AC-Tubeamp one chanel
My concept of a balanced dc-coupled class-A amplifier I used up to now I did not keep entirely for the benefit of a straightforward effort. To build a dc-coupled amplifier in tube topology means an enhanced effort and it is questionable if this enhanced effort will have negative effect on the sound quality. This amplifier gets by on only two coupling condensators in the signal path, and they are again integrated in the feedback loop of the output stage. In the periphery are two transistors to implement two current sources but they do not touch the audio signal.
dimension: 95 x 135 mm
Partlist you find here.
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. First put all pots to center position. Connect the input to ground and adjust the output voltage at +Out using R35 and -Out using R36 to +460 Volt. You have to repeat the adjustment until the voltages fit, because the output voltages are interacting a little bit.
With the recommended supply voltage of 800 Volt the amplifier can output 450 Volt eff (=1250Volt Vss from stator to stator) before clipping. The distortion is untypical low for a tubeamp without overall feedback, in the condition that you use matched tubes. If you use unmatched tubes distortion can increase up to four times higher. The 6S4-A can withstand 2KV peaks and has a maximum power consumption of 8,5W 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, Jecklin Float, Sennheiser, etc, just fit the BIAS to the desired value.
measurement results (supply voltage 800Volt, 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: 10KOhm
frequency response (-3dB): 0,3 - 40000 Hz