Recently I built the KGSSHV and it is also a stellar amplifier, but for me it was a bit of a struggle to build, very technical, complicated, costly and getting harder and harder to source components. I made a mistake on the first power supply and almost gave up a couple of times, but built another one and it worked. My very first amp was the opposite, very cheap to build, worked the first time every time, everything fit on one board and it performed well for the cost and simplicity. But there were some noticeable improvements that could be made. The Agave amp is somewhere in the middle. It has everything it needs to perform very well, with margin, without being overkill. The added cost, space and effort is targeted to places where it would make the most difference in performance.

One of the main challenges with electrostatic amplifiers is that they are driving a capacitive load (unlike an inductive load with traditional speakers). There is a misconception sometimes that since it is a capacitor that they “draw virtually no current” and don’t need much power. This is not true because the headphone impedance will follow the formula 1/(2*pi*f*C). Where f is the frequency and C is the capacitance of the headphone. Therefore the impedance will change proportionally with the audio frequency. At 20kHz and assuming a 150pF load, is about 53k ohms. At say a 400V peak going into 53k this is about 7mA. About 3 Watts, and multiply that by two legs (-R and +R) and then two channels (L and R), and you have quite a lot more power needed compared to a traditional headphone. And at high voltages, components can become quite large.

Unfortunately the current is just a bi-product of creating the voltage, but the voltage is what is creating the electrostatic force needed to drive the headphones. Some amplifiers, such as the SRM1/MK2, and also one of my very first designs, use “resistor current sources” meaning that pulled up to the high side rail is a large resistor (usually around 50k Ohms). This saves a lot on space and costs less but at high voltages, and high frequencies, the amp can’t keep up with the slew needed to charge and discharge the capacitance through a large resistor. This appears as voltage distortion that can be seen on an oscilloscope, and heard as sharp sibilance at higher volumes.  If there is enough open loop gain in an amplifier, one could add a large amount of global feedback, but this feedback requires compensation to ensure loop stability and this could make the signal a bit more sluggish. These are all normal design challenges every electrostatic amp has to deal with.

The Agave amplifier is inspired by the Nelson Pass “Zen” speaker amplifier where a single FET operating in Class A is used to drive the load, similar to how a Triode is used in a tube amplifier. This concept does not use any global feedback but a small amount of local feedback to stabilize the loop. A true top side current source is used instead of a resistor current source. Using a current source instead of a resistor adds a lot of open loop gain to the circuit, which makes the local feedback have much more headroom to work better, and it also provides enough bias current to keep up with the slew needed for high frequencies. Finally, driving a single FET can be challenging at high frequencies due to something called the Miller effect. Which is where the output capacitance of the FET gets multiplied by the gain of the amp, so the pre amplifier circuit needs to be able to drive a large amount of FET capacitance. The way I get around this is by doing what’s called a Cascode design, where another FET is added beneath the output FET to drive and buffer the output FET.

Where I saved space and cost is with the power supply. The high voltage supply is not actively regulated and is designed like a vacuum tube amplifier power supply. A large 600VAC transformer is used and RC’s are used to provide the +/- 390VDC needed to drive the circuit, and very large capacitance (overkill really) is used to remove any voltage ripple and ceramic caps are used to remove any high frequency noise. The supply is just as stiff as a regulated supply. The main “disadvantage” is that +/-390V can be slightly higher or lower depending on how high or low your mains 120VAC is. But this is not really an issue since current sources are used, the bias current will always be the same. The low voltage rails for op-amps and signal level audio are 14V and 7V and are fully regulated and filtered.

The +600V or +580V selectable bias voltage for the diaphragm  is done with voltage doublers and zener diodes. This is a very typical method but I made sure that the capacitors used in these doublers are large enough to really maintain that very high voltage under load (which is a series resistor and Zener’s) such that there is enough current to bias the Zener’s properly to give their true voltage output. So you really do get exactly a 600V or 580V with no ripple, depending on what you select with the circuit.

I use CL90 current limiters at the input of the supply to limit the inrush current so that the fuse can be properly sized for safety. I etch away some metal oxide so that metal on metal contact can be made to ensure full continuity in conductivity of the chassis. I then tie the chassis directly to earth ground for the mains plug.  The ‘secondary ground’ or signal ground (which is derived from the isolated secondary of the main transformer) is also grounded to earth via two back to back diodes and a 330 Ohm series resistor which is a common strategy to enhance safety and prevent ground loop hum.