Ultra Micro Amp: Why do I hear cross-talk between the two source inputs?

[schaqfu]

EXECUTIVE SUMMARY OF QUESTIONS HEREIN:
1) Is it normal to hear cross-talk between input sources or should I be concerned?
2) Is there potential for channel imbalance when the volume pot is turned all the way down?
3) Should I be concerned that I hear loud buzz when I merely touch the volume pot?
4) Do the different gain settings, compared at identical volumes, change the sound quality/character?
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Just got Ultra Micro Amp today; very impressed so far. But I noticed that if I switch to input #2, which is not connected to anything, and I keep a source (my Benchmark DAC1-USB) playing through input #1, I can hear #1 playing faintly through #2. The volume is affected both by the volume of the source, and by the volume/gain of the Ultra Micro Amp, which suggests to me that the signal paths are crossing/merging. Am I supposed to be hearing that? Aren't the two inputs supposed to be totally isolated?

Thanks.

EDIT: And, as long as I'm airing all my questions (can you tell I'm enjoying using this thing?!!): another quirk I discovered is that whenever I touch the volume pot with my finger, or even get in very close proximity to it, a significant buzz is produced. The buzz is at identical volume levels in both LOW and MEDIUM gain settings, and in HIGH gain mode it is still audible but much less, and it sounds more gurgly/grumbly, whereas in LOW and MEDIUM it's a very smooth, constant hum (but loud). I realize pots are finicky in general, but this one seems very well constructed and doesn't make any noise at just from turning the dial, so I'm surprised it buzzes when I merely touch it.

And one more question on the pot: does it suffer from channel imbalance or other imperfections at very low levels? I feel like the bass is richer on my HD650s when I have it at MEDIUM gain with the volume turned way down, but I can get the same volume on LOW gain with the dial closer to 11 or 12 o'clock. So really I guess that's two questions: is there channel imbalance at low levels, and does the quality/character of the signal change based on gain settings when comparing them at identical volumes?

Thanks.

[schaqfu]

And a separate question that deserves its own post:

Is there an ideal input level that my source should be outputting to the Ultra Micro Amp? I can precisely calibrate the RCA analog outputs of my DAC1-USB to a fairly wide range, but they're currently set by default to +2 Vrms (8.2 dBu) at 0 dBFS (or -10 dBV at -16 dBFS). Either in terms of audio quality or efficiency, is there an ideal input level for the Ultra Micro Amp to be receiving from its source? If so, what is it?

Thanks.

[schaqfu]

And another minor question I can't seem to find the answer to anywhere: what is the output impedance of the Ultra Micro Amp, and does it differ from the regular Micro? Are they both approaching zero ohms? I see from the product specification page that the input impedance is 70k ohms, which is great!

[Tyll Hertsens]

Originally Posted by schaqfu

EXECUTIVE SUMMARY OF QUESTIONS HEREIN:
1) Is it normal to hear cross-talk between input sources or should I be concerned?

It's normal to hear a little under the conditions you speak of. I'd be curious if the amount of cross talk was different when you have an input on the channel you're listening to and put it on pause while listening for crosstalk from the other channel. Having something plugged into the input you're listening to 'terminates' that input. If the input is left floating, it has a high impedance and is therefor more suceptable to EMI (electro-magnetic interference).

2) Is there potential for channel imbalance when the volume pot is turned all the way down?

Much less on the pots we now use (the Clarostat pots we used to use were significantly worse), but there's almost always a bit of imbalance at the low end.

3) Should I be concerned that I hear loud buzz when I merely touch the volume pot?

Do you have anything plugged in while you do this? Again, it is important to have inputs connects to terminate them. I wouldn't be surprised at all to hear the if you have no input and have the gains turned up that you'd hear hum touching, or even getting close to the unit.

4) Do the different gain settings, compared at identical volumes, change the sound quality/character?

We've had people comment that the 'high' position sounds a bit different than the 'low' and 'med' position. I really don't hear too much difference though.

[mikeg88]

What sort of environment are you operating your rig in?
Because I've got single coil pickups on my guitar that pickup EMI like crazy (like nearly anything in the room) - and they get amplified so I hear them through my amp when I'm not playing. I've found that some halogen fixtures and fluorescent ones have awful EMI nearby. I'm guessing it's because they've got big "ballast" transformers for the fixtures (inside the walls) that have big coils and lots of energy being exchanged.
Do you have either of said fixtures where you're listening?
If so, I'd recommend incandescent or LED lighting - it seems much less noisy for me, anyways.

Good luck

[mikeg88]

Oh, and I've heard that computer fans and monitors are pretty bad for this as well.
Just in case you've got either of those! (Who'd have thought!)