Help please! Troubleshooting LM3886 - diyAudio
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Old 8th July 2008, 06:00 PM   #1
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Location: Brooklyn, NY
Default Help please! Troubleshooting LM3886

Hi,

My p2p lm3886 chipamp was working very well for few days until I meessed up with the cable of one speaker and I installed a dodgy ebay dact type attenuator. The fact is that I must have shorted either the speaker cables or the signal cables and one channel is gone.

I just switched on the channel and it sounded a very very quiet, constant tic tic tic tic tic and caput!.

Furtunately the speaker was not damaged and I do not think there was any shorting involving the DC cables. And there was no smoke or any nasty smells or weird sounds and the chip was not hot.

So if there was a problem on the signal path I do not know were to start to correct it. Should I get a nother chip? Could the caps or resistors also be damaged- which ones? Some guideance on where to start would be very appreciated.

This are the schematics http://www.vikash.info/audio/p2plm3886/

Thanks in advance.

Antonio
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Old 8th July 2008, 07:25 PM   #2
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That ticking sound could be the Spike protection. If you are lucky it is protecting the IC, because the fault is still present. But may as well be the only thing left alive.

Here are some questions that you can try to answer yourself, and that may help you to find the mistake.

If you swap the channels, does the ticking swap as well? If not, the amp is okay and you have to check elsewhere, e. g. preamp, input buffer, attenuator, and so on.

Are both rail voltages present and at their correct values? If not check the fuses.
Fuses okay or not present? Disconnect the amp from the power supply and measure again.
Are the voltages too low with the amp connected and correct with the amp disconected? There is a short somewhere on the amp board, the LM3886 might be broken. Take it out, and check the voltages again.
Voltages okay? Replace LM3886.
Are the rail voltages still low with the amp disconnected? Fault is in the power supply.

Is that attenuator still connected? Disconnect it, and check again. Fault gone? Check the attenuator connections.

Check for shorts everywhere and correct them.

Is the ticking present only with something connected to the inputs? Search the fault outside of the amp.
Ticking always present? Fault is probably in the amp.

Measure the output voltage of the ticking amp. About the same value as the rail voltage? Replace the LM3886.
More than half the rail voltage? Replacing may or may not help, but worth a try.
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Old 8th July 2008, 08:28 PM   #3
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Quote:
Originally posted by pacificblue
That ticking sound could be the Spike protection. If you are lucky it is protecting the IC, because the fault is still present. But may as well be the only thing left alive.

Here are some questions that you can try to answer yourself, and that may help you to find the mistake.

If you swap the channels, does the ticking swap as well? If not, the amp is okay and you have to check elsewhere, e. g. preamp, input buffer, attenuator, and so on.

Are both rail voltages present and at their correct values? If not check the fuses.
Fuses okay or not present? Disconnect the amp from the power supply and measure again.
Are the voltages too low with the amp connected and correct with the amp disconected? There is a short somewhere on the amp board, the LM3886 might be broken. Take it out, and check the voltages again.
Voltages okay? Replace LM3886.
Are the rail voltages still low with the amp disconnected? Fault is in the power supply.

Is that attenuator still connected? Disconnect it, and check again. Fault gone? Check the attenuator connections.

Check for shorts everywhere and correct them.

Is the ticking present only with something connected to the inputs? Search the fault outside of the amp.
Ticking always present? Fault is probably in the amp.

Measure the output voltage of the ticking amp. About the same value as the rail voltage? Replace the LM3886.
More than half the rail voltage? Replacing may or may not help, but worth a try.
Thanks a lot Pacific. The problem seems solved but I have to keep checking to find the precise cause.

Voltages were ok and I removed the attenuator. Then I tried with a different pot and nothing. Not even ticking.

Then I checked all the connections and found that the power ground to pot and speaker and also the signal ground to input were either disconnected or making a false contact (my set up is provisional that is why they were not soldered but just joined with metal sockets).

Also I must have been making the wrong connections to the attenuator and pot because I removed the pot and connected the source directly without volume control and Voila! the amp worked well. I believe the IC did indeed trigger its protection circuits due to the false contact and possible connection errors.

I learned a lesson: always double check connections and make them proper even if temporary.

I am glad I did not kill my main speakers. I was told that a false contact can easily kill an amp and speakers.

I am sure your troubleshooting procedure will be of much help to other newbies as well.

Thanks again.

Antonio
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Old 8th July 2008, 08:49 PM   #4
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Location: Wirral UK
A Sanchez

Get yourself some cheap test speakers, or even just a couple of test drivers. Its better to fry Richersounds cheapies than your pride and joy.

I take it you are aware of the "in series lightbulb tester" as often recomended about these parts? It saved me when I accidentally shorted a new playstation power supply I was measuring. There was a spark and the bulb lit but no damage was done. Tape up your multimeter probes leaving only the tips exposed next time you have it out, it saves shorts if you are poking about in tight spaces!

John
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Old 8th July 2008, 09:27 PM   #5
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Quote:
Originally posted by john blackburn
A Sanchez

Get yourself some cheap test speakers, or even just a couple of test drivers. Its better to fry Richersounds cheapies than your pride and joy.

I take it you are aware of the "in series lightbulb tester" as often recomended about these parts? It saved me when I accidentally shorted a new playstation power supply I was measuring. There was a spark and the bulb lit but no damage was done. Tape up your multimeter probes leaving only the tips exposed next time you have it out, it saves shorts if you are poking about in tight spaces!

John
John

I do have cheap speakers for testing but I was negligent and did not use them because I thought changing the pot would not be a problem: mistake and another lesson learned.

I did however have the light "bulb tester" in place

Many thanks

Antonio
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Old 8th July 2008, 10:17 PM   #6
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That's a good lesson when it doesn't involve magic smoke!

Can I go OT just for a second? How tricky a job is it to re pad a sax?
I've seen it said on the internet that it's easy but we all know that what gets said on the internet isn't always true!

John
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Old 8th July 2008, 10:27 PM   #7
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Actually I will take the opportunity to ask another question in this thread rather than starting a new one.

Its about volume/gain:

Currently the amp has 20db gain because it is supposed to be used with an active pre, which it is not the case. So at the moment at full volume the amp is loud but not much and I feel the speakers could easily handle more power. Also I feel a bit weird turning the volume more than half turn to obtain a moderate volume.

So the questions are: will an increase of the gain to around 30db make a noticeable difference? If yes, should I go for a bit more than 30db?

Also, is it the same to alter either 2k RF (R3 in schematics see below) or the ground 100R resistor R2 to achieve the desired gain. Basically I could go for 3k for RF and leave R2 untouched or change R2 to 65R and leave RF untouched. So, is it the same to change one or the other? Or should I change both and go for 20k and 680R . The designer did suggest to keep the resistors low but I would like to understand the logic and benefits keeping values low or high if in the end the result (ie 30db gain) is the same.

If needed these are the amp and PSU schematics: http://www.vikash.info/audio/p2plm3886/ I am using 2x22 300v toroids (one per channel) and a 10k pot. My speakers are Jordan mltl full range driver : 6 Ohms and below 88 sensitivity

Many thanks

Antonio
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Old 8th July 2008, 10:43 PM   #8
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Quote:
Originally posted by john blackburn
That's a good lesson when it doesn't involve magic smoke!

Can I go OT just for a second? How tricky a job is it to re pad a sax?
I've seen it said on the internet that it's easy but we all know that what gets said on the internet isn't always true!

John
John

All I ignore about gainclones and electronics I know it about saxophones and I can tell you that you should not even try to re pad one.

Unless you want to learn, in which case I suggest you to buy the cheapest second hand chinese sax plus some good books and a mentor, it is not worth even trying. The cost of the pads and the tools, plus the time and the almost certain bad result should discourage anyone.

A sax is much more complex than it looks as soon as you start removing screws.

What I suggest you is to really shop around in order to find a good repairer with reasonable pricing (which is not easy in the UK- but I can help you if you want). The good thing is that a good re-pad and proper care of the sax can last many years.

Antonio
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Old 9th July 2008, 07:24 AM   #9
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Raising the gain from 20dB to 30dB means ~3,16 times higher output voltage, 10 times higher output power and the acoustic impression of double the volume for the same input voltage. This is of course only true as long as you don't reach the amplifiers voltage and power limits.

680/22k is widely used, and is a reasonable value for your rail voltage. It will give you nominal output power into 8 Ohms at somewhere between -0,4dB and 0,5dB input level, depending on the size and quality of your transformer and the amount of F in your power supply.
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Old 9th July 2008, 08:00 AM   #10
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Quote:
Originally posted by pacificblue
Raising the gain from 20dB to 30dB means ~3,16 times higher output voltage, 10 times higher output power and the acoustic impression of double the volume for the same input voltage. This is of course only true as long as you don't reach the amplifiers voltage and power limits.

680/22k is widely used, and is a reasonable value for your rail voltage. It will give you nominal output power into 8 Ohms at somewhere between -0,4dB and 0,5dB input level, depending on the size and quality of your transformer and the amount of F in your power supply.
Thanks Pacific,

So, increasing the gain would indeed make a noticeable difference. Can you guess why the designer of the schematics recommended not to go above 4.7k resistor values when increasing the gain?

By the way I have around 24 700 uf per rail and decent quality 300va toroids.

Antonio
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