Hello
I had a chance to buy two pairs of Stax ESTA 4u speakers.
Both pairs look nice, but all four had imbalance issue and I decided to check what is inside and if I can repair them.
As you can see, drivers are easy to open. The disintegrated foam's particles got into drivers and stick to the diaphragm (foam was layered with some glue).
I had to clean it all from this and use Licron on diaphragms (I did that only with one pair atm).
Now the one pair works well, bas is not so strong, but it is enough to work with some active sub.
The stereo sweet spot is really thin, but when you seat in the right place the image is sharp and precise.
I'm thinking about rebuilding the second pair, but to use drivers to make custom speakers, rather tall, just to use it as computer speakers.
What is the biggest surprise? This is 3 way project !
There are two diaphragms but 8 stators.
Looking from the back of the speaker:
the left driver is full playing bass and lower mids. The right driver is two way system. The very right part is playing bass and all mids. And the second part is driver with full signal from transformers.
The open area (I mean the window where stators are open to outside) is 161x92 mm.
The one stator is 104x171 mm (50x171 mm one part, mind the gap in the middle between stators in one driver).
I had a chance to buy two pairs of Stax ESTA 4u speakers.
Both pairs look nice, but all four had imbalance issue and I decided to check what is inside and if I can repair them.
As you can see, drivers are easy to open. The disintegrated foam's particles got into drivers and stick to the diaphragm (foam was layered with some glue).
I had to clean it all from this and use Licron on diaphragms (I did that only with one pair atm).
Now the one pair works well, bas is not so strong, but it is enough to work with some active sub.
The stereo sweet spot is really thin, but when you seat in the right place the image is sharp and precise.
I'm thinking about rebuilding the second pair, but to use drivers to make custom speakers, rather tall, just to use it as computer speakers.
What is the biggest surprise? This is 3 way project !
There are two diaphragms but 8 stators.
Looking from the back of the speaker:
the left driver is full playing bass and lower mids. The right driver is two way system. The very right part is playing bass and all mids. And the second part is driver with full signal from transformers.
The open area (I mean the window where stators are open to outside) is 161x92 mm.
The one stator is 104x171 mm (50x171 mm one part, mind the gap in the middle between stators in one driver).
Attachments
Thanks so much for sharing such good pictures of your STAX restoration project. I had acquired a few panels of this type some time ago, but did not get the electronics that went with them and they have just been sitting in the closet waiting...Evidently the STA-4U were also available in kit form.
Your post prompted me to search the web for information on them. I found one posting on the AudioKarma forum that had users manual and kit technical manuals posted. They were quite large files so I can't repost here, but attached is the schematic from the technical manual that I think matches your speakers.
Stax ESTA 4U | Audiokarma Home Audio Stereo Discussion Forums
The circuit is quite interesting, including a regulated self-bias HV supply and a limiter on the audio input to help avoid over driving the panels with powerful amplifiers.
Your post prompted me to search the web for information on them. I found one posting on the AudioKarma forum that had users manual and kit technical manuals posted. They were quite large files so I can't repost here, but attached is the schematic from the technical manual that I think matches your speakers.
Stax ESTA 4U | Audiokarma Home Audio Stereo Discussion Forums
The circuit is quite interesting, including a regulated self-bias HV supply and a limiter on the audio input to help avoid over driving the panels with powerful amplifiers.
Attachments
Thanks for schematic, now I can see clearly how it is made!
It was some time after the restoration and I'm still in love with them.
But I had to reorganize geometry of installed drivers.
Both speakers are made equally.
So there is no symmetry - when you put it on the bench, the driver with full signal (what is important with highs) is placed inside and the other is outside.
I decided to make it full symmetry and place drivers with full signal outside of the speaker's front.
It was good decision - now they are playing even better, with more holographic and precise stereo image.
It was some time after the restoration and I'm still in love with them.
But I had to reorganize geometry of installed drivers.
Both speakers are made equally.
So there is no symmetry - when you put it on the bench, the driver with full signal (what is important with highs) is placed inside and the other is outside.
I decided to make it full symmetry and place drivers with full signal outside of the speaker's front.
It was good decision - now they are playing even better, with more holographic and precise stereo image.
Good idea! And thanks for sharing the outcome.
I had wondered if the ESTA-4U speakers were built identical or as a mirror image pairs like Acoustat did with their Spectra 11 and 1100 models. The later STAX ELS-F81 and F83 used a symmetric layout with the full range segment in the middle.
Attachments
Oh, thanks for another schematic 
I would love to get some bigger Stax speakers in my hand.
I have Quad ESL-63 with wider dispersion on highs, but I think that it is kind of compromise between quality and comfort of listening.
ESL-63 are easier to listen outside of sweet spot, but cannot generate such sharp and precise image like Stax or any other conventional ESL, in my opinion ofc
I would love to get some bigger Stax speakers in my hand.
I have Quad ESL-63 with wider dispersion on highs, but I think that it is kind of compromise between quality and comfort of listening.
ESL-63 are easier to listen outside of sweet spot, but cannot generate such sharp and precise image like Stax or any other conventional ESL, in my opinion ofc
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