| jduffy |
I'm trying to get the parts together to build a Gainclone amp. Have the caps and resistors already, but I need the transformer and the Nat Semi LM3875TF. No luck finding a brick and mortar store yet in the Los Angeles area with this items.
Any ideas out there? I've asked the people at a couple of electronic parts stores but they don't know of any places.
Thanks for the help. |
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| CarlosT |
| Who sells Bicron toroidals? They make plain vanilla 60hz toroidals. |
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| grimberg |
Jduffy,
I live in the O.C. and have the parts you are looking for. I am not a merchant, just have extras. They are new and were purchased from reputable companies. If you are interested just send me an e-mail. |
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| jduffy |
Sorry but I can't send email because I'm a new member. I have to wait until I have X number of posts first.
Thanks for the offer though. |
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| jackinnj |
The fellow who sells torroids on EBay is worth a visit -- AnTek -- these are stepper motor/CNC toroids -- obviously from China and obviously "very strong stuff" -- put toroid in the EBay search engine. Bringing transformers from New Jersey to California is like coals to newcastle, however. AnTek also sells nice aluminum and steel cases.
As a newbie, I would try to dissuade you from using the TF version of the LM3875 or LM3886 -- reason being that the insulation is a real heat barrier. While the insulation makes it more convenient to use, the thermal impedance is something like 1 degree C/ watt. This translates to a whoppingly bigger heat sink for the same amount of power dissipation. I have found no difference between the T and TF versions when listening (but others have differing opinions.)
Try using the mica insulator vresion with silicon grease on each side -- you can use pretty high mounting pressure with these two chips. (something you can't do with the LM4780.) |
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| jduffy |
Thanks for the help.
A guy hooked me up with a nice transformer and a couple of the LM3875s. I went with the insulated variety but not a problem cooling. I was an overclocker back in the day and have plenty of heatsinks.
It took a couple of hours to build but no major hassles. It was crazy simple wiring it up.
Not a bad sounding amp, particularly when you figure it cost me about $65 and I had fun building it. Best part is that I can keep tweaking the thing when I get bored with it.
Good times! |
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| Spasticteapot |
| quote: | Originally posted by jduffy
Thanks for the help.
A guy hooked me up with a nice transformer and a couple of the LM3875s. I went with the insulated variety but not a problem cooling. I was an overclocker back in the day and have plenty of heatsinks.
Good times! |
I wonder when we're going to see the first watercooled gainclone?
Just curious, but why not use computer-grade thermal paste, like Arctic Silver? Computer overclockers swear by the stuff, and with heavy thermal loads (like a power amplifier or Prescott), it's been show to drop the temperature by a few degrees. |
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| falcott |
| quote: | Originally posted by jackinnj
I would try to dissuade you from using the TF version of the LM3875 or LM3886 -- reason being that the insulation is a real heat barrier. While the insulation makes it more convenient to use, the thermal impedance is something like 1 degree C/ watt. This translates to a whoppingly bigger heat sink for the same amount of power dissipation. I have found no difference between the T and TF versions when listening (but others have differing opinions.)
Try using the mica insulator vresion with silicon grease on each side -- you can use pretty high mounting pressure with these two chips. (something you can't do with the LM4780.) |
How much "whoppingly bigger" heat sink, I wonder? I'm building an amp using LM3875TF chips (bought back in the days of blissful ignorance) and will be running them on 35V. They are to be mounted (about 40mm apart, centre to centre) on a slab of aluminium measuring 12mm thick x 340mm long x 100mm deep (about 1/2" x 14" x 4"). There are no fins. Should this be whoppingly big enough?
If not, I have a finned aluminium heatsink that I am considering whether to mount on the back of that heatsink... :xeye:
I wonder also if clamping the chip by a piece of aluminium across the printed face, rather than using the mounting hole, will add to the cooling of the chip?
CarlosT, those stick-on RAM sinks look very cool!
(pun intended ;) )
Edit: Sorry to thread-jack! |
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| jackinnj |
| quote: | Originally posted by falcott
How much "whoppingly bigger" heat sink, I wonder? I'm building an amp using LM3875TF chips (bought back in the days of blissful ignorance) and will be running them on 35V. |
At +/- 35V The thermal impedance required for a LM3875T is 2.90 c/w, while the TF is 1.90 -- the heat sink's required surface area is about 70% greater, as a wet finger in the air guess.
You can figure this out by "backward engineering" the AavidThermalloy thermal resistance tool -- plug in values for length, width of the HS base, and height of the fins -- then play around with the number of fins required to arrive at the correct c/w. use 32 LFM for normal convection. The surface area is going to be a simple alegbraic function of the number of fins. I don't think that the Aavid Thermalloy tool takes into account bypass, or turbulence etc., |
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| Spasticteapot |
I'm running a LM3875TF at about 28v (off a rectified 20-0-20 transformer). A socket 370 computer heatsink looks to be plenty of cooling.
Those RAMsinks look perfect for an Amp3 or T-amp - just put two or three of them next to each other. |
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| CarlosT |
| How about an LM3875TF mounted to a plain Hammond aluminum box? I was thinking of than along with one of those RAM stickons on the other side. I'll be running 25-0-25 into a 4 ohm load (single LM3875 powering a single 4 ohm driver). |
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| falcott |
| quote: | Originally posted by jackinnj
At +/- 35V The thermal impedance required for a LM3875T is 2.90 c/w, while the TF is 1.90 -- the heat sink's required surface area is about 70% greater, as a wet finger in the air guess.
You can figure this out by "backward engineering" the AavidThermalloy thermal resistance tool ... |
Thanks! I did a google search and found it.
http://www.aavidthermalloy.com/technical/thermal.shtml
It's more useful than a wet finger on a hot chip, I would say! :hot: :clown:
Joining the heatsink to the aluminium slab (mentioned above), the combined surface area gives 0.814 deg C/W. I'm guessing that this is enough for a pair of hard-working TF cases...?
If 2 chips require double the heat dissipation of 1, do we just halve (double?) the C/W rating? I.e. if 1 chip = 1.90 C/W, do 2 chips = 0.95 C/W?
Again thanks! :) |
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| jackinnj |
you can double it for 2 devices -- but where you place them on the sink will affect the hs performance.
wrt using a slab of aluminum -- it is only conducting heat to the extent of its surface area and the flow of air over it -- the rest of the heat is stored in the material -- the problem with this type of heat sink is that it also heats the device you are trying to cool by virtue of its stored energy. |
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| CarlosT |
| quote: | Originally posted by jackinnj
At +/- 35V The thermal impedance required for a LM3875T is 2.90 c/w, while the TF is 1.90 -- the heat sink's required surface area is about 70% greater... |
Doesn't that fact that you need to add an insulating washer to the "T" version sorta cancel out with the perceived advantage over the "TF" version which does not need an insulating washer? |
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