I was testing a homebrew tube amp today, and got a strange result, pic below. A weird triangular spike in the FR. I ran the test again and got a different, negative spike. I’m using a Lenovo Yoga laptop with a USB powered hub for the QA403. I’ve run tests before without an issue, but this time I left the laptop on battery supply. When I plugged in the wall wart (which is ungrounded) the test ran normally.
. I’m just curious–is this a USB issue perhaps?
Energy saving settings?
Could be! I didn’t think of that.
Hi @zackthedog, the plot is derived from the trace(s) shown on the main display and those are derived from the time domain traces. So, you can usually see the disturbance in one of those traces to see precisely what the cause or at least the source is. If your grounding isn’t clean during a single-tone test, the artifacts can introduce wild swings.
Can you plot a single-tone test of the amp at 1 kHz, and then again around 1.3 kHz (where the bump seems to be)?
Thanks for that info, Matt. Unfortunately I couldn’t duplicate the blip, it moved around, sometimes positive and sometimes negative. Plugging in the laptop power supply removed the problem.
