dbx 223xs set up

[Mick C]

Our current set up is the mixer stereo L&R feeding to 2 x Electrovoice Sba760 powered subs, each feeding via the internal 100Hz crossover to Electrovoice Sxa360 powered tops. So a 2.2 system?
We'd like to remove one sub to make a 2.1 system (no need to lug around two big heavy subs) and am therefore looking at a dbx 223xs crossover to go after the mixer (QSC Touchmix 16). The mixer doesn't have a mono summed output - only has L&R outputs.
Adjusting the level of the total PA output in the 2.2 system is as simple as adjusting the mixer's main out fader. Built in x-overs in the subs = easy.
In the 2.1 with the mono summing of the dbx to the sub, presumably the Low output knob on the dbx needs to be a lower value than the High output knobs to achieve the same low/high balance, and the same overall loudness into the room, as I had in the 2.2?
Ignoring doing this by ear, is there a rule of thumb? For example on the dbx the 12pm position for the input gain and low / high outputs is 0, -10 and -10 respectively. I should set the input gain at 0, so not to affect the level of the incoming signal from the mixer, but what about the low and high outputs?

Yesterday's full PA set up trial was in the 2.2 set up, and it was only at the end when everything was perfectly done for the whole band that one of the guys (the one who lugs around 2 subs) announced the desire to go to 2.1 - so I'm keen to understand this better before committing to the change and buying anything, because the next set up will be in front of an audience and no time to fiddle around.

Here's the link to the dbx manual: https://dbxpro.com/en/product_documents ... nualpdf--2

Thanks everyone!

[James Perrett]

I may have missed something but I don't see the need to use an external crossover. According to the SBA760 manual, you can just feed left and right signals to the single sub and use its built-in crossover.

[Hugh Robjohns]

We'd like to remove one sub to make a 2.1 system (no need to lug around two big heavy subs) and am therefore looking at a dbx 223xs crossover to go after the mixer (QSC Touchmix 16).

Not needed!

The mixer doesn't have a mono summed output - only has L&R outputs.

No, but the subwoofer already does... Why spend money on something you don't need when all you actually need to do is read the sub's manual?

The SBA760, like many of its kind, already have the built-in capability for 2.1 operation.

Left and Right outputs from desk go to the Left and Right inputs of the single sub.

The Left Mid-High output from the sub goes to your Left top speaker, while the Right Mid-High output goes to the Right top speaker.

Job done.

The Sub automatically sums the low frequencies of both input signal together to feed the rumbly speaker, while high-pass filtering the two inputs and feeding them out to the two tops separately.

[Sam Spoons]

Same answer here, I have an SbA750 which is pretty much identical to the SbA760 and do this all the time (with two QSC K12's). If you don't need excessive bass it will sound fine (The SbA760 has a max output of 128dB SPL so a few dB down on the output of a pair of Sxa360s).

[Mick C]

We'd like to remove one sub to make a 2.1 system (no need to lug around two big heavy subs) and am therefore looking at a dbx 223xs crossover to go after the mixer (QSC Touchmix 16).

Not needed!

The mixer doesn't have a mono summed output - only has L&R outputs.

No, but the subwoofer already does... Why spend money on something you don't need when all you actually need to do is read the sub's manual?

The SBA760, like many of its kind, already have the built-in capability for 2.1 operation.

Left and Right outputs from desk go to the Left and Right inputs of the single sub.

The Left Mid-High output from the sub goes to your Left top speaker, while the Right Mid-High output goes to the Right top speaker.

Job done.

The Sub automatically sums the low frequencies of both input signal together to feed the rumbly speaker, while high-pass filtering the two inputs and feeding them out to the two tops separately.

I have read the manual, but here's a bit more info so you can understand why I'm asking

When we cabled the speakers as 2.2 we ended up with the Left sub fully anti clockwise, infinity, so it was basically acting as x-over doing its thing to the top with no bass output from that sub. Thereby the bass in the room was from the right sub only, therefore only the Right mixer output being used, so half of its level below 100Hz. This loudness and balance was good in the room.

When we cabled to 2.1 with mixer L&R into the right sub only, using the Hi outputs for L&R tops, it obviously mono-summed below 100Hz - so we effectively went from half of the desk’s bass output in the '2.2/pseudo 2.1' set up, to significantly more as a true 2.1
But when I turned the single sub down it still sounded too boomy on drums and bass to the guys out front. I'm 90% sure I went to -12 or even lower on the sub's level pot - so I don’t get how this didn't resolve the boominess.

I don't want to spend any money on a x-over, but my logic was to get easy 'at your fingertips' level balancing out front without fiddling around at the back of speaker cabinets trying to counteract the mono-summing on the sub back panel

[Wonks]

You have a digital desk with a 1/3 octave graphic on the outputs, so why do you need to fiddle with the rear of the speakers? You've also got a 6-band parametric EQ available.

Turning down the sub cuts all bass frequencies, but with a graphic you can reduce the problem ones in the bass region. It's probably room resonances causing the excessive boominess, but this won't be at all frequencies.

And have you got high pass filters on your mic inputs to reduce the chance of low frequency feedback? Even on the bass and kick they can come in useful.

[Hugh Robjohns]

When we cabled the speakers as 2.2 we ended up with the Left sub fully anti clockwise, infinity, so it was basically acting as x-over doing its thing to the top with no bass output from that sub.

So it was already running as a 2.1 system, but without delivering bass from the left channel (unlikely to be an issue if bass and kick were panned centrally).

But when I turned the single sub down it still sounded too boomy on drums and bass to the guys out front. I'm 90% sure I went to -12 or even lower on the sub's level pot - so I don’t get how this didn't resolve the boominess.

Assuming everything bassy was panned centrally, the difference between connecting one or both channels can't have been more than 6dB, so the level setting anomaly is a bit of a mystery. And as Wonks said, boominess was provably a room acoustic issue made worse by the higher input level.

Nevertheless, the principle of connecting both channels to the one sub is valid, and it's the way I work very happily with my own 2.1 (RCF) system.

...but my logic was to get easy 'at your fingertips' level balancing out front without fiddling around at the back of speaker cabinets...

Yes, I can appreciate that. But I'd have thought you could arrive at a good top/sub balance through a little trial and error, and once the optimum nominal gain settings are established you could fine tune at gigs through the output EQ/graphic controls in your (digital) desk.

Obviously, if you really want to use an outboard crossover at the desk you can. But you're duplicating the crossovers already built into the sub.

[Wonks]

And a crossover could well interfere with the sound at the crossover point. The tops run full range so an external crossover can be used with them (or fed the high pass signal from the sub. But these subs have a non-switchable built-in low pass filter with a 100Hz knee frequency.

So any external crossover would have to be set a bit below 100Hz, say 80Hz, to avoid too much interference between low-pass slopes. But that reduces the useful range of the sub from 43Hz to 80Hz instead of 100Hz, and makes the tops work harder as they now have to cover some of the bass frequencies, reducing their working headroom.

[AlecSp]

Nevertheless, the principle of connecting both channels to the one sub is valid, and it's the way I work very happily with my own 2.1 (RCF) system.

Same here. Depending on the venue, the transport I have, and how much I can be bothered, I'll take either one or two subs. A quick listen and it's dead quick to adjust the relative sub level

Obviously, if you really want to use an outboard crossover at the desk you can. But you're duplicating the crossovers already built into the sub.

Exactly. Pretty pointless to add another bit of kit, more cabling, another point of failure, and more complication - when what you've got is fully capable of doing the job.