LM3875 comes 'back to life' after turnoff transient suppression - diyAudio
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Old 27th August 2004, 11:26 AM   #1
Beggar is offline Beggar  United Kingdom
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Default LM3875 comes 'back to life' after turnoff transient suppression

Hi everyone, sorry if this has been asked before, but i really couldnt find anything.

Ive just built 4 GC based on the BrainGT PCBs, and on 3 of the 4 amps i get a strange phenomenom, where after power is removed the amps mute, however about 3-5 seconds later the amps then breifly (less than a second) unmute and play a distorted signal and then turn off properly.

I'm not even sure if this is a problem as such, and im confused why it happens on 3 of 4 "identical" amps, just wondered if anyone has this happen to their amps, and if so how they did, or did not fix it.

Thanks, Nik
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Old 27th August 2004, 01:52 PM   #2
zeno is offline zeno  Europe
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I have built NIGC and IGC, both have the same problem. NIGC on the boards of Brian.

Maybe a zobel stops this????


Marc
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Old 27th August 2004, 02:12 PM   #3
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Have you checked how the power supply voltage looks like? It may have something to do with the undervoltage protection. I'll believe that this protection has certain conditions in order to work well... my amps are completely silent....
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Old 27th August 2004, 02:23 PM   #4
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Which voltage have you on rails and which value has resistor on mute pin ?
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Old 27th August 2004, 02:24 PM   #5
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LM3875 hasn't got a mute pin
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Old 27th August 2004, 02:34 PM   #6
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Isn't that quite common behaviour? Most amps do that...like the caps that discharge...and then give a final effort...after which they are really discharged and the amp is then silent....
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Old 27th August 2004, 02:37 PM   #7
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hello

I have 26.8v on both rails, its very strange though, i am running 2 transformers, each powering 2 amps each. On one of the transformers, one of the amps makes the coming back to life problem, where as the the other does not. Im going to try and see what the rail voltages are when the problem occours, although with only a DMM i think this is going to be hard to do.

Nik
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Old 27th August 2004, 02:42 PM   #8
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Quote:
Originally posted by Bas Horneman
Isn't that quite common behaviour? Most amps do that...like the caps that discharge...and then give a final effort...after which they are really discharged and the amp is then silent....
Hey

Im begining to think it may have something to do with the caps, maybe as the thing approches a certain level of offness the amp actually 'thinks' its on again, maybe as the voltages on the rails become within some threshold of each other? Im not really sure, the main issue for me is if this is normal behaviour or my 3 of 4 amps are an exception?

Nik
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Old 27th August 2004, 02:50 PM   #9
zeno is offline zeno  Europe
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+/- 24 V AC, little bit more than 32 V DC.

I am not so sure that the caps can be the reason. Standard types (Philips), better ones (panasonic) and best (?) BG. All behave in the same manner. And it is only 1000mu.
Just to make it clear : I cut the mains power. The amp failrly quicly silences and than, all of a sudden, the sound comes back. Distorted.

It seems to happen when the caps are almost 'empty', so some voltage regulation activity could be the reason.

I do have the same behavior in four chips uptil now.


Marc
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Old 27th August 2004, 02:51 PM   #10
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It is not a common behaviour and the amps should be completely quiet when switching on and off.

It is probably cased by ground and power connections in your amps, try to change them.
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