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Old 23rd December 2006, 01:25 AM   #1
Mx is offline Mx  Yugoslavia
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Angry Hiss on LM1876tf amplifier

I have a problem with my Sony cmt-nez3 micro system. It has pretty good sound and recently i changed speakers with ones that have better tweeters. Now hiss is constantly present when there is no sound playing. Problem is that i sit very close to the speakers as i used them as monitors (30cm).
Amp inside micro system is LM1876 and voltage is 43,8V (Vcc-Vee). Output inductors are present. There is no motorboating just higher frequency hiss. Hiss is present when i select audio in and disconnect input cable from pc. What modifications do you suggest to get rid of it?
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Old 23rd December 2006, 01:58 PM   #2
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The sony AMP is a low cost item that may have cut a few corners. With that thought... the source & supply filtering as well as grounding may not be ideal. (Read as: Just good enough)
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Old 23rd December 2006, 06:46 PM   #3
Mx is offline Mx  Yugoslavia
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Thanks for answer
Considering the price (around 110$) the unit is built extreemly well. It's all in SMD and supply filtering is 2200uF bypassed with lower values of smd capacitors. Main board is very well organised, but offcourse it's not in ground star configuration. Do supply capacitors need to be bigger (again i must say that there is no motorboating effect)?
Or should i build separate LM3886 amp?
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Old 24th December 2006, 07:21 AM   #4
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Are the new tweeters more efficient than the old ones?
Maybe the hiss is just more noticable now because of higher efficiency drivers?
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Old 24th December 2006, 09:00 AM   #5
ash_dac is offline ash_dac  United Kingdom
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It's a Sony gainclone!

Which Sony has a LM3886 or LM3876 in it ?
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Old 24th December 2006, 10:48 AM   #6
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Well, speakers that came with the system were awfull 10cm fullrange units. Their frequency response was bad enough so hiss can't be heard on them. These speakers that i use now are much bigger 3 way speakers, that i had left from other sony mini system which is dead. Speakers are very good comparing to the more expensive hi-fi speakers. Tweeter efficiency is not that important because frequency response of the whole speaker is (lets say)near flat, and the whole speaker efficiency is maybe just a little better than small fullrange units. Hiss is now noticable because speakers have extended freq. range.
Well i should have bought bigger system in the start. Sony has great 2 way with 4 digital amplifiers system at around 230$, and philips even have ribbon tweeter at that price. I don't know any model with 3886 in it except some Aiwa 5.1 receiver which had very low price (around 300$).
So is there a way to make this little amp not to hiss, or should i build 3886 amp and listen on it (or insert 3886 in the sony ) ?
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Old 24th December 2006, 10:51 AM   #7
ash_dac is offline ash_dac  United Kingdom
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What's the transformer like ? (torroidal,etc)

I'd say look at the PSU caps and grounding.
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Old 24th December 2006, 12:45 PM   #8
Mx is offline Mx  Yugoslavia
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Since pictures tell you more than thousand words, here they are:
First is just view from the back, and behind the radiator on the right side is transformer (not torroidial).
Second is view from the back of the board which is on left on the first picture. Left on the board is backside of lm1876 and smd components around it.
And the last picture is just close view of the lm1876, power condensers and output inductors.
Any suggestions, now that you can see system inside?
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Old 24th December 2006, 12:46 PM   #9
Mx is offline Mx  Yugoslavia
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second picture
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Old 24th December 2006, 12:48 PM   #10
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