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The 990C+ discrete op-amp is the finest op-amp available for audio applications. If you want superior sound quality, the 990C+ can provide it. The 990C+ is used in the most critical audio applications. Leading recording studios, remote recording companies, disc and CD/DVD mastering facilities, and TV and radio stations use the 990C+ for superior performance. Several mic preamp products using the 990C+ are available from The John Hardy Company.


The circuit design of the original 990 is by Deane Jensen of Jensen Transformers. Deane was awarded U.S. patent #4,287,479 for aspects of this design. Every aspect of the design and performance of the 990 was optimized through extensive computer aided design and analysis. Each component of this discrete op-amp was carefully chosen for its specific task, providing superior performance compared to monolithic opamps and other discrete op-amps. For complete design theory, circuit details and specifications, please see the Jensen engineering report. The “C” modifications were developed by Steve Hogan of Jensen Transformers.


The packaging and production design of the 990 is by John Hardy of the John Hardy Company. The design enables this 49-component circuit to be constructed on a circuit board 1” square, with final module dimensions after encapsulation of 1.125” square by 0.600” high. The dimensions and pinouts conform to the API 2520 package, allowing direct replacement in most applications.

John Hardy Co. 990C+ Discrete Op-Amp

  • 990C+ 0dBu = 0.775V

    Open-loop gain, DC to 30Hz: 125 dB

    Gain error at 100dB gain: 0.4 dB

    Noise-voltage spectral density

               each transistor: 0.8 nV/vHz

               differential pair: 1.13 nV/vHz

    Noise current spectral density: 1 pA/vHz

    Noise index: 1kOhm source resistance: 0.6 dB

    Equivalent input noise voltage:

              - 20kHz bandwidth, shortedinput: 160 nV

              - Corresponding voltage level: -133.7 dBu

    Maximum input voltage at unity gain: 13.8

              - Correspondingvoltage level: +25 dBu

    Input impedance, non-inverting input: >10 M Ohms

    Input bias current: +2.2 µA

    Maximum output voltage:

              - RL = 75Ohm 13.8 V

              - Corresponding voltage level: +25 dBu

    Maximum peak output current: 260 mA

    Total harmonic distortion at 20kHz:

               - VOUT = +24dBu

               - RL=75Ohm, gain = 40dB: 0.06 %

               - RL =75Ohm, gain =20dB: 0.005 %

               - RL =600Ohm, gain = 40dB: 0.015 %

    Slew rate, RL = 150Ohm: 18 V/µS

    Slew rate, RL = 75Ohm: 16 V/µS

    Large-signal bandwidth:

               - RL = 150Ohm: 145 kHz

    Small-signal bandwidth:

               - at unity gain (ft): 10 MHz

    Gain-bandwidth product:

    Phase margin at 10MHz: >38 deg

    Phase margin at 60 deg

    Response time at unity gain:

    Supply current with no load: 25 mA

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