[Mezzo]

Hi everyone,
 
Having an argument with my friend who insists his system is set up correctly, but I need your help on a few things.
 
He tells me all his speakers are set to Large and he has set his subwoofer low-pass to 40 Hz. He has a Rotel RSP-1068 processor.
 
I looked at it here :
 
http://www.rotel.com/content/manuals/rsp1068.pdf
 
But I can't find any mention of being able to select a subwoofer low-pass if speakers are set to large. Can someone please just double check for me. He insists his system is set up this way. I want to prove him wrong.
 
I'm not sure what could be low-passed if all speakers were set to large anyway.

 

[xnor]

All LARGE speakers plus subwoofer:
 
Quote:

This system requires no bass re-
direction. All five speakers play the nor-
mal bass recorded in their respective chan-
nels. The subwoofer plays only the LFE
channel  bass.  Depending  on  the
soundtrack, there may be minimal use of
the LFE channel, so the subwoofer would
be under utilized. Meanwhile the normal
bass places higher demands on the capa-
bilities of the other speakers and the am-
plifiers driving them.

 
 
Crossover setting of speakers: (40Hz/60Hz/80Hz/100Hz/120Hz/150Hz/200Hz)
Quote:

This setting ONLY affects redirected bass and does not affect the
LFE channel in any way.

 
There's also a subwoofer crossover setting (same frequencies as above).

 

[Mezzo]

So if he is talking about a 40 Hz low-pass that must mean he either has all speakers set to Small 40 Hz, or has set his front speakers to Large and center and surrounds to Small at 40 Hz.
 
So then he talking nonsense to me. He kept on saying he set the subwoofer crossover in the processor to 40 Hz. But this can't possibly be.

 

[jaddie]

All powered subs have built-in low pass filters that can be set over a range from about 150Hz to some lower frequency like 40Hz.  That LPF is active in the sub itself.  Could be what he's talking about.

 

[Yahzi]

No, he said he set the sub LPF to 70 Hz (on the actual sub). He said he set the low-pass on the sub in his processor to 40 Hz. But all speakers are set to Large. This he confirmed for me.
 
So now my question is, if all speakers are set to Large then how could he be low-passing anything? The sub should be playing the LFE only, surely? The sub out is low-pass filtered but the manual does not say if you can adjust the low-pass for the LFE so I assume it must be 120 Hz, since that is the upper limit of the LFE.
 
But if he has set 40 Hz as the sub low-pass, I can't imagine how it could low-pass at that frequency if all speakers are set to large. Correct me if i'm wrong?

 

[jaddie]

Ah, had to read the manual a bit. Page 36, Subwoofer yes/no/max.  In the "max" setting you have all the bass in any large speaker also directed to the sub.  It's Large with the sub getting all the bass plus LFE.  The crossover setting would obviously be active.  40Hz doesn't make a lot of sense, but the max mode is well named.  

 

[Yahzi]

Jaddie, that setting sounds like a double bass feature. Still don't understand how a 40 Hz low-pass can work as he said he has it set to 40 Hz. There IS no crossover since if all speakers are set to Large there can be no crossover. The sub can ONLY handle it's own LFE otherwise what is 40 Hz low-pass going to do?

 

[Yahzi]

Also just to add, normally if there was a double bass function there wouldn't be a crossover setting. It would just send the bass from the mains (if set to Large) to the sub. So I really don't get how he has 40 Hz low-pass for the sub if all speakers are already set to Large.

 

[goodvibes]

His system is not setup as you mentioned. The crossover frequency only comes into effect when something in the system is selected to small. In this case it's likely the just the center and rears with the fronts being full. The woofer still operates in it full band on the effects channel and also plays the low bass from anything below the selected crossover frequency of the smalls in the system. He's lucky if his rears and center actually reach 40hz so he should likely raise the xover point for those. It doesn't seem to be a as big a deal in video but you actually want response an octave below the crossover point to get a proper transition so 80 tends to sound best if you've got a good woofer. Sometimes keeping the fronts full range can sound better by just not asking for more DSP in surround or (mediocre) woofer use in stereo but in theory, crossing the fronts should be better unless they are flat to 20hz.
 
If he listens to a lot of stereo music he may actually have a reasonable setup but he should still raise the crossover frequency for the center and rears when using surround.

 

[Yahzi]

So then what would the subwoofer preout be low-pass filtered to? If he disables his subwoofer LPF (on the sub) then what would his LFE low-pass be on the processor? 120 Hz? Because I don't see any section for adjusting the LPF of LFE.
 
BTW, thanks jaddie and goodvibes!!

 

[jaddie]

Quote:

Also just to add, normally if there was a double bass function there wouldn't be a crossover setting. It would just send the bass from the mains (if set to Large) to the sub. So I really don't get how he has 40 Hz low-pass for the sub if all speakers are already set to Large.

 
The "double bass" or in the case of the Rotel, "Max" setting there still has to be half of a crossover, even though all speakers are large/full range.  The half that is working is the LPF that feeds the sub.  You wouldn't normally want full range audio going to a sub.  In this, and most setups, there are two possible sub LPF functions, the one in the AVR/Pre-pro and the one in the sub itself.  Ideally, you'd want to only use one of the two and completely bypass the other.  
 
Even though "Large" or "Full range" speakers may have some extended bass, they aren't subs and can't hit the required maximum SPL at 20Hz, for example.  Yet they may do ok between 60 and 40Hz, depending on the design.  Hitting them with full range and allowing whatever energy they product below the crossover does two things.  1, it augments a possibly wimpy sub adding to the total maximum SPL, and 2, because the LCRs are in different locations, they could possibly even out bass response in a room that may otherwise be uneven because of modal distribution.  That's the up-side.  The down-side is that full range speakers usually can't produce high SPLs a sub frequencies without distortion. So, while you get additional bass, it's probably not very clean, or nearly as deeply extended.  It's a compromise, sometimes good, sometimes not.
 
Why pick 40Hz for the sub only?  Depending on the bass response of the main speakers, if they are really "large" with lots of woofer cone area and excursion in a cabinet designed to do some real bass, then they may well extend town to 40Hz and below.  But below 40Hz almost every full range speaker runs into issues where it needs help from a real sub.  Frankly, I'd have picked 80Hz, but I don't know the speakers.  

 

[jaddie]

Quote:

So then what would the subwoofer preout be low-pass filtered to? If he disables his subwoofer LPF (on the sub) then what would his LFE low-pass be on the processor? 120 Hz? Because I don't see any section for adjusting the LPF of LFE.
 
BTW, thanks jaddie and goodvibes!!

The Rotel manual is a bit vague on this.  The MAX mode description implies an LPF is present on the sub output, but they don't specifically say so.  Without interacting with the setup menu, particularly the advanced settings, there isn't much we can tell about this.  But logically, you would want an LPF on the sub out, and it should be settable.  It is a function duplicated by the subs own LPF of course, but there are a few situations where a sub may not have an LPF, specifically, a passive sub driven by a full bandwidth amplifier.  
 
As to the specific LPF frequency setting, that's going to be something best determined with a calibration, hopefully by someone who knows how to do it, though honestly with a pre-pro like that, our hands are kind of tied. 

 

[Mezzo]

Excellent points. Thanks for contributing guys.

 

[Mezzo]

Quote:

The Rotel manual is a bit vague on this.  The MAX mode description implies an LPF is present on the sub output, but they don't specifically say so

 
What if the Max mode is not selected? If the low pass filter was set to 40 hz for the subwoofer what would the subwoofer be low pass filtering? Thanks!

 

[jaddie]

There are 3 sub settings on the Rotel: yes, no, and max
 
Yes: you have a sub, and then get to pick large (sub gets LFE only, no crossover active), or small (sub gets all bass, crossover frequency affects both mains and sub)
 
No: you don't have a sub
 
Max: you have a sub and LFE plus bass from all channels to go to the sub, LPF active (probably) and settable as a crossover frequency, though it's only half of a crossover.  Mains get full bandwidth.
 
 
It gets worse.  Advanced settings let you pick stuff on per speaker, per surround mode basis.  Yikes. It's tweak-head madness.