Description
The magic of the GM-70 triode
With the GM-70 one of the very large directly heated carbonized tungsten monoplate triodes comes into operation in single-ended mode, but without fully utilizing its allowable plate dissipation. Compared to smaller directly heated triodes, such as a 300B, operated at the allowable plate dissipation limit, the lifetime of the GM-70 triode has thus been increased many times over. Since the GM-70 operates below the power limit, it clips earlier than possible, but lower-loss output transformers can also be used and the cathode can be heated slightly less which further increases the service life of the valve. Thus, the GM-70 is able to demonstrate its sonic advantages over the “smaller” competitors and still outperforms a 300B triode in its power output by twice as much. When using loudspeakers with an efficiency of 92dB/W/m or more, this stereo amplifier can generate a clean sound pressure level of 100dB in an average listening room. This corresponds to the peak sound pressure level of a symphony orchestra in a concert hall or the volume of a pneumatic hammer located one meter away. The Terrastrato Typ-1 thus marks the golden mean of an ideal single-ended amplifier: as “small” as possible (therefore less heat and lower-loss transformers), as large as necessary (sufficient headroom on efficient speakers).
With or without preamplifier
The Terrastrato Typ-1 does not in fact require a line preamplifier. Its input sensitivity has been designed in such a way that sources with an output voltage of 1V or more can drive the amplifier up to clipping. With the wired remote control, the motorized potentiometers of the two integrated mono amplifiers can be operated co rotating (for volume) and counter rotating (for balance). This means that only one potentiometer is located in the signal path of one channel. Thus, no additional gain had to be taken into account for the implementation of a balance control. The sound stage can be centered from the listening position. The discreet design of the two mono power amplifiers remains unaffected. The signal path from the volume potentiometer to the control grid of the first valve is desirably short. Impedance converting output stages of a preamplifier can also be omitted. The remote control practically replaces the line preamplifier.
Since the line connectors of the Terrastrato Typ-1 are located on the front of the control panel, as with an analog synthesizer, it is easy to interchange different playing devices. With the line-A/line-B switch, two already connected sources can be switched quickly, for example a digital-to-analog converter (DAC) with digital source selection and a phono preamplifier. If a line preamplifier with many analog inputs is used, the line A/line-B switch of the Terrastrato Typ-1 becomes a tape monitor switch which is rarely found on modern preamplifiers. The potentiometers of the Terrastrato Typ-1 can then be used for balance or gain adjustment.
Aurally compensated volume control
One of the very useful features of a line preamplifier or integrated amplifier is an aurally compensated volume control. The “loudness” switches of many commercial amplifiers have, however, discredited the aurally compensated volume control among music lovers. It is commonly associated with unnatural bass and treble enhancement. When the pioneers of high fidelity came up with the concept of an aurally compensated volume control, they had, however, something else in mind: to compensate for the losses of human hearing in treble and bass perception when recordings are listened to at a lower volume than that at which they were recorded live. The Terrastrato Typ-1 integrated amplifier achieves this goal by means of its aural circuitry. The sound stage opens up naturally, even at low listening levels. The subjectively felt air and especially the deep accents reappear. If the aural/linear switch is set to “linear”, the network for the aurally compensated volume control including the palladium contact points of the aural/linear switch is no longer part of the signal path. If a line preamplifier is also used, a static treble and bass enhancement can be set using the aural switch and gain adjustment on the Terrastrato Typ-1 amplifier.
Tone control – analog, minimalist, disconnectable
The enhanced-highs switch of the Terrastrato Typ-1 allows a slight increase of the highest frequencies of the audio band at the touch of a button – a useful function in a fully analog playback chain: An attenuation of the high frequency range at a high ambient temperature can thus be compensated without the use of an additional gain or a tone control stage. If full range drivers are used, a passive equalization network on the loudspeaker with all its disadvantages or a supertweeter can be omitted. (Different crossover frequencies can be ordered upon request.) If the enhanced-highs palladium switch is set to “off”, this network also completely decouples from the signal path.
Rectifier and power supply unit
Terrastrato equips all amplifiers with a heat dissipation of more than 300W with Schottky rectifiers as the tonal effect caused by a higher heat output on the listening room on warm days is considerably more significant than the sound effects of a tube rectifier. Irrespective of this, during listening tests with various prototypes, the low impedance power supply with a Schottky rectifier and a CLC filter in combination with the GM-70 triode proved to be superior in terms of sound.
Bi-Amping or subwoofer
The buffered-output “buff. out” allows the connection of an active subwoofer or a low pass filter and an additional amplifier. For this purpose, the Terrastrato Typ-1 can be equipped with smaller coupling capacitors for a high-pass with a freely selectable cut-off frequency and a rolloff of 6dB or 12dB/octave. Thus, it is not necessary to loop in a separate crossover network for bi-amping, if an extremely short signal path is preferred. However, the bass reproduction of the Terrastrato Typ-1 is so natural and precise that it only makes sense for very large listening rooms or main speakers with limited frequency range not to leave the bass amplification to the GM-70 triode.
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