Ok, well... Not my intention to "tease" about this, so here's the account. I would have thought someone might have suggested to "follow the chain" through the system.
DAC arrives and is installed. As stated before, the symptoms of cutting out and amp "clicking" begin the first time I use the DAC. The unusual nature of the speaker drop outs is that at very low levels (under 12 on a digital readout of 100 max) there were no drop outs.
I always power up equipment in a new rig in a safer mode, i.e. mute, then bring up volume slowly. So, in this case, as I bring up the level slowly everthing is fine, until I pass 12. Then the left speaker begins to cut out and the amp switch/click. The low level at which the speaker continued to play perfectly added to the difficulty in diagnosing the problem, as will be seen.
My assumption was that the DAC was damaged in transport; there was a pushed in corner where it had fallen, but no visible damage to unit and it was packaged extremely well.
I try the tube swap, to no effect. So, to rule out the DAC I remove it from the rig. I replace it with a different DAC and obtain the same results.
The problem persists (amp still clicking/switching), so I suspect the amp is damaged (by the DAC? Not likely, but can't be sure...). I remove the amp in question and replace with another pre/amp. Now the clicking of the amp has disappeared, but the left speaker is still cutting out! Can't rule out the first DAC as causing a problem in the first amp, but the odds of the amp being damaged now are greatly reduced.
Next, I test the cdp, since it's the source upstream and could be feeding the bad signal. Not likely to cause an amp to switch/click but must be ruled out to obtain a definitive answer. To check this source, I switch to streaming audio fed directly into the replacement pre/amp. The speaker cutting out remains, so the cdp has been eliminated as a potential cause.
I have at this point cleared only one component (besides the second DAC, which played no part in the initial problem), and that is the cdp. The DAC could still be the source of the problem, and the amp could still be damaged.
Now I am down to skeleton rig: streaming audio to the second pre/amp, the cabling and the speakers. I do NOT want to discover something shot with the speakers! But, I'm at the point of checking them now. Here is where the diagnostics got tricky.
I had listened to the speakers up close initially at a lower level, just enough to make them cut out. I assumed (here is where I was misled) that the entire speaker was dropping out - the speakers are biwired. I had put my ear up next to the mids/treble and could hear the drop outs. I did not hear any bass. That is because the low listening level (after all, I'm not going to blast a speaker that's cutting out and put my ear up to it!) and the extremely low frequencies emanating about two feet below ear level. The other speaker is playing fully and is masking the lowest bass output of the speaker I'm diagnosing.
So... I'm now down to the speaker as possible cause. I remove one cable and test one set of posts at a time, first the bass and then the treble. Now I hear each of them dropping out as the level is raised, not surprising since I had not detected the woofers still running prior. I then switch to the second set of cables and repeat the process. This might seem unnecessary, yet to rule out all possibilities it must be done. WALA! This cable performs perfectly at all levels!
To confirm the diagnosis of one faulty cable in a biwire set on the left speaker I replace the suspect cable and add back in each component. Perfect performance from every test. The speaker no longer drops out, the amp no longer clicks, the new DAC is flawless.
It was a speaker cable which I had not touched when I put in the new DAC! It happened to fail at the moment when the DAC was put in, so I had to work through the entire chain to find the bad cable! One would not initially suspect a cable because of the addition of a new component, which had been dropped/hit in shipping, introduced into the system.
In the end I am grateful that it was "just a cable" - the DAC, amp and all other equipment are fine. I happen to be deriving even more pleasure from the DAC than I had anticipated.
The Lesson for us all, and I reiterate for myself, is that we cannot make any assumptions when a problem arises. The testing has to be methodological. Even then, finding a cause can be tricky. One thing is for sure, I'm glad I didn't stop at concluding that the DAC was the cause. It would have been embarrassing to return a perfectly good DAC!
What are the odds that a cable would fail at the precise moment when a DAC is inserted into a rig? Or that the cable would work at low levels and cut out at high levels? Or that to diagnose at moderate levels I would not hear the woofers?
This diagnosis was a challenge! So, yes, I knew the odds of anyone guessing the correct problem was virtually nil. However, assuming that it was the DAC was incorrect. This problem would not easily have been found if I did not have an extra DAC and amp.(It could have been found, but how many of us honestly would have assumed it could NOT be a cable?) One of my points in this excercise is to reinforce that assumptions can mislead, and that one must think systematically (pardon pun) in diagnosing problems. So, forgive me if you felt I was teasing; it was meant only as a brain-teaser.
Next time there's an issue, I just may switch a cable FIRST!
:)
