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Old 26th March 2013, 12:05 PM   #1
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Default how to test a vi limiter

Please could someone explain to me how to test a vi limiter in a power amplifier without running the risk of causing harm to the output transistors?
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Old 26th March 2013, 12:12 PM   #2
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VI Limiters in Amplifiers
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Old 26th March 2013, 12:18 PM   #3
Mooly is online now Mooly  United Kingdom
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VI limiters are specific to each amp so we would have to see a circuit for exact details.

Given that most operate in the same way a procedure would be.

1. Confirm the emitter resistors on the outputs are the correct value by measurement.

2. Using ohms law calculate any pre existing "bias" on the limiter transistors. This would come from SOAR (safe operating area protection) components (ifthe design has them) and these are usually just a resistor network monitoring the supply and output. Confirm by measurement that the bias voltage is as expected.

3. Assuming that the remaining components are good then the circuit has to work as intended.

To test by actual operation involves loading the output stage with a lower than normal load resistor and monitoring with a scope to see that the output limits at values previously calculated.
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Old 26th March 2013, 02:22 PM   #4
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Mooly View Post
........................To test by actual operation involves loading the output stage with a lower than normal load resistor and monitoring with a scope to see that the output limits at values previously calculated.
I have never been brave enough to measure the effectiveness of the VI limiter.
Don't want to damage a perfectly good output stage.

But both Pass and ESP show a nondestructive measurement method that is supposed to not endanger the output stage.
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Old 26th March 2013, 02:37 PM   #5
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
I have never been brave enough to measure the effectiveness of the VI limiter.
Don't want to damage a perfectly good output stage.

But both Pass and ESP show a nondestructive measurement method that is supposed to not endanger the output stage.
I know what you mean regarding testing. Same for DC offset protection too.
We all hope it will work, but hope we never have to find out for real when connected to expensive speakers.
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Old 26th March 2013, 03:43 PM   #6
Vostro is offline Vostro  South Africa
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hehe I like the topic question.
I've been tinkering with diy amplifiers, and testing them for personal fun.

I've blown to many nice output transistors during testing of min output load, heavy clipping, testing amplifier stability.
Lucky to not have destroyed my test Speakers yet. (I try to use powerful resistive dummy load where I can).
And with a smile of my face I can say you'll smile if it works and be moody for days if it doesn't.
I've been lucky and never had testing my VI limiting blow my output transistors, but worry if it cuts in to early.
So I don't have any advice, but will follow this topic to see what others say

Regards
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