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
No comments:
Post a Comment