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JJ543 Integrated Tube Stereo Amplifier 2x35W

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The JJ543 is an integrated power tube amplifier. Used in conjunction with high quality input signal sources (CD, PHONO) and high quality stereo speakers, this amplifier is intended for medium sized listening rooms.

The operation of the amplifier is controlled by a microprocessor. The user communicates with the amplifier via a remote control unit and all functions are shown on the amplifier display. All electronic parts used in the JJ543 come from reputable manufacturers. Capacitors in the signal path are special custom made polypropylene types for audio applications.

The JJ543 has four linear inputs. Inputs 1, 2 and 3 have sensitivity 0.775 V; input 4 has sensitivity 2 V at full power. Input 4 is intended for sources with a higher level of signal as is the case of phone preamplifiers. In addition to the speaker outputs L and R, the JJ543 also has three low power level outputs.
The first stereo output is level controlled by the main attenuator. The second output is not level controlled.
The third output is L+ R sum to be used for sub-woofer applications. The level control for this output is accomplished by means of special internal circuitry (a resistor matrix controlled by the microprocessor). Therefore this is not the case of volume control in the feedback loop or a volume control using D/A converter. The level is shown on the display.

The signal path is comprised of four active stages. Only the last two stages are within the balanced negative feedback loop. The first stage amplifies the signal 10 times. The second stage is a split load phase inverter. In order to guarantee excellent phase balancing, this phase inverter was designed using a high accuracy anode and cathode resistors (0.1%). The first and second stages are not part of the global feedback loop. The third stage is the power tubes driver. Its cathodes are fed from a balanced feedback loop from the bifilar OPT secondary winding. The feedback loop is also symmetrically connected to the cathodes of the power tubes. The driver is not connected differentially, which eliminates the cancellation of even harmonics. The total feedback is only 10 dB.
The power tubes operate in high class AB (45mA bias per tube). The idle current is adjusted by a negative bias voltage source (fixed bias). Both the cathode currents and the output pair balance can be adjusted by trim pots (P1 and P2).

The output transformer is of a highly sophisticated design. The transformer is wound on a double C-core made of laminations 0.15 mm thick, with an air-gap of 4x0.05 mm. The transformer has 8 primary and 4 secondary bifilar windings, the UL tap is at 25%, which, together with the cathode feedback, results in a feedback ratio of 30%. Due to symmetrical feedback, the secondary windings have a grounded centre tap. Therefore the JJ543 cannot be used in a bridged connection.