Ultra High-End Hybridamp Alpha Centauri V7.0
circuit
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 is very high because it is running against the current sources T103,T203 The two units of the double triode 6SN7 is working in parallel with a load of 30K, that means a BIAS-current of about 13mA by a supply voltage of 400 Volt. The output stage has a gain of 14. 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 5p capacitors as local feedback to the output tubes.
starting up
WARNING!!!
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 R50. 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.
result
With a supply voltage of +/-400 Volt the amplifier can output more than 400 Volt eff (=1200Volt Vss from stator to stator). The 6SN7 can withstand 450V, up to 1500V peaks and has a maximum power consumption of 7W 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 6SN7 sounds completly different to the 6S4-A Version. While the V4 did sound more like a transistor amplifier the V7 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.
I recommend to use the Electro Harmonix 6S7N, they are 450V/7,5W and sound beautiful.
Schematics, partlist and layout download here.
Measurement results (supply voltage +/-400Volt, all voltages RMS)
distortion level vs. output voltage f=1KHz:
100Volt: 0,02%
200Volt: 0.04%
300Volt: 0,13%
410Volt: 1%
distortion level vs. frequency
Uout = 100Volt:
100Hz: 0,03
1KHz: 0,02%
10KHz: 0,06%
Uout = 200Volt:
100Hz: 0,04%
1KHz: 0,04%
10KHz: 0,2%
chanel seperation vs. frequency (Uout = 100Volt):
100Hz: >90dB
1KHz: 86dB
10KHz: 66dB
signal-to-noise ratio: 90dB
output impedance at 1KHz: 1KOhm
frequency response (-3dB): 0 - 50000 Hz
picture gallery Alpha Centauri