@ginetto61
When I did the SE loopback tests of the QA403 with the laptop connected to the mains and/or if the QA403 was deriving the 5V power from the laptop there was a small amount of 60Hz hum. With the power bank I do not see any. Since the loopback test is the theoretical best of what a QA403 can measure, I stopped my experiments at that point. Simple and easy as long as you don’t need to do measurements for extended periods of time.
Best,
Anand.
When I did the SE loopback tests of the QA403 with the laptop connected to the mains and/or if the QA403 was deriving the 5V power from the laptop there was a small amount of 60Hz hum. With the power bank I do not see any. Since the loopback test is the theoretical best of what a QA403 can measure, I stopped my experiments at that point. Simple and easy as long as you don’t need to do measurements for extended periods of time.
Best,
Anand.
Well so much for that theory. An additional 60dB of broadband gain lifted the 50Hz hum to a healthy mV level, but the 3rd harmonic is still there:
On the DSO rather than QA403, with the digital filter applied to band-pass 150Hz:
That rules out the QA403 - I'm verifiably sitting in a 150 Hz hum field here.
Nothing on my property is radiating this. Has anyone else experience this? Are any major appliances such as HVAC systems notorious for 3rd harmonic distortion on the mains? Electric lighting controllers? Is my neighbor doing something hydroponic in his adjacent shed??
I'm always fighting those elusive issues. First- To confirm its external short the input as close to the first gain stage as possible. This is a baseline. If the hum is there its internal. Possibly from ground routing. Then make a magnetic field probe- a chunk of iron or ferrite with 50+ turns will be quite sensitive to magnetic fields. It will help you find what is radiating. I discovered at one point a power cord under the bench was the dominant source. There may be wiring in the walss running an AC some where else radiating. These are elusive and tedious to run down but you will be haunted until you at least identify them
Another possibility is oscillation at very high frequencies. Those show their uglyness as hum all too often.
Another possibility is oscillation at very high frequencies. Those show their uglyness as hum all too often.
Yes. A relatively quiet one. I am using the QA403 to display its input-referred noise spectrum.
There aren't any stability issues with any of the amplifiers - this is purely hum pickup.
I decided to directly probe the mains. Ignore the amplitude scale, I am using a 100:1 1500V probe plugged directly into the ~100k input of the QA403.
Pretty ugly. It's funny how, aside from the fundamental, it's only the 3rd harmonic that is giving me a headache. 150Hz must be some kind of ULF sweet spot for the radiation efficiency of the mains wiring of my lab.
I looked at the mains waveform on the DSO and the tops of the sinewave are heavily flattened. In fact, the mains waveform here looks exactly like the mains transformer secondary voltage of a conventional rectified and filtered DC power supply with the peaks of the sinewave flattening due to winding resistance limiting current as the filter capacitor(s) are topped up.
There aren't any stability issues with any of the amplifiers - this is purely hum pickup.
I decided to directly probe the mains. Ignore the amplitude scale, I am using a 100:1 1500V probe plugged directly into the ~100k input of the QA403.
Pretty ugly. It's funny how, aside from the fundamental, it's only the 3rd harmonic that is giving me a headache. 150Hz must be some kind of ULF sweet spot for the radiation efficiency of the mains wiring of my lab.
I looked at the mains waveform on the DSO and the tops of the sinewave are heavily flattened. In fact, the mains waveform here looks exactly like the mains transformer secondary voltage of a conventional rectified and filtered DC power supply with the peaks of the sinewave flattening due to winding resistance limiting current as the filter capacitor(s) are topped up.
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I would like to be able to measure the maximum input and out levels (vs distortion) on various pro audio gear. Unfortunately, the QA403 goes only up to +18dbu which is rather low for this task. Therefore, I'm thinking of maybe building a simple (but quiet, low distortion and high bandwidth) circuit that can boost the signal by +10dBu (or thereabouts).
Has anyone done something similar? Any recommendations on where should I look at? Also, I'm open to some off-the-shelf options.
Has anyone done something similar? Any recommendations on where should I look at? Also, I'm open to some off-the-shelf options.
At least nothing obvious.
It could be that something in your building ground system is amiss. There could be higher than normal currents in the safety ground system.
FWIW, I once had a similar problem. I traced it down to a very high quality power transformer that was in the same room as the QA401 system. Instead of digging into why this transformer was a problem, I just got rid of it. It wasn't that large and was four feet away from the QA401. There was no other indication that it wasn't working right.
Have you tried physically reorienting the various pieces of the system? I found that turning the QA401 on its side nulled out the hum coupling, which was like yours at the mains fundamental frequency and its third harmonic. That might not be a long term solution, but it could give you more insight to the source.
As Damian suggested, you do have power wiring all around you. It wouldn't be hard for some power line current to couple into your QA403 and/or the associated cabling. Steel test bench?
I've gone out to the wilderness (well a layby on a rural road without power lines), and measured stuff with my laptop and QA403 - it helps, turns out a car (ignition off) plus laptop is not that electrically quiet though, but no mains spurs at least. My main problem was I had 100mH chokes acting as magnetic antennae.
A mu-metal room would be an expensive answer to the problem!!
A mu-metal room would be an expensive answer to the problem!!