Hybrid Electrostatic Amplifier


Posted on Apr 12, 2012

The genesis for this hybrid electrostatic headphone amplifier occurred when I was in Hawaii on vacation, at a fancy hotel on Maui. Sitting at the bar on the beach, drinking `Blue Hawaiis,` I drew the schematic for the amp on a placemat. The design is my conception of the mysterious and rare Stax T2 amp, which I have never been able to find at anything resembling a rational price. It also used 6DJ8s as input tubes with some solid state in the second and third stages. My design uses the first and second stages from my solid state electrostatic amplifier coupled with a third FET stage and then the final grounded grid stage. My design ended up with a fairly large amplifier pulling significant amounts of power which results in a very smooth and extended frequency range from DC to over 200khz (-3dB at 400khz)


Hybrid Electrostatic Amplifier
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Hybrid Electrostatic Amplifier - image 1
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Figure 1 is the amplifier schematic. The entire amplifier has a differential topology from input to output to get a balanced input and for lower noise, less ground loop problems. The first stage is a differential amplifier with feedback directly from the output stage. It works equally well with both balanced and unbalanced audio input sources. The step attenuators from Goldpoint make good volume controls for this stage. The JFET device (Q1) is a dual JFET all on one wafer. It is known for extremely low noise and excellent matching, and is used in a number of expensive designs, such as the Nelson Pass amplifiers. Q17 is a current source that sinks 3mA. Because the amp is totally DC coupled from input to output, drift in the input stage is a bad idea. Since the first two stages run in current mode, the JFET input is more linear than a pair of bipolar transistors. Dual transistors all on one wafer suitable for audio use are hard to find these days. The FETs steer current away from the current sources Q2 and Q3. Together Q2 and Q3 each supply 2mA or a total of 4mA. The Q17 current source takes away 3mA leaving 0.5mA in each of Q4 and Q5, but some of the sink current is coming from the output feedback, so each FET is actually using somewhere between 0.5mA and 1mA. The approximate voltage gain of this stage is 5; this stage really runs in current mode. The unit was designed to work equally well in both balanced and...




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