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Old 12th January 2009, 09:38 AM   #11
AndrewT is offline AndrewT  Scotland
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Default amplifier protection

Hi,
VI limiting should NEVER be triggered when a valid audio signal is sent to a valid speaker load.
If the VI never limits an audio signal then it can NEVER be audible.

Set out to design a VI limiter that achieves that audio target. It's not easy but it is very possible using linear limiting topologies.

Now, look at abuse of the amplifier.
This could be shorting the output, or using a speaker with too low an impedance or deliberately overdriving the amplifier.
In any of these situations some audible distortion of the audio signal due to the limiter is quite acceptable. However it would be nice if the resulting output did not damage any attached speakers.
The protection diodes from output to supply rails go some way to help, but as pointed out these very fast inductive back emf spikes are fast and will pass to the treble drivers. But I remind you this is only during amplifier abuse, not operation within specification.
Of the three abuse scenarios, the shorting of output will automatically take the back emf spike to ground. Result tweeter survives.
Of the two other scenarios these are down to deliberate operator misuse and the operator deserves all he/she gets. Call it learning by experience.

Now, let's look at VI limiting with diodes on the output.
The amp should pass DC current up to the the rated power level into the rated resistive load.
The amp should pass transient peak currents into reactive loads approaching three to four times the rated DC current. The higher this transient current the shorter the acceptable duration before the limiter triggers. i.e. a 1uS spike at 3 to 4times the DC level and/or a 1mS to 100mS spike at approximately 2times the DC level.

Take a 200W into 4r0 amplifier.
The peak output current for a continuous sinewave output is 10Apk (40Vpk into 4r0). The limiter must pass this without triggering.
For a 1uS pulse the limiter must pass 30Apk to 40Apk without triggering.
If the peak lasts longer than 1uS then the limiter can progressively reduce the delivered current. Note this is not instant cutoff and the back emf will not be as high as if an instant cut off had been triggered. I would expect a 100mS pulse to trigger @ ~20Apk.

Now look at two of the conditions just mentioned.
A continuous 10Adc into <=4r0 will dissipate an enormous power in the output devices and the transformer and cause excessive temperature rise in the smoothing capacitors. A second protection system drops in to alleviate this. Supply rail fuses are selected with values of F4A to F5A. These will rupture within a few hundred mS and protect all the susceptible components that could be damaged by long term overloads.

The second 100mS scenario at 20Apk may stress the temperature de-rated SOAR of the output devices. I would design the output stage to specifically survive this stress. This SOAR design will also ensure that the output stage will survive a long term overload into 2r0 provided the heatsinks are cool.

Do all this and your VI limiter should NEVER be audible with valid audio signals.
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Old 12th January 2009, 09:23 PM   #12
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i've seen a simple system in Peavey amps and some fender amps. if you're approaching the VI limits of the amp, a photocell circuit reduces the gain of the amp and levels it out just below where the VI limits are. on Peavey amps, the device can be turned off. inexperienced sound men turn it off when they find out it keeps the amp out of clipping. this accounts for many amps with shorted outputs, running an amp without the dynamic limiter and also running it into a squirrely load.
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Old 27th January 2009, 06:26 PM   #13
OMNIFEX is offline OMNIFEX  Jamaica
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Thank you Gentlemen for answering my questions.

I apologise for taking so long to reply.


Cheers!
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