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Old 26th June 2007, 11:06 PM   #1
pjpoes is offline pjpoes  United States
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Default Testing before starting my new amp up, please help

Ok I have checked, rechecked, and rechecked again every connection in my amplifier. Before I plug everything in and remove the resistors I'm using to test it for now, I want to make sure the results I'm getting are right. I'm using 100ohm 10 watt resistors inplace of fuses to test, as suggested by Anthony Holton. With out the amplifier connected to the power supply, I get 66 volts per rail, or 132 volts together. I get the correct negative 66 on the negative side, and positive 66 on the positive side. However, when I hook the amp to it, the voltage decreases to around 2 volts positive and 30 volts negative. Is this normal behavior? I've never used resistors to test durring setup, so I'm not really sure if they should be doing that. Is this caused by the amp trying to draw a bunch of current and with the resistors in series, the voltage is going down. The positive side more because its trying to draw more current? Is this a sign that everything is working ok, or a sign that something is messed up.

I also hooked an 8ohm speaker to the terminals of one amp to see if I heard any sound, I don't, but I did feel the speaker cone move. Again, I can't tell if that means its working correctly or not. I'm afraid to remove the resistors until I'm sure everything is hooked up ok and I can test it full bore. Again, I've checked, checked, and rechecked my wires and I can't find anything wrong with it, so I can't tell if this is a sign of a problem or not.
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Old 27th June 2007, 12:41 AM   #2
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If your rails are dropping as you say, then you have a problem with your amp. In idle state with no input and assuming it is connected to an 8 ohm load, with idle current of say 200mA (guess), then with the resistors in place of your fuses the rails should be about +/- 46V (200mA x 100R drop across the resistors). Lucky you didn't run it without the resistors or you may have seen a bit of smoke. Check your positive rail output devices - sounds like you have a short there - maybe between the transistor case and the heatsink, which would definitely cause what you are describing.
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Old 27th June 2007, 12:58 AM   #3
pjpoes is offline pjpoes  United States
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Yeah I think you are right, I think I do have a short. The amp uses the Buz900P range of devices for output devices. I think the center pin is the one I want to test to ground, if someone knows better, that could be helpful. None the less, two of the amp modules test like 5 mega ohms against ground, then the third module measures a dead short. Problem is, ever single output device measures that way. That doesn't make much sense to me, I'm trying to figure out what it could be. Is it possible that something else is causing that short, or if one output device was shorted, they all would act shorted?
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Old 27th June 2007, 01:07 AM   #4
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Without seeing the circuit, I couldn't say for sure, but for most output stages where devices are in parallel, then if one device is shorted the others will also appear shorted. You either have a dead device or it is shorting throught the heatsink. If possible unscrew them from the heatsink and see if you still have a short. This is normally the most common problem, otherwise you'll have to remove the BUZ900Ps in the thord module and check each one to see if it dead.
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Old 27th June 2007, 01:11 AM   #5
teemuk is offline teemuk  Finland
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According to datasheet pin 1 = gate, 2 = source and 3 = drain. There should be about 1 - 4 V between pins 1 and 2 and more than 2 V between 3 and 2. Resistance between pin 3 and ground should be high. gate-source and gate-drain junctions act like diodes. Resistance between gate and source or gate and drain should be nearly infinite. The circuit can interfere with measuring so lift some pins off the board before you measure.
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Old 27th June 2007, 02:03 AM   #6
pjpoes is offline pjpoes  United States
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Thanks guys, the amp is working now, hooray. I can't believe its finaly finished.
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Old 27th June 2007, 02:16 AM   #7
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Well done and enjoy!
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Old 27th June 2007, 02:36 AM   #8
pjpoes is offline pjpoes  United States
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Thank you, I'm very happy. I will have to start another thread to outline this thing, but the power supply is working wonderfully. Basically its a CLC supply and this amplifier is the quietest amplifer I have ever heard. The backgrounds are blacker than any amplifier I have ever owned, I achieved exactly what I wanted with that. I wont comment on its sound right now, I'm just not sure its good to test a cold amp, especially considering how much silver wire and what not is in this thing.
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