Originally Posted by morgiastic
Hi. Thank you for the guide, quite an accomplishment. I've been involved in Home Theatre for quite awhile, used to be quite active on another forum at one time, but not for many years (that every time I see MLP I think Meridian Lossless Packing, will give some of you an idea of the era that was). I have recently made some additions and upgrades to my system and that lead me to search out answers to some questions that ultimately, link after link, to Mike's guide. I have not yet read all of it, but, I believe, have read all the calibration information. This has raised some questions. I have also read much of this thread, and scanned through the rest, so I don't believe they have been asked yet. The first has to do with the following:
"The first thing to understand is that it is desirable to make the subwoofer amplifier send voltage to the driver, rather than having that voltage come from the AVR amp, because the subwoofer amplifier is much more robust and powerful than the amps in the AVR. This is an extremely important point, because using the subwoofer amp will help to prevent clipping. Clipping is a form of distortion, due to an alteration in the waveform. It can be audible in some cases, and if prolonged, can lead to overheating the voice coil in the woofer. When a waveform is clipped, the round top of the wave is squared-off---clipped-off. "
While I do seem to remember from the pre-digital room corrections days that it was better to level match with the sub-woofer gain, I would also have thought that the milliamps coming over RCA cables would not be able to tax an AVR amp to the point of clipping. Or, is it the sub amp that you're talking about clipping somehow?
First, welcome to the thread, and thank you for the compliment! I have very little expertise in electronics, but my understanding is that it is the voltage coming from the AVR amp that can cause clipping of the sub amp. For a more expert explanation of this you could consult someone like Mark Seaton, of Seaton Sound. He is probably the one I would turn to if I wanted to understand the actual mechanism better.
The whole gain/trim issue is a little more complicated, because different makes of AVR's may have different voltage structures in their sub amps. For instance, Yamaha AVR's have historically output a lower voltage signal to the subs than Denon/Marantz. That has changed in the last couple of years with at least some of the higher-end Yamaha's. The older, or lower-end Yamaha AVR's, have been less likely to induce clipping with positive AVR sub trim levels, but it has also made the subs less likely to turn-on from Sleep mode with lower trim levels.
Denon/Marantz and Onkyo AVR's, on the other hand, have never had a problem with outputting enough voltage to make subwoofers turn-on automatically, at most negative trim levels. But positive trim levels, especially combined with higher master volumes, have been more likely to induce clipping with those AVR's.
Another complicating factor is whether subwoofers can achieve their max RMS SPL with lower gain levels? Some subwoofers can't. I know a very experienced HT owner who used a Yamaha AVR, and who had resisted setting his AVR trim at a negative level. When he tried going down to about -3 in trim level, and increasing his subwoofer gain proportionally, he picked-up an additional 3db of headroom in his measured compression tests.
I don't pretend to have sufficient expertise to predict particular AVR/subwoofer/gain/trim/master volume relationships, or even to understand the exact reasons for the relationships. So, my default advice is simply to keep trim levels in negative numbers, and somewhere around -5 or -6 appears to be a pretty optimum range.
I think that trying to stay in about that optimum range kills two birds with one stone. First, it helps to insure that we won't be clipping the subwoofer signal. Second, it helps to insure that, if we want our subs to produce a lot of SPL, we will be increasing their gain settings in order to do it. For subwoofers which need
higher gain levels in order to produce max SPL's, that will be an advantage.
I have also heard that dropping the trim level too low (thereby reducing the voltage coming from the AVR amp) can also potentially degrade the sound. Ed Mullen, of SVS, referred to it as cutting the quality of the AVR signal. I don't pretend to understand that mechanism either. What trim level is too low? I'm not sure, but to be on the safe side, I probably wouldn't go below about -7 or -8.
Is a particular range of -5 or -6 really going to make a big difference in the headroom or sound quality of a particular subwoofer, in a particular room, at a particular listening level? I really can't say. But, from the standpoint of trying to give good general advice, and to establish a simple best practice standard, I think that shooting for about -5 or -6 in AVR sub trim makes some sense.
I know this answer goes well beyond your original question, but I thought it might be worth making a more complete answer for the benefit of others who may be silently reading along.