The project is built around harmonic movement and musical glue, not just the look of a vintage unit.
Mastering
Model 670 Vari-Mu Compressor
A vari-mu tube compressor built in reverence to the original 670 harmonic movement, aimed at the musical depth that made the classic so legendary.
Vari-Mu
Tube Compression
Stereo / Dual Mono
Time Constants
Sidechain Filter
Why It Matters
Four things that define the Model 670
The Model 670 is a vari-mu compressor built around classic tube combinations.
Training notes describe dual-channel operation with independent or linked behaviour.
The focus is musicality and harmonic richness rather than surgical control.
Workflow Reference
Use it for cohesion and harmonic movement before using it for force
A vari-mu compressor usually earns its place by how it glues, rounds, and deepens a mix — not by how dramatically it catches every peak.
- Vari-Mu Feel Expect smoother gain reduction and tube movement rather than the fastest clamp in the room.
- Stereo Or Dual-Mono Thinking Choose linked behavior when image stability matters; separate behavior only when the material really calls for it.
- Low-End Sensitivity Use the sidechain high-pass option carefully when bass energy is steering the action too much.
Quick Start
How to start before deciding the 670 is too soft or too slow
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1
Start with lighter compression than you would on a fast bus compressor and listen for cohesion before obvious gain reduction.
-
2
If the mix loses punch, check whether the attack behavior and threshold are simply too ambitious for the source.
-
3
Use linked stereo behavior first on bus work so transients do not pull the image sideways.
-
4
If low end is driving too much compression, re-check the sidechain behavior before turning the whole unit down.
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5
Write down the threshold, timing, and mode choice when the mix starts to sound more expensive rather than just more compressed.
Start simple, listen in context, and save only the settings you would actually want to recall.
Working Uses
Where owners usually reach for a vari-mu first
A natural first use when the goal is glue, harmonic movement, and a more finished stereo picture.
Useful when compression should sound expensive and smooth rather than hard and obvious.
Can work well when a source needs body and control without sounding pinned down.
A fit when the dynamic move and the harmonic move are both part of the decision.
Working Notes
Notes that usually matter once vari-mu stops being romantic and starts being useful
The Model 670 usually rewards starting with movement and cohesion rather than chasing obvious gain reduction immediately.
If the compressor feels lazy or too heavy, low-frequency energy may already be deciding more than you think.
Keep the image stable first, then only complicate the setup if the material gives you a reason.
A small change can sound expensive here. A large one can sound like the unit is now the song.
Program-dependent behaviour can still hit hard; it just does it with a different envelope than a VCA or FET.
Recall notes are stronger when they describe density, image, and movement rather than reduction numbers alone.
Specs
Key Specs
THD+N
< 0.05% typical · <1% at 20dBu
Noise Floor
< -83dBu (AP tested) · < -90dBu (tested via AD/DA in DAW)
Maximum Output Level
20dBu (1kHz, THD < 1%)
Maximum Gain
20dB
Power Consumption
300W
Dimensions (Unit Only)
482mm x 352mm x 300mm depth (including deepest point)
Last Updated
11/07/2025
Owner FAQ
Questions that usually appear once the first bus pass is done
Character Why does a vari-mu feel gentler than a VCA or FET compressor?
Because the design goal is different. Vari-mu compression is prized for smooth, musical gain reduction and tube movement more than for aggressive peak control.
This is one reason people reach for it on buses and mastering work.
Bus Use How much compression should I start with on the mix bus?
Start light. This kind of compressor often sounds best when it is unifying the mix rather than obviously leaning on it.
If you hear the compression before you hear the cohesion, you may already be too deep.
Stereo When should I link the channels?
Link them first for stereo bus work so single-channel transients do not shift the image. Separate behavior only when there is a clear reason.
Stereo stability is usually the right first assumption on a bus.
Low End Why is the bass making the unit feel heavier than expected?
Low-frequency energy can dominate the detector. If the design includes sidechain filtering, revisit that before assuming the whole threshold needs to change.
Bass-rich material can make a vari-mu feel slower and heavier than the rest of the mix wants.
Expectation Should I expect this to behave like a Fairchild-style peak destroyer on every source?
No. The owner-guide mindset here is glue and harmonic movement first, not brute-force transient catching on everything.
If the goal is aggression first, another topology may simply be the better starting point.
Tubes Why do small setting changes feel bigger on a tube bus compressor?
Because the dynamic move and the harmonic move are happening together. You are not only changing gain reduction; you are changing how the tube path is being asked to behave.
That is part of why light compression can already sound “finished” here.
Session What is the first thing to check if the 670 suddenly feels too heavy?
Check whether bass content or level into the compressor changed before rewriting the whole timing idea. Vari-mu behaviour often shifts because the source changed, not because the last setting became wrong.
That saves a lot of unnecessary knob chasing.
Downloads & Resources
Keep the key files and working tools in one place
Use the shared Setup Sheets library for now.
WIP: Dedicated setup sheet pending.
Open LibraryKeep a dedicated quick start PDF here once the shorter guide is ready for release.
WIP: Dedicated quick start guide pending.
Use Quick Start SectionUse this page for first-session workflow, quick specs, and owner FAQ while the session is live.
Keep a dedicated downloadable manual here once it is ready for release.
WIP: Dedicated manual PDF pending.
Use Manual SectionUse the support page when the issue moves beyond normal workflow and starts looking like routing, power, noise, current draw, phantom, or service.
Open SupportService
What to have ready before you reach out
Mode Context
Mention whether the unit was linked, dual-mono, or otherwise configured for stereo behavior.
Source Context
List whether the issue appears on mix bus, mastering chain, vocals, or another source.
Low-End Context
Include whether strong bass material was present, especially if the complaint is pumping or overreaction.