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Old 17th April 2008, 09:26 AM   #1
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Default amplifier testing

when looking at amplifiers for faults and using a sinewave generator do you have to have a dummy load connected or not?

thanks

chris
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Old 17th April 2008, 09:53 AM   #2
AndrewT is offline AndrewT  Scotland
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I would do both.
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Old 17th April 2008, 10:14 AM   #3
d3imlay is offline d3imlay  United States
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Most trouble shooting can be done without a load. Note that in DC coupled amps you can not troubleshoot from stage to stage with a scope.

I always bring up an amp, through a light bulb, without a load.
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Old 17th April 2008, 03:17 PM   #4
ttan98 is offline ttan98  Australia
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Default Re: amplifier testing

Quote:
Originally posted by reddish75
when looking at amplifiers for faults and using a sinewave generator do you have to have a dummy load connected or not?

thanks

chris

The steps I would take are:

1. Ensure the bias voltages at various are correct, most transistors(except protection transistors) should be forward bias, ie vbe = about 0.65v. The bias voltages at different point must be within design parameters. no load connected(*)
2. once ok the output voltge if it is dc ampl. should be less than 50mv. any larger should really be unacceptable, amp, maybe ok but o/p is too high(i/p transistors or jfets not matched). This is measured when the input is close to gnd, not left floating. no load connected(*)
3.apply i/p sinusoidal voltage to full power when o/p is loaded with 8ohm load.this ensures the o/p transistors can handle full power.optional test with 4ohm load if your speaker is 4 ohm.

hope this helps. cheers.

* amp must be unconditionally stable with no load connected.
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Old 17th April 2008, 04:33 PM   #5
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Default Re: Re: amplifier testing

Quote:
Originally posted by ttan98



The steps I would take are:

1. Ensure the bias voltages at various are correct, most transistors(except protection transistors) should be forward bias, ie vbe = about 0.65v. The bias voltages at different point must be within design parameters. no load connected(*)
2. once ok the output voltge if it is dc ampl. should be less than 50mv. any larger should really be unacceptable, amp, maybe ok but o/p is too high(i/p transistors or jfets not matched). This is measured when the input is close to gnd, not left floating. no load connected(*)
3.apply i/p sinusoidal voltage to full power when o/p is loaded with 8ohm load.this ensures the o/p transistors can handle full power.optional test with 4ohm load if your speaker is 4 ohm.

hope this helps. cheers.

* amp must be unconditionally stable with no load connected.
Agreed, but I would do 2) first. When the output sits close to zero, you'r almost sure that everything else is OK, because whatever goes wrong will always impact Vout, DC-wise.

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Old 17th April 2008, 05:11 PM   #6
Nordic is offline Nordic  South Africa
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..strap a cat to the loudspeaker....
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Old 18th April 2008, 06:57 AM   #7
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just a thought. If you have two identical circuits you can connect red lead of volt meter to circuit 1 and black lead to circuit 2, and ideally it should be zero volts if its working. If it is not working right you will know. Of course you have to probe both sides in the exact same place or part. You do not reference to ground when doing this. Its just a general quick check. I am talking about an amp with left and right channel. This info is just an idea to help in troubleshooting.
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Old 12th September 2008, 08:20 AM   #8
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Default DC coupled

I'm trying to troubleshoot a Crown PSA-2 which has a crackling/hiss in channel 1 after it warms up (about an hour). What's the best way to trace the signal with a scope? If the PSA-2 is DC coupled, how can I troubleshoot from stage to stage?

many thanks

Quote:
Originally posted by d3imlay
Most trouble shooting can be done without a load. Note that in DC coupled amps you can not troubleshoot from stage to stage with a scope.

I always bring up an amp, through a light bulb, without a load.
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Old 12th September 2008, 11:24 AM   #9
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Default it looks

like a highly sophisticated device ...proceed with caution
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Old 12th September 2008, 01:09 PM   #10
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when it starts crackling, begin spraying semiconductors (diodes and transistors, and op amps) with freeze mist one at a time (except for differential transistors, spray both of them evenly at the same time). move from input stage to output stage, and wait 30sec between each one. usually such popping and hissing is the input stage, but not always. the reason it's difficult to troubleshoot (not impossible, just difficult) a typical DC coupled amp with a scope is that from the colloctors of the diff amp to the base of the voltage amplifier stage, the signal is in the form of current, not voltage, and the viltages are usually about 0.7 volts less than one of the power supply rails.
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