What I was getting at though, for receivers that do not select a zero point, starting at the point at which the volume knob gives you zero for the first speaker means that for your room, that indicator on the dial now represents zero and the rest of the speakers are relative to that, allowing the most + and - flexibility (if needed).
I think I see what you are getting at, but am perhaps confused by what you mean when you say 'represents zero'.
The channel trims are like faders. 0 dB means 'no change'. Just for an arbitrary example to illustrate the fact that the different scales all interact, let's say you start with all the channel trims at zero and the master volume is at 82 out of 100 [forget about the relative scale and '0 dB' for a moment - it's the same thing with a different nomenclature].
Let's say that the receiver is putting out 82% of its max voltage at that setting. Say the output SPL for the front channels is at 90 dB and is therefore too high. You turn the channel trim to -5 to bring the SPL for that channel down. What it does is reduce the power for that channel by 5 dB (by reducing the voltage).
If the channel trims end up positive, it reduces the range of the master volume control because the receiver cannot exceed its max rail voltage. For that example receiver, the master volume control will now top out at 95 instead of 100.
If you use -30 on the volume dial and turn every speaker up to +9, isn't that the same thing as just making all of them zero and turning the volume up by 9?
Yes. The catch is what you alluded to in the last post. Due to the range of the channel trims, you may not have enough range to adjust to a higher SPL at a lower volume setting. I assume that is why thx receivers automatically choose the level for you.
For a real world example, on my lowly Onkyo 502 I chose 60 for the reference volume position (it goes to 79). My trims ended up at +6 for the fronts and +10 for the center. If I wanted the front trims at zero, my reference volume position would go up to 66 (and the center would have to be +4). When I lived in an apt and the room was much smaller, the trims were all at zero and I could achieve the same reference level at 50 on the scale. When I get around to buying a 703 for this house, I'm sure the channel trims will be wildly different.
Which brings up another question or two - Do all receivers adjust via the volume dial in a linear fashion? Ithought I read somewhere that some receivers increase volume more rapidly as you move closer to the 100% gain. Do all receiver's volume knobs adjust in 1dB increments?
Older receivers with analog volume controls definitely increase the volume in a logarithmic fashion. It was called 'audio taper'. Now we have 'digitally controlled' analog volume controls - so the processor calculates the amount of gain needed to increase the power by 1 dB. I think there are some receivers (some Yamaha models?) that advertise .5 dB volume increments.
On the older receivers the volume knob definitely had an endpoint. Now, the volume control is infinitely variable. Spin the volume knob so the volume is high, say 0 dB (or 80 or whatever) and then turn it all the way down with the remote. You can then go back to the volume knob and keep turning it up - there is no endpoint because it is controlled digitally and not by the physical knob.
Onkyo tells us in their manual that 82 on the absolute scale is equivalent to 0 on the relative scale. What they don't tell us (nor does any other manufacturer) is what that means in terms of the receiver's rail voltage. I'm sure thx defines a specific voltage (or percentage of max) for '0 dB' and I've been trying for years to find out what it is.
