[Tubegem]

Purchased a DAC-9 in order to have access to 2 channel amp and by-pass processor (Yamaha CX-5100).
Works fine with my existing 5 channel amp (Blue Circle). Also tried my Meitner 2 channel amp.
When the A5 is selected for HT bypass there is a slight high pitched buzzing which is minimum from seating position, and quite acceptable. Muting the DAC-9 eliminates all noise.

Purchased a STA-10 to have an additional amplifier and set it up as main front channel. When the A5 on the DAC-9 is selected the high pitched buzzing is quite loud and unlistenable.

I have tried different cables. Both XLR and RCA. Tried different power cables. Tried different wall plugs. With and without power conditioning.

The STA-10 and DAC-9 get along beautifully including a different 2 channel system upstairs, but not when the HT is engaged. A5 works fine as an analogue input by itself when not connected to the processor.

I'm totally at a loss in what else to try. Not sure where the fault is if it's the amp or the DAC.  Does anybody else have a similar combination with processor or receiver engaged?

[zwizardofoz]

Try to see if removing the cabletv coax feed helps. Sounds like you ht setup might be inducing noise...I might have the connectivity image my head wrong however so perhaps a diagram would paint a better picture.

[JackD]

Agree, it is a TV related cable if one is involved or if not a grounding issue between the three components if it only shows up when the connection from the DAC-9 to the Yamaha is open.  If not related to coax then try a cheater plug on the components one at a time and see if it goes away and which component is the source.  There is sometimes no rhyme or reason and it just happens.  I just went through the same thing when a new amp was introduced into a previously dead silent system. Seems to be a greater chance of occurrence when there is a mix of fully balanced and "faux" balanced components. 

[Tubegem]

Agree, it is a TV related cable if one is involved or if not a grounding issue between the three components if it only shows up when the connection from the DAC-9 to the Yamaha is open.  If not related to coax then try a cheater plug on the components one at a time and see if it goes away and which component is the source.  There is sometimes no rhyme or reason and it just happens.  I just went through the same thing when a new amp was introduced into a previously dead silent system. Seems to be a greater chance of occurrence when there is a mix of fully balanced and "faux" balanced components.

Hi Jack and zwizardofoz for your replies and suggestions.

I do not have a cable box as such. TV delivery is via a fiberoptic box (Bell), which is connected via a CAT6 cable and the power is two prong only. There is no coax connection at all.
Further to your suggestions, I have disconnected the Bell box totally (power, hdmi, cat6) > no difference
I have disconnected every single input/output individually on the Yamaha. > no difference
All of the connections are single ended (rca) to be consistent. Tried two sets of both DH Labs and Crimson RCA cables.

Learned something new.  As long as there is a physical connection (rca) between the processor and the DAC, even with the processor powered OFF, the hum appears.
The hum is of a high pitch electrical noise and not a low one usually associated with a ground loop which I had in the past.

The volume on the dac has an affect. If the volume is around 70 or lower, the noise is lower and tolerable. As the volume increases the hum shows up and increases with volume so that it's too loud to be able to listen/watch anything past 94.
Tired plugging in the OPPO 105D into the A5 on the dac instead of the processor. There was hum, but quite a bit lower in volume as well lower in pitch.

The Yamaha and Bell box are 2 prong only. The NuPrime duo, Blue Circle amp and Meitner amp has 3 prongs. I will have to get a cheaper plug and see if what affect there is. But why would the STA-10 be so loud and not the other two amps with exactly the same set up? 

Appreciate any other thoughts. 

Happy Thanksgiving to my American friends.

Ed