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Old 18th February 2013, 02:05 PM   #71
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Originally Posted by Mooly View Post
The offset might not be so bad with a load (try it with a 10 ohm or something). Amplifiers of all kinds can do strange things as the rails collapse and the real answer is always to provide a simple switched speaker delay, 4 volts won't harm the speaker but is would spoil the "user experience" of such an amplifier. Try it with the dummy load.

For the channel that saw reversed supply, I wouldn't use the LM3886 whether or not it appeared functional. It's just not worth the risk later down the line.
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Originally Posted by blu_glo View Post
I was going to post answer when I got into work (typing on my mobile on a train is a bit of a hit and miss affair) .... However I see Mooly has replied - with exactly the same answers I'd have given!

However in addition I went to check the datasheet because I had suspicion this device had more than one ground - it doesn't. But, there ARE pins with NC written by them Check there are no shorts (solder or copper whisks) to ANY of these pins sometimes NC does not necessarily mean "no INTERNAL connection" and you can get odd behaviour if they ARE connected to anything.....

Great, thank you both. I'll place an order for some new parts today. Heading out of town for the week, so I'll have to wait until next week to get back to it.

I'll double check the NC pins. Will a 10ohm wirewound resistor suffice for a dummy load? 5-10watt?
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Old 18th February 2013, 02:06 PM   #72
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I am personaly collecting the bits for a similar project; but 3 way tone control and probably 6 inputs or 5 and phono.
Cool, I'd like to see what you put together when you do.
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Old 18th February 2013, 04:19 PM   #73
blu_glo is offline blu_glo  England
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my 4 dummy loads consists of 8 x 1 ohm 25W wirewound resistors (from an ebay job lot of 50) connected in series and mounted in a metal case with 4mm banana plugs. Those are good for 200W continuous! You could do the same with 8 off 1 ohm aluminium clad type resistors mounted on a spare (!) heatsink....
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Old 18th February 2013, 04:35 PM   #74
Mooly is offline Mooly  United Kingdom
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10 ohm (5 or 10) watt is plenty for this. Even a 1 watt carbon would be OK just as a test.
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Old 18th February 2013, 04:35 PM   #75
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Originally Posted by blu_glo View Post
my 4 dummy loads consists of 8 x 1 ohm 25W wirewound resistors (from an ebay job lot of 50) connected in series and mounted in a metal case with 4mm banana plugs. Those are good for 200W continuous! You could do the same with 8 off 1 ohm aluminium clad type resistors mounted on a spare (!) heatsink....
Putting part order together now for everything... looks like I can get 2ohm 15w's for a good price. So I could do 4 of those for 8ohm 60w.
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Old 18th February 2013, 04:37 PM   #76
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10 ohm (5 or 10) watt is plenty for this. Even a 1 watt carbon would be OK just as a test.
Ah, maybe I'll stick with my original plan then since that will be cheaper and I'm not testing at full power.
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Old 18th February 2013, 04:51 PM   #77
Mooly is offline Mooly  United Kingdom
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If we're just testing for the offset problem at switch off then the 1 watt is fine. If you have thoughts of testing the amp with test tones then you need something far more substantial of course.
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Old 18th February 2013, 04:54 PM   #78
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If we're just testing for the offset problem at switch off then the 1 watt is fine. If you have thoughts of testing the amp with test tones then you need something far more substantial of course.
Got it. At this time, I'm just looking to test the offset. Thanks!
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Old 18th February 2013, 05:04 PM   #79
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Yup I admit I did equip myself wiith the future in mind! I've done lots of amps over the years......
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Old 19th February 2013, 03:45 PM   #80
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You may want to take a look at LM1036. I believe this device was engineered with the LM3886 in mind.
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