Caution: Electrostatic amp compatibility (HE90 owners in particular)

[Hirsch]

Thanks to Kevin Gilmore, Justin Wilson, and Mikhail Rotenberg for providing this information.

At the Saturday meet in Philadelphia, a channel drop out was noticed on the HE90. I've asked several people what might be causing this, and thought that this information should be posted. What was happening was arcing. It's caused by too much bias voltage, and can damage an electrostatic headphone. Stax amps are biased at 580 volts. However, the HE60 is rated to take 540 volts bias voltage, and the HE90 should be biased at 500 volts. This explains the listening results in Philadelphia, and also at the NY mini-meet. There were channel imbalances in the HE90's there because all amps but the HEV90 were putting out too much bias voltage for the Orpheus headphones. Any amp built for a Stax headphone can potentially be too much for the Orpheus.

The Gilmore designs and the Singlepower design can be adjusted to provide the proper bias for the Orpheus. It should also be possible to drop the bias voltage of the Stax amps (I've got a suggestion from KG to try out), but not recommended to run the HE90 out of an unmodified Stax amp.

Thanks again to the people above for researching this.

 

[spritzer]

Thanks for the heads up.

 

[kevin gilmore]

A simple 5 meg resistor in series with the bias line on the adaptors that
people have been buying may fix the problem in most cases, but is
certainly not ideal. Other solutions are possible. One is 2 resistors added
to the jack on the inside of the stax amp. A better solution is 3
additional resistors per channel to make up for the fact that there is
no easy place to get ground. More info soon. The resistors necessary
are of very high impedance and are not easy to get.

 

[Xanadu777]

Quote:

Originally Posted by Hirsch The Gilmore designs can be adjusted to provide the proper bias for the Orpheus.

I would be most happy if this is a procedure that is doable for a KGSS owner in step by step, understandable to a layman method.

Thanks for the heads up and any follow up info...

 

[cosmopragma]

Oh no, this is the end of the dream of a versatile amp for all modern electrostatic headphones.A member here even has ordered a custom McAllister tube amp with connectors for Stax Pro, HE 60, HE 90 and Koss.
Bad news for him.
How dangerous is a 580 V bias for a HE 60?
A weird idea comes to my mind.
I could get the appropriate bias from the crappy HEV 70 and the swing for music from my tube amp.
The solid state amp could run all the time this way keeping the electrostatic drivers charged and the tube amp only when I'm actually listening.
Is it doable or a flawed concept?

 

[kevin gilmore]

Chill!

If the amp has not left mcalister yet, it can easily be repaired with
2 resistors inside the chassis for the he90 connector, and another
2 resistors inside the chassis for the he60 connector.

Here are the real numbers.

Koss 600 volts (measured by me last night)
hev70 530 volts (measured by me last night)
stax high bias 580 volts (measured by me last night)

he90 now assumed to be 500 volts. I don't know anyone with
a real orpheus amp, otherwise i would have them measure it.

All headamp built electrostatic amps have a bias adjust pot on the
power supply board. You need a decent digital meter to measure
and adjust. You have to do this before the output resistor. If you
don't understand this, then contact justin for support.

There are other methods!

 

[mikeg]

I ordered this amp from McAlister Audio, but Peter McAlister has not yet begun to assemble it. I've sent him an e-mail, asking him to read this thread, and so, I assume, that things will work out OK.

Quote:

Originally Posted by cosmopragma Oh no, this is the end of the dream of a versatile amp for all modern electrostatic headphones.A member here even has ordered a custom McAllister tube amp with connectors for Stax Pro, HE 60, HE 90 and Koss. Bad news for him. How dangerous is a 580 V bias for a HE 60? A weird idea comes to my mind. I could get the appropriate bias from the crappy HEV 70 and the swing for music from my tube amp. The solid state amp could run all the time this way keeping the electrostatic drivers charged and the tube amp only when I'm actually listening. Is it doable or a flawed concept?

 

[mikeg]

Wmcmanus has an original HEV90 amp.

Quote:

Originally Posted by kevin gilmore Chill! If the amp has not left mcalister yet, it can easily be repaired with 2 resistors inside the chassis for the he90 connector, and another 2 resistors inside the chassis for the he60 connector. Here are the real numbers. Koss 600 volts (measured by me last night) hev70 530 volts (measured by me last night) stax high bias 580 volts (measured by me last night) he90 now assumed to be 500 volts. I don't know anyone with a real orpheus amp, otherwise i would have them measure it. All headamp built electrostatic amps have a bias adjust pot on the power supply board. You need a decent digital meter to measure and adjust. You have to do this before the output resistor. If you don't understand this, then contact justin for support. There are other methods!

 

[Hirsch]

So far, the 5 meg resistor in the bias line appears to be working fine. While not optimal, this solution would be great if it was enough, as it can be installed in the bias line in an adapter cable, and would not require modification to the actual Stax amp.

 

[agile_one]

hmmmm - guess we were just lucky at the last FL meet where we powered HE90's from an 007t. Glorious sound 100% of the time. Now that I think of it, however, I believe we always had a pair of Omega II's hooked up to the 007t at the same time, so the 007t was driving both the HE90 and OII. Perhaps that (2 cans in parallel) provided a margin of safety?

Two questions occur to me:

1. Was the 500V Bias for the HE90's confirmed with Sennheiser?

2. What is a "5 meg" resistor? I am not familiar with meg as a resister value - do you mean 5,000,000 Ohms?

 

[spritzer]

Quote:

2. What is a "5 meg" resistor? I am not familiar with meg as a resister value - do you mean 5,000,000 Ohms?

That would be 5 Mega Ohms

 

[fjf]

Yes, the HE60 is 540 volts:

http://buy.sennheiserusa.com/ASP/Sen...HE60_HEV70.pdf

 

[ayt999]

Quote:

Originally Posted by agile_one 1. Was the 500V Bias for the HE90's confirmed with Sennheiser?

http://buy.sennheiserusa.com/ASP/Sen...df/orpheus.pdf

 

[eric343]

Whoa!
This is a bit of an issue... was the HE90 whose channel dropped out repairable? Or was this a temporary symptom?

Quote:

Originally Posted by kevin gilmore A simple 5 meg resistor in series with the bias line on the adaptors that people have been buying may fix the problem in most cases, but is certainly not ideal.

If I remember correctly, there is already a 5 meg resistor in series with the bias output of the KGSS. How does this change things?

 

[Canman]

Thanks for the heads up guys - I have been driving the HE-60 with the Headamp KGSS and was getting some occasional noises which could possibly be arcing. Perhaps this is the problem. I would like to use the Stax 007t to drive my HE-60 so I'll be keeping an eye on this thread.