Sitting Room Build: Jack 12 Lite

[Bill Fitzmaurice]

Using this site http://www.the12volt.com/ohm/ohmslawcal ... .asp#power I found that inputting 400 watts at 8 ohms equals 56 volts. Therefore, if you want some protection without being barely quieter, set the Jack limit to 50 volts.

To get an appreciable level of protection tops should be limited at 1/2 the driver thermal rating. For a 400w 8 ohm driver that's 40v. The 3dB of additional output you might get by limiting at the full thermal rating isn't worth chasing, and with thermal power compression you probably wouldn't get more than 1db with that last doubling of power anyway.

[Tom Smit]

I fully understand that, Bill. It's somewhat of a different story for the 3012HO, though, isn't it. I mean, if my understanding is correct, it will suffer much less from power compression than the typical driver, right?

[Bill Fitzmaurice]

I fully understand that, Bill. It's somewhat of a different story for the 3012HO, though, isn't it. I mean, if my understanding is correct, it will suffer much less from power compression than the typical driver, right?

I don't know, as Eminence doesn't publish power compression figures. However, a general rule of thumb is that the higher the thermal rating the more power compression that will occur. That's because power compression results from the voice coil resistance rising as the voice coil heats. The hotter the coil the greater the resistance rise, and one can expect that the more power you put into the coil the hotter it will get. Based on that you can expect the least amount of power compression in a 100w driver, a moderate amount in a 400w driver, and the highest amount in a 2,000w driver. That's one reason why you have to do a lot of research to find out if the higher price of drivers rated for 800w and up is actually cost effective in terms of actual dBs gained. A driver may have 6dB of power compression at 1,000w, only 3dB of power compression at 500w, so that last 500w of power applied isn't giving you anything.

[Tom Smit]

Thanks, Bill.

[HugoMack]

Yes, and I could adjust it with the iPhone. Sorry - that was to Tom Smit...

Many thanks Bill. I'll work out (backwards) what a threshold of 40 volts would be in terms of the PA2's limiter. Then that's at least completely set.

[Bruce Weldy]

Yes, and I could adjust it with the iPhone. Sorry - that was to Tom Smit...

Many thanks Bill. I'll work out (backwards) what a threshold of 40 volts would be in terms of the PA2's limiter. Then that's at least completely set.

That is not how you do it.

Send a test tone of 100hz into your board. Run the board up to -0- VU. Turn the power amp up all the way. With no speakers attached, put a volt meter on the amp's speaker output.

This will register a voltage. Go into the DR and adjust the limiter until the voltage comes down to where you want it. Once it's there, slam the master fader on the board all the way up and see if the voltage remains the same. If it does, then you've successfully set the limiter.

You CANNOT set the limiter with math. It has to be with a volt meter.

[HugoMack]

Thanks Bruce. That's brilliant! Each amp will be different.... very clean and simple.

[Bruce Weldy]

Thanks Bruce. That's brilliant! Each amp will be different.... very clean and simple.

The driverack lets you set the limiter for the highs and the lows, so yes, each amp will be set independently.

[HugoMack]

Well first gig last night - a very loud jam night in a medium size pub. Five guitars at one stage, with one of them an idiot with an AC30 on the floor so he was standing above its beam.... who I nearly punched. And no fold back....

But the vocalists could hear everything through the peripheral from the Jack Lites, and the harp sounded so good that the player insisted on standing two feet in front of the the LH FOH.... No feedback thanks to the DBX PA2.

No bass or kick through it yet, and it was well below any sort of threshold even though very loud - using the Matrix GT800 (400w @4R) at @ 2pm.

I couldn't apply a 40V limit using the PA2 as for some reason I wasn't getting a sensible meter reading from the MAtrix. As it only has Speakon outputs, I was putting the red probe onto the tip of the speaker jack lead and the black onto the shaft . I'd previously broken in the speakers by measuring 8 to10 volts on the speaker terminals with the speakers connected. Now the cabs are all sealed up, I can't do that... Any suggestions gratefully received.

But the Jacks are really fantastic! A local promoter asked me if I'd do some gigs for him with the system as it is now - without the subs. So that will be useful money.

But with the singers being able to hear themselves so well, meant the guitarists felt they had to turn up, so it got rather silly. Usually with foldback, you can give them a blast of their guitar up though their wedge so they turn back down to a level where they're no longer competing with the FOH. But with just the two jacks out front I couldn't use that trick. (I'm a guitarist, so I understand the mindset!! Or lack of it....)

[Bruce Weldy]

I couldn't apply a 40V limit using the PA2 as for some reason I wasn't getting a sensible meter reading from the MAtrix.

If you were trying to set the limiter with the AFS on - that's your problem. The dbx sees the test tone as feedback. Turn it off, set the limiter, then turn it back on.

Don't ask how I know this.

[ripNdeb]

Don't ask how I know this.

http://billfitzmaurice.info/forum/viewt ... =4&t=21596

[HugoMack]

I'd worked that out - and had disconnected the DBX - having ascertained that the sine wave was coming through the speakers before pulling out the jack plug.

[Grant Bunter]

If you don't have spare NL4's get more!
Make up a pigtail NL4, an NL4 that has a couple of short pieces of different coloured wire coming out the back. Strip the ends of the wire and always keep them seperate. Remember which colour goes to +1 and -1. Plug the NL4 into your amp, and you can put probes from your multimeter onto the wires to +1 and -1 to check voltage...

[HugoMack]

Thanks Grant.

[HugoMack]

Yet more inexperience at work... Having made up an NL4 as suggested, my voltmeter is marching quite rhythmically through a series of 4 or five readings the median of which increases as I turn up the amp. I'm inputting a 30hz sine wave and reading in V DC. When I did this before with the speakers connected taking the reading from the speaker terminals, I got a steady reading.