Model 670 Vari-Mu Compressor

Our answer is simple: the real, original circuit. With almost any brand, you’ll read about which parts of the circuit they modified, usually because the original design was considered too extreme or impractical. Maybe all of them work—but we’ve tried them, and they are truly not the same.

Beyond the tubes themselves, the real secret lies in the transformers. Today, no company actually produces transformers that are identical to the originals, especially when it comes to core size. Tubes and transistors behave very differently; DCR and inductance subtly affect impedance, frequency response, and even distortion. So having the correct turns ratio alone is nowhere near enough.

That’s why we tried every possible option. Whether it was Cinemag or Sowter, none of them provided a truly correct solution. When the wrong transformers are used—especially in the sidechain—serious distortion and frequency response anomalies appear. This is why you see so many manufacturers modifying their designs. Just take a look at photos of the rear panel: if the transformers look small and “cute,” it usually means significant cost-cutting, or simply that no one has ever systematically recreated these crazy original designs.

But this is where we are different. In the end, we custom-designed every transformer from the ground up, specifying all core materials ourselves. By collecting and studying original transformers from around the world and reproducing the exact DCR, all of the problems disappeared. We were finally able to use the same main signal path and sidechain circuits as the original. They look absolutely insane—and they sound just as insane—but we believe it’s worth it.

In the end, we completed it with a completely unmodified original circuit. Aside from the added modern features, you can drop in 6386 tubes and use it exactly like the original. Both the gain behavior and the distortion characteristics are extremely close to the original unit.

This is the most expensive aspect of the 670. A single service for the 6386 can easily cost thousands of dollars. Beyond the cost, the more important issue is that in our testing, currently produced 6386 tubes are simply not the same as the original versions in any respect. So whether this is truly a worthwhile investment is something that must be experienced firsthand.

In the design of the 670, tube matching is absolutely critical. Once the balance is lost, the low-frequency weight and the rich, oily character disappear instantly. This means frequent calibration and ongoing maintenance are required, which is exactly why it is so expensive and so rare.

After extensive evaluation and compromise, we found that a TBAR MOD based on 6BA6 tubes offered the best overall balance, and this is what we supply as standard. Although it uses 16× 6BA6 tubes, they are affordable, readily available, and extremely stable and reliable in every respect. We will also provide convenient maintenance tube sets to make upkeep straightforward. All of our demo videos were recorded with the standard version, and we believe it sounds excellent—no less charming or musical.

Of course, if you are deeply committed to the original 6386 sound and are fortunate enough to obtain eight units in truly excellent condition, we have fully reserved the 6386 sockets. You can install them yourself and calibrate the unit smoothly. The choice is entirely yours.