "oversized" power transformers, "massive" power storage reservoirs with high-speed capacitor shunts that are largely responsible for the amp's excellent transient response. In their circuits they also feature, among other things, high-quality internal wiring, polypropylene, polystyrene and Teflon capacitors, except in the main power supply storage where high-speed electrolytics are used because of the extremely high capacitance required.

Placed
I used the conrad-johnson ET250 in my main system. I "placed" the amp (at about 65 pounds, it was more like I lifted with my knees, not my back) on the second shelf of the Arcici Suspense equipment rack. Some say that power amps are better served on a rock-solid surface such as a weighty amp stand, but since a PS Audio Power Plant takes up the fixed bottom shelf of the rack I wasn't in the mood to re-route the power cables of the entire front end of the system. Plus, the isolation provided by the air-cushioned hanging shelf wasn't at all detrimental to its sound — at least I thought it sounded slightly better than when the amp was placed on the commercial-grade carpeted floor. The amp's power cable was sent directly to a dedicated 20-Ampere line with a Virtual Dynamics wall receptacle. The analog front-end of the system consists of a Basis Debut V turntable with a Lyra Helikon phono cartridge mounted on a Tri-Planar VI tonearm. The phono preamps were either the outstanding on-board phono section of a Balanced Audio Technologies VK-3iX preamplifier or an excellent-for-the-price Lehmann Black Cube SE. When listening off-axis I use a modest but quite serviceable NAD 4155 AM/FM tuner. Arcam and Oppo players spin aluminum digital discs, the resident speakers are Sound Lab Dynastats, and late in the review period a pair of Thiel CS2.4SEs. Cables used throughout the system are by MIT and Virtual Dynamics. The amplifier reverses polarity, so I simply swapped the negative and positive leads on the loudspeaker cable. No fuss, no muss.

Cruising Altitude
Once the amp reached cruising altitude after its relatively lengthy break-in period, it was clearly apparent that the ET250S had no problem driving the somewhat difficult load presented by the Sound Lab electrostats. Yes, the output stage of the ET250S is solid-state, and that, of course, should make life quite a bit easier. But in the past I have run into some problems with less than capable amps — they have the potential of not only decreasing the speaker's treble extension, and although not as significant, the system's overall gain. There was very little evidence of this occurring when the C-J was in the system.

I could easily use Enjoy the Music.com's entire server space filling this review with all sorts of audiophile hyperbole regarding the ET250's wonderful sound, or lack thereof. Basically, it had all the neutrality and musicality as one should expect from an amp in its price range (and quite beyond). The hybrid C-J did not have the brilliantly sharp transient response that some lesser pure solid-state amps posses, and conversely, it did not sound at all sluggish as some lesser pure tube amps might have. So, did the conrad-johnson ET250S sound as if it was powered by a combination of both tubes and transistors? Yes and no, and I mean that only in the most positive of ways. I have much respect for Drs. Conrad and Johnson, and I assume that they know how to get the best out of both valve and solid-state circuits. So, I wouldn't classify the sound of the ET250s as having an abundance of tube warmth or solid-state snap, but appropriate degrees of both. And as I said, contrary to the simplistic description of the sound of the ET250S that conrad-johnson claims in their ad, neither its vacuum tube nor solid-state character called attention to itself. Again, I mean that in a positive way.

On Claudio Abbado's reading of Mahler's Seventh Symphony on DG, it was as if I was looking though a transparent sonic window at the Berlin Philharmonic laid out before me. I realize that not everyone is as taken as I am when it comes to Mahler symphonies, but that hardly matters when objectively listening to the reproduction of the orchestra going full tilt, or as Mahler is wont to do, when he reduces the orchestra to quadruple pianissimo for short moments during several passages. The amplifier's midrange was extremely neutral and silky smooth to boot, as was the entire extent of its high and low frequencies. Added to this was that I could seem to hear every iota of the decay of the hall sound. This piece showed the prowess of both the amp's macro- and micro-dynamic capabilities as well as its excellent transient response. Tympani whacks during the 3^rd movement's cartoonish waltz were excellently reproduced, with the sound of the mallets striking the skins, the midrange resonance of the drum, and its room-shaking low-end heft. When the piece quickly shifted to the quiet portions of the movement, every instrument's subtle shadings were clearly audible.

The ET250S' neutrality, and by implication, its transparency, was on display in full measure when I compared two different pressing on two different formats of Yes' Fragile album — the CD re-issue on Mobile Fidelity and the Analogue Production's reissue on heavy vinyl. Given its vintage, both sounded as good as a multi-tracked recording could, and I assume both were produced from the original master tape. But it was easy to hear that the Mobile Fidelity CD dug deeper into the signal that was present on the original tape. Those who know me would assume that I would prefer the LP pressing, and it should be quite surprising that I preferred the CD given that the analog front end costs about ten times as much as the digital rig. But the amp's high level of transparency made it easy to hear that even though both formats had a midrange that was unexpectedly similar, the mids were more articulate on the CD without sounding too forward (rock ‘n' roll forward — yes, CD glare forward — no). Even though the LP had a typically pleasant analog-like warmth, on the CD Jon Anderson's vocals were more intelligible, yet at the same time better integrated into the song's complex structure. Also, the ET250S' powerful bass was on full display here, and showed the CD's low-end went significantly deeper and was more powerful than the LP. Chris Squire's Rickenbacker bass guitar and Bill Bruford's kick drum shook the window frames of the listening room.

Of course the ET250S' soundstaging prowess is definitely worth mentioning. Whether it is because tubes are in the signal path or just because the amplifier is simply designed correctly, the ET250S is a soundstage champion. But what I do not want to infer is that the amp generates an overblown soundstage via tube euphony. When the recording was up to it the soundstage was naturally stretched across the horizontal span of the speakers, creating a believable soundstage that was drawn to scale. Instruments and groups of instruments were placed in their proper location within this ‘stage, extending well behind and to the sides of the speakers when appropriate. I feel uncomfortable talking about "imaging", since a more accurate term would be instrument "placement" within the soundstage. It was simply a reproduction of what was captured by the microphones during the original event.

When I started writing this review, I told myself I was going to try and steer away from the cliché that the amp was a perfect blending of the attributes of tubes and solid-state. But I have to eat my words, and not only because the ET250S displayed the positives and did away with the negatives of both. Great solid-state amps have no problem sorting out complex passages, and so should great tube amps — but what the ET250S goes one further is by increasing what I like to call "dynamic distance" between instruments – when the amplitude of two or more simultaneously played instruments are different, and the component accurately displays this difference. The ET250S has the tautness and macrodynamic slam with plenty of power to spare; and microdynamic shadings that give life to the music. Of course no amp is perfect, so I am forced to mention that the amp's soundstage perspective was as if one's listening seat was a bit as if in Row M rather than the perfect Row H. I felt as if I was placed just a little further back in the hall than I'd like. The other criticism is that the amp only has unbalanced RCA inputs. Deal with it.

Considerations
One would assume that the raison d'être of a power amplifier that combines a tube input stage with a solid-state output stage is to take advantage of both design principles. Hopefully, the designer of said amp would steer clear of any disadvantages that often crop up from combining the two. It is obvious that conrad-johnson have met this challenge with their ET250S hybrid power amplifier. But what I have discovered by living with this component for a while is that they have produced not just a great hybrid power amplifier — but a great amplifier. It allows not only a clear window into the sound (or lack thereof) of rest of the system, but more importantly, a window to the source. And if the source is up to it, it passes this signal to the speakers with as much objectivity as one could hope for, with enough power to drive just about any sanely sized loudspeaker. 

Specifications
Type: Hybrid stereo amplifier
Power Output: 250 wpc from 20 Hz to 20 kHz (400 wpc @ 4 Ohm)
Tube Complement: Two 6922
Sensitivity: 1.85V to rated power.
Frequency Response (at 10 Watts): 20 Hz to 20 kHz (+/- .25dB)
Hum and Noise: 104 dB below rated power 
Input Impedance: 100 kOhms
Dimensions: 16.125 x 19 x 7.188 (DxWxH in inches) 
Weight: 65 lbs.
Warranty: Three years parts and labor
Price: $7500

Company Information
conrad-johnson design, inc. 
2733 Merrilee Drive 
Fairfax, VA 22031

Voice: (703) 698-8581 
Fax: (703) 560-5360
E-mail: custserv@conradjohnson.com
Website: www.conradjohnson.com