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Problem with Mini^3. Sound is cracking/popping.

post #1 of 14
10/22/07 at 9:20am
Thread Starter 
Hey guys,

I recently bought a Mini^3 from a member here (KT88) but I am having some problems with it. It's been working fine for a few weeks, no problems at all. I don't know why this started happening, but for some reason the headphones are "popping" now when I listen to it. It's loud and makes the amp unlistenable. It is the high performance version by the way, not sure of the gain.

My sister told me this happened, she said it just started happening. As far as I know it hasn't been moved or messed with; it's just been sitting next to me on my computer desk. The crackling/popping happens when powered by both battery and/or power supply (15v, 300mA). It also happens with both my pairs of headphones, and the headphones work fine straight out of my computer's sound card (which is the source I'm using).

I have no idea where to start to diagnose the problem, so I'm asking for head-fi's help. If this is a common problem, sorry, I didn't really know what to search for.

Thanks
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post #2 of 14
10/22/07 at 10:09am
Scotty, stop listening to the amp now. Do you have any experience working on electronics?
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post #3 of 14
10/22/07 at 10:30am
Thread Starter 
Quote:
Originally Posted by n_maher View Post
Scotty, stop listening to the amp now. Do you have any experience working on electronics?
Not much. I did electronics in school and got a B, but that's about it. We only did basic things though. Mostly written theory and some soldering/making basic things like alarm clocks and electronic board games.

I haven't listened to it since it started happening of course.

Do you know what is wrong with it?
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post #4 of 14
10/22/07 at 10:50am
Did you contact KT88 for assistance? See this thread for related info. Need to know if the noise is in one channel or both, if I was a gambler I'd bet on U5.

You'll probably need to find someone locally to help due to shipping costs, etc.

Good luck.
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post #5 of 14
10/22/07 at 10:50am
The first thing that you'll need to have access to is a multi-meter of some kind. You need to check and see if the amp is putting out any significant DC offset. The easiest way to do this would be to plug a mini mini cable into the output jack and with your MM set to measure milivolts turn the amp on and measure from the right and left channels to ground. You can do all of this from the exposed end of the mini-mini cable. The tip is the left, the ring the right, the sleeve (large ring) is ground.

After that the next step would be to take the amp out of the case and measure the current draw (slight more complicated).

My guess is that the amp is oscillating and that's not a good thing.
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post #6 of 14
10/22/07 at 10:59am
reflow your opamps.
if you can.
if not maybe contact KT88 and ask him if he can rectify this.
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post #7 of 14
10/22/07 at 11:08am
Thread Starter 
Quote:
Originally Posted by n_maher View Post
The first thing that you'll need to have access to is a multi-meter of some kind. You need to check and see if the amp is putting out any significant DC offset. The easiest way to do this would be to plug a mini mini cable into the output jack and with your MM set to measure milivolts turn the amp on and measure from the right and left channels to ground. You can do all of this from the exposed end of the mini-mini cable. The tip is the left, the ring the right, the sleeve (large ring) is ground.

After that the next step would be to take the amp out of the case and measure the current draw (slight more complicated).

My guess is that the amp is oscillating and that's not a good thing.
I don't think I have a multi-metre. I'll ask my dad, he might have one somewhere.

Thanks for the info.

I was going to ask KT88 to repair it if I couldn't fix it on my own. But it looks like I may have to ask him now, because this seems a little too complicated for me.
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post #8 of 14
10/22/07 at 11:14am
I'd contact KT88.
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post #9 of 14
10/22/07 at 11:18am
Thread Starter 
I've just sent him a PM. Hopefully he has the time to repair it for me.

Thanks for your help guys.
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post #10 of 14
10/22/07 at 11:40am
Quote:
Originally Posted by n_maher View Post
The first thing that you'll need to have access to is a multi-meter of some kind. You need to check and see if the amp is putting out any significant DC offset. The easiest way to do this would be to plug a mini mini cable into the output jack and with your MM set to measure milivolts turn the amp on and measure from the right and left channels to ground. You can do all of this from the exposed end of the mini-mini cable. The tip is the left, the ring the right, the sleeve (large ring) is ground.

After that the next step would be to take the amp out of the case and measure the current draw (slight more complicated).

My guess is that the amp is oscillating and that's not a good thing.
the ground output line from the mini cable is an "active" ground, meant to sink current actively.

I know you already knew this but your post was a little confusing. He should measure all three outputs' dc offset (output ground, output left, output right) and using the passive ground as a reference, which in this case, just tap the input grounds. The Mini^3 is a three channel amp with a differential output. DC offsets have to be measured this way.
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post #11 of 14
10/22/07 at 6:13pm
I don't know what kind of an op-amp is acting as the virtual ground driver in your Mini^3 but if it's OPA690 you might have it blown with any voltage higher than 13V. I saw you mentioned 15V PSU so use your multimeter to see if your active ground is still alive. What you need to do is measure the voltage between ground and one of the rails, and compare it to measured battery voltage. All voltages have to be read when the amp is turned on. If the ground voltage is different from a half of the battery voltage more than a couple of mV, your ground driver is dead.
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post #12 of 14
10/22/07 at 6:59pm
The voltage is divided into rails just before it reaches either opamp. Amb specs a walwart anywhere up to 24V.
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post #13 of 14
10/22/07 at 8:18pm
The voltage is regulated to 12V max internally. There's a regulator on the PS rails.
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post #14 of 14
10/22/07 at 8:30pm
I'd say contact KT88.

Here's another thread.

http://www.head-fi.org/forums/showthread.php?t=258353

His ferrite beads are very high off the board. For myself, I had to tip the L1G ferrite bead inwards, because of the screw rail in the Hammond case which juts in. I mounted my ferrite beads much lower, the bottom of the ferrite bead just above the height of the MLCC caps.

Anyhow, it's possible that the leads of the ferrite are touching the case and have just started making electrical contact, or the battery contacts are. The case is input ground. The other possibility is that one of your opamps have blown.
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