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GM 20 OTL Power Amplifier


The GM 2O power amplifier uses an OTL (output transformerless) and OCL (output capacitorless) configurafion. The benefits of this type of circuit are very widely recognised and may be summarised as providing unrivalled transparency, speed and dynamics. OTL / OCL designs avoid the need to couple the loudspeaker load by means of an output transformer. In the GM 2O therefore the output stages are directly coupled to their respective loads. The circuit in each of the two power stages is based on a bridge in which the 6C33C tubes represent two of the four sides. The GM 2O uses a fully differential and balanced configuration (i.e. symmetrical) and is DC coupled between each of its stages with great benefits in terms of immunity to noise, including noise from AC mains.

Circuito.jpg (13029 byte) Great care has been taken to ensure stability in all the working stages of the amplifier, making them practically immune to variations in the characteristics of the active components (thermic, static and dynamic) and maintaining their special character unaltered over time. The power supply is made up of six distinct sections, four of which float with respect to ground and provide power for the output stages. The remaining two sections provide power for the driver and gain stages. This method of supplying energy to the various stages of the amplifier is an excellent solution to the problems inherent in providing many different voltage values which are commonly used in valve amplifiers.
Two 6C33C triode valves are used in each of the two channels' power stages, conceived in such a way as to make the best use of the ability of these tubes to give high current with a low voltage power supply. These power tubes, not normally used in less sophisticated circuits, represent an ideal choice for an OTL design thanks to their great robustness and surprisingly good sound quality.
The power amplifier input stage uses two 6922 double triodes which are not only responsible for the voltage gain but also operate as impedance adaptors. The driver stages, working with two triode-coupled

EF 1 84 pentodes, take advantage of the differential circuit mentioned above and work also as phase splitters. This guarantees good driving ability for the power tube grids and perfect symmetry between the two signals, which are in push-pull mode, and are used from the power stages.
It's important to add that each channel voltage gain and driver create together a double differential cascade stage which is able to create a high CMRR.
The stabilizing offset and bias circuits fitted to this power amplifer ensure completely trouble-free setting-up. In the GM 2O loudspeaker protection is guaranteed by a sophisticated circuit which does not make use of series relays or current limiters, since the tubes themselves are naturally current-limiting.
Retro72.jpg (9772 byte) A small amount of feedback (6dB) is used to allow the GM 20 to drive even difficult loudspeaker loads without problems.
The particular circuit design described above allow excellent results to be obtained from the GM 2O.

As can be seen from the attached specifications, the performance is of a very high order. For example, the bandwidth extends from 1 Hz to beyond 35OKhz!
As befits an amplifier of this class, it is possible to use the two inputs L and R in balanced mode by means of XLR plugs or in unbalanced mode via RCA phono plugs.
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Technical Data:

Output power : 20 W + 20 W RMS
Output Impedance : 8 16 ohms - 4 16 ohms mono
Input Sensivity : 600 mV
Input Impedance : 50 Kohms
Total Harmonic Distortion(THD): < 0,4% a 1 W
Noise/Signal Ratio

lineare 85 dB pesato 95 dB A


60 dB

Frequrncy Response : da 1 Hz a 350 KHz (-3 dB)
Damping Factor : 4
Negative Feedback : 6 dB
Power Requirements: 105 120 / 210 240 V (50 60 Hz)
Power use : in riposo 450W
Dimensions (L x H x P): 350x 215 x 440 mm
Weight: 22 Kg
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