John Hardy Company Twin Servo 990 1-CH
1-Channel Microphone Preamplifier

OUR PART #:  990-1CH

The Jensen Twin Servo 990 Mic Preamp is considered the world's finest mic preamp by audio experts.
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Our Price: $1,537.60
List Price: $1,550.00
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John Hardy Company Twin Servo 990 1-CH
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The Jensen Twin Servo 990 Mic Preamp is considered the world's finest mic preamp by audio experts. It's the ideal integration of advanced design software, hardware, engineering know-how, and carefully controlled 'golden ears' tests. It has earned high marks for sonic performance since its introduction in 1988.

The basic circuit was originally published as an application note by Jensen Transformers for their finest microphone input transformer, the JT-16-B. Each channel uses two precision 990C operational amplifier modules, developed and patented by Deane Jensen, to achieve one tenth the distortion and twice the bandwidth of a single stage design.

The Jensen Twin Servo 990 mic preamp has a 19" wide mainframe rack-mount chassis (1.75"H x 19"W x 8"D) and this model offers 1-channel. Additional channels can be easily added later up to the total capacity of the mainframe.

Note: Image shown above has been modified to better represent the 1-channel model, actual appearance may vary. 


  • JT-16-B Input Transformer. Jensen's best mic input model.
  • Two 990 Discrete Op-Amps Per Channel. Faster, quieter, more powerful and better sounding than typical monolithic op-amps.
  • Elimination Of All Coupling Capacitors From The Signal Path. Results in less degradation of the audio signal.
  • JT-11-BMQ Output Transformer. Jensen's best line output transformer.
  • Input Bias Current Compensation Circuitry And DC Servo Circuitry. Eliminates signal degrading coupling capacitors.
  • Fully Sealed Conductive Plastic Gain Control. Provides smooth continuous gain adjustments from 16dB to 60dB.
  • 20 OHM MIC switch or 20dB PAD switch.
  • PHANTOM POWER switch.
  • All Front Panel Switches Are LED Illuminated.
  • Gold Plated XLR Connectors for maximum reliability.
  • Ground Lift Switch On Each Channel. Helpful for eliminating ground loops.
  • VU-1 meter. Very accurate and informative 20 segment LED meter, with separate Peak LED, for monitoring preamp output.
  • Toroidal Power Transformer With Additional Shielding. Eliminates hum problems so no external power supply is needed.
  • Universal Power Supply. Internal switch provides six primary voltage choices to accommodate use anywhere in the world.
  • Chassis Ground Isolation Switch. Helpful for eliminating ground loops.
  • Pin 2 Or Pin 3 High. Internal jumpers make changes in the field easy.

Specifications, typical (0dBu = 0.775V):

Voltage Gain Range +1dB 16 to 60 dB
E.I.N., 20-20kHz bandwidth, unweighted, 60dB gain, 0dBu = 0.775V 150 Ohm source -128.7 dBu

Frequency Response, ref: 1kHz 20Hz -0.05 typ, -0.1 max dB

20kHz -0.05 typ, -0.1 max dB

0.4Hz and 140kHz -3 dB

Time Domain Response

     Deviation From Linear Phase (maximum) 20Hz to 20kHz +2 deg
     Group Delay 20Hz to 120kHz 2.16 +0.05 µs
     Squarewave Risetime
2.2 µs
     Squarewave Tilt 20Hz 5.8 %

Total Harmonic Distortion (note 1)

     Output Level +4dBu, any gain setting 100Hz to 20kHz 0.001 typ, 0.003 max %
  20Hz 0.036 typ, 0.100 max %
     Input Level +8dBu (note 2) 100Hz to 20kHz 0.001 typ, 0.003 max %
  20Hz 0.120 typ, 0.300 max %
Common Mode Rejection Ratio 60Hz 125 dB

1kHz 100 dB

10kHz 80 dB
Common Mode Input Voltage
+300 V-pk
Optimum microphone Impedance (switch selectable)
150/20 Ohms

Input Impedance, +15% max 20Hz to 20kHz 1.27 k Ohms

Phantom Power

+52 VDC
     Current (maximum)
9.5 mA

Maximum Output Level 75 Ohm load +24 dBu

Output Load Impedance Range minimum/nominal 150/600 Ohms

Output Source Impedance 20Hz to 20kHz 70 +5 Ohms

Power Requirements, 4 Channels +15% 100, 120, 140, 200, 220, 240 VAC
48 to 66 Hz

25 Watts

Dimensions (note 3)
1.75H x 19W x 8D Inches

Notes :

     Unless otherwise specified : 25°C ambient, 150 ohm source, 600 ohm load.
     (1) Low frequency response distortion is almost entirely 3rd harmonic.
     (2) Worst case of maximum possible input at minimum gain with unclipped output.
     (3) Handles extend 1" beyond the front panel.
     "Twin Servo" and "Comtran" are registered trademarks of Jensen Transformers, Inc.

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