The GlassWare Noval ES HPA-2 kit. The FR-4 PCB is lovingly and expensively made in the USA. It is extra thick, 0.094 inches (inserting and pulling tubes from their sockets won’t bend or break this board), double-sided, with plated-through heavy 2oz copper traces. The boards are 6.4 by 3.4 inches, with four mounting holes. Each PCB holds one electrostatic headphone amplifier for one channel; thus, two boards are needed for stereo use.
The circuit is relatively simple. A differential input stage (usually filled with a 12AX7) cascades into a differential output stage (usually filled with an ECC99) that requires a +/-300Vdc bipolar power supply. The balanced output sits near ground potential and is DC coupled to the electrostatic headphone's stators. The circuit accepts either a balanced or an unbalanced input signal. In addition, the 10uF capacitor provokes some Aikido mojo that enhances the PSRR.
With electrostatic headphones, you simply hear things that you never hear with any loudspeaker and with few other headphones; for example, classical musicians turning their sheet music over, floorboards creaking under the bass player and his fingerprints rubbing against the strings. In addition, you hear flubbed notes and clumsy recording-room edits that you never heard before. In other words, bad recordings sound bad, whereas many other headphones homogenize all recordings, smoothing over such defects, but at the cost of smearing clean recordings.
If you have never given electrostatic headphones an extended listen, you owe to yourself to do so—but please use a tube-based headphone amplifier, as most solid-state amplifier sound crazy cold and brittle in comparison.
The ES HPA-2 can be used as a power amplifier frontend, where it can drive inefficient push-pull output tubes, such as 6AS7s in a circlotron output stage or 845 output tubes in huge push-pull power amplifier. The circuit can also drive cathode-follower output stages. This explains why mono PCBs were made rather than a single stereo PCB. It also explains why the boards include added pads for optional output coupling capacitors.
The ES HPA-2 kit includes the PCB, four sets of hex standoffs and screws, all the resistors and capacitors (5 not the ones shown in the old photo), and two ceramic sockets. Kit comes with user guide.
Two kits are needed for stereo. Requires a +/-300Vdc bipolar power supply, such as the PS-8.