Earthquake 200dhc with hum in output - diyAudio
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Old 20th November 2010, 07:48 AM   #1
spooney is offline spooney  United States
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Default Earthquake 200dhc with hum in output

Got this amp in a trade and although it appears to work properly it has an annoying low level hum that is being produced through the speaker outputs. It appears as if somebody has worked on this amp before . I'm guessing that at least the ps fets were replaced at one point and most likely the outputs went at that time as well . It looks as if somebody may have changed out a resistor near the speaker outputs as well. Every pic I've seen of the inside of this amp shows a 2-3 watt banded resistor (not sure of the value right now) right behind the speaker outputs which I believe I read somewhere is part of some sort of filter. My amp has a large white .1 ohm 5 watt wirewound resistor in place of that banded resistor. Here is a link to a pic of an amp similar to mine showing the original resistor. The resistor can be found in the lower left hand corner of the pic just to the left of the two output inductors. Google Image Result for http://img191.imageshack.us/img191/6808/img2651yq.jpg

Could this reistor be part of the issue? Those color bands don't look like .1 ohm to me and that is what somebody put into my amp.Also was surprised to see a fuse soldered directly to the underside of the board across the positive speaker outputs. I know it was recommended to fuse the speaker line in these amps but it looks like somebody just cut the trace leading to the positive speaker output and soldered a 20 amp atc fuse right across it. Weird looking for sure. I added lengths of wire and made the fuse accessible from the top side of the board so you don't have to remove the board from the sink to change the fuse if needed.
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Old 20th November 2010, 03:02 PM   #2
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I think it's a 0.2 ohm resistor.

Does it hum with no RCAs connected?
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Old 20th November 2010, 04:53 PM   #3
spooney is offline spooney  United States
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Haven't had a chance to see if it hums with no rca's yet. I'm in the process of replacing all the electrolytic caps in this amp with some nicer panasonic FC caps.. As soon as I wrap that up and can get it back into the heat sink I will see if it hums with no rca's. Maybe the new caps will clean up that noise a bit. We will see shortly. Thanks for the suggestion on the rca's.
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Old 26th November 2010, 02:21 AM   #4
spooney is offline spooney  United States
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Well after replacing the caps it seems as though most if not all of the hum I was experiencing is gone. There is virtually none to speak of at this point. Just the slightest bit of hiss which more than likely would be inaudible in a car if it isn't being caused by my power supply. RCA's in or out of the amp seem to make no difference. Even though it works ok I am still not convinced there isn't a small issue somewhere in this thing. I noticed that if I touch the heatsink or rca shield with my bare hand it seems to alter the noise I am hearing. Touching the heatsink doesn't alter the sound as much as touching the rca shield does though . Touching the rca shield with no rcas attached and a sub hooked to the speaker outputs causes a nice pop of the speaker cone. Not sure if this is normal or not. I kind of accidentally touched the rca shield the first time I noticed it. Never noticed this on other amps.
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Old 26th November 2010, 02:46 AM   #5
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You probably already checked it . But if you touch the rca shields and the speaker pops sounds to me like you have a broken leg for the ground shiled on the rca jacks
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Old 26th November 2010, 02:58 AM   #6
spooney is offline spooney  United States
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Seems like I did verify already that the shield ground was still intact but I will check it again. Thanks Mike. Sometimes I just need someone to point me in a direction.
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Old 26th November 2010, 04:02 AM   #7
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Is there any DC on the heatsink?
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Old 26th November 2010, 04:36 AM   #8
spooney is offline spooney  United States
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There is about .6 volts of dc on the heatsink whether I use secondary or primary ground as a reference( there is no continuity between primary and secondary ground though). The voltage slowly trickles down while I am measuring it. Not sure if that is my meter stabilizing its measurement or if its really going down but it always starts right about .6 volts when the meter leads are first placed on the amp.
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Old 26th November 2010, 05:28 AM   #9
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0.6v is insignificant.

If it only pops the first time you touch it, it could be static electricity.

What are you referring to as the secondary ground?
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Old 26th November 2010, 05:47 AM   #10
spooney is offline spooney  United States
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I was referring to the rca shield ground as secondary ground. Perhaps my terminology is off.
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