Aux feed subs?

[Per Søvik]

Re: Aux feed subs?

Years ago I changed from Aux subs to dual PA for larger shows. One PA is full range including subs tuned for instruments (read flat or neutral) and is taken from my main LR out. The other PA is just top boxes tuned for vocals and is taken from the highest sub-master or 2. Any delays or secondary feeds are from a matrix mixer. I find this leads to a nice sound with lots of head room, simplified wiring, and an easy to mix on system for “brother of the drummer is our sound guy” who have no clue about aux subs and other such magical audio things.

I prefer it that way, you get vocal clarity even when you are absolutely trashing the instruments PA, you can optimize the vocal PA for vocals, and you get a more open sound with the vocals sitting in their own part of the three-dimensional aural space, much more like "real" acoustic sound untouched by bits and volts.

 

[Chris Johnson]

Re: Aux feed subs?

So 4 posts later, Chris was 100% correct.

Acoustical Crossover is not some term I made up, but rather a term that is widely used in the industry. I can almost guarantee Chris was referring to the acoustical crossover in his post.

Indeed, I should have been more clear.

As with all these things, what we do in the electrical domain is merely a means to an end when it comes to influencing the acoustical domain, which is what our listeners experience.

I still use 'aux fed' subs regularly, but not in the way you typically think about them:

First off, I only feed the subs from a fixed-level group that is, within the console, phase aligned with the main outputs. I may also do the same thing with front fills, as an example. This group is setup such that having a source assigned to L,R,Sub yeilds exactly the same result as having it assigned to L,R and having the subs crossed over with the mains.
Then, instead of thinking of things I want to add to the sub, I'm really thinking of things I want to remove from the sub.
What I find is that this functions as an EQ tool, without affecting the overall system response at all. It allows me, for example, to get great low-end performance out of a vocal mic with only the 4 bands of EQ I have to work with on the console, because I'm not wasting a band tuning the effects of the sub.
Secondly, it helps in a practical sense because subs are difficult to get right. Getting great low end performance in line with the performance of the main arrays (in a typical system where you might assume the main arrays crossover at around 60-90Hz) requires lots of boxes, lots of amps, and proper positioning of sources. This is not always an available option. Sometimes because lesser system techs don't really know how to achieve these kinds of results, but often because practical considerations won't allow you to have your subs accross the front of the stage, only a LR stacks will work, or maybe you can't fly any subs, etc...

So, thats my approach. I do feed the subs a seperate console output, but its not a channel aux, and its constructed in a way that doesn't negatively impact the system as a whole.

 

[harrybrilljr]

Re: Aux feed subs?

This thread is like which kick drum mic is best. I feel like I've read this quoted post a hundred times. If I had the internet when I was starting out I would have made use of it. Brian, it's NOT the same. Set the subs up on an AUX. Send every single channel to that AUX post fader. Balance the PA. Soundcheck and use your channel highpass filters. MUTE the PA but not the subs. Listen. Now MUTE the Bass, Kick Drum, Keys, Floor tom (the instruments you need the subs for). Do you hear anything coming out of the subs? If the answer is YES, then you now know it's not the same as just not sending that stuff to the subs.

What would the difference be in using aux fed subs versus using high pass filters on channels that you didn't want to get into the subs? Wouldn't that be essentially the same thing?

I could see aux fed subs to be used when there are additional subs needed for an effect, but the added complexity for most applications doesn't appeal to me at all.

With today's digital boards, you can get a pretty accurate high pass filter, and adjust the frequency where you want it to be. I could see maybe with older consoles that didn't have that flexibility, but not now.

I'll actually often do a high pass and a low pass filter on instruments to match their range needed, and keep the unwanted sounds out of the system. Each type of instrument is going to be different, but once set up, it can really help keep noise levels down.

 

[harrybrilljr]

Re: Aux feed subs?

pictures

 

[Art Welter]

Re: Aux feed subs?

pictures

I see a picture of a 2000 Hz electrical crossover with the lows boosted 24 dB, and another picture with a dip at the crossover point.
What are the pictures supposed to tell us?

 

[Per Søvik]

Re: Aux feed subs?

I see a picture of a 2000 Hz electrical crossover with the lows boosted 24 dB, and another picture with a dip at the crossover point.
What are the pictures supposed to tell us?

I guess it tells you that if you have a crossover with x dB/oct steepness and you boost one of the outputs by x dB, you move the acoustical crossover point by one octave? ~;-)~:wink:

That is of course assuming that both drivers have linear response and so on and so forth........

 

[Evan Kirkendall]

Re: Aux feed subs?

Hi Evan,
No thats not what I am saying at all. Sorry for the confusion. What I am referring to is strictly the relationship between two components in a PA System. Because the question was about sub on aux. So lets say I have a System flown in the Air and Subs on the ground. If I control the Subvolume separate from the Main Pa Volume and turn them down for example. A certain point on the slope of the crossover filter will now move down on the frequency scale. David named that the acoustical crossover earlier on. This must have an effect on my phase alignment and time alignment between the Subs and the flown PA. So once I have tuned the system and aligned all components with each other I should not move this relationship between the cabinets anymore. So to me it seems wrong to have the subs on a separate Fader and to move them up and down during a show. Changes should, in my opinion, then only be made by EQ and to the entire System. But I am aware that the negative effects depend on many factors. What filters are used and the ideal crossoverpoint for the cabinet. But I would like to stress that I was not refering to any channel eq or Low cuts in desk channels. I am also aware that opinions here vary and some people swear by SUB on AUX. :razz:

Sorry that it's taken me so long to reply back to this thread.... Finally got around to proving my point today. Below you will find a SMAART trace of a house system with the subs on an AUX.

The purple trace is with the subs/lows phase aligned at a "nominal" level. The blue trace is with the sub AUX at +10. The phase shift is almost nonexistent, and the slight change I would blame on the room, not the change in amplitude. Yes, the acoustical crossover point has changed, but NOTHING electronically has changed. Furthering my point: phase deals with time, NOT amplitude.




Evan

 

[Ivan Beaver]

Re: Aux feed subs?

I guess it tells you that if you have a crossover with x dB/oct steepness and you boost one of the outputs by x dB, you move the acoustical crossover point by one octave? ~;-)~:wink:

That is of course assuming that both drivers have linear response and so on and so forth........

It is amazing how many people do not "get it" when talking about the difference between acoustical and electrical crossovers.

They just "assume" the sound stops at the xover freq-wrong.

And they also assume that the loudspeakers have a flat response-wrong again.

Once you start to realize what is happening in the REAL WORLD-all sorts of things start to change-and it get harder the more you understand.

 

[Silas Pradetto]

Re: Aux feed subs?

Sorry that it's taken me so long to reply back to this thread.... Finally got around to proving my point today. Below you will find a SMAART trace of a house system with the subs on an AUX.

The purple trace is with the subs/lows phase aligned at a "nominal" level. The blue trace is with the sub AUX at +10. The phase shift is almost nonexistent, and the slight change I would blame on the room, not the change in amplitude. Yes, the acoustical crossover point has changed, but NOTHING electronically has changed. Furthering my point: phase deals with time, NOT amplitude.




Evan

Evan, Gert is correct. No, adjusting amplitude of a passband does not affect the phase of that passband, but it will change the acoustical crossover point, and therefore, the phase relationship at the acoustical crossover point may no longer be aligned after the amplitude changed.

So, if you're all set and aligned at 100Hz and the acoustical crossover is at 100Hz that's great. But if you boost the subs bringing the acoustical crossover up to 130Hz (which happens all the time!!!), then you have to check the phase at 130Hz now to ensure it's still aligned.

Any system tech worth their salt tries to get the phase responses of the subs and tops to align as far as possible on either side of the acoustical crossover, for just this reason. I like a minimum of one octave either way from my usual 80Hz crossover, but often try to go as high as 200Hz.

The phase relationship of the subs to the tops is only relatively constant if the subs and tops are co-located. If they're separated, any and all alignments are a compromise.

 

[Phil Graham]

Re: Aux feed subs?

Evan,

Silas' post is a good one. If you have good phase alignment about the nominal crossover point, then you'll be in good shape for phase alignment as the effective XO point changes. This isn't always achievable, but the situations where you might have a hard time doing this would make for a very long post.

As the effective XO point get raised, and the wavelengths get shorter, the resulting tolerances for speaker placement and overall phase alignment are reduced. A few ms is not a big deal at 50Hz, but it is at 200Hz.

In several instances where I had sufficient DSP resources, I have used an allpass to help broaden the range of phase alignment between the subwoofers and main speakers, with the goal of making the aux fed subs and the tops in phase alignment over a larger frequency band.

 

[Gert Sanner]

Re: Aux feed subs?

Thank you Silas for clarifying the point I was trying to make. I thought no one understands me.

In several instances where I had sufficient DSP resources, I have used an allpass to help broaden the range of phase alignment between the subwoofers and main speakers, with the goal of making the aux fed subs and the tops in phase alignment over a larger frequency band.

Yup that is a great way of doing it. Also when you have many different types of cabinets like Subwoofers, flown Mains, small Frontfills and maybe medium Sidehangs.

 

[Silas Pradetto]

Re: Aux feed subs?

Thank you Silas for clarifying the point I was trying to make. I thought no one understands me. View attachment 6898




Yup that is a great way of doing it. Also when you have many different types of cabinets like Subwoofers, flown Mains, small Frontfills and maybe medium Sidehangs.

A good example of how an allpass filter helped me out recently:

I use RCF and JBL active subwoofers, and they both unfortunately use high-order lowpass filters. I also use Fulcrum Acoustic boxes, which have a 24dB/octave highpass integrated in their greybox processing. When you try to align a box with a 24dB highpass and a box with a 48dB lowpass, it is just not going to happen nicely. Fortunately, the processing the Fulcrum boxes use has allpass filters, and I was able to double-stack second-order allpass filters at the same frequency as the highpass, effectively giving me the phase shift of a 48dB/octave highpass with the rolloff of a 24dB/octave highpass. Things aligned easily after that.

A very simple example, but I hope it helps.

 

[harrybrilljr]

Re: Aux feed subs?

It's not that the phase is changing with amplitude, it's that the crossover is changing to a place where the phase is NOT aligned. It's rarely possible to align the phase 3 octaves wide.

Sorry that it's taken me so long to reply back to this thread.... Finally got around to proving my point today. Below you will find a SMAART trace of a house system with the subs on an AUX.

The purple trace is with the subs/lows phase aligned at a "nominal" level. The blue trace is with the sub AUX at +10. The phase shift is almost nonexistent, and the slight change I would blame on the room, not the change in amplitude. Yes, the acoustical crossover point has changed, but NOTHING electronically has changed. Furthering my point: phase deals with time, NOT amplitude.




Evan

 

[harrybrilljr]

Re: Aux feed subs?

Some folks prefer an impulse alignment, which is not to say you use the impulse response to do the alignment. You still use the phase trace. I got this straight from the source, Dr Don, in person. In this case the phase traces only CROSS, not overlap. The overall impulse response is much tighter, and the system as a whole sounds tighter. If you then shifted the crossover up or down you would definitely be in trouble.

 

[Loren Aguey]

Re: Aux feed subs?

So those of you who do use aux fed subs: Do you set the aux master to unity and input sub sends to taste or all input sub sends to unity and set master to taste? I do the former not for any consideration of phase, but because I find I don't want everything I'm sending to the sub at the same level. And depending on the signal I get, the sub send generally replaces my low shelf boost or cut unless I have an issue with a specific frequency range.

 

[harrybrilljr]

Re: Aux feed subs?

I prefer to use a MONO bus, like if you have a desk that has a STEREO bus and MONO bus, along with the groups. I assign everything to stereo, and only the stuff I want in the subs to the mono. If I have a desk that doesn't have a mono buss, I'll use an AUX, and set them all to unity. I go to the DSP to turn the subs down. They are always too loud. I still drive them just as hard, but I do it in the mix.

So those of you who do use aux fed subs: Do you set the aux master to unity and input sub sends to taste or all input sub sends to unity and set master to taste? I do the former not for any consideration of phase, but because I find I don't want everything I'm sending to the sub at the same level. And depending on the signal I get, the sub send generally replaces my low shelf boost or cut unless I have an issue with a specific frequency range.

 

[Rob Timmerman]

Re: Aux feed subs?

I prefer to use a MONO bus, like if you have a desk that has a STEREO bus and MONO bus, along with the groups. I assign everything to stereo, and only the stuff I want in the subs to the mono. If I have a desk that doesn't have a mono buss, I'll use an AUX, and set them all to unity. I go to the DSP to turn the subs down. They are always too loud. I still drive them just as hard, but I do it in the mix.

This.

 

[Tim McCulloch]

Re: Aux feed subs?

I prefer to use a MONO bus, like if you have a desk that has a STEREO bus and MONO bus, along with the groups. I assign everything to stereo, and only the stuff I want in the subs to the mono. If I have a desk that doesn't have a mono buss, I'll use an AUX, and set them all to unity. I go to the DSP to turn the subs down. They are always too loud. I still drive them just as hard, but I do it in the mix.

Yes, yes, yes.

"Who needs channel EQ when I have the kick sub on an aux?" - guest engineer.

 

[Loren Aguey]

Re: Aux feed subs?

I prefer to use a MONO bus, like if you have a desk that has a STEREO bus and MONO bus, along with the groups. I assign everything to stereo, and only the stuff I want in the subs to the mono. If I have a desk that doesn't have a mono buss, I'll use an AUX, and set them all to unity. I go to the DSP to turn the subs down. They are always too loud. I still drive them just as hard, but I do it in the mix.

That's what I do with our X32 and link the mono bus to the stereo bus. But it still has a variable send for each channel that I use and adjust depending on the signal I'm receiving. Any particular reason you turn the subs down on the DSP vs the master aux/or mono bus send on the desk?

 

[Silas Pradetto]

Re: Aux feed subs?

That's what I do with our X32 and link the mono bus to the stereo bus. But it still has a variable send for each channel that I use and adjust depending on the signal I'm receiving. Any particular reason you turn the subs down on the DSP vs the master aux/or mono bus send on the desk?

Keeps the desk at unity for easy reference.