Volume control secrets with DACs


Press release | Fall is here in the Northern Hemisphere, the season to enjoy the colors of the leaves changing, going more inward and taking time for introspection. The changing of seasons touches all of us, so if you could use a little cheer-up please remember that every mood can be improved by beautiful Music. Time to take good care of yourself, use a Metrum or Sonnet DAC to be closer to your musical friends or even share the experience with your real-life friends. For us, this is also an opportunity to go inward and explain a little more about the inner workings of our volume control.
Traditional volume control: an imperfect situation
In the ideal world, volume control doesn't add or remove anything from the original signal. Also, the ear and hearing system would be perfectly linear (which is not the case. Thank you, Fletcher & Munson, 1933). We'll stick with the electrical part for this newsletter. Traditionally, a volume knob (potentiometer) with a buffer amplifier was implemented to regulate volume. The buffer was added to avoid coloration of sound when volume was low.

Volume control secrets with DACs

As a potentiometer (or other resistance element) and buffer amplifier introduce noise and distortion, these need to be as neutral as possible. A buffer that is a "straight wire with gain" is needed. Unfortunately, wires with gain don't exist and every electrical component can make for a different sound. For this reason, tweakers and DIYers switch opamps or roll tubes, while only exotic resistors and capacitors lead to audio Nirvana.

A philosophy that is common in the audio world is called "out of the signal path". When electrical components are "out" of the signal path, like capacitors, they don't influence the sound as much as when they are "in" the signal path. As an audio company, this is a design choice, partly defining the sound-signature of the audio devices. A short signal path with as few components as possible is preferred. Therefore, tube amplifiers are usually simple in setup and sound well. We digress into the analog world... back to volume control.

Digital volume control: a worse solution
Another option for volume regulation is in the digital domain. This implementation introduces quantization (round-off) problems leading to a loss of fine resolution. The original signal needs to be "recoded" with a calculation to fit the new – lower volume – waveform, with round-off errors as a consequence. The most ubiquitous form of this type of volume regulation is in the volume control built into. We understand that the user experience is also important, but when listening critically it just doesn't fly: bit-perfect playback is no longer possible.



Volume control secrets with DACs

In conclusion, controlling volume was always about taste and choosing the lesser evil. Until... you don't have to choose between evils. So, how can we regulate volume using the "straight wire with gain" and "out of the signal path" philosophies without changing the bits?

Metrum volume control: bit-perfect resolution
The volume control in the Metrum Jade and Adagio, as well as the Sonnet Morpheus and Pasithea, use a different paradigm. In our digital pre-amplifiers, the volume control is completely out of the Analog signal path. The Metrum way gives an unaltered bit-perfect signal with full resolution that arrives at the DAC modules (unlike in DSP-based volume controls), regardless of the volume setting.



Volume control secrets with DACs

How does it work? Our R2R ladder DAC technology transforms the art of volume control by adjusting its input reference voltage of the DAC. Traditionally VREF is a fixed voltage, but the R2R architecture allows for a variable voltage reference. The reference voltage input is managed by a potentiometer-buffer combination, ensuring no filtering in the signal path.

It's a volume regulation technique that is not found in off-the-shelf audio DAC chips; many are not using the R2R architecture or require fixed reference voltage. Our Non-OverSampling (NOS) and filterless architecture ensures that the integrity of each bit is preserved, delivering a True to Nature sound. At very low volumes, our design minimally affects switching noise, maintaining a noise floor better than -140dB, which attests to the clarity and precision of our DACs.

Please note that our volume control is in the Analog domain, but in our implementation can't be used to control an analog musical input since it already controls a DC reference voltage. Therefore, it is not so easy to implement an extra analog input that also has volume control. For this reason, the Adagio, Jade, Morpheus and Pasithea are called digital pre-amplifiers.

Closing words
Enjoy the colors of autumn without coloration, paint your musical palette with all the colors of the wind. Take five and invite Brubeck and friends for some five-quarter genius. Musicians and their Music make the Autumn season so much more enjoyable. Pour yourself a cup of something wonderful and take good care of yourself by immersing yourself in a musical treat that our filterless NOS DACs can deliver; with neutral volume control. You now know how that works...
If you are interested, our new order portal supports all of our brands (Metrum, Sonnet and Acelec) so you or your dealer can order any device we carry, even in the mix. So you can order a Metrum Ambre alongside a Sonnet Pasithea without problems (they happily interconnect too by the way). We made sure to have enough stock for all of our most popular models and will produce even more equipment before the end of the year. Holidays are coming, and so is Winter...

For further info: to Metrum Acoustics website

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