Related to this, I haven't forgotten it, got the move done back to the normal apartment last weekend, quick testing, quick measurements with the oscilloscope with screenshots a week before that, clearly showing the 50Hz hum in tube mode but not in the two others, tried another new thing this week (more on that later, but the problem got fixed, but it's not really an sensible option with a 1700€ set), and doing further testing with another power amp instead of Vidar with some other ideas. I'll report back as soon as I can.
Schiit replied me with forwarding my questions to a "senior tech" along with a request to contact the distributor - Sonority Audio, haven't gotten around to that yet until I've exhausted all of my ideas. No reply from that senior tech from Schiit; but I'll keep them updated about my findings when I get around to that.
As promised, my measurements, both units in the same grounded power strip, both 230V versions, Freya muted
- Schiit Freya with supplied power cord
- ViaBlue NF-A7 RCA interconnect
- (GLI-2RCA RCA ground loop isolator when mentioned in the description)
- Schiit Vidar with supplied power cord
- Blue Jeans Cable Ten White speaker cable
- Tannoy XT 8F (91dB sensitivity 2.83 volt @ 1 m), measurements from the link plates of the speaker posts.
As mentioned in my previous posts, I have several dozen matched pairs of tubes, with some tube pairs the hum is very loud, and with some it's so faint that you can't really hear it from the listening position unless you really concentrate, (albeit heard more clearly from various places in the room due to room resonances).
All 18 measurements with descriptions are found here:
https://imgur.com/a/DxhZicV
Measurements from #1-15 done with a tube pair where the hum is very loud,
(measurements #10-15 from the splitted RCA signal, the level of the signal is so low that you can't really see anything, but they're in the album if you're interested)
and measurements #16-18 are among my quietest pairs, where you really have to concentrate to hear the hum.
As mentioned, I'm not an EE, so a friend took the measurements.
Here's a quick comparison:
- #1: XLR -> Vidar in monoblock mode, measurements from right speaker post, tube mode:
- friend thought it was merely an RCA ground loop problem, where the balanced differential signal would get rid of the hum, but no.
- #4: RCA -> Vidar in stereo mode, measurements from right speaker post, tube mode
- the hum is a lot louder vs. XLR/monoblock configuration
- #5: RCA -> Vidar, measurements from right speaker post, passive mode
- very, *very* low noise floor, you can't even tell if the amp is on or off, with your ear by the tweeter
- #6: RCA -> Vidar, measurements from right speaker post, JFET
- from my previous post: "... with your ear directly by the speakers' driver, you can hear the noise floor rise with JFET even so slightly"
- #7: RCA with a cheap RCA "ground loop isolator" -> Vidar, measurements from right speaker post, tube mode
- my friend wasn't 100% certain if the 50Hz hum was seen by the oscilloscope or not, in real life the hum disappeared in my opinion, but was it due to the fact that the cheap isolator attenuates the signal a bit so the hum disappears in the inherent noise?
- see the album images #8 and #9 for passive/JFET measurements, they're just as noisy?
- in RL the noise floor did rise a bit with the isolator, almost inaudibly, but tbh the measurements look a lot worse? I seriously thought the problem was solved with the isolator - it was that quiet.
- don't use a cheap RCA ground loop isolator, as it *really* messes up the frequency response, my unit got rid of the bass
- #16: bonus measurements with a "quiet" pair, RCA -> Vidar, measurements from right speaker post, tube mode
- a pair among the quietest tubes but the 50Hz hum is still faintly audible as mentioned above
I'll send the measurements to Schiit and ask, is this working as intended (designed this way?), or could my Freya (or Vidar?) be faulty, and I'll contact Sonority about the problem after their response.
Before these measurements in my previous post I thought I had solved the problem, as the hum did, and does disappear with the (cheap, low-quality, frequency response-messing) RCA isolator (IRL, IMO).
But now after the measurements, I'm not 100% certain if it the ground loop isolator does the trick or not;
and I can't say if a better quality, Radial Engineering/Jensen transformed DI/isolator box (which doesn't mess up the frequency response) can help or not with my problem (and if so, will it help due to isolating or attenuating?).
Also, they're quite expensive - dropping 200-400€ for a box between the Freya and Vidar shouldn't be the way to go, eh?
My friend thought that perhaps an RCA -10dB attenuator would get rid of the hum as well, in my case.
Any comments would be appreciated.
Edit/addendum 28.10.2018, 20:40EET:
I forgot to mention in the post, that we confirmed the hum with another amp instead of Vidar, an old AVR Yamaha acting as the "power" amp.
Additionally, in my post #14511303 I mentioned how I tried different grounding schemes before, along with an isolation transformer before/between the units, lifting ground manually, grounding the chassis of both or either units (as suggested a post after that) etc., to no avail.
This time no isolation transformer was used, but there were no other electric appliances plugged in the fuse in my living room, other than Freya and Vidar (and Yggy for a while), for the measurements - as mentioned, both grounded in the same power strip.
