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An adapter to connect elctrostatic amplifiers to audio measurment equiqment

Hase

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Hi,

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.

fate-front.jpg


Ideas:
* 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.
 
Last edited:

imrul

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I'd be interested in seeing some results. I have a hunch that electrostatic amps measure poorly given even high-end ones have tubes.
 

Wombat

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I take this to mean amplifiers for electrostatic loudspeakers.
 

Sir Sanders Zingmore

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I take this to mean amplifiers for electrostatic loudspeakers.
I think he means amps for electrostatic headphones (like Stax).
They need to be plugged into special amps in order to generate the voltage required (they don't work with conventional headphone amps)
(apologies if you already know this)
 

maverickronin

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I'd be interested in seeing some results. I have a hunch that electrostatic amps measure poorly given even high-end ones have tubes.

Electrostatic headphones are high impedance loads which require high voltage drive so tubes will work just fine as output devices.

That said, given the much higher gain electrostatic headphones need I wouldn't expect the same kind of SINAD you can get on a normal amplifier.
 

pkane

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Electrostatic headphones are high impedance loads which require high voltage drive so tubes will work just fine as output devices.

That said, given the much higher gain electrostatic headphones need I wouldn't expect the same kind of SINAD you can get on a normal amplifier.

I have one of the older Stax energizers that hook up to the output of a power amp. I’d be interested to see what kind of additional distortion such an energizer adds to what’s output by the amp itself.
 

kevin gilmore

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70K load per side is way to low for accurate measurements. Most stax tube amps have an output impedance of 47K ohms.
Even the mighty T2 has an output impedance of about 22K. And most stax amps have 5.1K safety resistors in addition.
An appropriate load is probably something like 1M per side. Even that is going to have a 5% effect on the signal.
 

solderdude

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Hi,

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.

View attachment 21566

Ideas:
* 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.

Do you think 150k resistive load only is a representative load mimicing a 100pF capacitor (stat + cable) that doesn't have a resistive character ?

Edit: I just saw Kevin Gilmore also addressed this. He is perfectly right of course.

It needs to have a very high resistive load where the voltage divider isn't affected by the input capacitance of the measuring gear and there must be a representative capacitive load directly over the high voltage side. The 50pF after a way too low-Ohmic voltage divider is not going to give meaningful results.
 

RayDunzl

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I take this to mean amplifiers for electrostatic loudspeakers.

I have Martin Logan Electrostats.

There is no particular specification for amplifiers.

There is a transformer as part of the high-pass circuit that steps up the voltage (50:1? or more?) from the regular old amplifier to the voltages required to be fed to the stators to push and pull the statically charged (2500V?) membrane.

1592475235753.png 1592475544850.png
 

solderdude

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Most ESL speakers are connected to regular power amps.
No need for special measurement gear other than a representative complex load.
As most stats have quite different loads (because of the used transformers, correction circuits and capacitance) you would need to use a very extreme load.
In this case the demands could be that the amp should be able to handle 2 Ohm loads and high capacitance.

For headphones driven directly without a power-amp transformer based energizer the demands differ immensly.
 

Frank Dernie

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I have Stax Lambda Pro headphones and their contemporary amp, a T1 iirc, in storage and Kingsound H03 with their little battery energiser.
I have never had cause to question their performance due to noise but I suppose distortion measurement would be interesting though I am genefally sceptical that the electronics will be a concern compared to the transducer.
 
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