Under any circumstances, repair notes should be expected as common courtesy, even if to say “no problem found”.
I sent an email to Darren and Paul, since I talked to them previously.
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The team replaced all the boards inside and so you have a new unit. I think Darren emailed you that the next step will be to have a look at your setup. We’ll be glad to help.
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Yes, thank you; Darren and I are going to video conference to see if we can troubleshoot it.
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OK all - Paul and Darren got back to me and explained the repair work done (replacement of the guts, basically). Darren video chatted with me and we walked through some additional troubleshooting but we could not determine what the source of the hum was, even taking the signal path down to the phono stage and power amp (connected with good balanced cables). The best that we can come up with at this point is that it’s an environmental situation on my side which I haven’t figured out yet, since it’s pretty much a brand-new phono stage. The hum cannot be heard at normal to higher listening levels (and certainly not while music is playing). Thanks Darren, Paul, Morris, and those who offered suggestions in assisting me to try to work this out.
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You’re not just listening to Glenn Gould LPs, are you? 
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Why, yes - how did you know? Sometimes I hum along, too!
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You never mentioned whether the hum is still there with shorting plugs in the inputs. When a turntable is attached and the gain is high enough you will always start to get environmental hum. The goal is that it not be audible at listening levels. Testing with shorting plugs differentiates hum that is induced via the turntable/cables and that directly into the phono preamp. This is why SNR tests use shorting plugs to determine internally generated noise.
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Have you tried a different power cord on the Phono Pre?
Yes, I tried it with shorting plugs on Nov. 22nd, shorting the unused inputs and unplugging all turntables and shorting ALL inputs. Shorting the inputs either way didn’t change the hum and shorting the outputs caused a severe drop of output because I suppose the bal/unbal outputs were now coupled.
I did - tried the standard OEM cords I had around of various lengths and Pangea AC-14 ones. Moved them around so they weren’t passing over other cords, moved the unit totally away from everything else and ran longer RCA/XLR interconnects, no difference.
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Hooray for me! Thank you.
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Based on that test, then the only remaining suggestions is to check whether you really have a true ground on the third pin of your AC outlet using an outlet tester and whether the AC polarity is the same between your SPP and preamp. This can be done with a ground cheating adapter and measuring the AC voltage between the ground of your AC outlet and the chassis ground of each component. They both should show the lowest voltage with the same orientation.
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Thank you, had not tried that yet; lost my outlet tester and never got around to finding it again until now. Checked the power at the outlet, correct. Checked the power at the long surge strip I’m using, correct. I have already tried another outlet on another circuit, plugging directly into the wall, running the phono stage directly to my power amp (bypassing the preamp entirely), cheater plugs on various equipment. None of these seemed to change the characteristic of the hum, only the level of noise relative to the gain provided. I’m still leaning towards it being something in the house doing this because the entire PCB board of the phono stage has been replaced and I’ve cut the signal path down to the SPP, the amp and the speakers, and using any other source (CD transport, streamer-DAC) through the preamp results in silence.
This may have been mentioned previously in this thread, but I’ve found some light dimmers can raise hell with phono preamps in the form of hum, buzzing, etc. If not already attempted try turning off all lights at least in your listening space or try adjusting light dimmer level while listening and see if the noise changes in amplitude or characteristics.
Good luck. These kinds of problems can drive you a bit nuts.
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You said you have a spectrum analyzer. If this is induced hum directly into the SPP, then you should see a change in its magnitude between placing it horizontal versus vertical. If you see no magnitude change then it’s a grounding issue. If you could post a picture of your analyzer output, that would help in understanding the magnitude of the hum versus the noise floor.
Yes, thank you. I have no dimmers on this line or on that floor, but I do have fluorescent lights which I turned off (as well as everything else in general vicinity) to see if I heard a difference: turned off laptops and all lights of any kind, unplugged a refrigerator, unplugged the wireless modem. None.
I don’t have an actual spectrum analyzer unit - I’m using the iPhone SPL Spectrum Analyzer and Decibel X apps. This is the best I can do with what I have.
The 1st picture is the SA app showing the frequencies when the phono input is selected.
The next three pictures were recorded with the Decibel X app, portrait and landscape modes;
the third picture is the phono input with the preamp nearly maxed out from 1 foot away, and the 4th picture is another input selected - also the reading I get when I have the volume down.
Thanks for these as they are confirming what I suspected. I am not sure how familiar you are with phono circuits so I apologize if you already know this. Phono preamps have 40-70db of gain compared to CD, Tape Streamer, etc. They need to amplify a .0002V too .05V signat up to 2V which you can see is 200-1000x. In addition, they apply an RIAA filter which rolls off from 20 Hz to 20K Hz a total of 40 db. The consequence of this is that all AC-powered phono circuits will exhibit higher low frequency noise. The net is that it is normal to hear more noise and especially hum from the phono output than any of the other line outputs.
If you set your SPP gain appropriate for your cartridge (which if the Zephyr should be MM and Low) and set the volume at levels that you actually listen to a record at, then you should not hear any hum when the tonearm is queued up from your listening seat. I expect that will be the case based upon your tests. Therefore, you should accept this as normal and sit back and enjoy the music.