GRAAF GM 20 OTL

Pre owned. In excellent condition. No visible signs of wear. Original packaging not included.

Three units available, all in the same cosmetic condition.

The GRAAF GM 20 audio power amplifier is born from the experience and passion of its designer, Giovanni Mariani, who conducted in-depth research on the most suitable vacuum tubes for the project. Through rigorous testing and careful circuit design, the GM 20 is engineered to maximize the sonic qualities of the tubes while ensuring full compatibility with any type of loudspeaker without compromise.

The output configuration combines OTL and OCL architectures, fully differential and balanced, with direct current coupling. The power supply is divided into six separate sections, four of which are “floating” relative to the main ground and feed the power stages, while the remaining two sections supply the driver and gain stages, ensuring stability and optimal circuit separation.

The power circuits employ two 6C33C triodes per channel, designed to take full advantage of the tubes’ ability to deliver high currents at low supply voltages. Voltage amplification is handled by a 6922 dual triode, which also functions as a phase inverter, followed by two EF 184 pentodes connected as triodes that drive the output tubes. Special attention is given to the offset and bias stabilization circuits, which remain stable even under mains voltage fluctuations.

Speaker protection is ensured by a sophisticated dedicated circuitry without the use of current limiters, as the intrinsic characteristics of the vacuum tubes already perform this function. A very light 6 dB negative feedback allows the GM 20 to drive even highly reactive loads without issues, maintaining precise control and natural signal reproduction.

Every detail of the GM 20, from the design to the materials used, has been carefully selected to ensure quality and reliability. The amplifier is hand-assembled, reflecting the dedication and passion of the GRAAF team. The result is a product that combines technical research with craftsmanship, offering enthusiasts the chance to experience true hi‑fi without compromise, with high performance, musicality, and precision in every sonic detail.

Technical specifications:

• Output power: 20 W + 20 W RMS
• Output impedance: 8–16 Ω; 4–16 Ω mono
• Input sensitivity: 600 mV
• Input impedance: 50 kΩ
• Total harmonic distortion (THD): <0.4% at 1 W
• Signal-to-noise ratio: linear 85 dB; A-weighted 95 dB
• CMRR: 60 dB
• Frequency response: 1 Hz – 350 kHz (-3 dB)
• Damping factor: 4
• Negative feedback: 6 dB
• Power supply voltage: 105–120 V; 210–240 V, 50–60 Hz
• Power consumption: 450 W at idle
• Dimensions (W × H × D): 350 × 215 × 440 mm
• Weight: 22 kg