Audio N00b, blew a new amp... Help me figure out what went wrong. - AVS Forum
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post #1 of 7 Old 10-01-2013, 04:14 PM - Thread Starter
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Hi. I'm not an incompetent person; I do motorcycle mechanics on old jap bikes for a hobby, have been screwing around with computers (up to light programming) for 15 years, and have used a soldering iron once or twice. I've used basic wiring diagrams to troubleshoot motorcycle mechanic issues before.

Anyways.

I bought a pair of Dayton B652's new off of Parts Express, an external USB DAC (Creative X-fi SB1240) off of craigslist, and a lepai 2020A+ off of craigslist. The lepai didn't come with a power supply, so after a little research (people saying the 12v 2amp that comes with it is underpowered, should use a 5amp for its full potential--which 5amp? one guy recommended a particular one off of ebay, Li Shin, that did not produce noise on his, after buying another 12v 5amp off Amazon that did produce noise and did not increase volume on his) I got this:

12V 5A AC DC adapter power supply Li Shin LSE9901B1260

power supply on ebay.

All the parts finally came in the mail.

I inserted the wires into the back of the speakers, and then to the amplifier. (It's only a 2.0 system, so yes, I did this part correctly. I've hooked up many speakers and amplifiers over the years, just never a 'home-built' system) I then plugged the power supply into the wall. Then to the amplifier. Flipped the switch on the amplifier. Blue light! Good! :D

I turn the volume all the way down. I plug in the included RCA-to-phone cable's phono side into the Lepai's "mp3" input. I plug the RCA ends to the output of the X-Fi DAC. 

Lastly, I hooked the external sound card to the computer--originally, the USB cable I was using to hook it up seemed to be a little less than par (the guy who sold it to me gave me two cables in the box, one that looked 2nd rate with a little bit of insulation torn away, and another nicer one. The nicer one was being used elsewhere, so I grabbed the second rate one, figuring it would at least work if he included it in the box). I think this is not the problem, but I am mentioning it for the sake of being thorough and not leaving any detail out.

The reason I think it was actually less than par is that the blue LED light on top of the sound card was dimly/partially lit, maybe even flickering, but using the nicer wire a minute later produced a full, solid blue light.

Anyways.

Hooked that up. Windows Vista SP2 identified and started installing the drivers. I'm sitting there wondering about the quality of this cable, when all of a sudden, I smell something--christ, what is that smell? I panic, because I have smelt this before at some point, maybe soldering, maybe breaking something else in my childhood, but I know that's a bad smell. I immediately unplug the sound card, wondering about that cable, smelling the sound card--it smells like it on the outside, but it seems fine. I go to grab the better USB cable from the table nearby. I walk back. Smell is stronger. And I notice the amplifier light is out. 

****.

I disconnect everything but the power from it. Turn it off, turn it on. Blue light comes on, then quickly dims. I do this two or three times and get the same result. I unplug the power. I walk outside to try and air out the smell. My roommate walks out a minute later and asks if something is burning.

What the hell happened?

I used only a setup thoroughly vetted by many others with a great deal of experience. My only guesses are:

1. The power supply was actually a piece of ****, and not correctly spec'd
2. The amplifier, which I got off of craigslist, was actually dysfunctional
3. Something with the DAC?

I consider #2 to be unlikely, because the guy seemed really honest, and even told me that (I asked him if he was SURE it worked, because I had recently bought an amp for a friend that didn't work off of CL and the guy claimed it 'must have been damaged in transit') "If it doesn't work, come back to me and I'll give you back your money. That guy was full of ****, sorry you had to put up with that." He was a well dressed, nice guy. 

#3 is a long shot; I highly doubt an error there would blow the amp. I didn't even send any music through it, and the cable error seemed to be undepowering the unit, not shorting anywhere.

Any ideas? What should I look for?

Here are some pictures from when I took it apart. 

The whole board, for context:


Back side of the power input here, looks fine:


The power switch; doesn't look like the best soldering job, but looks fine:


Power knob (with a blue glowing LED around it)... I think it's ok too.


Some of this looks a little suspicious:


Pretty clearly some burned solder joints here:

 


The top side of those burned solder joints, where it connects into the Tripath chip:

 

:(

WHY ME? I was so excited. I guess I should be thankful the least expensive part is the part that blew. I only paid $10 for it.

Diagnosis?

Thanks,

Kyle

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post #2 of 7 Old 10-01-2013, 04:29 PM - Thread Starter
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For reference, here is a picture of the power supply I purchased on ebay and used:






And here is a picture of a stock power supply (picture taken from amazon listing):
 



Here is a sample comment from Amazon that is relevant:

 

Quote:
2) The shipped power supplies were to be a 2 amp one and a 6 amp one in the second case.
3) If you run this amplifier from a 2 amp power supply - here is some news for you. The theoretical max watts your system can deliver would be 12X2 = 24 watts - However, since the amp is running around 80% efficiency - you can get approx 24X0.8 = 18 watts max from this amplifier (best case scenario) or around 9 watts per channel. The problem comes as you drive the amplifier harder. The power supply voltages will start dipping, and as it does so, the amplifier will distort. So your clean sounding amplifier, won't sound so clean after all once you drive it past 7 - 8 watts.
4) To really use this amplifier with low distortion at high volumes, you should use a switching powersupply that delivers 12 to 13.5 volts at 4 amperes. 6 amps is a bit of an overkill, but recommended if you will be using 4 ohm speakers.
5) You cannot use this amplifer with an un-regulated transformer type powersupply rated at 12 volt. Their no-load voltage can be anywhere from 15 - 20 volt and can even be dangerous if connected to this amp - since the supply capacitor is rated at 16 volt.


And here is the comment I based the purchase of this power supply on:

 

Quote:
I bought this adapter to replace a 12V 3.5A Li Shin adapter to hopefully provide (5A) a little extra headroom for powering a small 15 watt class-T Sonic Impact amplifier and later another Lepai LP-2020A+ class-T amp.

The original Li Shin (3-prong plug) adapter produced no hum and added no noise. This Amazon-purchased ADP-15HB (2-prong plug) added a lot of hum and noise making it useless for my particular audio application. I have the Sonic Impact amp connected to a small M-audio Fast Track Pro mixing device connected to a series of guitar effects. When hooked up to my electric guitars, there was no hum problem but when connected to an amplified ukulele or an amplified acoustic guitar, both would exhibit major hum and noise when using the 2-prong ADP-15HB adapter but not when using my original Li Shin adapter.

For me, this adapter is worthless for audio but for someone else, it may or may not cause major hum problems.

BTW, the "new" Amazon-purchased 5 amp adapter provided no noticeable extra headroom over my "old" 3.5 amp adapter.

UPDATE: I just received a 12V 5A "Li Shin" replacement PS that I bought on Ebay. This 3-prong grounded PS, unlike my purchase on Amazon, is without the noise and hum of the Amazon-purchased PS. I can also clearly hear the added headroom of the 5 amp PS vs my older 3.5 amp PS. This tells me that the 2-prong Amazon purchased PS not only added extra hum and noise but it is also underpowered and not really a real 5 amp PS.

--http://www.amazon.com/Replacement-adapter-charger-Benq-Monitors/dp/B003Z6ZR5O/ref=pd_sim_e_8

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post #3 of 7 Old 10-02-2013, 04:23 AM
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Quote:
Originally Posted by fintip View Post


Hooked that up. Windows Vista SP2 identified and started installing the drivers. I'm sitting there wondering about the quality of this cable, when all of a sudden, I smell something--christ, what is that smell? I panic, because I have smelt this before at some point, maybe soldering, maybe breaking something else in my childhood, but I know that's a bad smell. I immediately unplug the sound card, wondering about that cable, smelling the sound card--it smells like it on the outside, but it seems fine. I go to grab the better USB cable from the table nearby. I walk back. Smell is stronger. And I notice the amplifier light is out. 



What the hell happened?


I used only a setup thoroughly vetted by many others with a great deal of experience. My only guesses are:


1. The power supply was actually a piece of ****, and not correctly spec'd

The Lepai 2020 is based on the Tripath 2020 chip. I have another amplifier that is based on the same chip. As long as you feed it something that is close to being 12 volts, they seem to be happy enough.

However, a wall wart power supply can be far enough off spec to perhaps hurt the amp. Some wall warts put out AC, not DC. The kind you seem to have do not have the best voltage regulation, but IME as long as it stays below 18 volts which is likely, no problem.
Quote:
2. The amplifier, which I got off of craigslist, was actually dysfunctional

Probably the most likely answer. $10 is pretty cheap. Lepai 2020s run around $20 so replacement is the obvious fix:

http://www.amazon.com/LP-2020A-Lepai-Tripath-Class-T-Amplifier/dp/B0049P6OTI/ref=sr_1_1
Quote:
3. Something with the DAC?

Unlikely

The Amazon unit comes with a 12 volt 2 amp power supply. You will no doubt hear that upgrading the power supply improves its sound quality. Unless you run it hard into a low impedance speaker, not so much. If you want a really heavy duty power supply cheap, pick up a cheap PC power supply and run the amp off of its 12 volt leads. A local chain computer chain is selling refurb PC power supplies for under $10. You can find wiring diagrams for them on the web.

If you want to "push" the amp and get more headroom at some risk, see if you can find a discarded laptop power supply. They generally put out 16-19 volts. If you go the laptop power supply route, amps like this respond well if you "stiffen" its power with a "stiffening cap" that are common in carsound realms.
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post #4 of 7 Old 10-02-2013, 04:10 PM - Thread Starter
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Thanks. Good to know I'm not crazy or missing something obvious.

How could I use a multimeter to test the power supply and verify that's not the problem?

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post #5 of 7 Old 10-02-2013, 04:30 PM
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Originally Posted by fintip View Post

Thanks. Good to know I'm not crazy or missing something obvious.


How could I use a multimeter to test the power supply and verify that's not the problem?

Put on a voltage range that is > 12 volts and measure the output of the power supply. It might be as high as 15 volts with no load, which is OK. If its over 20 volts then not so good, but still shouldn't have fried the amp, maybe.
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post #6 of 7 Old 10-03-2013, 02:24 AM - Thread Starter
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Thanks. I ordered another Lepai, this time off of Amazon. $20, and includes a power supply. I will test both power supplies with a multimeter, and will be very careful about plugging the new one in (just in case!).

One more note: I did not use a wall wart style power supply. The Li Shin 5amp power supply I purchased is a brick style, like a laptop power supply. Does that pretty much put the nail in the coffin on the power supply being the culprit on its own?

Cheers.

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post #7 of 7 Old 10-07-2013, 03:23 PM - Thread Starter
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Sure enough, it was the amp. I replaced it with a brand new lepai 2020A+, and no problems--either with the included power supply, or the 5amp one I bought.

 

Further, the 5amp Li Shin does not produce buzz, and seems like a quality item. So Thumbs up there.

 

Thanks again.

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