Graaf GM 70 Phono Preamplifier

The philosophy of GM 7O phono stage design, is the most typical for GRAAF, followed for its whole production. It regards mainly the insertion of the active elements in an environment that is able to let them express their complete potential. The same importance is given to the full exploitation of each component own characteristics, so to avoid the use of additional circuits, which could interfere on the musicality of the performance. Therefore, RF filtering is obtained by the inner characteristics of active components used, and not with the recourse to a passive network. In the same way, in order to obtain the correct input sensitivity for MC cartridges, there is not any additional gain stage. Instead, a variable gain Implement has been chosen, because it has a simpler lay out and is more transparent to music.

RIAA eq is made by a completely passive network, placed after the gain stage. Its components have been calibrated with the greatest care to maximize the response precision and the purity of the musical signal, with the help of critical listening tests as well. Power supply implements the same transformers section of GM 13.5 B II, quite huge, followed by independent filtering for gain and RIAA stage, and for output stage.

The correct electric interface between cartridge and phono stage has been carefully regarded. The knob on the back side of the unit is for the selection of six different impedance loads. Wishing to have a custom load, it is possible to use the upper RCA plugs to connect a resistance of the proper value.

The input stage is based on two pairs of PC 9OO tubes. Gain stage and output buffer stage use a pair of 6922 tubes each, chosen for their linearity, low noise and musical characteristics. The active components used by GM 7O and the whole GRAAF production are subject to a very hard selection, that guarantee the full correspondence of each unit to the reference specifications.

Built manually one by one, with components selected with the greatest care, tested by a severe burn in procedure that go on no stop for several days, each one presented to the definitive approval of the designer in person, given only after a careful listening test, GRAAF electronics have all the qualities to fulfil the most discerning of the listeners, first of all for purity of sound and luminousness of timbre.

 SPECIFICATIONS

Frequency Response:  20 Hz - 20 kHz

50 KHz Attenuation:  28 dB.

100 kHz Attenuation:  34,8 dB

Input Impedance:  38, 56, 100, 500, 1000, 4700 Ω

High Gain:  56 dB

Signal-to-Noise Ratio:  73 dB

Separation:  60 dB

Low Gain:  46 dB

Signal-to-Noise Ratio:  82 dB

Separation:  70 dB

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.

SPECIFICATIONS

Function:  OTL/OCL Tube Power Amplifier

Output Power:  20+20 W RMS [Stereo], 65 W RMS [Mono]

Output Impedance:  8 - 16 Ω , 4 - 16 Ω [Mono]

Input Sensitivity:  600  mV

Input Impedance:  50 K Ω

Total Harmonic Distortion:  0.4% at 1W

Signal-to- Noise Ratio:  unweighted  85 dB,  weighted  95 dB A

CMRR:  60 dB

Frequency Response:  1 Hz to 350 KHz (-3 dB)

Rise Time:  1 µsec.

Damping Factor:  4

Negative Feedback:  6 dB

Using tubes:  6922 x 2;  EF-184 x 4;  6C33C x 4

Power Supply:  120/220 V, 50/60 Hz

Power Dissipation:  450 W

Dimensions (W x H x D): 350 mm x 215 mm x 440 mm

Weight:   22 Kg

Finish:  Ferrari Black

GRAAF GM-50 New

GM 50 Integrated Amplifier

GM-50 is the most recent GRAAF creation, an integrated amplifier born to complete our production of the highest class. GM-50 includes in a single frame the features and the level of performance that in the past were an exclusive of GRAAF preamplifiers and power amplifiers. Construction qualities, sound performance and the highest finishing level are joined with the lower cost and the ease of use of an integrated amplifier. The "GM" monogram on the front panel is a synonimous of the greatest sound quality, until now exclusive for top level electronics like GRAAF GM-200 power amplifier and GM- 13.5 B preamplifier. The goal of GM-50 is to establish new quality standards between top class integrated amplifiers.

As usual for GRAAF electronics, GM-50 aestethics are very accurate even in the slightest details. They rely on the coupling of a massive 15 mm aluminium panel with a transparent crystal that allows the vision of the internal circuit. Led indicators show through the crystal the source selected by the user, making the front panel even more attractive. Large rotary knobs give a comfortable access to listening level control and source selection. They look solid and sturdy; the pleasant feeling given by their use is quite what the most demanding audiophile can expect of a GRAAF electronic.

The careful manifacturing and the plentiful sizing of the most critical components are suggested by a frame depth of almost 50 centimetres. GM-50 has also a remote control for on/off switching, source selection, listening level control and mute switching. The amplifier can manage up to six line sources: two of them with balanced outputs, the others with single ended ones. Two tape outputs are provided as well.

High quality binding posts allow the connection of a couple of loudspeakers. Elegance and refinement, joined with versatility and ease of use are just a preview for the exclusive GM-50 internal features. They have been considered with the greatest care in the aim to reach new limits for sound and quality in top-class integrated amplifiers. To attain this object some of the solutions typical of the OTL Series electronics, conceived by Giovanni Mariani, are used. The fully balanced configuration from input to output is between them. Its realization concerns the complete doubling of the electronic circuits, the only acceptable solution for the GRAAF quality standards, until now used only for very few electronics of higher cost. At the power switching, GM-50 enters in mute condition, until tubes reach the operating temperature.

The project of GM-50 integrated amplifier is based on a lay out of great simpleness, an essential rule to obtain the best sound quality. The first stage, for gain and impedance adjustment, serves the level control, in order to reach the most correct operation. Audio signal then enters a stage made of two differential circuits in cascade, with the second of them that works as the actual driver for the output stage. At this point the signals are compared each other and subject to a further level gain. Output stages are made from a push pull of KT 88 tubes, biased for A-AB Class operation. Each tube has its own automatic bias control system, a choice that allows to obtain the best performance in any operating condition. Another exclusive feature of the output stage regards the use of transformers with ground connected central end in the secondary windings. Such a solution is to have a true balanced configuration for the output stage. Also, this allows to obtain a balanced configuration for the feedback circuit. Usually, in audio amplifiers, the feedback circuit brings back to the input only a part of the output signal, in a point defined as comparison knot. With this solution GRAAF GM-50 has two comparison knots for each channel, allowing to have a balanced feedback net, more linear, precise and stable in its correction purposes. GRAAF GM-50 has a feedback rate limited to 6 dB only.

Specifications

Power Output:  50 W + 50 W RMS

Input Sensitivity/Impedance:  500 mV /100 k Ω

Output Impedance:  6 Ω, 4 - 8 Ω

Frequency Response;  10 Hz - 100 kHz (0,5 dB, 1 W)

Signal-to-Noise Ratio:  83 dB A

Total Harmonic Distortion:  0,3% (10 W)

Negative Feedback:  12 dB

Damping Factor:  4

CMRR:  50 dB

Power Requirements:  105 - 120 / 210 - 240 V , 50/60 Hz

Power Consumption:  360 W

Dimensions (W x H x D):  450 x 160 x 500 mm

Weight:  30 kg