Headphone electrostatic JFET Amplifier
Posted on Sep 16, 2012 6436
Under: Audio Amplifier Circuits
The amplifier operates primarily in the current domain. The first stage is a voltage controlled current sink. The second stage is a current-controlled voltage source. The fourth stage is a constant current sink. The main advantage of current domain amplifiers is speed. Standard voltage gain amplifiers with lots of gain are affected by the Miller Effect which prohibits extended frequency response. This solid state amp is so much better than my tube amp that I no longer listen to it. I'm not a solid state snob; it's just plain better. The people who have listened to this amplifier (some of whom were giants in the industry in their day) love it, much more than my tube amp. I love it too. I can't stop listening to it. The tube amp has moved into a secondary position in my listening rack.
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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 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. 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 approximate voltage gain of this stage is 5. But it really runs in current mode. The unit was designed to work equally well in both balanced and unbalanced mode. For single-ended signals, ground either the + or - input and apply signal to the other. The much higher impedance of the JFET works better when one side is grounded for unbalanced inputs. The second stage starts with a constant current source. The current source feeds a common base amplifier. The common base amplifier feeds a modified Vbe multiplier. I believe a famous designer is now calling this circuit a current tunnel. Its the most linear way of translating the voltage down to the bottom rail. The voltage gain of this section...