CD distortion through preamp

Which inputs on the preamp are you using for your experiments to isolate the problem?

I see the preamp has a "gain" setting that is input-specific. You've tried adjusting that? (It's not clear to me from a quick view of the manual if it should go higher or lower.)

Twoleftears, I’ve tried a few different inputs, but didn’t think to try using the dac or phono preamp’s inputs. I’ll check that out. Thanks!

Mike_in_nyc, you are correct. there are 3 gain settings for each input. I have the CD player on the lowest setting. I’d think higher would be worse, but I’ll give it s shot to see what happens when I do. 

Puzzling problem, especially given that while it does sound like the preamp is responsible, whether or not the issue appears depends on whether a CD player (used as a transport) or a computer is driving the DAC.

One thing I would try as an experiment, if you already haven’t, is disconnecting the headphone amp when you are listening to speakers, and vice versa, rather than simply turning off the one that is not being used at any given time. As is the case with many ARC preamp designs the two XLR main outputs of the LS25 for each channel are simply jumpered together inside the rear panel, and the RCA main outputs (which it sounds like you are not using) are simply provided with the same signal that is provided to pin 2 of the XLR connectors. And although this is just a shot in the dark I’m thinking that a possible contributor to the issue might be driving the uncontrolled and unknown input impedance that is presented by whichever amplifier is turned off, that might even vary as a function of signal amplitude.

Also, fyi, ARC recommends a minimum load impedance of 20K for the LS25 and for most of their other line stages and preamps, although their specs don’t make clear if that applies to the balanced outputs or the unbalanced outputs or both. And it appears that your headphone amp has a balanced input impedance of only 10K, while the Pass amp has a balanced input impedance of 30K.

With both amps connected to the preamp’s XLR outputs that means the preamp is being presented with a combined load of 7.5K (30K in parallel with 10K), which is well below the recommended 20K minimum. And that is when both amps are powered up; we have no way of knowing what that figure is when one of them is turned off, and as I indicated the input impedance presented by the unpowered amp could conceivably vary as a function of signal level, with unpredictable consequences.

Good luck. Regards,
-- Al

Al,
To clarify, both the amplifier and the headphone amplifier are connected with balanced cables, not rca for amplifier, and balanced for headphone amplifier. 
With my little knowledge of impedance, I would not have thought it would be affected by connecting 2 amps. I’m interested to see if that makes a difference! Thanks!

@tripg   I glanced over manual and found warning, that mkII uses different tubes than mkI.  Are you sure you have 6H30 tubes installed?

Manual say that single ended inputs can handle up to 14V (28V for balanced) at the lowest gain position (6dB).  I cannot imagine it being overdriven. Likely there is something wrong inside (wrong or faulty tubes etc.)

As I glanced through the manual, the function of the gain control was not crystal clear, by a long shot. I thought it said that the preamp would limit the signal above the "gain" chosen; i.e., that something with high output (like a CD player) should be run at the higher "gain" settings.

Mike_in_nc,
if I understand you correctly, I don’t think it works that way. When you raise the gain setting from lo to med or high the volume definitely jumps up a notch. 

Al,
you were correct!!!!

And looking back, it makes a lot of sense.

I haven’t always experienced this issue and I’ve been using this preamp/amp combination for years. I switched the headphone amplifier to a variable output about a year ago. If I had a better memory, I’d likely recall that’s when the problem started. Although I still think it’s odd the DAC didn’t distort like the CD player. I’d think their outputs would be the same. I’m just happy it works correctly now!!!

I can’t express how grateful I am. Thanks AL!!!!

Luther