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An adapter to connect elctrostatic amplifiers to audio measurment equiqment


New Member
Jan 5, 2019

The field of electrostatic headphones fascinates me greatly. A good Stax earspeaker system is always pretty high on my wishlist, but I have one problem with buying such a system: I can't judge the quality of the amplifiers.

For the earspeaker itself there are quite a few measurements, which lets me decide on their objective quality, but for the electrostatic amplifiers on the other side there are hardly any measurements. Stax themselves only provides pretty broad measurements (They specify their THD as either <0.025%, <0.02% or <0.01%). There is really no way to objectively verify any of the available amplifiers other than subjective listening tests by people with "golden ears". I also find the usage of tubes in some "high end" gear highly disturbing...

Since the amplifier outputs a high voltage, I can understand that standard measurement equipment can't be used. But I want to change that!

I have some knowledge and experience creating electronic projects my github account contains some projects of mine) and want to take a sidestep of mainly digital based projects to an analog project:

My goal for this project:
Create a device that lets people with audio measurement hardware to safely measure electrostatic amplifiers.

Electrostatic amplifiers (STAX Pro Bias compatible) create a differential signal up to 1600Vpp (Blue Hawaii) and send it with a bias voltage (+580V) to the earspeakers. Stax earspeaker have an impedance of around 145kOhm.

I first though that I would need to create a differential amplifier with a unity gain and an attenuator as a frontend (using three OPA1611 as differential amplifier with a buffer stage) to lower the voltage to a nice 2 Vpp line level (I already simulated such a device in TINA-TI). But since we would need to create a load to lower the voltage anyhow (145kOhm in our case) and all good audio measurement equipment can handle balanced signals, we could remove the differential amplifier and just create a passive attenuation adapter box.


* Open hardware project form the start (BOM for Digikey and Mouser once finished)
* Universal high voltage input connector (Suggestions highly welcome!)
* Create a custom cable to connect a amplifier to the adapter box (If somebody has an old stax cable lying around that isn't needed anymore... I would be open for donation!)
* 2x XLR (3pin) or 4x RCA output (differential signal)
* 3D printable case (I will use polycarbonate in my final build)
* Max 1600 Vpp input voltage between one differential pair
* Currently 40 Vpp max. output voltage (APx555 and QA401 should be able to handle these voltages)
* I tried to make the design as low noise as possible (using wirebound resistors) and as safe as possible (low capacitance diodes for overvoltage protection)

Links to my current design and the current simulation.

Feedback highly welcome! (Maybe there is already such a project out there or maybe I made some stupid error in my thoughts)

After getting some feedback, I will finish the design and will later build one or two prototypes. Finally I would provide/donate them to people interested to test electrostatic amplifiers.
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