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LM3886 Chipamp Kit and optional Ci (feedback capacitor)
http://diyaudioprojects.com/Forum/viewtopic.php?f=10&t=366
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Author:  the.sabs [ 06 May 2009, 16:15 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

18V. then to a pair of 8ohm drivers.

Author:  Gio [ 06 May 2009, 21:25 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Ok, so 18VAC is about 25VDC once rectified. Therefore your Vcc is 50V (|V+| + |V-|). From the plot, go up from 50V till you hit the 8 ohm line and then go across and you will see Pd is about 16-17W. Keep going across and if you guess it will be about 40°C or 50°C in the enclosure, you can see that you will want a heatsink with a maximum rating of about 5°C/W.

So your sink is too small.

Author:  the.sabs [ 07 May 2009, 16:13 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Hey,
Took me a bit to get that info logged, but with you 100% now. Good eyes gio. Off to RS for new sinks.
Laters

Author:  the.sabs [ 02 Jun 2009, 14:27 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

i think maybe i went mad on the sink!!!! is this too big??? or could i attach two chips to one sink?
Attachment:
LM3886-Chipamp-PCB-Heatsink.jpg

Author:  RiGuy [ 02 Jun 2009, 18:00 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

That sink looks fine for a LM3886. I KNOW you wont kill your chips if you put 2 properly insulated chips on there. The sink might get hot depending on the load as far as Ohms and sensitivity and listening level. So try just one channel on it and push it hard for a while. If it gets HOT like 5-10seconds with hand on the sink and then you cant stand it then you are fine with one chip per sink. If it gets warm and not hot then pop the other on. Its not to sophisticated but really what we want is a chip thats not burning up right? My bet is its fine for 2.
RI

Author:  Gio [ 02 Jun 2009, 20:33 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Hi Sabs,
Do you have a rating for the sink? It is definitely big enough for one chip. If you mount that sink outside of the enclosure you may be able to put both chips on it. Can you should us a photo with the enclosure and where you were thinking of mounting the heat sink.
Cheers

Author:  the.sabs [ 04 Jun 2009, 16:01 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Ok got some time to have a think and take some pics.
This was my original layout, imagine that there are wires an all, this is just a lay out of transformers, psu, the preamp and the two channel LM3886.
I was planning on putting a sink on each of the board, but these were too small as gio saw.
Attachment:
P6040016.jpg

This with the top on. I chose a ventillated top to aid convection
Attachment:
P6040017.jpg

With the new sinks (that are quite a bit larger) this is one inside the case with the cover on. There is just enough room.
Attachment:
P6040018.jpg

So i could use a sink on each channel, but i'm wondering is the psu will be too close or if it's overkill. I have space as the case is 45x39cm and 2U high.
Attachment:
P6040019.jpg

Ri said there was a possibility of putting the two channels on one sink. I thought like this.
Attachment:
P6040021.jpg

And in the case thus so. Have on channel upside down, towards the cover.
Attachment:
P6040022.jpg

How does this look? and is one way better than another?
cheers
sabs

Author:  the.sabs [ 04 Jun 2009, 16:09 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Sorry gio,
the spec on the sink is:
Thermal Resistance R 1.3°C/W
Length 75mm
Width 120mm
Height 37mm

Author:  RiGuy [ 04 Jun 2009, 17:07 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Just from guesses I think you are on the edge of needing two sinks. Without doubt that sink is fine for one channel. I would say, if you want to save sinks, try both on one. If it goes into protection mode (you will think it is dying) then try two. You could of course play it safe and use both. I dont see why not since you have both. But I feel you will probably get away with just one. Sorry, but what is the efficiency of your speakers? Dont know if you posted this before.
Ri

Author:  Gio [ 04 Jun 2009, 20:41 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

You may be able to do both chips on the one sink. If you want to try that, you shoulf test it out before you start to put the enclosure together.

The ventilated top is good. Drilling holes on the bottom plate right below the heatsink will also help.
Cheers

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