Q. Why might an equipment designer prefer unbalanced I/O?

Sound Advice : Recording

I recently purchased a Thermionic Culture Little Bustard based on the review that appeared in Sound On Sound. All the inputs and outputs are unbalanced on the unit, which seemed strange to me. The review suggested that the designer prefers the sound this way. Why might this be?

Bob Endsleigh via email

SOS Technical Editor Hugh Robjohs replies:

Balanced inputs and outputs are common on professional equipment, but they are not an inherently perfect interface solution under all circumstances. A well-engineered balanced interface does provide enhanced rejection of EM and RF interference, and should provide freedom from ground loops. These are very important benefits in temporary or unpredictable installations.

However, depending on the specific circuit implementation, there can also be significant drawbacks, such as the use of balancing transformers in the signal path, or dual-output circuitry, resulting in higher cost, noise and distortion than would be the case for an unbalanced interface.

It is not unusual, in a well-designed and controlled installation, to prefer unbalanced interfaces on a sound-quality basis. Indeed, Bob Katz designed one of his mastering rooms this way because he believed it sounded better than a more conventional approach, and it usually works well enough for high-end hi-fi systems, after all! The key is in paying proper attention to system grounding, signal levels and headroom margins.

Returning briefly to the specific situation of the Thermionic Culture Little Bustard, adding input and output transformers or employing dual‑differential circuitry to accommodate balanced I/Os would have added substantially to the cost and changed the sound character in a way that didn’t appeal to the designer.

Balanced inputs and outputs aren’t always the ideal. Depending on the specifics of a device’s circuitry and, potentially, the use of balancing transformers, balanced I/O could produce more noise and distortion. In the case of the Thermionic Culture Little Bustard, balanced I/O would also have resulted in a higher cost and could have changed the device’s sound character.

As I said in my review, the unbalanced outputs shouldn’t cause any problems at all in a well-organised system, and I’ve learned subsequently that all current production LBs are equipped with a small resistor in the cold side of the inputs to prevent problems with active balanced output stages. The same is also true, apparently, of the Fat Bustard design.

(To read our full review of the Thermionic Culture Little Bustard, go to www.soundonsound.com/sos/nov10/articles/littlebustard.htm.)

Published in SOS July 2011