This is the discrete line stage used in the X-Altra Mini II line level preamplifier discussed in detail elsewhere on this website. The presentation linked to above discusses the design and presents the measurements of just the discrete line level amplifier.
This is a paired down, simple design that nevertheless offers excellent measured performance and an interesting alternative to IC based opamp stages in line level amplifier applications. By running the VAS at a high current (c. 20mA), it is able to remain in class A up to 9V RMS into a 2K ohm load with distortion no more than 30ppm – at less output levels, its even lower – at 2V RMS out into 2k it falls below the measurement gear noise floor of about 3ppm. This is a perfect line stage for DIY builders that do not want any opamps in their signal chain. Further, by inserting or removing a jumper, the compensation scheme for the amplifier can be changed between Miller or Transitional Miller Compensation, the latter offering lower distortion at HF. The output has a mute relay which can be controlled by the Mute output on the Standard PSU or by the users own circuit. The design features 1kHz spot noise of -136 dBV.
Here is the BOM (updated 25th July 2022 to include TI JFE2140 option for U3 and U4)
Here are some performance measurements using the Texas Instruments JFE2140 dual JFET in place of the LSK389 originally specified. The TI device offers better device matching and slightly lower overall noise – its also readily available from Mouser
You can buy a PCB for the Balanced Line Stage in the Shop
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