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Old 15th January 2012, 01:42 PM   #1
tvrgeek is offline tvrgeek  United States
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Default Amp testing SOP

I managed to do some damage to an amp while testing. OK, fried everything past the diff input. This gives me a great excuse to play with the circuit and newer parts. I don't have a big auto transformer yet, so let's leave that obvious answer aside.
I would like to power this up without the outputs installed as I make sure everything is as I intend it to be. This causes a bit of an issue as no source for global feedback. Any "standard" recommendations so one can bring up the first two stages without drama? Or is it just easier/safer to go ahead and bring it up with several hundred ohms load on the outputs? What is SOP?
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Old 15th January 2012, 02:02 PM   #2
DF96 is offline DF96  England
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In a typical SS amp almost everything is connected to almost everything else. However, you might get away with temporarily moving the feedback point from the output to the VAS collector. If the output stage is still present then you will get a much larger DC offset so don't connect a speaker - use a dummy load of some sort.
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Old 15th January 2012, 02:12 PM   #3
tvrgeek is offline tvrgeek  United States
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Sounds reasonable. My main motive is to be sure I get everything fixed before I order the not very cheap MOSFETS. It amazing what rectifier breakdown can do. Need to check my junk box for hundred ohm or so load. My test load only 2 to 8 Ohms.
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Old 15th January 2012, 03:59 PM   #4
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Originally Posted by tvrgeek View Post
...What is SOP?
SOP is as I know it 'Standard Operating Procedure', and represents a general set of guidelines to follow when performing some task.

As for SOP for testing an amplifier, some general suggestions:

1) Always power up via a current limiter of some description, be that a current limited bench supply or the amplifiers own PSU through a light bulb (lower wattage=lower current limit).

2) If possible (and it can be done most of the time) do a current limited power up without the output devices in place to check that the rest of the amplifier is behaving as it should.

3) When powering up with the outputs in place for the first time, continue with the current limited PSU and I like to use relatively high value emitter resistors and continue functional evaluation.

4) Once all seems well, install the normal emitter resistors and recheck with the current limited PSU. Adjust and evaluate DC offset, bias and any drifting of these parameters.

5) Once satisfied that basic functionality is alright, then one can power up with the full capacity PSU with only the usual safety measures in place. Now proper load testing can begin.

I'm sure you'll encounter variations of this general outline, but the idea is to bring the unit up in gradual stages for your safety and to prevent a minor error from releasing the magic smoke from the rest of the unit being evaluated.
---Jason Kuetemann---
Power is only granted to those willing to lower themselves enough to pick it up.
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Old 15th January 2012, 06:24 PM   #5
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If you have the schematic and post it, I am sure we could guide you through a repair procedure
I generally remove all output transistors and take basic measurments across all in place active elements
I, not owing a variac, use a light bulb in line with the power transformer. A 100 Ohm (or so, depending on the amp) load on the output
Than pray.... E
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Old 15th January 2012, 06:25 PM   #6
tvrgeek is offline tvrgeek  United States
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Bid on a variac.

I forgot about the old light bulb trick. I should know better as I built on into a 2 Farad power supply I once built for a sub amp. Larger emitter resistors, that makes sense. Thanks.
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Old 15th January 2012, 06:47 PM   #7
tvrgeek is offline tvrgeek  United States
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The amp is just a Hafler DH 120. Schematic on their site. Pretty much about half the transistors past the input LTP and half of the output protection zieners are toasted. Maybe only two of the outputs, but as they are not available, ( Famous Hitachi L-MOSFETS) I have to put in a substation anyway,so I have to buy all four. All the rest of the parts are really, really generic. All of them total about $10 from Digi-Key. I may just shotgun the entire boards. I am glad the Q1-Q2 survived so I don't have to match any of them.

If you do take a look at the schematic, please explain the function of R10, the 6.1K 2W resistor that BTW, runs at 1.8W! I put in a 3W on longer leads as it was cooking the boards. I don't quite understand the details of the VAS yet.
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Old 16th January 2012, 04:43 PM   #8
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For other readers, the light bulb tester:

Light Bulb Tester
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