Ultra High-End Hybridamp Alpha Centauri V6.0
Here you see one chanel of the balanced DC-coupled Class-A amplifier with tube output. The input stage is a differential amplifier in cascode topology in order to reduce the Miller-effect. To compansate the low voltage amplification of the tubes, the lion share of the voltage amplification is generated in th first stage. The voltage gain of this stage can be calculated by the quotient of 390K/300 = 1300, reduced by the finite value of the transconductance of the JFet to about 1000. The EL84 works in triode mode with a load of 25K, that means a BIAS-current of about 14mA by a supply voltage of 350 Volt. The output stage has a gain of 14, together with the gain of the first stage we have an open loop gain of 14000. The feedback circuit consits of 4x150K resistors that feed the output signal back to the drains of the JFets and reduces the closed loop gain to a value of 1000. In order to minimize TIM I placed 10p capacitors as local feedback to the output tubes.
Remember that you are handling with high voltages that can be dangerous to life. So if you are working on powerd on devices use only one hand in order to prevent electrical shock.
If the amplifier is connected to the power supply you can begin with adjustment. Connect the input to ground and adjust the output voltage at +Out to 0 Volt using R57. Now use R56 to adjust the voltage between +Out and -Out to zero. Repeat this procedure until there is no longer a difference to measure. The amplifier is now adjusted and ready for operation. Remember that BIAS tends to drift away if using virgin tubes, so adjust the amplifier after 10 hours ontime.
With a supply voltage of +/-350 Volt the amplifier can output more than 350 Volt eff (=1000Volt Vss from stator to stator). The 6S4-A can withstand 600V and has a maximum power consumption of 13W so it will operate within proper limits. The power consumption of the amplifier is 70 Watt. It can drive all STAX Pro headphones (580Volt BIAS), if you reduce BIAS to 280Volt it can drive all STAX headphones. Certainly you can drive other electrostatic headphones like KOSS, Sennheiser, etc, just fit the BIAS to the desired value.
The sound of the Alpha Centauri with EL84 sounds completly different to the 6S4-A Version. While the V4 did sound more like a transistor amplifier the V6 sounds like a typical tube amplifier. Very clear in the midrange with a warm sound down to the lower frequencies. Compared to the Sirius, it sounds more relaxed, suitable for long time listening, on the other hand the sirius has a stronger deeper bass for powerfull listening.
Measurement results (supply voltage +/-350Volt, all voltages RMS)
distortion level vs. output voltage f=1KHz:
distortion level vs. frequency
Uout = 100Volt:
Uout = 200Volt:
chanel seperation vs. frequency (Uout = 100Volt):
signal-to-noise ratio: 90dB
output impedance at 1KHz: 1KOhm
frequency response (-3dB): 0 - 50000 Hz