Am I overrunning my amp???

[discoian]

I am a mobile DJ - please don't shoot me! I can't seem to get decent advice anywhere else, so I'll try here :)

 

I have a 4 box speaker system built by Deltec in east sussex. I have a pair of BX16F's - a 600w RMS sub built around an Eminence 15" LF driver. It is low pass filtered at 250hz and has an SPL of 100db. I also have a pair of CX1F's which are a v.small 250w RMS full range speaker which are high-pass filtered (at what I do not know).

 

I can bypass the filters on all cabs by using 4-pole speakons, but I use 2-pole at the minute. All cabs run at 8 ohms so if I link them in pairs, I.e sub to top on each side then each side runs at 4 ohms yes? So, as I understand it I needed an amp offering 850w per side at 4 ohms.

 

When I bought the speakers, I also purchased a QSC PLX2 3602 which offers me 1250w rms per side at 4 ohms in stereo.

 

Now, I have heard this 4 box system with a QSC PLX 3402 much louder yet my 3602 peaks and compresses much sooner. Could there be a reason for this? Could it be that my system is running at 8 ohms?

 

Obviously I think I could benefit from bi-amping with a crossover and run them a little harder and not pressure my amp too much.

 

Any help offered very much appreciated.

[Shez]

If your tops & subs have passive filters in them, and you're connecting them to the amp in parallel, the impedance will still be 8 ohms. Think in frequency terms - at 100Hz, current can only flow to the sub as it's blocked by the top's HPF; result: 8 ohms. At 1000Hz, the current can only flow to the top as it's blocked by the sub's LPF; result: still 8 ohms. It may get a bit messy around the crossover point but then impedance varies with frequency anyway so any figure is nominal.

[wmcalin]

Hi

 

Sounds like your power amps is seeing an 8ohm load when the passive xovers in the bins are used, you need to check with your supplier or get someone to check yfor you, but I've come across this before with some Peavet kit.

 

Have you considered purchasing a DBX drive rack, nice little cross over with RTA/limiting EQ etc, I use the drive rack 260 but I think the driverack PA will be ideal for your requirements.

You could then use one amp if used in mono, i.e. chA running your tops 4ohm load, and chB running your subs again 4ohm load? Or use both amps if you require stereo, or another option use 2 amps run the sub out in mono from the drive rack into chB on your amp, then chA is free for a monitor for you if required?? Just a thought.

I think you will find a big difference using a decent xover, the drive rack PA is worth a look.

 

Good luck

 

:)

[smalljoshua]

the drive rack PA is worth a look.

 

Good luck

 

:)

You will need all of the luck you can get with a Drive Rack PA. Have a search on here for reasons why. Basically it has the tendency to destroy HF drivers if its power is pulled. Accidentally or on purpose. The Behringer DCX2496 is a much better box for the price in my and other member's opinions.

 

Josh

[discoian]

I will check with the guy who builds the speakers about the 8ohm load with the passive xovers - that could be an issue.

 

And yes, I've heard many bad things about the drive rack, I may get a DCX. In fact, I've had one before, then I got rid of it. Silly!

[Rob_Beech]

As much as I dislike the DRPA and the principles behind it's use from the company. It would work for this application. There is a passive crossover within the mid/top box that will protect the HF driver in this instance.

 

I can't RECOMMEND the product myself as I wouldn't recommend any product from a manufacturer with such ethics, however, I cannot say that it is unsuitable for this application.

 

I too would look at the DCX2496 in it's place. There are no units that I'm aware of that beat it without spending 4 figure sums on the NEW market.

 

There is no reason that the same system should sound any different on the 3602 than a 3402. There is not enough power difference (if the figures are accurate) to give you any significant difference in volume. It would seem odd that a newer designed amp with similar output figures wouldn't drive a set of speakers as effectively as an older model. Surely the new amps haven't been built down to a price? I've always been fond of the PLX series for small to medium sized stuff like this.

[wmcalin]

I heared about the issue blowing drivers when they first come out, but thought this was fully resolved.

[mervaka]

yeah, they brought out the driverack 260

[discoian]

There is no reason that the same system should sound any different on the 3602 than a 3402. There is not enough power difference (if the figures are accurate) to give you any significant difference in volume. It would seem odd that a newer designed amp with similar output figures wouldn't drive a set of speakers as effectively as an older model. Surely the new amps haven't been built down to a price? I've always been fond of the PLX series for small to medium sized stuff like this.

The only assumption I can come to is that the system is running at 8ohms. At the minute I run Left Sub and Left mid/top on CH1 and Right Sub and Right Mid/Top on CH2 - I was convinced this would be 4ohm.

 

The guy with the 3402 had unfiltered cabs and used a Xover, maybe that's the key.

[Rob_Beech]

Unfiltered, with active Crossover? In Mono? 1 amp?

[discoian]

Yup...I had to pinch myself to make sure. It sounded sweet and loud whereas mine sounds sweet but not loud, but in stereo.

[Rob_Beech]

Well, no, the scenario you state would probably explain it being louder.

 

The crossover in the sub seems very very high. Probably nearing an octave higher than I'd expect, even with a small driver handling the MF.

It's a known fact that passive crossovers take up power. The lower the crossover point, generally the more power they take (although design plays an important role aswell). I'd suggest 2 amps and a crossover, but a crossover and use your 1 amp in mono as in the other system would be a good start.

 

A well designed crossover will give the impedance of the box the signal is going to at a given frequency. For example, at 1kHz all of the signal is going to the top box. So the amplifier sees 8ohms at this frequency. At 100Hz it is going to the sub, also 8ohms. Just because 2 are connected in parallel doesn't mean you'll see a 4ohm load when using a passive Crossover.

 

Hope that helps.

[wilkee]

I would get yourself a DCX, run the two subs mono Bridged on the QSC and get yourself an 500w/ch into 8 ohms amp to run the Mid tops, crossed at a suitable frequency probably 120-150hz . This will give you a lot more headroom than at present and should sound quite a bit better.

 

You will also get a lot more flexibility for future changes.

 

Because of the drive units used it is never going to be a powerful system due to the efficiency of the 8" co-ax mid drivers.

 

Tony