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Chip Amps Amplifiers based on integrated circuits

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Old 28th May 2010, 06:34 PM   #41
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Quote:
Originally Posted by regiregi22 View Post
I didn't knew that, why C17, C18, C19 and C20 are not being used and supplied with the kit?
As I've said several and several times:

that caps are NOT part of Mauro Penasa's design (added later by TP), he thinks they disrupt the MyRef 'floating' rails and most important:

it sounds better without...
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Old 29th May 2010, 10:46 PM   #42
billo44 is offline billo44  United Kingdom
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Default Mounting chip to heat sink

Just a quick note to ask if there would be a problem using an aluminiumk shim pad between the chip and the heat sink proper?
Even with relatively simple tools (small hacksaw, file, drill) it would be possible to make a shim, I would use a section slightly bigger than the chip, cut from one face of a piece of extruded aluminimum angle, which is pretty flat both sides. Plenty of heat sink compound on both faces.
I can't open Bill P's pictures from post 31 (sadly), but if the chip will mount with the legs bent 'a bit' then 3mm or 1/8" (3.17mm) should give plenty of clearance...
Even half that perhaps...

Thanks everyone for all the good input...

Bill O

Last edited by billo44; 29th May 2010 at 10:57 PM. Reason: speelling ewrrors!
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Old 30th May 2010, 02:25 AM   #43
Bill_P is offline Bill_P  United States
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Originally Posted by billo44 View Post
Just a quick note to ask if there would be a problem using an aluminiumk shim pad between the chip and the heat sink proper?
Even with relatively simple tools (small hacksaw, file, drill) it would be possible to make a shim, I would use a section slightly bigger than the chip, cut from one face of a piece of extruded aluminimum angle, which is pretty flat both sides. Plenty of heat sink compound on both faces.
I can't open Bill P's pictures from post 31 (sadly), but if the chip will mount with the legs bent 'a bit' then 3mm or 1/8" (3.17mm) should give plenty of clearance...
Even half that perhaps...

Thanks everyone for all the good input...

Bill O
You can use a shim but it adds an extra thermal interface that degrades the heat flow path. To make the degradation small the shim must be very flat with no burrs. Heat sink compound goes on one face, the one that mates to the heatsink. A little compound goes a long way - apply too much and the thermal conduction becomes worse. The shim side facing the LM3886 needs to be electrically insulated. No thermal compound used there - use the insulator in the kit. You really need an insulating shoulder washer so the mounting screw does not short the LM3886 tab to the heatsink. Some people use nylon screws to get around the shorting issue but they are not capable of anywhere near the torque that should be applied in mounting the LM3886. Use a real screw, the right insulators, and check for short circuits with an Ohmmeter when you are done.
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Old 30th May 2010, 04:25 AM   #44
billo44 is offline billo44  United Kingdom
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Default Mounting chip to heat sink

Thanks Bill P,
I have to confess I have a couple of insulated LM3886 lined up for this build.

Looking at your very clean build Bill P (I can see the pics now) it looks like an insulated chip might actually have just enough extra thickness?
Has anyone tried it yet?

Still I am sure there are any number of good solutions for this mounting issue.

Bill O
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Old 30th May 2010, 04:43 AM   #45
billo44 is offline billo44  United Kingdom
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Default Caddock at R3

The MP915-0.5-1% caddock seems to have quite long legs.
Is it a crime to leave them long and mount further off the board to give more space to mount the sink?

Bill O
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Old 30th May 2010, 12:32 PM   #46
Bill_P is offline Bill_P  United States
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According to the National datasheet for the LM3886 the distance from leads to the back of the chip is 4.29mm for both the T and TF packages.

The heatsink has a hole for mounting the Caddock so the position of the resistor on the heatsink is determined. If you want to raise the whole heatsink off the PC board and support the whole assembly with the resistor leads, I don't recommend that. As it is the assembly is wobbly with the back of the heatsink resting on the board. Once the amp is finalized here I plan to put a bead of glue along the back of the heatsink where it touches the board to stabilize the assembly against vibration.
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Old 30th May 2010, 10:13 PM   #47
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Originally Posted by Bill_P View Post
The heatsink has a hole for mounting the Caddock so the position of the resistor on the heatsink is determined. If you want to raise the whole heatsink off the PC board and support the whole assembly with the resistor leads, I don't recommend that. As it is the assembly is wobbly with the back of the heatsink resting on the board. Once the amp is finalized here I plan to put a bead of glue along the back of the heatsink where it touches the board to stabilize the assembly against vibration.
Unless you drill it. Might be a quick solution. I haven't got the heatsink in my hands yet, though.
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Old 31st May 2010, 12:34 AM   #48
Bill_P is offline Bill_P  United States
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Originally Posted by regiregi22 View Post
Unless you drill it. Might be a quick solution. I haven't got the heatsink in my hands yet, though.
The heatsink is made of 1.25mm thick soft aluminum and the greatest care must be used if you drill it yourself. With such thin material it would be easy to ruin the surface flatness. There also must not be a burr left around the newly drilled hole. As with the LM3886, the Caddock resistor needs an interface between it and the heatsink - either a thermal pad or thermal grease.

The position of the existing hole in the heatsink is good enough so I used it to mount the resistor.
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Old 31st May 2010, 12:03 PM   #49
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Thank's for your reply Bill, I will keep that in mind. I will see what to do when I have them here
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Old 1st June 2010, 08:53 AM   #50
shark70 is offline shark70  United Kingdom
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Got my parcel, very well packed Uriah, THANKS you are a gentlemen. I will be getting stuck into it later
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