If you ever feel really good about your measurement rig and your skills, volunteer to show someone how it works and how to test a piece of gear. For some strange reason, for me this always reveals how far I still have to go. When I work by myself I typically have good results.

So this is what happened most recently. I was doing a show and the house audio personnel asked me a question about their rig. I explained how I would go about trouble shooting what they described and said I was glad to help them. They brought me their Mackie mixer & their audio snake. I setup my SpectraFoo rig and configured it to test the mixer. When thought I was ready I sent pink noise to the mixer and back out of the main mono output. I got a very strange reading.

To verify that my rig was working correctly I bypassed their mixer and looped out of my rig and back in again. Same behavior, different gain difference.

Notice the delay finder impulse response:

Like a skipping stone. I’ve seen this sort of behavior before (outside of the FFT instrument) when a signal is clipping. I also noticed some weird behaviors in the Mio Console itself. Trying to adjust the gain for the return channel was very touchy.

To make a long story short, it turned out that I was creating a feedback loop by NOT muting the return channel in the Mio Console.

Here is what I had. Notice that my return channel is feeding the MAIN buss. Perfect feedback loop.

Once I muted the return channel like this:

Everything was stable and this is the measurement I was able to get measuring the internal signal generator against the D/A A/D physical loop:

This was a 15 minute detour and once again reminds me that the key to measuring is to build a rig, label everything and how you use it, build whatever configuration files you need in advance and test. Otherwise one routing mistake and your rig doesn’t work.

It turns out that the Mackie input to output was all good. We discovered some broken solder joints inside the snake box due to a failed strain relief. I tried to resolder things but the snake proved to be of such poor quality that the fix didn’t work.