I bought this amplifier board, a 68W per channel LM3886 based amplifier with integrated power supply. With the board I am using a 420VA 25-0-25 toroidial transformer.
2 x 68W Watt LM3886 + NE5532 Audio Amplifier Board on eBay.ca (item 330375561759 end time 11-Nov-09 22:00:52 EST)
I have spent the past few months working on trying to get it to work correctly. So far I have blown up two boards trying to get them to work, and am on my third one now. The only reason I bought any more boards is because the chassis is built around the board's design and I don't want to re-do the chassis. Hindsight is 20-20, so please dont ridicule me about buying chinese junk off ebay. I'm on my last thread of sanity and I just want to get this thing going.
The problem with the first two boards was that I hadn't isolated the LM3886 chip. There were no instructions or any documentation included with the amplifier board, so I didn't think of isolating the chip. Now that that issue is fixed, there isn't any more sparks, but I have managed to liquify two 3A fuses at the 120V mains input. The amplifier will work for a while with a small 10W 8 ohm test speaker, but when i connected a 40W 8 ohm bookshelf speaker the fuse blows like its a dead short. The speakers are fused with 1A fuses, which did not blow. I am not sure if its the speakers that are causing this or if its something else. One of the 40W speakers is now damaged from this amplifier.
If anyone has any idea what I'm talking about or what is going on with my unfortunate amplifier, please post. Thank you.
2 x 68W Watt LM3886 + NE5532 Audio Amplifier Board on eBay.ca (item 330375561759 end time 11-Nov-09 22:00:52 EST)
I have spent the past few months working on trying to get it to work correctly. So far I have blown up two boards trying to get them to work, and am on my third one now. The only reason I bought any more boards is because the chassis is built around the board's design and I don't want to re-do the chassis. Hindsight is 20-20, so please dont ridicule me about buying chinese junk off ebay. I'm on my last thread of sanity and I just want to get this thing going.
The problem with the first two boards was that I hadn't isolated the LM3886 chip. There were no instructions or any documentation included with the amplifier board, so I didn't think of isolating the chip. Now that that issue is fixed, there isn't any more sparks, but I have managed to liquify two 3A fuses at the 120V mains input. The amplifier will work for a while with a small 10W 8 ohm test speaker, but when i connected a 40W 8 ohm bookshelf speaker the fuse blows like its a dead short. The speakers are fused with 1A fuses, which did not blow. I am not sure if its the speakers that are causing this or if its something else. One of the 40W speakers is now damaged from this amplifier.
If anyone has any idea what I'm talking about or what is going on with my unfortunate amplifier, please post. Thank you.
You're right, it has a lot of headway, but the circuit should not be drawing 3A @ 120V, thats 360 watts. The board shouldn't require that much power. Also, the way that the fuse blew and the conductor vaporized into little copper balls indicates that much more than 3A passed through that fuse very quickly.
On power up the tranny will supply as much as its capable of until the power capacitors are charged. That 20,000uF acts like a short until charged. Also looking at the pictures from the auction, I only see 2 diodes so I'm really confused on how they get the +V and -V. half wave rectification on each half the center tap??
Not sure why it would act differently with differnet speakers attached.... I'm probably barking up the wrong tree.
Not sure why it would act differently with differnet speakers attached.... I'm probably barking up the wrong tree.
It's possible to stress or even open a fast acting fuse with large filter caps and no in-rush limiting, but I'm concerned that you were actually able to bake a speaker with a 1A inline fuse that didn't blow. You got something going on there. Once that amp is powered up you should be able to change speaker loads with no signal and nothing real fancy should happen. I'd power it up with no load and check for offset at least. If it's the caps inrush current blowing your rail fuses and you don't have a short or oscillation problem, it's going to happen no matter what is connected to the output of the amp.
Full-wave bridge!
The fuse will blow because of inrush current.
The speaker damage could be due to oscillations.
Andy
Whoops, I neglected to mention that the speaker died before the 1A fuse was put in place. Sorry.
As far as I can hear, there is no oscillation problem. When the unit is functioning correctly, there is no audible hum out of the speaker. I haven't put it on a scope yet though.
Checking for DC offset is still a good idea though, I think. I will check it out tomorrow.
The trouble with rail fuses is they are "designed" to protect the amplifier, not the load. Depending on the behavior of the IC one rail might make it to the load if that's all it's getting. So without knowing anything about the IC ('specially if it's shorted), if you use rail fuses, use an inline fuse too, or watch out.
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Well, now I have a fuse on the live wire from the wall, and I have 1A fuses on the positive of each speaker terminal, hopefully that will protect the speakers from a number of unpleasant situations.
Your torroid voltage is too high. The ebay ad calls for 28 volts DC which needs 20 volts AC although 22 volt AC would probably work. How big are your heat sinks and did you use Artic Silver or something similar? As others have pointed out you must have low DC offset. If you use your own bridge rectifier and a regulator to drop the voltage your torroid will be fine.
Good luck,
Ted
Good luck,
Ted
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How Now Brown Cow?
Voltage: AC 28V-0-28V 5.5A
My transformer outputs AC 25-0-25 and 420VA = 8.4A, tons of spare current on the amplifier side.
I could see only 2 To220 power diodes..and a small bridge rectifier (3 amp) probably for the preamp IC....
Try a bridge rectifier and 2 regulators for the +- op-amp supply
Andy
BUT THAT WILL GIVE U A CATASTROPHIC CRACKING SOUND
!!! if there is DC at the output and the 2200uf charging thru the voice coils !!!
Here's the LM3886 datasheet from National Semiconductor.
http://www.national.com/ds/LM/LM3886.pdf
You will find the output cap on page 6.
http://www.national.com/ds/LM/LM3886.pdf
You will find the output cap on page 6.
I have looked at the circuit, and following the trace from pin 3 (output) pin of the op-amp goes directly to the screw terminal of the speakers; there is no output capacitor. Could this be a source of trouble?
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