What is stereo?

There are now two system of high fidelity, monophonic (monaural) and stereophonic. Monophonic is a system that starts from one microphone and is fed through a single high fidelity set. Stereophonic is a double system. Two separate microphones are placed at different sides of the orchestra and two different systems are used to keep the two signals or channels separated. Two separate speakers are used, placed on different sides of you room. Stereo is much like 3-D photography, two slightly different sound reach your ears giving you a new dimension in sound.

H.H. Scott '59

Graaf GM 20 OMT

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.
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.
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.

Function: OTL/OCL Tube Power Amplifier
Output power: 20+20 W RMS/ Stereo, 65 W RMS/ Mono
Output impedance: 8~16 Ohms, 4~16 Ohms Mono
Input sensitivity: 600 mV
Input impedance: 50 K Ohms
Total harmonic distortion: 0.4% at 1W
Signal/ Noise ratio: unweighted 85 dB, weighted 95 dB A
CMRR: 60 dB
Frequency response: 1 Hz to 350 KHz (-3 dB)
Rise time: 1u sec.
Damping facto:r 4
Negative feedback: 6 dB
Using tubes: 6922 x 2, EF-184 x 4, 6C33C x 4
Mains: 120/220V, 50/60 Hz
Power dissipation: 450 W
Dimensions: 350 mm x 215 mm x 440 mm (W x H x D)
Weight: 22 Kg
Finish: Ferrari Black

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