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Amarok2 - electrostatic STAX headphone amplifier

WARNING! Circuit works with dangerous voltage!

PSU schematics:
Electrostatic STAX amplifier schematic (power supply)

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:

Electrostatic STAX amplifier schematic

Amplifier could be with symmetrical inputs or asymmetrical inputs (in that case IN- is 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 approx. 900V.
Wrognly placed input cable (e.g. near MOSFET heatsinks) can cause amplifier instability (2kHz square - same as below) even when using shielded cable! Moving cable (under PCB for exaple) ensures complete stability.
PCB in 300DPI suitable for CNC milling.
Dut to high voltage, it is necessary to apply electrically isolating coating on PCB!!

I strongly recommend to use two boxes - first for PSUs second for amplifier board (reason - hum, ...).

Maximum output is 480Vrms (750V supply) BIAS is selectable between PRO (580V) and NORMAL (230V). Output MOSFET stage works in class A with 6mA quiescent current. Voltage gain is approx. 54dB. 

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.

THD at 450Vrms/1kHz is 0.05%, IMD 250Hz+8kHz 0.01%.
THD at 350Vrms/1kHz is 0.03%, IMD 250Hz+8kHz 0.005%.
Frequency response 8Hz - 25kHz (-1dB). Noise 10mVpp, 20mVpp hum (SNR = 91dB).

First FFT plot shows THD @ 1kHz, next shows noise spectrum.

2kHz quare response (20V/div, 100us/div, input square tr=1us)
Electrostatic amplifier - square response

Impulse response (20V/div, 100us/div)
Electrostatic amplifier - impulse response

Impulse response - input signal (1V/div, 100us/div)
Electrostatic amplifier - impulse response - input

PSU +-15V:
Electrostatic amplifier - +-15V PSU PCB

PSU 800V + BIAS:
Electrostatic amplifier - 800V PSU PCB

Amplifier board:
Electrostatic amplifier - amplifier PCB

Finished amp and STAX SR-3 NEW, second socket is PRO bias, both sockets are DIY made from two 3-pin XLR:
Electrostatic amplifier

Inside (old photo - only one headphone connector borrowed from STAX SRD-5 and exserted potentiometer buttons):
Electrostatic amplifier - inside

Questionare hint ;)
Tell me your opinion!:
1) Electrostatic headphones are useless for me;
2) Electrostatic headphones are more available to me thanks to this amplifier;
3) I rather buy proper electrostatic amplifier (e.g. STAX) than this one.

Author: Luboš "Amarokcz" StÝíteský