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Old 25th November 2010, 11:12 AM   #701
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nah, I can visualize the fets and resistors pressed down against the board..

Perhaps the one sink is the way to go, and I'll case the boards upside down. I need square footage for the toroids. Gravity says they want to be on the bottom! My wallet is saying it doesn't want a sink large enough for everything on the bottom. My eyes are saying it needs to look pretty atop my Aleph.
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Old 25th November 2010, 01:23 PM   #702
opc is offline opc  Canada
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Hi Guys,

The basic board design allows quite a bit of flexibility, so there shouldn't be a need to run wires to side mounted fets. If you absolutely have to, then it will work, but I'm not sure how well. Keep in mind that although this dissipates as much power as an amplifier, it's still a low level circuit, and should be treated as such.

I've attached a sketch of exactly how I will be mounting my I/V in a chassis. You'll have to excuse the terrible perspective drawing, but it gets the point across.

I have two 4"x12"x1" heatsinks that are going to be the sides of the chassis. I'll be using a 1/4" thick aluminum base, and connecting it to the two side heatsinks with 1" aluminum bar stock. I have done this in the past, and it results in a chassis that is both solid, heavy, and transfers heat quite well to the side sinks. I'll be using three bolts into the heatsink, three holding the base to the bar stock, and then I slather both joints with heatsink compound and bolt it all together. If you have a non-ferrous blade and a table saw, you could build something like this in an afternoon for less than $50.

I'll probably be mounting my transformers either on the front plate, or on the base depending on how much room is left. Either way works.

As for mounting the board itself to the heatsink, it's pretty straightforward. Either use thermal pads (thin ones) or use mica pads and thermal grease. The Ohmite resistors are exactly the same height as the FQA fets, which works really nicely. They don't have mounting holes though, which means they need to be mounted such that a bolt can still be put in above them.

For the Caddock resistors, I used a scrap piece of FR4 PCB as a shim, which makes them exactly the same height as the fets. I drilled a hole in it, and glued it to the top of the resistor. Use a good amount of clamping force to hold all the parts down.

For mounting, I just took the PCB before mounting any PTH parts, and traced the holes with a pencil. I then used a centre punch to set each hole, and then drilled an appropriate size hole for the tap. You can use either M3 or 4-40 hardware. M3 is a perfect fit, which requires accuracy in drilling and tapping, and 4-40 is a loose fit which gives some wiggle room if your holes aren't exact.

Cheers,
Owen
Attached Files
File Type: pdf D1 chassis.pdf (127.2 KB, 226 views)
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Old 25th November 2010, 01:56 PM   #703
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ahh.. now I get it! Here I was picturing rubber feet mounted on a heatsink.

That's a no-sweat winner. Thanks,
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Old 26th November 2010, 02:53 PM   #704
qusp is offline qusp  Australia
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guys, anyone who is getting R's etc through me that hasnt got themselves the 200R caddocks and wants me to grab them for them with my digikey order, please drop me a line in the next hour or 2, i'll be in bed by then and would like to sort this out tonight. sorry for the lack of notice
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Old 26th November 2010, 04:25 PM   #705
tkwou is offline tkwou  Canada
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Hi qusp

I noticed from your previous post that you had to buy more JFET's due to being shorted. Could I get on your matched FET list? I got the SMD package from Owen but I haven't got the 200R Caddocks. Could I get in on your order? Thanks.
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Old 27th November 2010, 03:02 AM   #706
qusp is offline qusp  Australia
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I emailed u pretty much streaight after you posted this last night, what happened? I have covered your resistors in my order, please check your email and PM's
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Old 27th November 2010, 12:45 PM   #707
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Dear Qusp,

I had one of the leftover board of the group buy. Can I also get the extra components ?

Thanks,

Davide
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Old 27th November 2010, 03:08 PM   #708
Horio is offline Horio  United States
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That's great news Qusp (for those of us who are hoping to get in on the GB). Hopefully I am already on the waiting list...
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Old 27th November 2010, 03:18 PM   #709
qusp is offline qusp  Australia
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yes mate, your sorted, still cant know for certain how many precision matched fets I can get, but there will be devices either way, if not ideal match, probably still better than buying precision matched from elsewhere, because they will be matched at the correct operating point. pretty sure its all good though, you would hope so matching out of 170 fets
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Old 27th November 2010, 03:38 PM   #710
opc is offline opc  Canada
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qusp:

You won't have a problem finding matched pairs with a sample size that large. Keep in mind that you're not actually matching to a set value, you're just matching between devices, so I wouldn't be surprised if you were able to use at least 150-160 of the 170 devices. If you want to go to the extreme, then you could match into groups of four, but channel to channel variation will likely be dominated by the resistors, and not the fets themselves. Just the same, if you're using 0.1% resistors everywhere, then you could probably make an argument for matching all 4 devices carefully.

If you're running dual mono, then you should definitely match all four devices.

You'll get a feel for it when you start matching, but I expect you'll see pretty good results. Just be meticulous with labeling each fet, and noting the values you measure in a spreadsheet. It's best to hold off on matching until you've measured everything you have. Then use the spreadsheet to separate the parts into groups.

Cheers,
Owen
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