What is this device in STAX headphone?

I have an old STAX Lambda Nova Signature made in 1995. I enjoy using it very much with a home made tube amplifier. It is old and some parts were damaged, but I have DIY repaired them as far as I could. I found one device in the bias circuit (pointed by an arrow in the photo). It looks lika a SMD resistor just attached with the electrostatic sound element. If so, it may separate the bias circuit effectively between channels. I tried to measure its resistance using a digital multimeter, but it showed out of range. I appreciate it very much if anyone kindly tell me about this device. If it is a SMD resistor as I guess, how much is the resistance? It will be very helful, because I am planning to build a ESL headphone by myself!
STAX_bias_device_small.jpg
 
Member
Joined 2009
Paid Member
Hi,
If your multimeter is unable to tell it's value and you can unsolder the part: make a simple rc highpass filter with a known capacitor value, run white noise through it and from there you can guess it's value ( from -3db point and 1/(2xPIxFxC) formula).
 
Last edited:
  • Like
Reactions: 1 user
Why do you think it will matter if it is 100M or 200M in a headphone?
I have no clear answer about the value. The resistor may limit the movement of charge and at the same time suppress undesirable arching that sometimes occurs on ESL57 panels. QUAD ESL57 has no resister in the high voltage circuit, ESL63 has 10 Meg ohm and a Neon lump//capacitor, and old Acoustat series have 500 Meg ohm!. So, there are different policies about the values. I am not sure but It might or might not depend on the conductivity of the membrane coating. Someone has reported that the panel type STAX ESS-4A/6A has only 5.1 Meg ohm. Standard bias (~230 V) STAX SR-5 and SR-X MK3 has no such a resistor on each sound element but has only one resister of several Meg ohm inserted in the bias line of the adaptor or amplifier. Do all professional bias (580 V) STAX headphones have a resistor on each channel and why is it so high value?
 
old Acoustat series have 500 Meg ohm
I have these speakers, this is standard for electrostats with low(er) resistive membrane coating. The old B&W model 70 had 100M resistors as well. The high value is required to keep the membrane charged but must not be a firm voltage as then the membrane would get attracted stronger when moving closer to the electrodes when playing music and naturally would cause distortion, so the higher the resistance the better this works just not too high as then humidity of the air would make this unstable. An other way (better way) would be to make the membrane more resistive but that is difficult to produce and not stable over time (look at Quad or ML). Acoustats use low resistive membranes therefore the charge resistors need to be higher and vice versa...
 
Such a resistor is only built into very few STAX headphones.
It probably depends somewhat on the design of the individual model in combination with the amplifier used whether it is needed or not.
If you want to build one, I think you can do without it.
I also build electrostatic headphones and have never used a resistor ...
 
  • Like
Reactions: 1 user
Thank you very much. Your explanation is readily understandable. When restoring junk ESL57s, I used soluble Nylon (Elvamide®) successfully. For very old STAX SR-5 and SR-X MK3, I diluted an ESD-safe spray (Licron Crystal®) with purified water and painted thinly on the membrane with a brush, and got satisfied. My old SR-Lambda Nova Signature had no problem with the membranes, but needed to clean and replace sponge dumpers that had deteriorated to tatters. Then, I found the SMD device in this topic beside each sounding element. This gave me a new idea. So, I have put a resistor of a 6.2 Meg ohm in the bias line for each element of SR-5 and SR-X MK3. It sounds well at least for me. I'd like to check and measure the difference between a common resistor for both channels and an independent resistor for each element; no difference or possibly some difference in channel separation, or if any. And I hope to know the adquate value of the resistance. First, I will try 100 Meg ohm. Thanks, again.