Product Review -
Graaf GM-200 OTL Stereo Tube Amplifier - February, 1997
By John E. Johnson Jr.
Click to see
Graaf GM-200 OTL (Output
Transformer-Less) Stereo Tube Amplifier; Push-pull
amplification; 200 watts rms/ch into 8 Ohms; Input sensitivity
850 mv; Input impedance 100 kOhms; THD 1%; S/N 87 dB unweighted;
Frequency response 7 Hz - 350 kHz -3 dB; Slew rate 123 V/µsec;
Damping factor 12; Negative feedback 17.5 dB; Size 8 1/2"H x
17 3/4"W x 16 1/2"D; Weight 63 pounds; Black gloss
finish; $12,500; Graaf, Sonic Signatures, 30 Villa Avenue, San
Rafael, California 94901; Phone 415-456-1632; Fax 415-453-3136;
Graaf s.r.l, Via Venturi, 66/68, Modena, ITALY; Phone 59-22-5223;
Most tube amplifiers have output transformers between the output stage and the speaker. This stabilizes the amplifier's ability to drive low impedance loads. Otherwise, the amp can fail. However, a transformer is another circuit component in the path of the music signal, and some degradation of the signal quality occurs. The OTL amplifier does not have output transformers (OTL means Output Transformer-Less). By necessity, the amplifier circuit is more complex, and thus, more expensive. Design is critical for top performance. So, OTLs have to be carefully thought out, or not done at all.
Graaf, of Modena, Italy, has been making amplifiers for about 15 years, and are now introducing them into the US. OTLs are just one of the tube amp types they make, and we decided to review the GM-200.
Because tube amps date back to the beginning of audio, many tube designs look like they were built by hobbyists, with rather bland chassis, and meters on the front that might have come out of the cockpit panel from a Flash Gordon movie. Tubeophiles, including me, don't really mind this. In fact, it lends a very different, conversational-piece look. However, it is nice to have a choice of looks in this matter. The Graaf is exquisitely beautiful, with high-gloss black chassis, and chrome tube cover racks. Instead of a meter to indicate bias or other tube parameter settings, there are two electric eyes ( EM81 "Magic Eyes" tubes), that indicate power output. [Click here for photo.] Their green glow moves up the inside of the tube along both sides, and when the two parts meet, maximum power output has been delivered. The green against the candlelight yellow glow of the other tubes is quite extraordinary. Two small screw holes on the top front are used for adjusting the bias and offset if and when the tubes are replaced.
The power on/off button is on the top of the chassis near the front, and it too has illumination. Power capacitors (two Kendeil 6,800 µF, 160V, one 10,000 µF, 160V, and one 3,300 µF, 350V), and the toroidal transformer cover are gloss black like the rest of the chassis. The amplifier is essentially dual mono, and the circuit is fully DC-coupled from the input to the output. There are three stages of amplification. For each channel, the input stage consists of one 12BZ7, and a 5965 handles the unit gain. Two EFL200s are used as drivers and for bias and offset control. Sixteen PL504 pentodes power the output stage of each channel, eight per push-pull leg, and there is a small red indicator at the base of each tube which will glow if that particular tube becomes defective. These pentodes have their plate supply voltage wired to a connector at the top of each tube. Although most of the chassis is enamelized, touching one of the plate supplies and the chassis at the same time could deliver a 150V shock. The tube covers slide over chrome rails to prevent this from happening, as well as keeping the curious from getting burned. However, because there are so many tubes, even the covers get too hot to touch, so small children should be cautioned (and big children too). The amplifier is biased for 20 of the 200 watts per channel into Class A.
The rear of the amp has one pair of gold plated RCA input jacks, one pair of balanced input jacks, and two pairs of gold plated 5-way speaker binding posts. [Click here for photo.] Fuse access and the grounded AC input socket are also located here. Inside the chassis are a number of printed circuit boards, [click here for photo] so this amp is not of the "hard wired" variety.
We tested the GM-200 with our McCormack CD Transport, DAC, and Passive Preamp, Nordost and AudioQuest Cables, Anthony Moore Ribbon Speakers, and Mark IV Silver Quasi-Ribbon Speakers.
The Graaf has four solenoid switches that turn on in sequence as the amp powers up. No turn-on thump was noticeable, although a whooshing sound was heard at turn-off. The amp needs about 45 minutes to fully warm up to its best sound quality.
What was immediately noticeable about the sound of the GM-200 was its full bodied mid-range. This is typical of tube amplifiers, and frankly, part of this is probably due to audible harmonic distortion of the "good" kind. These harmonics characterize the tube sound. A listener either likes this or not. I happen to like it. After all, it is the sensory experience of listening to an audio system that really counts. Distortion can add or subtract from the pleasure. Tubeophiles will definitely say that it adds. The rich, warm mid-range was not mushy however, and unlike some tube designs, the treble was crystal clear, while the bass was deep. However, the treble and bass do take a back seat to the lush mids. Perhaps a bit distinctive to this particular amp, and not everyone's choice, but certainly there are some readers who will really be pleased with it.
One of the most enjoyable types of music to listen to with tube amps is singing. Whether it be the velvet vocals of Enya on her Watermark album, the raspy night club voice of Tony Bennett Unplugged, or a choir oozing Evocation of the Spirit, the GM-200 was quite a pleasure to behold . . . er . . . behear. Barbra Streisand's Collection never sounded more collected (Columbia CK 45369), and Maria Callas' Master Class (EMI CDC 7243 5 55572 2 3) raised nape hairs. Symphonic pieces thundered, and Rock shook the walls, but truly, it was the combination of tubes and vocal chords that shone most brightly. The GM-200 celebrates the human voice. Some might prefer triodes in pure Class A, but 200 watts per channel in Class A would produce enough heat to run a copper smelter. The tubes would have to be replaced more often, as well. The GM-200 tube life expectancy, on the other hand, is 10,000 hours.
We measured the pink noise output of the GM-200 into an actual speaker load (the Mark IV Silvers). At the edge of clipping, the amp produced 18.4 Volts and 3.27 Amperes of current (left channel measured, both channels operating). As we have said before, this is a more realistic approach than measuring output into an 8 Ohm resistor. The speaker is a reactive load, while the resistor is not. The back EMF from a speaker will counteract the voltage produced by the amp, so measuring full spectrum sound at the speaker is a real world test. With our calibrated microphone placed between the two speakers about 8 feet out, we measured 98.7 dB of SPL with pink noise, at the edge of clipping. This is substantial, especially since the Silvers are not easy loads to drive. I had to use earplugs (I always do with SPL tests over 80 dB). To top it off, OTLs have the characteristic of decreased output with lower impedance loads, just the opposite of conventional amplifiers (solid state amps don't usually have output transformers either, but their output increases with decreasing load impedance). So, all in all, very good performance into tough speakers. Electrostatic speakers are notorious for low impedances, and the GM-200 handles these very well too (that is the circumstance where we first listened to the Graaf and decided to request it for review).
In summary, the Graaf GM-200 OTL Amplifier is an astonishingly beautiful product with a terrific sound, especially if you like vocals. The 200's performance and good looks come at a hefty price tag, but then so does a Ferrari.
John E. Johnson, Jr.
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