The Bottlehead Stat is a zero global feedback, class A tube electrostatic headphone amplifier. It features both 580V Stax Pro Bias and 230V Stax Normal (legacy) bias jacks. There is a single input, a balance control, and level control. It has a universal power transformer. Because of the high voltages present inside the chassis this product is only offered in assembled form.
The circuit is rather unique as far as electrostatic headphone amplifiers are concerned, as it is a simple cascade of two differential amplifiers per channel, with the second differential amplifier directly coupled to the headphones themselves. This amplifier is a tube roller’s dream! A short list of tubes that can be used in the first diff amp position are: 12AT7, 12AU7, 12BH7, 6211, 6201, 5965, 7062, 6829, 6679, 7728, CV4024, 7062, E180CC, 7728, and many others. There are fewer tubes that can be used in the second diff amp, as the plate voltage is twice as high as the first stage and the operating current is also higher. We recommend either 5965s or 12BH7s in this position, with the 5965s providing more gain and the 12BH7s providing more bandwidth. 12AU7 and compatible substitutes will work in this position, as will 6SN7GTA and 6SN7GTB used in adapters.
Towards the end of 2019 Sajeev Ranasinghe of Nectar Sound contacted me asking if we had plans to do an ES headphone amp, and after some conversation he sent a demo pair of his very interesting headphones for us to listen to. Once our development backlog was clear PB set about building up a direct drive headphone amp design.
No global feedback is used in this design, which provides a smooth, open sound. We have listened to Nectar Sound, Stax and Koss ES headphones and they all work as expected.To further enhance this design, each differential stage uses our Bottlehead C4S constant current source. These provide a very high impedance under the cathodes of each differential amplifier, providing very tight AC balance in each stage.
Quality polypropylene capacitors are used in the signal path, long life high temperature electrolytic capacitors are used in the power supply, and a very generously over-sized power transformer was chosen to ensure a long lifetime. The headphone output jack is custom made and very gentle with both stock Stax plugs and the Moon Audio 5 pin plug. The jack pin contacts are recessed for added safety when plugging and unplugging headphones.
The bias supply for this amplifier is of the Cockroft Walton voltage multiplier type with additional series resistance between each output of the supply and the headphones themselves. This supply is unregulated, as it is designed as an ultra low current power supply for safe headphone operation. A 12AL5 diode sits at the bottom of the voltage multiplier and slowly turns on as the amplifier warms up.
The high voltage power supply in this amplifier is a bridge rectified split rail type. This provides a positive and a negative voltage rail that allow for an enormous swing at the output stage without voltages that are any higher than what you would find in a 300B amplifier. The positive side of the high voltage rail is soft started by a 6CG3 damper diode that provides a controlled application of the positive rail voltage to the amplifier to further reduce undesirable behaviors during startup. There is an additional small 12V DC power supply to heat the 12AL5 and the first gain stage tubes to maintain proper heater cathode voltages.
Output Voltage: 200Vrms
Bandwidth: -3dB 6Hz-54kHz unloaded, -3dB 6Hz-21kHz 100pF load 12BH7 V2, -3dB 6Hz-19.5kHz 100pF load 5965 V2.
Input Impedance: 50K with balance pot centered
Gain: varies with tube selection
Power Consumption – Approximately 65W per amplifier
“Sounds awesome!!! Lots of bass, smooth, and can handle these headphones.”
-Sajeev Ranasinghe of Nectar Sound.
“I have the prototype and it is my favorite headphone amp – sounds fantastic with the nectars!! And I am sure the one they are selling is better than the prototype!”