The  SB4001 500 Series Stereo Bus Compressor from Serpent Audio is a quad VCA based compressor, and can be used for either stereo or mono applications. It can be used for gain reduction and is suitable for a number of signal types and musical applications.
 
This stereo compressor has six Ratios, ten Attack Times, High Pass/Boost/Slope Sidechain Filters, Wet/Dry Blend Control, External (Key) Input, Classic Stepped Release Settings, Classic and Alternate Auto Release, Variable Release Control, and "Grind" Mode (a valve style harmonic drive adding vintage color).
 
The Features
- Electronically balanced
- True bypass (relay controlled)
- Protection from 48V phantom power
- Powder-coated front panel for extra durability
- All handmade through-hole main audio path
- Wet/Dry blend (for parallel processing)
- External/Key input accepts balanced or unbalanced inputs
- Premium Grayhill rotary switches
- Sealed pots
- All machined aluminum knobs