Pineville, NC, United States
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Stereo Variable-Mu Tube Tone in a 500 Series Module
The Heritage Audio GRANDCHILD 670 is a stereo variable-mu tube compressor, a 500 Series module built with four NOS tubes and three custom transformers across two slots. A variable-mu compressor uses the tube itself as the gain-control element, delivering the smooth, program-dependent leveling and warm color that made the topology a studio legend. Fitting a true stereo tube compressor into a two-slot 500 Series format is rare, and the GRANDCHILD 670 was the first to do it. It brings authentic vari-mu tone to any lunchbox-style rack.
How Variable-Mu Compression Works
Variable-mu, or variable-gain, compression relies on a special tube whose amplification changes with an applied control voltage. Because the tube gently reduces its own gain as levels rise, the response is organic and forgiving rather than clinical, clamping softly at first and building density when pushed. Set lightly, the GRANDCHILD 670 adds glue and subtle warmth; driven harder, it lends color and musical saturation. This is the classic sound engineers reach for on vocals, bass, drums, and stereo mixes.
Four NOS Tubes and Three Transformers
At the heart of the GRANDCHILD 670 are four 6BA6 (5749) NOS tubes, two per channel, running in a real all-tube circuit. New-old-stock tubes provide the warm, characterful gain that solid-state emulations only approximate. Three exclusively designed transformers, one per channel plus one for the sidechain, couple the signal and control path for analog depth and low-end authority. It is a genuine tube compressor shrunk into a modular format, not a solid-state stand-in.
Input Gain and AC Threshold
Hands-on control over how hard the tubes work comes from two front-panel controls. An input gain control with a 20 dB range sets the drive into the circuit, while the AC Threshold determines where gain reduction begins. Pushing more level into the tubes yields richer harmonics and denser compression, while backing off keeps things clean. This interplay between input gain and threshold is central to dialing in the classic vari-mu character.
Progressive Ratio via DC Threshold
Like its larger siblings, the GRANDCHILD 670 offers a progressive ratio through its DC Threshold control. This is a continuously variable adjustment that blends ratio and knee, sweeping from firm, hard limiting to gentle, transparent compression. Rather than choosing from fixed ratio steps, you shape the exact grip the compressor applies. It puts nuanced control of the compression curve directly under your fingers.
Five Selectable Attack and Release Times
Shaping how the compressor feels, the GRANDCHILD 670 offers five selectable attack and release settings. Faster times catch quick transients on drums and percussion, while slower times let the compressor breathe with a vocal or a full mix. Because both channels track together, the stereo image stays stable as the settings change. That range makes one small module useful across a wide variety of sources.
Clean Specs for a Tube Compressor
Despite its vintage-style circuit, the GRANDCHILD 670 posts modern performance numbers. It offers a 15 kohm input impedance and a low 50 ohm output impedance, with an output level up to +29 dBu and frequency response of 20 Hz to 20 kHz within half a decibel. Self-noise stays below -78 dBu, and harmonic distortion remains under 0.5 percent at levels up to +24 dBu with no compression applied. Those figures let it sit transparently in a chain until you ask it to color the sound.
Built for the Modern 500 Series Rack
Occupying two slots in a standard 500 Series enclosure, the GRANDCHILD 670 draws its power from that rack. Because it needs at least 400 mA per slot at plus and minus 16 volts, it pairs best with a well-specified enclosure that can feed a tube module. Its compact format makes genuine stereo tube compression portable and easy to integrate alongside preamps and EQs. For engineers building a modular studio, it delivers flagship vari-mu tone in a format that goes anywhere.
Price$2,299
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