New tape analog volume controllers

It was a busy week leading up to "the move". A lot of planning was required, we were testing various new configurations, measuring distances for cabling, ... and somewhere in between, we installed the new volume controllers for both tape machines.

Those of you who visit us regularly or chat with us on the Discord server have noticed it right away, but we haven't made a fuss out of it on the social networks and the like - yet. So now that the dust has settled on the gear rack at the new location, let's see what's new and special about the new companions to our beloved reel-to-reel ladies.

analog volume controller and studer tape at mixanalog
The gain staging around the Studer A812 at mixanalog just got waaay easier!

Intro

It all started with a few complaints from one of our very perceptive users who noticed a faint noise in the 20kHz region on the Telefunken M15 bounces. Unless you're very young and lucky with your hearing, you weren't going to hear it. But if you use saturation or distortion in your processing, the noise could cause all sorts of problems if aliased or inter-modulated lower into the spectrum where it becomes that much more apparent.

So in keeping with our mission to bring you the best audio experience, we set out to find the cause and the remedy. That turned out to be a bit bigger challenge than expected, as we figured out the old volume controller will just not perform to spec no matter what we do.

The solution - design a brand new one!

The Volume Unit V.2

While we're at it, why not make it much more useful than the first model?

analog volume controller PCB design render
The design render of the new volume controller
  • Instead of just controlling the input level, it now allows for level adjustment before and after hitting the tape
  • Both stages offer an extended range of attenuation and gain up to +9dB
  • The headroom is MUCH higher at +27dBu. Everything else we have in the chain will clip sooner than this unit!
  • All active stages are automatically bypassed at unity gain setting
  • Upgraded audiophile-grade components are used to offer pristine transparency

About that transparency...

Yup, this one gets its own chapter. We're kind of proud of it.

The inputs and outputs are handled by high headroom THAT line receivers and drivers. Those are the next best thing to transformers when it comes to common-mode rejection* but with only 0.0006% THD.

*CMRR or common-mode rejection ratio is a metric that tells you how good the chip is at ignoring radio signals and other electromagnetic fields that sneak into the balanced cable while going from one device to another.

The actual volume control is handled by a passive stepped resistor cascade. It's hard to get more audiophile that that.

The buffer and +3dB gain stage for that passive attenuator are handled by nothing less than a SoundPlus™ OPA1642, a high speed, low distortion audio-specialized operational amplifier.

And to top it off, the whole chain is DC-coupled with no capacitors in the signal chain. It goes down to 11 (and below) and doesn't incur any phase shift or distortion.

Conclusion

Finally, you have full and proper control over the level you send into the tape machines to finetune the amount of drive and saturation you can get from them! Keep in mind that these are not peak meters but VU and when they go full tilt you are not actually clipping, just driving the tape real hard.

Speaking of them, the VU meters on the interface correspond to the calibration levels on the machines. If in doubt, keep an eye on them and you're good to go. Easy!

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