Noise in the 788T headphone output when used for return monitoring, demystified.

There have be many reports, recently, of mysterious and unacceptably high noise levels when using the Sound Devices 788T headphone output as a send for return monitoring with a mixer. The reports went something like this:  “When using the headphone output of the SD-788T as a send for “tape return” monitoring, there is considerable hiss, but when I listen to the headphone output with headphones, there is no hiss”.

It seemed to be another case of audio voodoo. If I was a medical doctor I could have passed it off as “just one of those things no one knows the cause of”, but audio professionals are not that easy. And like thunder and lightening, electrons no longer work in mysterious ways.

With the assistance of Trew Audio ghost-busting staff, when I listened to a duplicated setup and heard the noise, it was like seeing a video of a ghost on one of those metaphysical [un]Reality TV shows. When an isolation transformer didn’t solve the problem, it was all but confirmed that the noise was from a 5th dimension. From the sound of the noise, I’m guessing it’s digital converter noise (but this in not yet verified).

It took a while, but I finally busted the myth:

The 788T headphone output has some noise that is [presumably] in the acceptable range when listening with headphones plugged into the 788T headphone output. But, this noise is independent of the headphone volume control; if the headphone level control is low or all the way down, the noise level stays the same. When using the headphone output to feed the line level return monitor input of most any mixer, with the return input trims set where they normally would be, the 788T headphone output needs to be turned way down to achieve unity level between “direct” and “tape”. With the 788T headphone level turn down, the noise level is about 20dB higher than it would normally appear to be if using headphones. This is why hiss is being heard in the return monitor.

There two ways to solve this issue:

1)    Turn the 788T headphone level control all the way up, then turn the return inputs down low enough to each unity between “tape” and “direct”, or….
2)    Turn the 788T headphone level control all the way up, and us a -20dB padded stereo cable between the headphone output and the return inputs. This would be the preferred method for anyone who did not want to be adjusting the return input trims radically different from what they be with other  setups. The shop at Trew Audio has the pad values for this cable and can make it by request (it is not yet a standard stock item).

Either of the two solutions above will result in signal-to-noise ratios similar to what you would experience if listening directly to the 788T headphone output with headphones. Since the Sound Devices 788T designers may have never been predicted that the headphone output would be used in any way other than with headphones, it can’t really be thought of as a flaw. But at least it’s not a ghost, either.