The physics and limits of relative loudness perception

@kofibaffour 

Thank you for the detailed response and the links to Erin’s videos. You provided me with a better vocabulary to describe what I experienced. Before I reframe that, the speakers are DALI Epicon 6, with a pair of asymmetrically placed Rythmik F12G subs. They have been carefully set up, including active crossover with the subs, and phase matching of the signals at the crossover points. All this can be viewed on my Virtual System page.

The facts were:

1. With the 0.8dB (unintentional) right channel gain boost and the Primaluna amp, I did not perceive any noticeable change in the location of the center vocals.
2. With the Apollon swapped in (no change to speaker location or angle), and still with the 0.8dB right channel boost, the vocals shifted perceptibly to the right
3. With the Apollon still in, but removing the right channel boost, the vocals shifted back to the center, where they should be.

Based on the new vocab you have given me, what I think was going on is:

1. The Primaluna tube amp has good imaging, but relatively poor focus. Very likely, the 0.8dB did shift the image of the vocals in the Primaluna just like in the Apollon, but with a spread out focal point, some of the location data overlapped with the center phantom image, and so the voices were "close enough" to center that psychoacoustically, the brain puts the vocal image in the center.
2. The Apollon has good imaging too, but better focus than the Primaluna. Thus, with the 0.8dB right channel gain, no part of the image of the vocals overlaps with the center of the soundstage, and thus the perceived shift of the focal area to the right.
3. I suspect that, if I were to swap the amps back, now knowing what to listen for, I would perceive a slight shift in the vocals using the Primaluna, but not to the degree of precision / noticeability of the Apollon. 

I find the psychoacoustics interesting, in that the shift wasn’t really perceptible with the Primaluna, but was very apparent with the Apollon. 

I was pleased with the soundstage and imaging before, but even more pleased now.

I found this interesting site: https://www.audiocheck.net/blindtests_level.php?lvl=0.5

I can, with 100% reliability, distinguish sound levels of +/-1.0dB. But only about 60% reliability for +/-0.5dB

Clearly no "golden ears" here 😬

@sfgak glad to see you've gotten to the bottom. And no one has golden ear in this space full of people nearing or in retirement age zone who can't hear pray 10kHz effectively for the most part. Hell music itself lacks any hearable content of good volume past 14kHz so don't worry about not detecting ±0.5dB differences 100% of the time. Happy listening

Left-to-right channel imbalance is very noticeable at 1dB, and can be annoying even at 0.5dB and a bit below. Yes, I’ve had aysmmetric rooms cause it, usually perceived as between 0.5 - 1 dB. But it can also be caused by components. Of course, sometimes it’s not just a flat SPL difference - it could be specific frequencies or phasing / timing issues that enhance our perception of the bias.

Since I’m more analog based and generally don’t use DSP, I’ll swap L/R cables along certain components to allow imbalances to cancel out where possible - I don’t really care if my absolute L/R channels end up flipped; music is not like movies. BIG difference if you let 0.25 + 0.25 + 0.25 = 0.75dB versus leveraging cancelling to net 0.25.

Suspect cable contacts / connections can also *easily* cause imbalances. Keep them clean, and periodically ensure you don’t have a cable with internals going bad. I think one of the more common culprits is those litttle DIP switches often used in phono stages. I HATE those, ugh. Tubes are another one of the more common sources of imbalances - so it might seem odd you got the imbalance after going from tubes to SS - unless you had an imbalance in the tubes that was actually offsetting the room (I’ve had this happen)!! Cartridges too, though not as common as you’d think - I’ve had some 40+ MC cartridges, and precious few with notable imbalances. Speakers too. 

I think the old "1dB is the minimum discernible by humans" is imperfect science. 1dB of absolute level might be negligible to casual listeners, but we’re not that. And 1dB of imbalance is far more discernible than 1dB of absolute level. This is surprisingly effective.

Ooof, channel imbalance has driven me crazy at times. I’ve thought about having dual (mono) stepped attenuators with say 0.25dB steps to allow for balance correction - e.g. from Khozmo or Goldpoint. Some preamps will throw in a balance pot but it’s inevitably one of those awful sounding Alps "Blue Velvet" RK27 things which is a no-go for me. 

When I have my Levinson No. 32 preamp in my system (which allows for very small incremental changes of volume and/or balance by remote control), I can hear very subtle image shifts with as low as a .2 db change in relative balance, with .5 db shifts being clearly discernable.  This surprised me because I cannot reliably hear a full 1 db change in overall level.  I consider balance control an necessity in a system.  My favorite linestage is a tube unit I own which has a balance control, but not with such fine steps.  Still, having some control is WAY better than not having such control.  This is particularly true with tube gear where level differences are much more likely to occur because of slight differences in tube gain and effects of aging.