Amarok2 - electrostatic STAX headphone amplifier
WARNING! Circuit works with dangerous voltage!
Entire amplifier is power by toroidal transformer. Its secondaries are: 3x230V/0.05A and symmetric 20V/0.1. Between primary and secondary there is aluminium foil connected to protective ground, primary is connected to mains via RFI filter. All MOSFETs (except U4 in PSU) are on heatsink approx 15K/W. All trimmers (except VR4 in PSU) are multiturn. VR4 is single turn trimmer 15x15mm and its purpoise is to set BIAS (PRO should be 580V and NORMAL 230V for earspeakers STAX). Trimmer VR3 sets V+ to 750V. Trimmer VR1 and VR2 sets ±15V.
One channel schematic:
Amplifier could be
with symmetrical inputs or asymmetrical inputs (in that case IN-
connected to GND). VR4 is volume control and VR3 is balance. Trimmers VR1 and VR2 sets voltage
at U1 and U2 drains exactly to half of V+. Output MOSFET
stage works in class A with 6mA quiescent current. IRFBE30 can be
replaced with IRFBG20 (or another HV MOSFET), V+ then can be up to
I strongly recommend to use two boxes - first for PSUs second for amplifier board (reason - hum, ...).
All following measurements are done with STAX SR-3 NEW as load, measured not differentially but on OUT+
Keep in mind that measuring THD at this high voltage is quite difficult and not very accurate in DIY conditions.
2kHz quare response (20V/div, 100us/div, input square tr=1us)
Impulse response (20V/div, 100us/div)
Impulse response - input signal (1V/div, 100us/div)
PSU 800V + BIAS:
Finished amp and STAX SR-3 NEW, second socket is PRO bias, both sockets are DIY made from two 3-pin XLR:
Inside (old photo - only one headphone connector borrowed from STAX SRD-5 and exserted potentiometer buttons):
Questionare hint ;)