Summer Audio Work
Summer is almost over with classes starting up again next week.
I completed my Blue Hawaii amplifier over the summer, made a number of prototype electrostatic headphones (working on a final version now), and built a tube phono stage.
I ended up having some thermal issues on hot days after a long on-time. I recently put a bunch of holes into the bottom plate and in empty space on the PCB to hopefully allow for convection to occur more. Hopefully that’ll help, won’t have a chance to test it until I get the new headphones finished.
As for the headphones, there was a lot of trial and error learning done there. I’m sure I won’t remember even half of what I had to figure out, but here are some details on that part of the project:
Electrostatic headphones operate by moving a thin charged film between two high voltage differentially driven stators. The distance between the stators is 1mm in my drivers, many retail headphones go even lower. This film has fewer resonances than a conventional driver, but it has one pretty severe resonance in the range of 100-300hz, since after all it is basically a spring and mass system. This resonance can be damped out by having a good seal between the driver and the listener’s ear. In fact, this drives the resonant frequency lower by creating that volume, but more importantly damps it. Below the resonance, however, there will be some bass roll-off if the resonance is too high. This means the tension of the film (in this case, I’ve been using both 2uM and 6uM mylar, will probably stick with 2uM) has to be low enough to keep that resonance low. But, the tension must be high enough to give it enough spring to not just stick to one of the stators. Since there’s only 0.5mm between the film and either stator, this provides the opportunity for the film to pretty easily stick if there isn’t enough tension to pull it back to the center.
The stators I made are 0.062″ FR4, which I manually drilled with a small drill-press using a perforated steel sheet as a drill template to ensure a clean pattern:
Through lots of trial and error, I found a good tension just above the point of instability where the film hit a stator and stuck. To accomplish the tensioning, I hang weights from the film as seen in this picture:
Frames are then glued down onto the film to hold the tension.
Next, these films must be coated in a partially conductive material to allow the placement of a charge on them. There are quite a few methods people use to accomplish this. I started out using a white-glue/graphite/water mixture which did work, but is somewhat hydrophilic which would not lead to long term reliability. I ended up using an Elvamide solution, which leaves a coating of nylon behind after drying which is just conductive enough to carry the charge out onto the film.
To accomplish this coating I have just been brushing it on, but in my final revision I am going to airbrush it on since I’ve had trouble getting a uniform coating with a brush. When the coating is not uniform and equal on both sides, it can lead to channel imbalances.
A mylar film glued to the frame and coated with Elvamide:
Next in the process was figuring out how to make a protection film for the inside. Since sweat can build up while listening, this could lead to moisture getting into the driver if run with no protective film. Initially I just put another tensioned mylar between the driver and the ear, but found that to cause bass dropoff (green without protective film, red with):
Having even enough tension to make the protective film structurally stable caused some amount of bass roll-off.
I noticed almost all retail electrostatic headphone drivers use a wrinkled material as the sweat protection screen, and figured I’d give that a shot. This seems to greatly lower the material tension, but allows it to still stay structurally rigid.
Over the last week or two, I’ve been learning solidworks to design some more reliable headphones. In this revision I will be making all parts by CNC (with the exception of the stators which have been drilled with more precision using a complete drill mask for all holes including mounting holes)