I own a GM-100, which I use with Kharma 2.2's. The sound is very rich, dynamic and well layered. My only reservation is that the amp is alittle noiser than some. The amp has alot of gain, and the 6dj8/6922 input tubes are run very hard, so getting a quiet pair of tubes is quite a challenge. However, the signature of the 6dj8/6922 input tube is immediately obvious and easily disinguished.
The following is a review for the GM-20 by Ken Kessler - Hi Fi News Sept '99:
"With a decade's experience in output-transformerless design, Graaf's latest OTL, the GM20, should be something really special . . . . . .
Graaf's Giovanni Mariani is one of but an handful of electronics wizards brave enough to carry on with output-transformerless circuitry. Mariani has entered a new phase of excellence managing to produce something totally out of the ordinary yet completely in character with the amps of his which preceded it.
Yes, the GM20 is an OTL amplifier, like the GM100 and GM200 it's built to standards surpassed by no other valve amp and contains Italian parts wherever possible. This particular model uses a rather special tube which I don't believe has been used in OTL's before.
The benefits of this type of circuitry may be summarised as unrivalled transparency, speed and dynamics. That's because OTL/OCL designs are as close as it gets to direct injection of signal into speaker. In the GM20 the output stages are directly coupled to their loads, and the output tube in this case is the Russian military workhorse; the 6C33C, known for the bunch of 'nipples' on it's top.
Like it's predecessors, GM20 uses a fully differential and balanced configuration (i.e. symmetrical) DC-coupled between stages. This explains the low levels of hum by OTL standards and general immunity to noise, including noise from AC mains.
GM20's power supply is made up of six separate sections, four for the output stages and two sections for the driver and gain stages. Two of the aforementioned 6C33C triodes are used per channel configured to exploit the ability of these tubes to deliver high current with a low voltage power supply. Completing the valve complement is an input stage using two 6922 double triodes responsible for the voltage gain and operating as impedance adapters. The driver stages employ two triode-coupled EF184 pentodes to take advantage of the differential circuit and to work as phase splitters. Loudspeaker protection is guaranteed by a novel and sophisticated circuit which avoids the use of series relays or current limiters.
The GM20 is so damn cool looking that even if you don't like tubes let alone OTL's - you'll still fall in love with it.
Contained within the dimensions of the 350x215x440mm is a look somehow more svelt - still brutal and purposeful, but more 'styled'. There's a bold green light built into the on/off button, the cage is plated in some exotic 'black chrome' and the gloss-black paintwork was applied in the Ferrari restoration facilities.
Quite clearly, OTL's offer an immediacy and an intimacy which can only be present when the signal path is uncluttered and direct. OTL's deliver speed and detail a level of 'snap' in the upper mid and treble transients and a neutrality that almost disavows what the coverted feel are valve virtues. Warmth remains - that much is indisputable.
Despite power limits which can easily be reached if you use hungry speakers, the GM20 sounds big and powerful regardless of the actual SPL's. You get the full works whether you're playing at 65dB or 95dB.
But what sold me on the GM20, beyond the gloriously like-like midband, the trademark OTL clarity and detail, and the sheer scale of the musical recreation is the sense of what I can only describe as 'presence'. The sound of the GM20's possesses something else, a palpability which gives the performers more substance and body than I've experienced from any amplifier this side of the Marantz Project T1, the Audio Research Reference 600 and the 80k Krells.
What makes the palpability so remarkable via the GRAAF is the GM20's price tag: £2750. Merely as an objet d'art, the GM20 is worth double that.
The GM20 is too damned good for the majority of the world's audiophiles. Period."
