[linux-audio-dev] live pa questions

[Pieter Palmers]

Andres Cabrera wrote:

>I think the main problem that can occur from DC offset is overheating of
>the amp, and then heat protection turns the amp of....
>  
>
Overheating of the amp is nonsense. Most amplifiers operate in class A 
or AB anyway, and maybe in class D. Heat dissipation is independant of 
the input signal for these amps. The types that might build up a problem 
with heat (e.g. class H) aren't used in PA enviroments. And before the 
amp overheats, your speakers will be dead.

The biggest problem regarding DC offset (and clipping) is the fact that 
speakers are not made to cope with DC signals. They heat up very quickly 
under DC load, and on top of that there is no air movement in the 
speaker itself to cool it somehow (athough this last effect isn't that 
important).

the following links provide quite some info regarding distortion, 
clipping and DC offsets:
http://sound.westhost.com/clipping.htm
http://sound.westhost.com/tweeters.htm

The remark regarding the dynamic range is right though: the only time I 
ever seen a blown up PA speaker was due to the dynamic range of the 
music involved. It started with a very silent intro of a minute or so, 
but then it contained a full-scale bassdrum. Nice to get a 'surprise' 
effect, and probably nice in a silent studio enviroment with moderate 
listening levels. But in a live situation there is a lot of enviroment 
noise, which tempts the PA engineer to crank up the volume on silent 
intro's so that the people are able to hear them. You can guess what 
happens once the bass drum kicks in. Even when using a limiter this can 
destroy speakers, because of its attack time. It's also not very healthy 
for the public.

My recommendations:
- Be sure to do a decent sound-check: have a full-scale piece of music 
ready for the PA engineer to set the PA desk incoming level, and be sure 
not to change your volume when soundcheck is done.
- Adapt the dynamic range of your music to the live enviroment, e.g. by 
using a compressor plugin just before the soundcard output.

Greets,

Pieter