GRAAF GM20 OTL

Pre owned. In good condition. It has some signs of wear, also visible in the photos. Supplied with the original packaging.

Tube-based power amplifier designed by Giovanni Mariani, founder of GRAAF, around a configuration that eliminates output transformers from the signal path. The OTL, Output Transformer-Less, designation identifies one of the key features of the design, developed to use the power tubes directly to drive the loudspeakers. The circuit also adopts a fully differential and balanced OCL, Output Capacitor-Less, configuration with DC coupling.

The power stage uses two 6C33C triodes per channel, tubes originally developed for military applications and characterized by their ability to deliver high currents at relatively low supply voltages. The voltage amplification stage uses a 6922 dual triode, also employed as a phase splitter, followed by two EF184 tubes connected as triodes and used as drivers for the power tubes. The various stages are designed to maintain linearity and stability during amplifier operation.

The power supply is divided into six independent sections. Four of these are floating with respect to the main ground and are dedicated to the power stages, while the other two supply the gain and driver stages. This separation limits interaction between the different circuit sections and contributes to reducing interference. The circuits dedicated to offset and bias regulation also maintain their calibration as the mains voltage varies.

The GM20 uses a limited amount of negative feedback, at 6 dB, and features dedicated loudspeaker protection circuitry that does not employ current limiters. The OTL configuration eliminates the influence of output transformers on the signal, components that are particularly critical in tube amplification both in terms of their design and their effect on the overall behavior of the electronics. The frequency response extends from 1 Hz to 350 kHz at -3 dB.

The absence of output transformers therefore represents a key design choice in the GM20. The design aims to achieve reproduction characterized by the specific qualities of vacuum tubes without introducing an element into the signal path that can affect the response and tonal characteristics of the amplifier. This configuration nevertheless requires careful loudspeaker selection, as the amplifier's behavior depends significantly on the characteristics of the connected load.

The GM20 performs at its best with loudspeakers offering good sensitivity and a nominal impedance of 8 ohms that does not exhibit significant drops below this value. Under these conditions, the amplifier delivers smooth and clear reproduction, combined with a fast response and a good ability to follow the dynamic variations of the musical program. The stereo soundstage also presents a three-dimensional arrangement of the sonic information.

The entirely hand-built construction and circuit design make the Graaf GM20 OTL a distinctive model among tube amplifiers. The transformerless output configuration, the use of 6C33C tubes and the separation of the power supply sections are the main elements of a design that emphasizes speed, clarity, smoothness and transparency, while maintaining careful attention to compatibility with the loudspeaker used.

Technical specifications:

• Type: tube power amplifier
• RMS power per channel: 20 W
• Reference load: 8 ohms
• Tubes used: 2 x 6C33C per channel; 1 x 6922 as phase splitter; 2 x EF184 connected as triodes as drivers
• Configuration: OTL; OCL; fully differential and balanced
• Negative feedback: 6 dB
• Analog inputs: balanced RCA
• Input impedance: 50 kOhms
• Input sensitivity: 600 mV
• Frequency response: 1 Hz - 350 kHz (-3 dB)
• Total harmonic distortion: 0.4%
• Signal-to-noise ratio: 95 dB
• Dimensions (W x H x D): 350 x 215 x 440 mm
• Weight: 22 kg