Portable Electrostatic Headphone Amp

I’ll be going home for winter break, and don’t want to carry my big electrostatic amplifier, so I decided to make a Class-D portable amplifier.

Here are a few schematics… Eagle is too much of a pain to make nice schematics, so I’m not going to post a full schematic at this point.

Output stage:

Input stage:

Aside from the actual amplification stage, I need to make 300V rails and and a 350V bias rail (configurable for up to 580V) to charge the diaphragm in the headphones. To do this I’m using the LTC3803 flyback controller. This is a current-mode control chip with an external compensation network in a convenient SOT-23-6 package. I wanted to test this out before ordering the full board, so I built a prototype converter for the 300V rails (if that works, the DC bias flyback will almost surely work).

Tests show it working fairly well, although it draws quite a bit of no-load power. I attached a BJT as the load so I could vary the output current to plot efficiency vs. output, which resulted in the following data:

I fit the equation

which represents a constant controller current (including core losses with the output load of the feedback resistors). This fit results in a controller power draw of 1.25 0.05W and an efficiency of 0.78 0.04. This is a bit high on the controller power draw, which I may be able to get a bit lower by not using the shunt regulator in the controller. Efficiency could be better, but is quite tolerable given that the amplifier really won’t draw much of any current on the 300V rails.

With summer comes another few weeks at home to spend time in the mountains (and in the snow since it was a pretty heavy winter for snow so lots still around).

All trips were solo, just the way I like it!

First up, Mt. Daniel (7/21/2012):

A failed summit but an awesome trip. Was feeling sick 1000ft below the summit, so I turned around. Still a great place, definitely will return sometime and probably make it a two-day climb.

Next, a quick run to Kendall Katwalk (7/24/2012):

Quite a bit of snow left for so late in the season. A number of very steep snow crossings before getting to the Katwalk, although there were good boot paths through everything. After the Katwalk, almost all snow covered.

Last, Mt. Adams before heading home to Boston (7/29/2012):

Did this one as a 2-day trip, and made it to the summit. Of course, the one thing I somehow forgot to bring was my camera since it was on the charger overnight. Oh well, iPhone will do!

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.

Success!

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)

Blue Hawaii

I’ve been working on making a Blue Hawaii electrostatic amplifier (designed by Kevin Gilmore). I’ve spent a lot of work on the chassis which I built myself. I’ve been having some trouble with premature clipping in the amplifier which I still need to sort out, and also need to figure out why there seems to be bandwidth issues above 10khz. Other than that, the project is well on its way to completion.

Power supply board

To do toner transfers for double sided boards, I hold the papers up to a light and align them, then tape the papers together to form a sandwich, and slide the board in before transferring. I’ve never had it off by more than a quarter of a drill hole. Usually it’s practically dead on.

Amplifier board after populating

Constant current sources as plate loads

All the boards (bottom right is a slow warm-up for the filaments and a delayed B+)

Milling the heatsink for the back of the power supply to fit a mil-connector and IEC input

Heatsink after cleanup and painting

Since paint isn’t the best of a thermal conductor, I took the heatsink back down to aluminum where transistors will mount. Fastest way was to just mill the surface off.

Covers to go over holes for tubes to exit the chassis

The chassis is built by having bars at each corner that faces screw onto. These were milled to be exactly 90* angles on all sides, so just screw it together and it forms a perfect box.

Drilling mounting holes for the base plate. I laid out the parts in cad (something I rarely do) to get drill positions so I could get all this done before getting boards ready to position.

Power output and IEC

The nicest box I’ve ever made. All the aluminum panels were polished to near mirror finish with a slight texture

More to come as I put more work into the project.

Live MIDI Interrupter

I had a fairly crude monophonic interrupter built for big-coil operation, but I wanted to improve on its design. It had no software control over pulse width so every note had the same width, and of course polyphonics is more fun.

One critical design requirement was that I need hardware protection against the controller accidentally going CW, even if the microprocessor somehow screws up. This is relatively easy by using a 555 as a one-shot on each rising edge, then AND gating that to the actual signal so that if an abnormally high duty cycle (or more likely a duty cycle of 100%) is attempted, the one-shot will turn off at a pre-set time (determined by trimpot R4). It is worth pointing out that an inverter could be used to trigger the 555, which requires a downward-going edge to trigger. I chose to use a second microprocessor output since it saves on components (and I wanted as many gates buffering the output power as possible).

I also wanted to have a fixed oscillator in the interrupter so that I can still use it as an easy optical oscillator for bench testing coil controllers without the need to setup a MIDI device. I threw in another 555 for this. A SPDT switch on SV1 allows for selecting between MIDI or this fixed oscillator.

Another goal was to have the controller programmable over the MIDI interface to control parameters such as pulse lengths specific to note (allowing low notes to be on for a longer bang than higher notes), MIDI channel, and multipliers to control pulse width during polyphonic playback. This can all be programmed over SYSEX, a communication means within the MIDI protocol that is designed for flashing MIDI hardware with firmware or updates.

I put together a quick python script to allow for changing parameters, although I may make a GUI at some point.

This project was also the first time I’ve tried 0.4mm isolation on ground planes, which allows for ground planes to pass between pins of a DIP socket. It etched fine using the laminator for toner transfer. The etch precision is getting so good that I need to look into solder masks.

Laminator Etching

I setup a laminator at MITERS for etching a few months ago, but forgot to write about it until now. So here’s some pictures and a little info on the modifications to make it into a fantastic PCB etcher:

It is a GBC 9″ laminator (pretty cheap, $45 on amazon usually, I’ve heard you can get it for $10 or even less if you get lucky on sales or search around a lot). Out of the box, it’s an incredibly low quality laminator that can’t do much more than paper. If you put a circuit board through it, the gears in the motor strip themselves out and it quickly destroys itself. You can kind of force it to keep going by pushing the board in, but it’s totally not worth it after how great it performs with some modifications.

All I did was take it out of its case and put a new motor on it. I put a slower motor on, something like 2RPM instead of the stock 6RPM motor (I forget the exact numbers). You can’t really go too slow since you want lots of heat and pressure to get a circuit board to transfer well, and multiple passes aren’t ideal since it can lead to blurring if thin paper is used.

I threw together some mount brackets which weren’t quite strong enough to prevent the motor drive gear from skipping, so I made a little bearing to keep the motor shaft lined up with the gears. That prevents the gears from slipping away from each other, and it easily feeds standard PCB thicknesses.

The capacitor on the front is a motor start cap which is overkill in size for this motor, but it was laying around so why not use it.

Transfers come out 100%, every time. There’s no longer any guess work involved with the time and pressure of using a clothes iron. It makes it a real treat to etch boards.

Adding Better Diodes

I’m currently bypassing the slow reverse recovery diodes in the IGBT bricks with much faster minibrick diodes.

I am adding a diode to the IGBT path to turn it into a switch that blocks both directions, and conducts in one direction when on, then adding a fast diode across that assembly.

The blocking diode doesn’t need to have a voltage standoff rating of the full bridge, because it is in series with the IGBT brick which can stand off the full voltage. Ideally I would use the highest current and fastest diode available, with no regard to standoff voltage. Since a friend had some laying around, I used some DSEI2x101 minibricks for this.

For the bypass diode, full voltage standoff is required. For this position, I’m using ST 12012TV1 diodes.

I’m a bit concerned about passing all this current through these diodes, but it is well within the pulse and I^2S rating of the components so hopefully it will be fine. All the minibricks will be heatsinked to busplates.

Some shiny pictures of the modifications in progress:

Polyphonic Hatcoil Controller

I’ve been pretty busy with class work so I haven’t been building a whole lot recently. I did get a polyphonic controller working for hatcoil, though. By using both timers on the Atmel based controller, it can simultaneously play any two notes making the songs much more interesting to listen to.

Also did a bit of dry ice overclocking, although the board seems to be damaged and won’t get basically any overclock stable (used to run 3.6ghz stable on water and can’t even get it up to 3ghz anymore)

Hat Coil Updates

Another safe avalanche-free winter at home with about half a dozen summits, but enough mountaineering, back to power electronics!

As promised, hatcoil is being brought back to life. Instead of repairing a questionable design, I decided to start over and make the whole thing better. This time it’ll be all on one heatsink and control board with a full bridge plus an over-current/inrush protected boost converter.

Since recently I haven’t been real into using TL494′s for moderate or high power boost converters, I just put a bang bang hysteresis controller on the board with voltage feedback. I also put a hall effect current sensor on the boost inductor, since I think inrush was killing my old boost. Basically this is what goes wrong without this protection:

When the battery pack is plugged in, the coil’s bus cap is uncharged, and will quickly charge through the boost inductor. There is no problem with this and it is normal behavior, except that if the boost logic is running during this phase, the boost FET might blow up since it has to take this current too. So to solve this, I just put a current sensor on the inductor, and at the start of every boost bang it checks with the sensor and makes sure inductor current is below a set threshold level (a few tens of amps at most, or probably even lower since this is a discontinuous boost design). This also has the pleasant side effect of making sure inductor current doesn’t build up and become continuous at the early stage of charging the bus-capacitor, where -di/dt during off time might be less than charging di/dt (leading to possible fault modes at a 50% duty cycle).

Both of these issues can be seen in the following simulations, the first without protection and the second with. Green is inductor current, and blue is transistor current.

I also did a bit of math on the boost inductor this time rather than just winding onto a random ferrite and crossing my fingers (it’s so tempting in power electronics, but it’s such an incredibly bad idea!)

I first considered conservation of energy in the inductor, which says that assuming discontinuous mode. can be replaced with where is duty cycle. This says that which is the equation you can use to figure out an appropriate inductor (ignoring any losses which could be added to account for less than 100% efficiency). Substitute battery pack voltage, desired duty cycle, desired frequency, and solve for inductance. Then all that is needed is to make sure the inductor won’t saturate. I came out with ~5uH for 50khz operation.

To etch (on dextrin paper, I can’t wait to try that magic!):

Iron

I gave in to peer pressure :/ 60hz Iron gate drive supplies