Power Amps

[littleboo]

Hello,

 

New poster with what I hope isn't a stupid question. I have a small system that I use for various charity events etc and also loan all or parts of it out from time to time. I'm looking to replace the power amp, my speakers are 250W RMS so I'm thinking something around 400 - 450W per channel.

 

My question is, if I go for a higher output amplifier say 600 or 700W per channel, is there a way ( apart from the gain control on the amp ) of limiting the output of the amp to protect the speakers ? I think I read somewhere that hire companies do this so they don't have to worry about stocking loads of different types of amps.

 

Many thanks

[Girlyboy]

Hello

 

Limiting in front of power amps does 2 things - it protects your amps from being overloaded/overdriven at the input, thereby protecting your speakers too. Rental companies use limiters to prevent people who don't know what they're doing (or just don't care) blowing up their gear.

Most companies use digital crossovers which allow them to store multiple programs for different amps and speakers, allowing quick setup times.

The easiest and cheapest way to limit a fullrange system, is to put a compressor between your desk output and your power amp. Set the ratio to infinity (that little sideways 8 symbol on the far right of the dial), set the attack and release times to their fastest. Now, you have to adjust the threshold to a point where it prevents you overloading the system. With the threshold at maximum, turn up the output of your mixing desk (you'll want to unplug your speakers for this, or it will get loud!) until the peak lights on the amp glow - then dial the threshold backwards until the peak lights just come on with the biggest peaks. The gain reduction meters on the compressor should show how much is being removed. If, once you reconnect the speakers, you hear any distortion, you'll need to reduce the threshold even more, until the distortion goes completely.

Like I say, this is the cheapest and easiest - there are more graceful ways but more expensive.

 

Let me know if you need more info. Hope this helps...

 

Simon

[Rob_Beech]

Hi, First off, welcome to the Blue Room.

 

Secondly, I'm afraid things aren't always as simple as they seem, and this is one of those occasions.

 

Lets look at the program material first. If you are going to be faced with loud music (be it bands or pre-recorded material) Then you have to be careful with th size of your amps. too small and clipping will destroy your speakers, too big and you can suffer from over excursion and the likes. In most cases you can power a speaker with a level higher than the rms level for most program material, you just have to know what you are doing and be careful.

 

As for limiting. Some amps have variable limiters in them to limit the power although these tend to be few and far between and can often be expensive. Most people (myself included) will use DSP to limit the input to the amp. A Loudspeaker management system from the inexpensive behringer DCX2496 to the much higher quality but much higher priced KT, BSS and XTA offerings. This is the easiest way of doing it, often the racks need to be locked at the back (with ventilation) and the DSP units "locked" using their security features. This stops people fiddling.

 

The last point I'd like to make is that you cannot limit the power of an amplifier by turning down the controls on the front. In nearly all cases this is just an input attenuator control. turning this down just means that you need a hotter signal to get the same output back. theoretically you can still get full amplifier power with the controls just above its minimum point. this of course is frowned upon as you need a very hot input signal and this would also increase the noise floor significantly.

 

 

Rob

 

Simon is quicker than me..........although what I will say is sometimes amplifiers peak/clip lights can vary somewhat dependent on the load on the amplifier. So having no load may cause them to clip earlier or likely later than say with a 4ohm load on each channel.

[Bacher]

theoretically you can still get full amplifier power with the controls just above its minimum point. this of course is frowned upon as you need a very hot input signal and this would also increase the noise floor significantly.

 

Uhm, wouldnt it lower the noise floor, as long as the output of the desk/crossover isn't clipping?

[Simon Lewis]

Uhm, wouldnt it lower the noise floor, as long as the output of the desk/crossover isn't clipping?

 

Yes it would.... and it would potentially increase the system's dynamic range too.

[Rob_Beech]

Erm.........I'm unsure what triggered me to write that the way I did, I'm trying to figure out if I MEANT to write something else or not......

 

If you are having to drive the desk (heads and everything else) harder you can easily introduce more noise into the system....

So the noise.....would be... higher..... and the gain structure could well be a mess.

 

Rob

[Simon Lewis]

So the noise.....would be... higher..... and the gain structure could well be a mess.

 

Have a look at this...