JJ828, High Class A1, Push-Pull...
JJ828, High Class A1, Push-Pull Parallell Ultralinear KT88, with Cathode Feedback, Integrated Stereo Amplifier remote controlled, 2x70W, 4@8 Ohm.
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|Dimensions||480 x 180 x 350 mm|
|Input sensitivity||0,775 for full output|
|Input Impedance||25 kohm|
|Output impedance||speaker output 4 or 8 ohm|
|Input||5 x RCA line (unbalanced)|
|Output||1 x Tape out|
|Power||2 x 70W at 4 - 8 ohm|
|Frequency Response||20 Hz - 25 kHz (-1dB)|
|Tubes||KT88 x 8 ; ECC99 x 2 ; ECC82 x 4|
|Bias||Adjustable for each tube with LED indication|
JJ828 is an integrated power tube stereo amplifier with an independent power supply for each channel.
The beam tetrode KT88 with an anode dissipation of 35 W has been selected as an output tube for this amp.
The choice of output circuitry is the result of compromise between output power, distortion and damping factor. Two KT88 used as a pentode in class B can deliver 120W, or, on the other hand, when used as a triode in class A, 22 W. We have decided on ultra linear circuitry using four KT88 with cathodes feedback.
The output power of 70 W per channel may seem to be low but it is justified in accomplishing the final outcome. When the KT88 current is 70mA and Ub= 420V (Pa ~30W), a distortion of approximately 1% is achieved at full power without any feedback. For comparison, distortion at Ia=40mA is about 5%. Introduction of feedback of 10DB (3X) sets the output impedance to a satisfactory low level.
The bias current is set individually for each output tube and it is indicated by LED diodes. At the moment the diode light goes off, the cathode current is 70mA. This accurate setting is important when tubes are replaced, because of their high tube conductance slope (up to 18mA/V). Improper bias, for example by 4V, can increase Ia current by up to 70mA and this could destroy the tubes in a few minutes. The output transformer is based on a C-core with air gap 2x0.005 mm, with two OT coils and 20 windings.
The driver consists of three stages. The first stage amplifies the input voltage and the overall feedback is applied to its cathode. We decided on a split load phase inverter. It features low distortion, the amplitude magnitudes are determined only by accuracy of the cathode and anode resistors (0.1%), and it’s unaffected by replacement tube quality. Even when the tube replacement differs by more than 50% from the original, it will not significantly affect this “concertina” phase splitter. By contrast, this is not at all the case with the differential phase splitter, which will suffer greatly from any tube mismatch.
Next follows a driver with the ECC99 tube, which amplifies the signal from the phase inverter and drives the KT88 power tubes. It has sufficient amplitude swing, low internal resistance (~4kOhms) and therefore can drive the KT88 as triodes.
The input section (5 inputs), controlled via a remote control unit, uses Meder relays for input selection and a blue motor potentiometer from ALPS for volume control. Signal from the selected input can be routed at the output “Tape”. Input selection, volume control, turning amplifier standby on, can also be done via controls on the front panel. The front panel is made of wood with brown, black or natural finish.