Originally Posted by johnson636
Currently, all speakers are set to small. So your statement holds true for the 2x4 HD as well? Are you saying neither the regular 2x4 nor the HD is the correct piece of equipment for bass duty? In regards to input clipping avoidance.
The 2x4HD can take 4Vrms on the input, which is better, and can output 2Vrms. So it's probably just adequate for many people. The balanced 2x4 can take 4Vrms input and can output 4Vrms (only using a balanced output).
You also need to keep in mind that many people are trying to boost the signal in their MiniDSP at some frequency or frequencies (house curve & EQ'ing), so that means they need to dial back the overall gain on the MiniDSP in order to gain headroom for the frequencies they are boosting. notnyt touches on the other point of whether you can get the full rated power from your amp with the signal or not. When combining these two factors you could fairly readily be in a situation where there isn't a combination of gain and level trim settings where you can avoid clipping the input or output signals from the MiniDSP, be able to drive your amp to it's full rated power, while simultaneously setting the subwoofer to the right level for 85dB playback with a -30dBFS signal.
The 2x4 Balanced and 2x4HD will clip their input at about the same point as most Denon receivers clip their output. They will actually just barely clip a 0dBFS signal in the LFE channel played at reference with the subwoofer trim at 0dB (all processing bypassed). To allow for redirected bass (if you listen at reference) without clipping the Denon should probably have the level trim set to about -6dB. This is then safe for the input of the MiniDSP 2x4HD (4V setting) or the 2x4 Balanced. However, if you're applying any filters in the MiniDSP that add gain at any frequencies you need to reduce the overall gain in the MiniDSP by a corresponding amount. So if you have some filters for EQ or a house curve that give you say 8dB of gain somewhere in the frequency range of 5-120Hz you need to pull down the gain in the miniDSP on the input by 8dB to prevent internal clipping. This now means you've reduced your LFE chain by 14dB from what's coming out of the receiver (which itself is dialed back from 0dB). 6dB from the 4V input to 2V output loss just going through the miniDSP, and 8dB from the internal MiniDSP gain adjustment. The overall efficiency of the subwoofer system and gain of the amp have to overcome all these gain reductions.
The output of the MiniDSP is what it is. Sure, you can say, "Well, I can get 2Vrms (balanced 2x4HD) out and that's more than my amp needs to make full rated power into the load." but that doesn't consider if you can still calibrate the sub to 85dB with a -30dBFS .1 signal. My opinion is that the latter is more important than making full power. If you can set the "gain" knob on your amp to any setting and achieve the correct SPL output level after you've set everything to make sure you don't get clipping in the signal chain in front of the amp I say you're done. IMHO, whether you can still make your full rated power from your amp is largely irrelevant. Maybe you'll clip the output of the amp from a lack of amp power or maybe your amp will never make more than half it's rated power, but the amp will be getting a clean signal on the input and your sub will playing back at the right SPL up to reference (assuming the amp isn't under powered and clipping the output) and that's the best scenario.