A "clipping light" Q

[RoboPimp]

Sorry if this is yet another meaningless thread, and I might have already asked this but....

 

I am really pretty new to having a "real" PA system, I am unsure of what the "clipping" light means on my amp. Does it mean "turn down now!" or is it OK for it to blink on loud parts? I am in the practice now of never letting them even blink if possible and I wonder what the protocol is for that?

 

the amp says it has a limiter: "comp. :THD?0.5%"

 

what does that mean exactly? Does that mean that when the clip light comes on it's using that limiter? I can remember my friend's peavey amp had the DDT and it seemed that you could run with that coming on pretty steadily with no problems...

[kevinnem]

you have to read the manual as to what your clip light means, .. some of them are at a true clip, some are at 3 db to. some come on what teh "protection " turns on, and that can be at like -6.

 

it should say in the manual.

 

Kev.

[RoboPimp]

Originally posted by RoboPimp the amp (manual) says it has a limiter: "comp. :THD?0.5%"

 

[boseengineer]

Unfortunately, decoding this snippet from the manual is beyond my modest skills.

 

In practice, the clip light means different things for different amps. In the last amp I designed a (analog) limiter, the clip light means something like this:

 

"If its turning red, you have reached the limit of what the amp can do without adding any artifacts or distortion (say

 

For most quality amps, some occasional (as in once every 2 or 3 seconds) clipping is okay IF the speakers are rated at least the same as the power amp. For the budget variety, it's hit or miss.

 

But then again, I don

[agedhorse]

As Boseengineer mentioned, there are several ways to design a clip detection circuit.

 

1. loss of feedback indication... where the output no longer represents the input (gain corrected of course) which happens during clipping, the amp's global feedback will go "open-loop" trying to correct for an uncorrectable situation. By measuring this signal, you can determine when this occurs. An advantage ofthis type of detector is that it also detects shorted speaker lines since the output will be "zero" for a valid input. A disadvantage is thatthe threshold may not be linear with frequency, since the global feedback must decrease with increasing frequency (actually the Av decreases which requires more feedback to keep the amp linear) so the feedback signal is by definition non-linear.

 

 

Another way is to look at the difference between the power supply rail and the top/bottom of the waveform. We know what the saturation voltage is (how close the output signal can come to the rail) and under load the rail sags so this differential measurement also clearly shows when you are approaching clipping. An advantage of this method is that it allows for choosing a value prior to clipping for the LED to light. I choose about 1dB or so. Another advantage is that the response is very linear, but will not indicate a shorted load.