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24.4.2 » Aux fed subs issue - HELP! » Go to message
One of the best sound engineers in Central Indiana sent me this email. I don't understand some of it but it's quite a read! If any of you can translate this into more Sesame Street terms, MANY would benefit!


Hi Joseph,
Theres a very good article on aux fed subs by Tom Young at http://www.prosoundweb.com/article/a_detailed_explanation_of_the_aux_fed_subwoofer_technique/P1/

here's an excerpt
Question: What happens when I do a fade-out from the master faders ? Doesn’t this leave the subwoofers on?
Answer: If you do fade out the masters on the console, you must also fade out the master aux send for the subs. Consoles with VCAs obviously provide a much better means for this.

"post" means that the aux level follows the channel fader and not the master fader.

The most important point on aux fed subs is the calibration with the mains. Here's a simple explanation of the affect of calibration. Say you are running normal with a 2-way crossover, subs-mains with all your amps running at the same gain. The electronic crossover is set at 80 Hz and the time delay is set on the mains outputs so that at the crossover point, the upper frequencies of the sub is in perfect phase allignment with the lower fequencies from the mains. This provides perfect summation at the crossover point and is calibrated....as opposed to phase cancellation at the crossover point which no amount of EQ can fix. Now a paradigm shift in the meaning of crossover point. It is always the frequency point at which the subs and mains produce the same volume in the air(physically). This can be a different frequency than the electric crossover point set in the amp. And that is the point where phase alignment is critical. It's even more critical if the bandwidth is wide such as crossing the sub at 120 Hz and the mains at 60 Hz (one octave of overlap). In a complete system, the true crossover point is the "Acoustic" crossover point, rather than the "electric" crossover point. So now say you take the perfectly calibrated system and increase the sub level (by increasing the gain at the amp in a conventional system, or in the case of aux-fed, increasing the gain of the aux send at the channel or anywhere else). Suddenly the point at which the subs and mains produce the same volume (the acoustic crossover point) has slid upward in frequency. This both widens the bandwidth of the crossover and increases the true crossover point frequency which throws the phase calibration (time alignment) out of whack creating a notch in the response.

So be careful, because aux-fed subs can degrade the sound more than it can help if your calibration is off or if the technique is misused. In a calibrated aux-fed system, the channels aux send is calibrated at unity. Once set and calibrated, the send level should never be adjusted unless turned off (all the way down) for that channel. For instance, the kick channel should never run the aux send at +6 while the bass guitar runs at . The proper way is all channels using the aux-sub should run at unity, and those that don't use the sub, should be all the way off. Else, each channel that is not running at calibrated unity has a different true crossover point creating both phase induced frequency notches and unintended frequency overlap summation for that channel. In addition to keeping the channels aux send at unity, the gain relationship between the aux master send and the mains send must be maintained.

hope this helps
24.4.2 » Aux fed subs issue - HELP! » Go to message
roblof wrote:
bnski wrote:If your mix is not or does not have to be stereo - can you pan subs left and the rest right... and control both from the main fader? Or will this cause an issue somewhere else in the flow?

I assume he's using the FAT on the main bus (compressor, limiter, eq and like) and it's in linked operation, i.e. stereo.

You can't separate its functionality into dual-mono and this would be required for correct operation.

Only geq can be separably controlled on the main bus...


Yeah, I'm thinking about trying this if I can't get the GEQ issue figured out.
If I "pan feed" the subs, and assign GEQ's 7 and 8 to Main left and right, with those GEQ's work for the MONO output? I've read through the manual quite a bit searching for this answer but I haven't come acrossed it yet.
24.4.2 » Aux fed subs issue - HELP! » Go to message
MikeRivers wrote:Ah, so he's talking about two separate amplifiers, not two channels of the same amplifier, one for the main speakers and the other for the subwoofers.



Yes, I have an IT-4000 for the top speaker (the inputs are y'd) and an IT-8000 on the subs (the inputs are y'd as well).
24.4.2 » Aux fed subs issue - HELP! » Go to message
Oh yeah, here's the reason for all this. The HPF just seems to be kinda useless. It only cuts by 6db? This past Saturday, just for a test, I only had the sub amp on. I asked the singers to sing together and you could hear all kinds of unwantedness in the subs. I had their channels high passed at 180. Just to see, I then high passed at whatever is next up (240 I think?) and you could still hear them in the subs! I've gotta get this aux fed subs thing figured out!
24.4.2 » Aux fed subs issue - HELP! » Go to message
Well, here's where I'm at. I aux fed the subs last Wednesday. The vocals sounded better than ever! I got many compliments! It was a singing competition. I didn't get to experiment much. I just kinda hooked it up and went for it.

So, I get ...all set up for Friday night. I'm checking the kick drum in a room that seems to just suck away the kick drum. There are a few bands that double up on subs just for this room.

Anyway, I hear and feel I need to make a small cut at 100Hz. I didn't hear the change at all? So I re-think through things. Something isn't right.

I come out of the SL24.4.2's MONO output,through the snake, and directly into the I-Tech 4000 that powers the tops. I come out of the consoles' aux 10 output (I use aux 10 to feed the subs), into the snake, and into the I-Tech 8000 that powers the subs. There's no DriveRack. The crossover(s) are set in both amps.

Then I figure it out. See, I don't think the GEQ actually works if you use the MONO out. You certainly can't assign one of the 8 GEQ's to the MONO out. There's no "actual" choice for MONO when assigning the GEQ's.

Also, I totally didn't think about properly assigning the correct GEQ to aux 10. This hit me all the sudden during the first song of the night! So I go to the GEQ's and sure enough, I still have the GEQ's assigned as if I'm just running the rig in stereo:

GEQ # 1 - Aux 1
GEQ # 2 - Aux 2
GEQ # 3 - Aux 3
GEQ # 4 - Aux 4
GEQ # 5 - Aux 5
GEQ # 6 - Aux 6
GEQ # 7 - MAIN LEFT
GEQ # 8 - MAIN RIGHT

That's why I wasn't hearing a change in the subs when I cut 100Hz! I should have left auxes 1 through 6 as they were and then assigned GEQ # 7 to "MONO" and GEQ # 8 to aux 9. BUT YOU CAN'T FREAKIN' DO THAT ON THE SL24.4.2!!!!!! The GEQ's are only assignable in pairs!?That is so dumb! Man, I hate that crap! Why does it HAVE TO be this way? If only I could have:

GEQ # 1 - Aux 1
GEQ # 2 - Aux 2
GEQ # 3 - Aux 3
GEQ # 4 - Aux 4
GEQ # 5 - Aux 5
GEQ # 6 - Aux 6
GEQ # 7 - MONO
GEQ # 8 - Aux 10

Man, these Presonus consoles sound great and have some great features, but there are some things about them that just make you so mad! I wonder if GEQ # 7 assigned to "MAIN LEFT" would really work on the MONO output? Or would I have to use GEQ's 7 AND 8 just to have a GEQ for the MONO out?

I'm too sleepy to think clearly now. Somebody, figure this out for me! I need GEQ's 1 through 6 to remain as they are. I'm falling asleep. Help a brutha git sum aux fed subness.

Thanks!
24.4.2 » Aux fed subs issue - HELP! » Go to message
OK, last night I tried to aux feed my subs. I have a Presonus StudioLive 24.4.2. I came out of the MONO out and into the input (channel 1) of the ITech4000 that powers my tops. Then I came out of aux 10 and into the input (channel 1) of the ITech 8000 that powers the subs. The inputs were Y'd on both amps. There's no Driverack in the mix of things. The crossovers are set in the amps.

So, I played a track (channels 20 and 21) from an Ipod. I raised those channel's faders a little past half way. With aux 10's master at "unity" AND SET TO "POST", I pushed aux 10's "MIX" button and turned up channel 20 and 21 to the center position in aux 10. I then heard the low end being added to the main mix. Everything seemed to be working as it should!

But when I turned the main fader down all the way the low end remained! I thought that as long as I had the aux 10 master set to "POST" it would follow the main fader? You know, the same as if I were using aux 10 for reverb or something. So I set things back up the way I'd been running the rig before because I didn't want to do the show that way untill I found out what was going on.

If I wanna turn the main fader up or down, I want the low end (the aux fed subs) to go right along with it. I never tried to aux feed my subs on my analog console (the 'ol trusty Mackie SR24-4!), but it would seem that if I used aux 6 (which is fixed POST) to feed the subs, it would follow the main fader?

"Hey, you guys are too loud! Can you turn it down a bit?" Does this mean I will have to turn down the main fader AND the aux 10 master? Is this normally how aux fed subs work? What am I missing here?

Thanks!
24.4.2 » GEQ questions... » Go to message
I picked up a StudioLive 24.4.2 today. I've read page 58 (GEQ's) over and over and I just don't understand how to insert them where I want them. HELP!

I want GEQ #1 on aux 1
GEQ #2 on aux 2
GEQ #3 on aux 3
GEQ #4 on aux 4
GEQ #5 on aux 5
GEQ #6 on aux 6
GEQ #7 on "MAIN MIX L
and GEQ #8 on MAIN MIX R.

What exactly do I need to do?

Also, how do I "link" GEQ's 7 & 8 so I don't have to adjust both of them individually?

Thanks everybody.
 
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