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Author Topic: DB reduction cable or adapter?  (Read 219 times)

David Allred

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DB reduction cable or adapter?
« on: Today at 12:20:07 pm »

I often have set-ups that require in or out fills for audience spaces that wrap around the stage.  Because the side seating is sometime closer to the speaker location than the front seating, I have to run more amps with more speaker cable runs to reduce output to certain speakers.  I want a quicker easier set-up with fewer amps and cables.
Is there a way to jump (See pic below) from a full output speaker and get a 6db (ish) reduction at the second speaker?  A typical application would be a graduation ceremony.  Mostly speech and a little music.
Speakers are 8 ohms each.
Amp that would be used PLX3102 (approx. 600w @ 8)
Would a simple L-pad be the most practical solution?  Does an L-pad alter the load on the amp?



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Ivan Beaver

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Re: DB reduction cable or adapter?
« Reply #1 on: Today at 12:38:17 pm »

I often have set-ups that require in or out fills for audience spaces that wrap around the stage.  Because the side seating is sometime closer to the speaker location than the front seating, I have to run more amps with more speaker cable runs to reduce output to certain speakers.  I want a quicker easier set-up with fewer amps and cables.
Is there a way to jump (See pic below) from a full output speaker and get a 6db (ish) reduction at the second speaker?  A typical application would be a graduation ceremony.  Mostly speech and a little music.
Speakers are 8 ohms each.
Amp that would be used PLX3102 (approx. 600w @ 8)
Would a simple L-pad be the most practical solution?  Does an L-pad alter the load on the amp?
An L pad will keep a constant load on an amp, but you must be careful with the wattage rating.

The largest generally available Lpad is rated for 100 watts.  So you need to be careful.  They can also get VERY VERY hot, remember that the wattage has to go somewhere----
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A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
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David Allred

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Re: DB reduction cable or adapter?
« Reply #2 on: Today at 12:44:52 pm »

An L pad will keep a constant load on an amp, but you must be careful with the wattage rating.

The largest generally available Lpad is rated for 100 watts.  So you need to be careful.  They can also get VERY VERY hot, remember that the wattage has to go somewhere----

Would you say that a 100w rms L-pad (worst case) ran at a consistent 100w rms would get as hot as a 100w incandescent?
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Geoff Doane

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Re: DB reduction cable or adapter?
« Reply #3 on: Today at 01:19:21 pm »

Would you say that a 100w rms L-pad (worst case) ran at a consistent 100w rms would get as hot as a 100w incandescent?

It depends (doesn't it always) on what the crest factor of the program is.

If the RMS rating of the L-pad is 100W, and you had it turned down all the way, you could probably put 1000W peak (10 dB  crest factor) into it an expect it to survive.  And yes, it would get as hot as a 100W incandescent lamp (it would generate the same amount of heat, anyway).

But since we don't really have to worry about giving our amplifiers a constant load these days, I would look at a simple box with a series resistor to drop the voltage to that speaker.  Maybe put a couple different resistors in it, with a switch to select between them.  An 8Ω resistor will drop the level to an 8Ω speaker by 6 dB, and the resistor has to dissipate only a quarter of the input power.  If you stick with the 10 dB crest factor assumption I made, that means you would need a 15W resistor.  A 25 or 50W resistor won't cost a bundle and should give you a good degree of safety margin.

The damping factor for that speaker goes all to hell, but it's questionable how important that is anyway for speech or a "little music".

GTD
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Jay Barracato

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Re: DB reduction cable or adapter?
« Reply #4 on: Today at 02:09:52 pm »

It depends (doesn't it always) on what the crest factor of the program is.

If the RMS rating of the L-pad is 100W, and you had it turned down all the way, you could probably put 1000W peak (10 dB  crest factor) into it an expect it to survive.  And yes, it would get as hot as a 100W incandescent lamp (it would generate the same amount of heat, anyway).

But since we don't really have to worry about giving our amplifiers a constant load these days, I would look at a simple box with a series resistor to drop the voltage to that speaker.  Maybe put a couple different resistors in it, with a switch to select between them.  An 8Ω resistor will drop the level to an 8Ω speaker by 6 dB, and the resistor has to dissipate only a quarter of the input power.  If you stick with the 10 dB crest factor assumption I made, that means you would need a 15W resistor.  A 25 or 50W resistor won't cost a bundle and should give you a good degree of safety margin.

The damping factor for that speaker goes all to hell, but it's questionable how important that is anyway for speech or a "little music".

GTD
An l-pad is just a self adjusting voltage divider. One resistor in series and one in parallel.

A quick calculation shows a 4 ohm resistor in series with a 10 ohm resistor in parallel will knock 30% off the voltage while leaving the total impedance of the load about 8.5 ohm.

Bad news is 1000 watt resistors are probably $100+ each and 2000 watt, $200+ each.

Even better, just using a pair of 4ohm would knock 50% off the voltage leaving a touch under 7 ohm as a total load.

Sent from my Moto Z (2) using Tapatalk
« Last Edit: Today at 02:27:01 pm by Jay Barracato »
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Jay Barracato

Mike Caldwell

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Re: DB reduction cable or adapter?
« Reply #5 on: Today at 03:46:50 pm »

I'm assuming you have a total of four speakers, two for mains and two for the out fills and also assume they are passive.
You could just use one channel of the amp for both mains and the second channel for both out fills. That takes a little more speaker cable but gives you level adjustment you need and you could process/EQ the out fills separate from the mains if needed.

Mike Pyle

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Re: DB reduction cable or adapter?
« Reply #6 on: Today at 04:44:30 pm »

I would use a 4 conductor cable to the first speaker position, break it out there and use 2 conductors for the first speaker and then run another cable to the second speaker. Then you can use separate amp channels for each.
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Geert Friedhof

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Re: DB reduction cable or adapter?
« Reply #7 on: Today at 10:57:59 pm »

You could use two speakers in series for 16 ohm, and connect that parallel to one of the 8 ohm main speakers, and place one at each side of the stage. This will give you -6dB per speaker.
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