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Old 5th August 2002, 01:38 AM   #1
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Default Pleas Help!!! PSU for LM3886?

I've built an amp based on the LM3886 chip, and I'm quiet pleased with it. Now I'm thinkin of taking a more seriouse approch and build a couple of monoblocks for surround use, based on the 3886.

So, my quesion is; How large torodial do I need for it to sound good. I used one 250 W torodial xformer/channel in my previous amp, with 6800 micro Farad/ rail.

Is this going to be sufficient or will a larger psu with more caps improve the sound?
I also don't know weather to use 2 bridge rectifiers/powersupply or juste one. That is, if it will make any difference?

Any answers are very welcome,
Best Regards
/Christian V
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Old 5th August 2002, 02:01 AM   #2
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Default Dual Secondaries.

Hello Christian,
Although not specific to the LM3886, running seperated power supplies gives good benefits IME.
By this I mean if you have seperate secondaries (as opposed to CT secondary), use a bridge rectifier for each secondary, and make the earth common close to the amplifier stage.
This will give double the supply refresh rate WRT CT secondary and single bridge, and the PSU common is at the amplifier and not the transformer.

Regards, Eric.
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Old 5th August 2002, 03:11 AM   #3
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Default Re: Pleas Help!!! PSU for LM3886?

Christian,

The Amp Chip DIY Forum is expressly concerned with the building of gain-klone/chip amps. There is a lot of collected experience already posted there.

The usually recommended supply is a 160 VA transformer, with 1,000 uF of cap per rail.

dave
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Old 5th August 2002, 03:40 AM   #4
sam9 is offline sam9  United States
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I used a 160VA toroid fro Avel-Lindberg to power a pair of LM3886s. An ordinary 25A bridge and 4 4700UF caps plus the usual additional components. Seems more than adequate.
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Old 5th August 2002, 03:42 AM   #5
sam9 is offline sam9  United States
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I forgot to mention, I believe the LM3886 has an upper limit on voltage of about 40V. So choose your transformaer accordingly.
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Old 5th August 2002, 03:59 AM   #6
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Default Re: sam9

Quote:
Originally posted by sam9
I forgot to mention, I believe the LM3886 has an upper limit on voltage of about 40V. So choose your transformaer accordingly.
When choosing the ultimate rail voltage have a look at the voltage vrs power output curves. On the 1875 & the 3885 these flatten out for 4 ohm loads at about 25 V rails. Anything higher and the amps ability to drive complex loads (ie almost any speaker) starts to fall off.

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Old 6th August 2002, 01:26 AM   #7
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So it will be fine with just a 150 W torodial/channel and 10*100 micro Farads per rail. I'd thought I would use many smaller caps because they will be faster, won't they?

My second question concerns the input without a pot, because these monoblocks are supposed to pure power amps.
In my last amp I used a pot, but I noticed that If you leave it out there will be a hiss or light hum to any sound amplified.

So I thought on putting a large resistor over the signal to ground, just so it won't amplify at full level ( maybe 8 kOhm or so).
Is this a good idea?
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Old 6th August 2002, 01:49 AM   #8
JoeBob is offline JoeBob  Canada
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You shouldn't need to do anything really besides removing the pot from the circuit. I've got an LM3875 power amp in service being used by my parents and it shows no hiss or hum at all.
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