New setup for our d&b system

[Rob K]

In my current job I've inherited a d&b system that gets used constantly on the road. It's all E15 subs (the old ones) and MAX15 wedges, which get used both as tops and as onstage wedges. The full rig uses four E15s per side and two MAXes. They are driven passively by two QSC PLX3402 amps in parallel input mode, one for each side - that is, each side of the amp is driving three boxes. I've been looking into the power specs of this stuff and am a bit concerned that there's a mismatch here.

 

The MAX and E15 boxes are all 8 ohms so when you have three in parallel, the overall impedance is 2.67 ohms (I think). Power handling is given as 250w RMS for the MAX and 300w RMS for the E15s. So in total, two E15s and a MAX should handle 850w...? However, the PLX3402 will give out about 1500w per channel when presented with this load. Are the boxes in danger? Last time I had them out, I thought I was hearing distortion...

 

I'm planning on getting a controller, probably a Behringer DCX2496, and rearranging the system a bit. I was thinking of using the existing PLX amps to drive four subs each, and getting another matching PLX to drive the tops. However, I'm now wondering if another 3402 will be too much for the tops anyway. Ditto the subs! What should I do? Keep using the 3402s and limit the input in an effort to protect the bass bins? Buy a lower-powered amp to drive the tops? Or just stop worrying and turn it up loud? :)

[Rob_Beech]

Hi,

 

I'm unfamiliar with the E15 subs, do they have a passive network built into them? I can only assume they do.

 

Running them as they are shouldn't cause too many problems, it's certainly not over powered, it's just a case of being sensible. They'll handle 850 watts all day, but the higher power the less time they'll handle it for before they overheat.

 

The other thing to remember is that passive networks are built in many different ways, the majority will show the varying impedance of the imbalanced sections. EG, the amplifier will see a 4ohm load for reproduction of frequencies within what the E15's handle, and an 8ohm load for where the MAX work. So in actual fact, your amp may not see a 2.67 ohm load at all.

 

Some form of redundancy is always nice and with this you have none if the amp fails, but its always nice to know that you can run the system from one amp should the need arise.

 

If you got a controller like a DCX2496 or one of the many other offerings on the market that isn't a Drive Rack PA (Pardon my language) then you could get a second amp and give yourself some redundancy and give the amps an easier time.

 

I'd let each of the amps do the same thing though. The worst thing in the world is to have one amp doing sub and another amp the same size doing the rest. It's such a waste of available supply. Most amps (the plx included) dont' REALLY have supplies big enough to run flat out on both channels all day everyday, so if you run the subs on channel 1 and the tops on channel 2 and use one amp for left and the other for right, you are balancing the heavy load of the sub, and the light load of the mid top equally between amps. Impedance mismatch is absolutely no problem at all.

[rusty999]

.

 

. The worst thing in the world is to have one amp doing sub and another amp the same size doing the rest. It's such a waste of available supply. Most amps (the plx included) dont' REALLY have supplies big enough to run flat out on both channels all day everyday, so if you run the subs on channel 1 and the tops on channel 2 and use one amp for left and the other for right, you are balancing the heavy load of the sub, and the light load of the mid top equally between amps. Impedance mismatch is absolutely no problem at all.

 

Does that mean that 2x Xti4000 amps are not a great match to run 2 s18s and also 2 F15s

 

Just tried this system out and it sounds first class

 

Just wondering

Thanks Rob

 

Rusty

[nb705]

Also, something worth noting is that the 15" Max series are Bi-wired as standard, with pins 1+/1- going to the woofer through a passive network and 2+/2- to the compression driver through another passive network. I don't believe that they have much of a low end roll-off through this network, so some form of active processing will probably be very useful to improve the efficiency of your rig.

[Rob K]

Thanks Rob and nb, some really helpful info for me there. So Rob, you're saying that rather than running two PLXs with four subs each and one other amp with two MAXs on each side, I should look at running each PLX with four subs on one side (2 ohm load) and two MAXs on the other side (4 ohm load), and that this is better because the amp will be able to draw more current on the "sub side" at the expense of the "top side"?

 

That would certainly help as I wouldn't have to buy another amp, though for the sake of safety I'd probably look at buying one anyway! I was only really considering another amp for the sake of using the controller, but using the layout you suggest I could still add the controller without adding another amp. General consensus on the web does suggest that the PLX 3402 will run fine with a 2 ohm load for ages, as long as I ventilate them properly, which shouldn't be a problem.

 

BTW yes, the E15s have a passive network, and nb705 - the MAX page on the d&b site suggests using a HP filter at 25hz so I always use the 30hz filter on the QSCs, but you're right - thinking about it, an excess of LF going into the MAXes might have been responsible for the distortion I was hearing.

[Rob_Beech]

Not quite, I wouldn't recommend running the amp at 2ohms by default, I'd recommend the second amp and running 2 subs on channel A of each amp and 1 top on channel B of each amp, however should the worst happen, you have redundancy to use 1 amp. This will make the amp work much harder but it'll get you through the show.

 

 

Rusty, the amp combination is fine, but I'd run each amp with a sub on one side and a top on the other to balance the load out. The internal processing makes this easy, and you just need your left XLR plugged into 1 amp (channel A) and the right XLR into the other (channel A) and the amp set to "Y SPLIT" mode. You won't necessarily have any issue running one amp for the sub and one for the top, but you are giving the sub amp a harder time.

[Rob K]

Ah, I see. I haven't made myself clear. The rig at the moment is eight subs and four tops. I've been running one amp for each side, ie one amp drives four subs and two tops (two stacks of two subs/one top). Output A of the amp goes direct to a sub, then to another sub, then to a top. Output B does the same, so each side of the amp is doing the same thing. The other side of the system is wired the same way.

 

It does sound like wiring it the way you suggest (althought at half your suggested impedances) might make some sense though, and make the system more controllable.