Graaf GM 200 OTL power amplifier
Jonathan Scull, September, 1997
I'm always eager to fulfill my prime Stereophile directive: "To go where no audiophile has gone before," as JA often quips. As it happens, I've long suffered an itch to audition OTL (output-transformer–less) amplifiers, wondering how eliminating the output transformer might affect the sound. Enter the Graaf GM 200, with nothing but wire between its power tubes and the crossover.
Of course, nothing's free; eliminating the output transformer (and its distortions, claim rabid OTLers) engenders other problems. For instance, the primary purpose of an output transformer (the bad puns never stop) is to reduce the intrinsically high source impedance of power tubes to one more appropriate for the amp/speaker interface. This accounts for the thicket of PL504s in the GM 200: each doubling of the number of tubes halves the total source impedance. And, as you might guess, choosing the right speaker is of great concern.
Importer/distributor Gary Kaye counters that while the amp sure does run hot, no arguing about that, an entire set of 32 PL504 output tubes costs an almost-reasonable $500. And, he added enthusiastically, "They're easy-to-source TV tubes with an expected lifetime of around 10,000 hours!" Unsurprisingly, Gary also reminded me that nothing's free; output transformers have their own well-documented problems. Too true.
Arte e industria...
I even found an item on the GM 200 while flipping through the magazine Graphis at Barnes & Noble ("Industrial Design Highlights," Jan./Feb. '97, No.307 Vol.53). And no wonder—the amp is, as they say, drop-dead gorgeous, a single-chassis stereo unit somewhat smaller than you'd imagine from the photos. On paper the eye is drawn to the bristling carpet of power tubes. But PL504s are actually rather petite, and the whole affair, when seen in the metal, is beautifully proportioned and takes up surprisingly little room.
Nevertheless, at just over 66 lbs it proves a hefty two-person lift. The polished roll-bars fore and aft of each set of 16 push-pull pairs of output tubes provided a useful way of hauling the amp around our listening room. (The hoops are meant to locate and support the tube cages, of course.)
When seen head-on, the uncaged power tubes resemble the exposed top-end of a racing engine. But let me give you a serious warning about running the amp without its cages: If you touch the anode at the top of an exposed power tube and the chassis at the same moment, you'll frolic'n'cavort to the tune of 150 Italian volts. For those less inclined to risk, the amp, with its polished tube cages in place, remains an icon of elegant industrial design.
Why the PL504 output tube? Manual: "This tube is able to supply a high value of current not obtainable with the most blazoned [sic] of tubes." I suppose Graaf is in a position to know about such things; they've been making amplifiers of one kind and another for almost 15 years.
On the rear apron I found two sets of high-quality WBT gold-plated 5-way binding posts, and a choice of balanced or single-ended inputs. The IEC mains-in is protected by a whopping 20 amp Slo-Blo. Set into the rear panel are a pair of test-point receptacles for adjusting bias and DC offset.
If you go in balanced with XLRs, the input signal traverses an additional "class-A Buffer Separator" module using a single 12BZ7 per side, then to a single 5965 per side to split the phase for push-pull operation. This stage also serves as the input when running single-ended. I got the best sound by avoiding the additional tube and circuitry of the "Buffer Separator" module and going in single-ended.
The voltage gain to drive the power stage is derived from a pair of tall, skinny EFL200 double-pentodes per side, mounted inboard of the squat 5965s. There are no solid-state devices in the signal path, or in the offset and bias controls. Power output is signaled by a pair of Telefunken EM81 Magic Eyes, one mounted each side of the industrial-strength power switch. In the dark, the fiercely glowing power tubes and jumping green Tellies are quite a sight.
The amplifier is a fully DC-coupled OTL (output-transformer–less) and OCL (output-capacitor–less) design. Scanning the complete and very technical manual (including parts list and schematics), I found the following: "The use of only one high-quality capacitor in the input stage avoids any unbalancing of the offset caused by undesirable DC voltages unintentionally produced by the preamplifier."
Since there's no blocking capacitor on the output, protection circuits are vital to avoid exploding tweeters and other speaker-related mishaps. As a result, firing up the amp is accompanied by a cascade of chattering relays (plug-in modules for easy field servicing). The circuits are protected by two current-limiting systems, and there's another protection circuit on the output. Importantly, the first limiter works on the power-tube filaments, thus improving longevity. All circuits are deactivated by a timed relay during normal operation.
Protection goes down to board level on the output tubes. Here's Graaf's somewhat idiosyncratic explanation: "In the soldering part of the output circuit board there are 1 ohm fuse resistors, connected in series to the cathodes of the power tubes. In case of tube trouble these resistors will break themselves and light an LED diode on the upper part of the circuit board near each power tube."
If a couple of tubes do go down, you can still operate the amp and enjoy music. Total power levels are somewhat reduced this way, but it's actually hard to tell the difference. How do I know? A cranky early-production GM 200 blew its way through three power tubes and shut down one channel seemingly at random. This beat and scruffy demo unit was sent for a brief compatibility check with the Joseph Audio RM-50s used as evaluation tools in the review. (This speaker's relatively high impedance curve—it never drops below 7 ohms—makes it eminently suitable for use with OTLs.) The second unit, an up-to-date example from current production, proved altogether better-sounding and behaved like a gentleman.
The amp held its bias and offset well. However, one evening I was taken
aback by a suddenly grainy and rather bright treble. I quickly found the
culprit: a touch of DC drift in one channel. A few moments later, tutto
Its overwhelming sonic characteristic is a very special clarity and purity of presentation. Clarity divorced of musical context can, in my experience, sound analytical and hard. But I found myself enthralled by the Graaf's openness and lack of grain or mechanical artifact. It was easy to feel "close" to the music in that special way I've so far experienced only with single-ended amps.
This fundamental clarity permitted a wonderful sense of transparency and air to develop. While the presentation of the GM 200—in cahoots with the RM-50s—was generally soundstage-forward, that soundstage was always transparent to its deepest corners and beyond, and absolutely as airy as any audiophile could ever hope to experience and enjoy. We're talking Italian sensuality here...la dolce vita.
(Credit must be given to the YBA 6 Chassis preamplifier for its outstanding ability to generate the most enormous, detailed, and pellucid soundfield of all the preamps on hand. The synergy was undeniable. Graaf's own $5500 13.5B tubed line-level preamp was no slouch either. Its softer focus and voluptuous, sexy presentation mated superbly well with the GM 200's clear, fast, powerful sound. Overall, I preferred the more extended, linear, and neutral YBA.)
The next Major Concept is the way individual performers "couple" to the soundfield in the listening room. This the GM 200 managed with great style and panache. On good recordings, each artist existed in a unique, beautifully developed, open surround of air that coupled to the other performers and the larger soundstage in a natural and coherent manner.
And one could never achieve such sparkling performance without the GM 200's hurtling, unimpeded speed. Think Mille Miglia: Jenks and Moss roaring out of Brescia on trailing throttle oversteer. Think Ferrari howling down the Mulsanne straight at Le Mans, two-thirds through and still accelerating at over 220mph. The blisteringly fast presentation made for a dynamic, impactful sound, impelled by a searing pace and razor-sharp timing. Twin turbos, no waiting.
"Fast" sound might be associated with a thin, analytic presentation. Not so the GM 200; it delivered a full range of seductive tonal colors and harmonics within an appreciably wide and linear power band.
Listening to Cassandra Wilson's Blue Light 'Til Dawn (Blue Note CDP 7 81357 2) was a revelation. Wilson shows wit, innovation, and style when refashioning the classics, and her own songs are quite superb. On "Sankofa" I noted a seductive openness in the mids and above, linear- and smooth-sounding right to the top. Wilson works three separate vocals parts; they harmonize and interweave with each other like dolphins skimming the ocean.
Wilson has a wonderful range, and in this set piece exults in her talent. What struck me was the singular way in which all three vocal lines came across as distinct entities of timing, rhythm, harmony, and, most of all, timbre. Take "I Can't Stand the Rain," featuring Chris Whitley on a National Resophonic Guitar. His expressive, fast-paced, Fahey-esque riffs bloomed out into space with good focus at the instrument's "launch plain." Wilson's soaring vocals emerged stage center, a lightly tapped foot marking the beat. Separate integrities flowing together, creating a single acoustic construct.
Try Chesky's new The Desmond Project, by the John Basile Quartet (JD156). Basile's wonderful musicianship, and that of his group, are without peer. I can listen to this recording all day. I have listened to it all day! "My Funny Valentine" struck me as familiar and yet totally fresh. The drumset shimmered in a vibrant cushion of air. The naturalness of the initial transients and the follow-on impact startled me; the palpability was of a very high order. While the bass was powerfully acoustic, it was betrayed by a mild thickness in the bottom octaves. I found myself largely undisturbed by this. Musically, it worked.
So how did we make out with the Joseph Audio RM-50s? Let's start with Susanne Vega's new Nine Objects of Desire (A&M 31454 0583 2) for its stupendous low-frequency content. In the likeably vapid "Headshot" the bass goes down powerfully—deep, ambient, room-energizing—then hits bottom. But rather than dropping away, it bloats out ever so slightly. The net result can sound a bit thuddy on some material, but at the same time quite powerful. The overall effect worked rather well at conveying the heft and weight of the bottom octaves.
In fact, the bass could sound positively menacing. Take the soundtrack for David Lynch's latest cinematic bauble, Lost Highway (Nothing/Interscope INTD-90090). Please. You'll find Smashing Pumpkins, Nine Inch Nails, and Angelo Badalamenti pumping out psychedelic bass that'll give your liver a massage. And it works, making for an impressive, room-filling foundation that really sets the table. Try "Mr. Eddy's Theme," by Barry Adamson. My notes: "The soundstage is huge, fantastic, full of amazing sound and color. If anyone needs proof that you can achieve virtually wraparound sound from two-channel stereo, it's in this recording. The bass sets the music in cement!"
This is your life...Bill Evans!
I couldn't believe how fine Sara K.'s new Hobo (Chesky JD155) sounded. Listening to "Brick House," I sensed her presence behind the microphone, singing, I was sure, with her eyes closed as she feelingly grooved to the beat (and much as we'd seen her at the Bottom Line here in the Village). I felt an intense sense of inner exploration, one that I felt she must have experienced as well during the recording. It was magical.
Listening to the recent Classic Records release of Ravel's Rapsodie espagnole (LSC-6094 (2)), I jotted: "Stunning! Nitpick if you must the Bernie Grundman Highs. Not me, I love 'em. The amp lays out the treble with such clarity and boldness as to be almost piercing at times. Like...live music?"
What does it all mean? Well, it's not quite a full-fledged paradox. You couldn't call the GM 200 a "sweet" amp, really—it's got too much of the crystal-clarity thing going. Yet...within that clarity and purity of sound I heard all the colors and resonances of the musical rainbow. Very, very glamorous.
A word about composure...
Here's what I think. You might like the GM 200 if you're the type who deeply appreciates style, craft, and art. (Presupposing, of course, that you can afford to indulge those tastes (footnote 1).) If being close to a perfect Ferrari 250 GT SWB or 275 GTB/4 Cam makes you weak in the knees, you might enjoy owning the GM 200. If you'd go into hock for a day in the current F50 (4.7 liters/12 cylinders/60 valves/513bhp), you'll love the GM 200.
If you're something of a connoisseur and there are no little kids with curious, sticky fingers about, the GM 200 is entirely recommended. It's not an amp to leave on all day playing background music, but for a breathtaking blast up the autostrada, it's hard to beat.
Tube stereo power amplifier. Tube complement per side: 32 PL504, 2 12BZ7,
2 5965, 4 EFL 200/6Y9, 2 EM81/6DA5. Output power: 200Wpc into 8 ohms (23dBW).
Input impedance: 100k ohms. Damping factor: 12. Negative feedback: 17.5dB.
Power consumption: 800W idle, 1600W at rated output.