The “it does everything” box.
The 517 provides a world-class transformer-coupled mic preamp, one-knob compression, DI circuitry with Vari-phase, Silk saturation and DI + mic blending capabilities – all in a single module.
 
SPECIFICATIONS
 
FREQUENCY RESPONSE
Mic Pre, Main Output, no load.
-0.1 dB @ 10 Hz
-1 dB @ 120 kHz
DI Input, Main Output, no load. 
-0.1 dB @ 10 Hz
-1 dB @ 120 kHz
NOISE
Measured at Main Output, unweighted, 22Hz - 22kHz, Terminated 150 Ohm. 
With gain at unity: better than -100 dBu
With gain at 66 dB: better than -60.5 dBu
Equivalent Input Noise: better than -125 dBu
DI
With Gain @ Unity: Typically better than -100 dBu
With Gain @ +30 dB: Typically better than -75 dBu
HIGH PASS FILTER
Frequency: 80 Hz
Slope: 12 dB/Octave Bessel
Microphone Input Impedance 
10,000 Ohm
Instrument Input Impedance 
1.6 Meg Ohm (3.3 Meg Ohm balanced)
Maximum Output Level 
Maximum output from 20 Hz to 40 kHz is +23 dBu.
GAIN
Mic Pre: Unity to +66 dB in 6 dB steps
DI: Unity to +30 dB continuous
Total Harmonic Distortion and Noise: 
@ 1 kHz, +20 dBu output: Main Output: Better than 0.001%
@ 20 Hz, +20 dBu output: Main Output: Better than 0.002%
Silk Engaged: Better than 0.2% Second harmonic
VARI-PHASE
@ 100 Hz: 2.9 degrees to 57.4 degrees
@ 1 kHz: 33.5 degrees to 156 degrees
@ 10 kHz: 149 degrees to -175 degrees
PHANTOM POWER
+48 Volts DC +/- 1% (provided by 500 Series Rack)
POWER REQUIREMENTS
Supplied by 500 series rack with 110-125 mA @ +/- 16V
COMPRESSOR
THRESHOLD
Continuously Variable from -20 dBu to +10 dBu
RATIO
Fixed at 2:1
ATTACK & RELEASE
Fast Mode: Attack 5ms, Release 50ms
Slow Mode: Attack 250ms, Release 500ms
TOTAL HARMONIC DISTORTION 
Measured @ Main Output.
MIC PREAMP AND DI
@ 1 kHz, +20 dBu output: Better than 0.002%
@ 20 Hz-20 kHz, +4 dBu output: Better than 0.02%
SILK
@ 20 Hz, +4 dBu output: Better than 0.5%
@ 50 Hz, +4 dBu output: Better than 0.1%