What am I hearing (or not hearing)?

[illkemist]

The time had come for me to shop for a new DAC, so I decided to try the HR Micro and Ultra Micro DACs. Tonight, when I hooked them up to A/B against my current DACs, both Headroom DACs trumped my existing DACs (Moodlab Dice, iBasso D1 stock). However, I noticed that when I removed the optical cable from my Macbook Pro, the Micro DAC (still on) had a very audible hiss in my headphones (connected to a Slee Solo amp). The Ultra, when tested under the same circumstance, was dead silent.

What am I hearing? Is this just the difference in the design of the two amps? I know this is really not a practical test of any normal listening conditions. When both units are connected to the MBP and I'm not feeding any audio, I get perfect silence.

Again, I'm not really concerned that anything is wrong, but I'd like to know if this is indicative of the added quality of the Ultra Micro DAC, or perhaps just a production flaw in the Micro DAC.

[TheSloth]

The older DAC sends that hiss when it doesn't have a lock from a digital source. It's a minor 'flaw', which I assume has been corrected on the newer DAC boards. As you guessed, when there is a lock from a digital source, there is no difference in noise floor.

[illkemist]

Sorry. I should have mentioned that these were both '07 HR DACs. But thanks for the explanation.

[drummerboy47]

I hear the same hiss with my 2007 Desktop Portable using USB.