Short of my high school fusor, this has been the most time intensive and significant project I have ever undertaken. Initially, I was using no CNC tools to build any of this, and I was basically just experimenting and figuring out tricks and methods for making electrostatic drivers. The designs later became much more elaborate.
What is an electrostatic driver? It is essentially just two metal plates (stators) with a thin film of plastic (diaphragm) spaced between them. The plastic is given a coating which allows charge to build up when a DC bias is applied to it. The plates are then differentially driven with a high voltage audio signal, and the plastic film is forced by the electric field to move.
Early Drivers and Headphones
My first drivers were hand drilled in 1.6mm FR4. The drilling was accomplished by using a sheet of perforated steel as a drill template. A small drill press was then used to drill them under water, since FR4 isn’t so good to breathe.
The plates were then painted. By painting the stators, it is less likely that the diaphragm film will stick to the stator. The film needs to remain centered between the stators, but they are electrically inclined to stick to a stator. The tension of the film is all that prevents it from sticking to one side.
This shows the elements of the driver. On either side are the stators. The middle left is the diaphragm charger, which is nothing more than a copper ring that is in contact with the diaphragm to give it a DC charge. The middle right is the diaphragm holder, to which the plastic film will be attached.
Stretching the film is a difficult process and needs a lot of experimentation. If there is not enough tension, the diaphragm will stick to one of the stators, a condition known as the driver going unstable. If there is too much tension, there is almost no bass since there isn’t enough electric force to overcome the tension of the material.
The stretching was accomplished by hanging weights from a mylar film then gluing the holder to it using contact cement (quite a few adhesives were tried, and contact cement is without a doubt the best to use – this is what most DIY electrostatic driver builders use).
And finally, here is the first electrostatic driver I ever made.
Some quick headphone frames were put together. No, they’re not pretty, this was an early prototype. The headphones actually sounded surprisingly good. Here they are next to Sennheiser HD600 headphones.
I made some crude leather pads, and added dust and sweat protection screens. The driver itself is very sensitive to dust or moisture since there is a high electric field between the two plates and the diaphragm. If dust or moisture gets in, it can cause the diaphragm to go unstable or induce squealing noises.
I was then introduced to laser cutting, which began my experience with CNC cutting. I made some slightly nicer frames out of ABS plastic using the laser, and the first headphones were complete. They actually look kind of like Stax Lambda headphones.
Sadly I disassembled them. I wish I could compare them to what I have now.
At this point, I moved onto my next build which was a clone of the Stax SR-007 headphones. These ended up with not only a laser cut chassis, but also CNC milled stators and diaphragm holders. This was necessary for the circular shape, and also provided a great opportunity for me to learn about CNC!
After painting, four stators were ready for assembly
And then the diaphragms were stretched
And here is the first version of my SR-007 clones
I later found that smaller holes tends to have slightly better bass response, and the SR-007 has only the center of the stators drilled leaving the outside area present. I made both of these modifications and the sound was slightly improved.
The biggest stumbling block at this point was my diaphragm tensioning. I wasn’t getting enough tension, nor was it a very repeatable process. I made a much better tensioning jig out of acrylic and lead weights. I’ve been using this ever since and it does great for repeatable stretching.
I needed some HD-600 replacement earpads anyways, so I figured I’d make some electrostatic headphones that use that earpad. I don’t have many pictures of this one, it was a pretty quick build. While they physically came out quite well, the sound just wasn’t good. They severely lacked bass due to their small size. In general, electrostatic headphones need to be large to have good bass response.
These were made of wood instead of ABS plastic
Here is a view of the driver stackup. Drivers consist of a protection plate, dust screen, stator, diaphragm, stator, dust screen, and another protection plate. These are all separate by spacers.
Bigger is better, right? These are clones of the famous Sennheiser Orpheus headphones, and are my most recent project. MITERS got a CNC mill which made fabricating parts much faster and easier. I’ve also been experimenting with making stainless steel stators, but that’s still a work in progress and currently the headphones have FR4 stators.
This time I made fairly nice earpads with real leather. The frames are made of Paduak wood, which is also CNC milled.
Where are all my pictures of this build?
And finally, here is all my electrostatic stuff at the New York Head-fi meet.
Based on the feedback I received, I have improved the bass. I’ve switched to a new stator design, changed from 0.8mm to 0.5mm spacers, and moved from 2um diaphragms to 1um. The thinner stators make more volume per volt since the electric field increases. I also decreased the bias voltage a bit which allows for a lower diaphragm tension resulting in more bass response.
I’ll add to this post when I build more headphones!