Loudspeaker Attenuator

[Ben Lawrance]

Afternoon All,

 

I'm just thinking out loud about a project that I need to do.

 

I need to run 4 passive loudspeaker cabs in parellel (2ohm load) but be able to control the volume of each box. The cabs will be spread over a distance, and each cab will be in a different area, so will require varying levels of sound.

 

 

The way I have run the event before, is to use powered boxes, and run a ###### load of XLR and 16a across the site, however this is not really an easy task. Total distance is about 300mtrs ish. I got quite a lot of volt drop (even using 2.5mm cable) at the last box, and it wasn't too happy about it.

 

What I want to do, is run passive cabs on NL4, and make some sort of box with a pot on it. Signal in, and parellel out, with an aditional output from the pot to vary the level of said output. I will then account for the loss in the cable by using a bigger amp than I need to.

 

Before anyone mentions 100v line, I did this the first year, and by the time music is put through the system it sounded awfull, thus why I changed it the next year.

 

 

A question I have is, if I put a pot in line between the amp and cabinet, will the line impedance change as I adjust the pot?

 

I know this all sounds a bit rubbish, but saying it in words is harder than I am thinking it in my head.

 

Unless someone can suggest an alternative?

 

Cheers

Ben

[Simon Lewis]

Ben,

 

I'd forget the attenuator idea. You'd need a high power potentiometer and you'd lose precious power with it in circuit. 300m of speaker lead will simply lose you even more power, and shoot your damping factor to pieces.

 

Either: Get heavier duty mains and run the powered cabs again, or, revisit 100V line, but with powerful amps, speakers and transformers designed to deliver a reasonable low frequency output.

 

Simon

[Rob_Beech]

I take it the 300 meters is also 300 meters away from local power.

[Ben Lawrance]

Yep, power is supplied via a genset which we hide behind the hedge.

 

The distro is sat in the control tent, which is where the mixer, playback, radio RX's etc are.

[Dan Appleby]

Hi Ben,

 

We do this exact same thing for several events at various trade shows etc where the client requires a distributed sound system for announcements and (decent quality) background music...

 

You will need to use an 'l-pad', these are specific 'pots', if you like, that will still present an 8 ohm load to your amp. We made our own volume control boxes using these.... http://www.theelectronicsshop.co.uk/Access...info%20page.htm (AT-25H), a little abs project box with 2 NL4 connectors on (loop through) and a 2m trailing lead that goes up your speaker stand and goes into your box. Bobs your uncle. It's a little crude I guess but it does work!

 

I have to say that we do only run speaker cable a maximum length of about 120m and as Simon mentions by the time you run 300m of NL4 cable (even with 2.5mm cable) you're going to end up with a big mushy mess coming out of your boxes. We just about get away with it at 120m!

 

Something to mull over I guess

[J Pearce]

Can't you use an amp channel per speaker instead of trying to alter levels post amplifier?

 

Increasing each amp channels impedance should also reduce the effect that 300m of cable will have on the damping factor. (You want the load to have a lower impedance than the cable, not vice versa).

[Ben Lawrance]

That then means running 4 cables............. (or multi to the first box.....)

 

This is starting to sound expensive?

[J Pearce]

Most hire companies stock 4 core cable, you can run two circuits on that. Get some circuit splitters and you're down to 4 shorter cables.

 

A lot of amps will also output both outputs onto a NL4, so you might not need circuit splitters at the amp end.

[Killyp]

An amplified signal is surely going to drop more relative voltage than a non-amplified one? How about sticking a pair of powered DI boxes half-way down the cable?

[Charlie Jeal]

An amplified signal is surely going to drop more relative voltage than a non-amplified one? How about sticking a pair of powered DI boxes half-way down the cable?

 

 

Because a DI drops line level down to mic level and therefore isnt the correct tool for the job.

 

Charlie

[Killyp]

Surely this is only if the pad is engaged?

[Rob_Beech]

Certainly not. Without the pad you'll just overload things. Then you'll be outputting on XLR. The idea behind using a DI to drop from speaker level is useful, but that is what it does. DROP FROM speaker level. To line / mic level. Additionally the input level required even with the pad can sometimes be overloaded, not much output is required, this is something we can't do as we need the end result to be at speaker level.

[jamesperrett]

Yep, power is supplied via a genset which we hide behind the hedge.

 

The distro is sat in the control tent, which is where the mixer, playback, radio RX's etc are.

 

The first question that needs to be asked is what sort of power are we talking about?

 

I suspect that you're going to suffer from more losses with 300m of speaker cable than 300m of power cable and speaker level attenuators aren't going to be cheap if they have to handle a reasonable power level. If you really want to go with your idea then building a switched attenuator might be better rather than a fully variable one.

 

Cheers

 

James.

[Killyp]

Certainly not. Without the pad you'll just overload things. Then you'll be outputting on XLR. The idea behind using a DI to drop from speaker level is useful, but that is what it does. DROP FROM speaker level. To line / mic level. Additionally the input level required even with the pad can sometimes be overloaded, not much output is required, this is something we can't do as we need the end result to be at speaker level.

 

Whoops, I didn't actually state it in my original post. I was referring to a DI box for powering a pair of powered cabs.

 

This (as pointed out) wouldn't actually work though so ignore me. ;)

[Mixermend]

Although in principle the high power 'L' pads seem like a very good idea - my experience of passive monitors fitted with these is that they are unfortunately not reliable.

Because of the power they might have to dissipate - they run hot, and as we all know heat is the enemy as far as reliability is concerned!

One poster mentioned mounting them in an ABS box - please don't, as they need to be ventilated to get rid of the heat. I can almost hear someone saying 'just where is that funny plastic burning smell coming from.......!'