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Chip Amps Amplifiers based on integrated circuits

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Old 28th July 2011, 09:53 AM   #21
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Alright then. I measured the resistance of the 50K pots, and adjusted them all to 25K according to my multimeter. From then i measured the voltage across pin 1 and pin 3, got 36.2VDC across all the LM3886 chips.
I measured the VDC across pin 3 and (ground) on all the chips, got between +30mV and +40mV offset. Now i dunno if it matters, i'm measuring +16mV across the chips now, but in coming this far i've reached the extreme of the pot adjustment. I'm measuring a resistance of 51K ohm on the pots now, can go no further, leaving the DC offset to 16mV. I'm running them at the moment, no problems. But i'll do a second measurment tomorrow to concur with what i have.

rick
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Old 28th July 2011, 10:10 AM   #22
AndrewT is offline AndrewT  Scotland
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An AC coupled single chip amp can be set up for near zero output offset. From cold at switch on (power up) to warm to hot the output offset will vary. The further away from zero offset, the worse the drift with temperature.

A DC coupled chipamp needs extra help to avoid risking damage. Don't use DC coupled nor mixed AC & DC coupled until you have learned how to cope with excessive output offset.

In the meantime, the 3 parallel chipamp is overall an AC coupled chipamp. But inside the AC coupling the chip are DC connected. There is the problem.
The output offset of each chip amp must, as closely as possible, match it's partners not only at start up but throughout all it's operating temperatures.

I suggest since you have 8ohm speakers that you experiment with a single AC coupled chipamp and find out what you can do to adjust output offset, how to adjust to zero offset and examine how much drift in offset is typical of a well set up chipamp.
When you are familiar with what to do and what to expect, then you are ready to look at adjusting your parallel chipamp amplifier.
If you do it wrong now you will either damage the amplifier or worse damage the speaker. Hell there is worse, you could damage both !

WARNING
paralleled (PA100) and bridged (BA100) and bridged paralleled (BPA300) chipamps are not for beginners. Never ever, unless they are guaranteed to survive beginner use.
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Last edited by AndrewT; 28th July 2011 at 10:13 AM.
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Old 28th July 2011, 10:44 AM   #23
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well i believe i have come as far as i can for this exercise and this chip amp. The lowerst offset i'm able to achive is 16mV -/+0.5mv. I'm finding trouble when i adjust one pot, it throws one of the other two out. I've come this far and have a 16mV offset with a -/+0.5mV variance between the three chips. I dunno, how am i doing? :\

Last edited by richard.C.; 28th July 2011 at 10:51 AM.
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Old 28th July 2011, 11:06 AM   #24
AndrewT is offline AndrewT  Scotland
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You can adjust the output offset lower. But, experiment on a single chipamp to find out (and learn) what you need to do, before you damage something.
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Old 28th July 2011, 11:19 AM   #25
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Well i have some more LM3886Tf chips coming my way if something does go haywire, as long as the amps don't blow the VC in my ScanSpeak drivers i'm not worried. I couldn't care too much about the amp chips, they cost me $2.50 each, ScanSpeak drivers cost me $210 each.
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Old 28th July 2011, 11:26 AM   #26
AndrewT is offline AndrewT  Scotland
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then when you finally connect a load, make it a cheap dummy load.
8off ohm 600mW resistors would do just fine. Monitor what happens to them over an extended test period, through a variety of different ON to OFF periods, cold and warm and hot start ups, overnight running to experience a bigger range of mains voltages.
Find out how it performs before you connect any speaker and certainly before you connect expensive speakers.
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Old 28th July 2011, 12:35 PM   #27
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Hi Richard,
I'm a surprized at the results you're getting. Did you ever receive a reply from the seller for a schematic? Without that we're just shooting in the dark. In a well designed circuit you shouldn't need to have the trimmer all the way to the end stop to just get it close. It might be a good idea at this point to remove two of the three amp chips and see if it's possible to adjust the offset correctly on just one of them, it seems you're getting a lot more interaction than would be reasonable. And I agree with Andrew on this, until you are totally confident it's working right, you shouldn't hook it to expensive speakers, use a dummy load or some cheap speaker you're not afraid to sacrifice.

Mike
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Old 28th July 2011, 08:54 PM   #28
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Hi Mike, still waiting for them to send the schematic, also maybe the muiltimeter is giving wrong reading, i'll get hold of another today to make reference. And yes i too thought of removing one or two of the chips to see if i could get it down further. You stated it was not needed to have three of them, if removing one makes my situation easier i will remove one. Could this problem have something to do with my floundering around previously with the power supply?

(off to work back in 8hrs)
rick
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Old 28th July 2011, 09:20 PM   #29
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I recommend removing two of them and leaving just one, for driving an 8 ohm load that's all you need anyway. That way you will eliminate any potential for interaction between chips, which will make it much easier to evaluate circuit performance. I don't know if anything was damaged or not, but these chips tend to be fairly rugged and not easily harmed. I'm also concerned about fake or relabeled chips, it has been an issue, especially when they are bought so cheaply.

Mike

Last edited by Michael Bean; 28th July 2011 at 09:23 PM.
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Old 29th July 2011, 09:24 AM   #30
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update on this. I've removed one of the chips (middle one) from the circuit, this helped improve the DC offset so far, but unable to reach 0mV on both chips, it's possible to reach 0mV with one but i throws the other one off target still, but not as bad as it was with three chips. Two chips are much more stable to set than three, and i've managed to get an offset of 4.5mV on all the remain 4 chips.
And what i'm thinking instead of removing the last chip i may replace the 50K pot's with a 100K pots, or try to add some extra 25Kohms to the circuit along with with the 50K pot in place and try again from there. It seems that the more resistance added with the pot, the more balanced and tighter the two chips measure to zero. I'm getting there.............

Last edited by richard.C.; 29th July 2011 at 09:28 AM.
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