LM1785 - enclosure and heat sink

Leolo · 5th January 2010, 05:59 PM

I'm planning on building a LM1785-based chip amp, once chipamp.com is back with the living. I have a few questions:

Everybody seems to be making enclosures out of aluminum and wood, very modern design looking. I find this boring. I want to do something steampunk inspired: copper tubbing, brass, stained oak and leather, maybe some mapple/cherry inlay.

1- Does the enclosure need to be shielded? I have a roll of shinny metalic tape with a peel-off back. DVM reading of 0 ohms, so I could use that to line the enclosure, if needs be.

2- The LM1785 datasheet talks about a 1.2 C/W heat sink. These seem to be on the $$ side. Also, not very elegent sticking outside of the case. I have access to a large sheet of 1/4 inch aluminum. Would a largish piece of this (say 17x4-5 inches) be adequate? I'm thinking of putting it cross-wise on a slant with many air holes or fretwork top and bottom.

3- To isolate the heatsink, I found the BER168-ND on digikey, which seems to be too good to be true, price-wise and heat transfer-wise. Are these OK?
Digi-Key - BER168-ND (Manufacturer - HF115TAAC-58)

Thank you all,

-Leolo

dave_gerecke · 6th January 2010, 03:25 AM

Bonjour Leolo, shielding on an amp is more dependent on the environment than anything else. If you have a location that has a lot of EM energy, then you may be best to shield it. Also, if you find the amp for some reason puts out some RF interference, then you should shield it. It would also be good to figure out why the amp emits RF.
If you are going for a steampunk sort of appearance, use copper as the heatsink for the amp chip. Copper is better at pulling the heat away than aluminum. If you really want to make it pull the heat, have the chip on a copper plate inside of the amp, direct contact between the chip and the copper heatsink. Then figure out a way to pull the heat out of the case that also electrically isolates the copper heatsink from the outside. Keep in mind here that if you do this the copper plate that the chipamp is mounted to will be electrically live, so you will need to provide electrical isolation from any other metals in the enclosure. You will also want to be careful when poking around inside the amp.
If you do go the conventional route, those thermal pads seem to have a pretty good reputation. Bergquist has been at it for a while.

Peace,

Dave

N-Brock · 6th January 2010, 05:18 AM

Are you talking about the LM1875? I'm not sure but I think the 1875 is what's offered at chipamp.com
I built one of the kits inside of a large power resistor. Not sure if this is what you're looking for, but I think it looks pretty neat. The power supply is separate, an advantage and drawback, less interference but two boxes.
I have had no problems with outside interference although I do live in the country. Here are some pictures of the amp. Turning a power resistor into a power amp (sorta)
My kit included isolators for the chips but they were rather thick, maybe not the best. Still, the chips and sink get just pleasantly warm running off +/- 18v.
Be sure to post pictures of your build and completed amp.

Leolo · 6th January 2010, 06:27 AM

Quote:

Originally Posted by dave_gerecke

Bonjour Leolo, shielding on an amp is more dependent on the environment than anything else.

How would I find out if my amp or environment is giving out EM energy? I suspect the answer is going to be "build it an listen".

Quote:

Originally Posted by dave_gerecke

If you are going for a steampunk sort of appearance, use copper as the heatsink for the amp chip.

Copper is also much more expensive than aluminum. And using a fan is 100% not an option. I was hoping to keep the heatsink hidden. But I'll look into various possibilities.

Leolo · 6th January 2010, 06:55 AM

Quote:

Originally Posted by N-Brock

Are you talking about the LM1875?

ACK! Yes, of course I mean the LM1875, but now my dyslexia has made me look like a fool.

Quote:

Originally Posted by N-Brock

I built one of the kits inside of a large power resistor.

Woah! That is really nice!

jrn77478 · 6th January 2010, 09:46 AM

Quote:

Originally Posted by Leolo

1- Does the enclosure need to be shielded? I have a roll of shinny metalic tape with a peel-off back. DVM reading of 0 ohms, so I could use that to line the enclosure, if needs be.

2- The LM1785 datasheet talks about a 1.2 C/W heat sink. These seem to be on the $$ side. Also, not very elegent sticking outside of the case. I have access to a large sheet of 1/4 inch aluminum. Would a largish piece of this (say 17x4-5 inches) be adequate?

Steampunk is cool. Be sure to post pics as you build!

Shielding is a good idea, especially between power supply and amp sections. If your design ideas permit, housing them in separate enclosures is a good choice.

If you keep the supply voltage reasonable, that area will be more than enough heat sink. Mine is a 10x10 inch sheet and it keeps the chips happy.

91art · 11:05 AM

The best enclosure I have seen for 1875,the guy who is worker use cnc build this.

91art · 6th January 2010, 10:59 AM

more detail

5th January 2010, 05:59 PM	#1
Leolo diyAudio Member Join Date: Dec 2009 Location: Quebec	LM1785 - enclosure and heat sink I'm planning on building a LM1785-based chip amp, once chipamp.com is back with the living. I have a few questions: Everybody seems to be making enclosures out of aluminum and wood, very modern design looking. I find this boring. I want to do something steampunk inspired: copper tubbing, brass, stained oak and leather, maybe some mapple/cherry inlay. 1- Does the enclosure need to be shielded? I have a roll of shinny metalic tape with a peel-off back. DVM reading of 0 ohms, so I could use that to line the enclosure, if needs be. 2- The LM1785 datasheet talks about a 1.2 C/W heat sink. These seem to be on the $$ side. Also, not very elegent sticking outside of the case. I have access to a large sheet of 1/4 inch aluminum. Would a largish piece of this (say 17x4-5 inches) be adequate? I'm thinking of putting it cross-wise on a slant with many air holes or fretwork top and bottom. 3- To isolate the heatsink, I found the BER168-ND on digikey, which seems to be too good to be true, price-wise and heat transfer-wise. Are these OK? Digi-Key - BER168-ND (Manufacturer - HF115TAAC-58) Thank you all, -Leolo

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6th January 2010, 03:25 AM	#2
dave_gerecke diyAudio Member Join Date: Dec 2007 Location: Vermont	Bonjour Leolo, shielding on an amp is more dependent on the environment than anything else. If you have a location that has a lot of EM energy, then you may be best to shield it. Also, if you find the amp for some reason puts out some RF interference, then you should shield it. It would also be good to figure out why the amp emits RF. If you are going for a steampunk sort of appearance, use copper as the heatsink for the amp chip. Copper is better at pulling the heat away than aluminum. If you really want to make it pull the heat, have the chip on a copper plate inside of the amp, direct contact between the chip and the copper heatsink. Then figure out a way to pull the heat out of the case that also electrically isolates the copper heatsink from the outside. Keep in mind here that if you do this the copper plate that the chipamp is mounted to will be electrically live, so you will need to provide electrical isolation from any other metals in the enclosure. You will also want to be careful when poking around inside the amp. If you do go the conventional route, those thermal pads seem to have a pretty good reputation. Bergquist has been at it for a while. Peace, Dave

6th January 2010, 05:18 AM	#3
N-Brock diyAudio Member Join Date: Oct 2007 Location: Cambridge, MA	Are you talking about the LM1875? I'm not sure but I think the 1875 is what's offered at chipamp.com I built one of the kits inside of a large power resistor. Not sure if this is what you're looking for, but I think it looks pretty neat. The power supply is separate, an advantage and drawback, less interference but two boxes. I have had no problems with outside interference although I do live in the country. Here are some pictures of the amp. Turning a power resistor into a power amp (sorta) My kit included isolators for the chips but they were rather thick, maybe not the best. Still, the chips and sink get just pleasantly warm running off +/- 18v. Be sure to post pictures of your build and completed amp.

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