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Old 22nd March 2013, 01:41 AM   #851
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Default Alternate resistor possibilities

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Originally Posted by xrk971 View Post
I will check out the FETAudio but it looks like a private party type sale? What about the resistor value change for the mu follower network? I can't believe 3W resistors are that hard to keep in stock. Also, has anyone tried a generic low noise JFET n-channel 400 mW TO-92 transistor from an "in-stock" part at Digikey as a replacement for the LSK170? I want to order all my stuff from Digikey in one swoop.

Digikey has nothing like it.

I just found it in stock at Mouser for $0.47 ea.
-40V .01A .4W Dgs Toshiba Fet TO-92 | TOSHIBA *SS | Distributed By MCM
You might consider using one higher wattage resistor of half the value (two .68 ohm in parallel equals .34 ohms). Digikey has a .33 ohm 7 watt wirewound here: AC07000003307JAC00 Vishay BC Components | PPC7W.33CT-ND | DigiKey. Not sure if it will fit but you might find another that would.
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Old 24th March 2013, 12:48 AM   #852
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With 19v supply and 1 amp bias is 19w per channel.
If resistors R1,R2,R3 and R4 are same as original schematic the current bias will be 1.13 A. The two mosfets Q1 and Q2 would contribute a total of about 20.7w of heat to the heat sink (for one channel). The remaining heat being lost by the resistors R1 thru R4.
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Old 24th March 2013, 01:12 AM   #853
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In a pinch, R3/R4 can safely be replaced with a single 0.33 ohm 2W flameproof resistor. The dissipation at idle is less than half a Watt. R1/R2 can be replaced with a 0.22 ohm 2W. It's always a good idea to mount power resistors up away from the PCB so they won't discolor the board.
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Old 24th March 2013, 05:50 PM   #854
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I ended up ordering the LSK170's (only type B available, hope that works) from Linear systems direct.
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Old 24th March 2013, 06:47 PM   #855
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You want the BL grade. If Idss is below about 4.5mA, you won't be able to set the quiescent output voltage.
The "BL" grade of 2SK170 is same as "B" grade for LSK170, so you are good.
BTW, when I ordered the 0R68 3W resistors last October from Digikey they were out of stock also. I just had to wait a couple of weeks for them to restock and send order.
Hope you decided to go with a regular passive heat sink instead of water tank thingy.
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Last edited by cochleus; 24th March 2013 at 06:53 PM.
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Old 25th March 2013, 01:46 AM   #856
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Hope you decided to go with a regular passive heat sink instead of water tank thingy.
xrk971,

Sorry for the smart*** remark.

I considered water cooling also, and did the design calculations, etc.

I ended up with a passive heat sink. If you want info on heat sink sizes let me know.

Regards,
cochleus
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Old 25th March 2013, 02:29 AM   #857
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Quote:
Originally Posted by cochleus View Post
xrk971,

Sorry for the smart*** remark.

I considered water cooling also, and did the design calculations, etc.

I ended up with a passive heat sink. If you want info on heat sink sizes let me know.

Regards,
cochleus

What's wrong with my Amp Camp Tea maker anyway ? I just think it would look kind of wacky and cool. I should get a teapot and bolt the MOSFETs to it, then I can fill with water and it won't spill or splash and i can pour it out to have some hot water for a cup of tea! Not sure if 40 watts will be warm enough....

I looked at passive heat sinks and if went that route I will probably use PC CPU cooler heat sink. I could order extruded heat sinks, I think they sell them by the inch and it would end up costing maybe $20 for two.

Where did you end up buying your heat sinks from and what size did you get?
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Old 25th March 2013, 02:35 AM   #858
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Quote:
Originally Posted by cochleus View Post
The "BL" grade of 2SK170 is same as "B" grade for LSK170, so you are good.
BTW, when I ordered the 0R68 3W resistors last October from Digikey they were out of stock also. I just had to wait a couple of weeks for them to restock and send order.
Hope you decided to go with a regular passive heat sink instead of water tank thingy.
Glad I accidentally got the correct ones. It was not clear from the instructions from Nelson Pass which ones to get. So you don't want quiescent current too low then? Maybe someone can explain what the requirement is and how you figured that out.
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Old 25th March 2013, 02:56 AM   #859
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Quote:
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i can pour it out to have some hot water for a cup of tea!


Where did you end up buying your heat sinks from and what size did you get?
Hot beverage? Hmmm, now that sounds useful.

The DIYAudio Store had heat sinks, you can get those if they are in stock, or get the size info. My nephew and I are building the Amp Camp and we are putting both boards on one heat sink. It is a 5 inch length of 10.08 inch profile from HeatSinkUSA. I have a 6 inch piece of that profile on a different amp that stays 22.6'C above ambient (measured next to mosfet) with 45.5W heat input.
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Old 25th March 2013, 03:40 AM   #860
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I bet I am the only one to ever think of a combo amp and tea maker! Just because it is so cool (or warm ) I have to make it now. Just to do it. Kind of like I had to make a foam core amplifier so that I have an all foam core audio system....
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