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Old 19th September 2011, 02:23 AM   #11031
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Hi
The temp is normally 25 Degree C in summer and about 15 in winter , with some ( a few days per year) hot days , I will place it on top to improve ventilation , as uaual with all modern apartment , it getting smaller .

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Old 19th September 2011, 02:29 AM   #11032
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Location: Melbourne, Australia.
Default F5 Monoblocks

As I write, the ambient in the vicinity of the F5's is 19 degrees C and the temperature in the middle of the heatsink is 41 degrees. The amps are behind each loudspeaker in a corner. They have been on for a few hours. Had the heatsinks been mounted vertically they might be a degree or two cooler.

I would expect that a heatsink half the size would get to a bit more than twice this temperature which to my mind would be verging on too hot for human safety let alone the occasional inquisitive cat, especially if your room was 30 degrees C in summer. Have a look at the Conrad site as there is usefull info on application of heatsinks. Cooling function of heatsinks is not linear either and their C/W/degree ratings are for an already elevated temperaure. You could use a smaller heatsink if you are prepared to use a fan but quiet fans are scarcer than hens' teeth.

Whatever might be the sonic benefits of running the FET's at higher temperatures I am more than happy with my sound.

As an aside, the temperature compensating elements (thermistors?) in the F5 are often seen mounted in free space where they get a sniff of the ambient in the vicinity of the FET. I chose to mount mine so that they actually touch the heatsink adjacent to where the FET's are mounted. I suppose they would track the temperature better but there may be some obscure reason why I shouldn't have done it this way.

David.
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Old 19th September 2011, 02:41 AM   #11033
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Plus, do you control the room temp to below 30*C?

From the Conrad website, the MF35-75 has a thermal efficiency of 0.32 *C/W at the regulation measuring temp of 80*C, so you have to derate this by about 15% for the 45->50*C you probably want to run it at - so your lower efficiency is now 0.35*C/W.

And with about 70watt dissipation each channel for a standard build F5 (25Vrails, etc) this will raise the heatsink temp by about 25*C (0.35 X 70) IN FREE AIR for each channel, and if you have a room temp control at about the usual 28*C, the heatsink will be about 28 + 25 = 53*C sitting on the floor.
If not, with summer similar to Melb, 35+ is often the case which would make that heatsink pretty hot indeed, (35 + 25 = 60) but not unreasonable.

So this is where the CanAm words question is critical.

Adding 2 x 70 watts to a closed rack isn't a viable proposition without an exhaust fan, and having an open rack in a corner is better, but still...

The Conrad MF35-100 has 0.28 efficiency at 80*C, derated to 0.32*C/w - the MF35-151 has 0.21 derated to 0.24*C/w - you can do your own calculations.

Incidently, the "Elna for Audio" caps (12,000uF/63V) need to stay as cool as you can manage.
Also suggest you get some of those Keratherm fet isolation washers from the Group Buy here - makes a noticeable difference.
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Old 19th September 2011, 03:12 PM   #11034
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Default sudden drop in bias

After playing flawlessly for over a year, one of the channels has developed a problem. The voltage across the output(r11/r12) has dropped to about 250 mv and the offset is about 175mv. The other one is steady around 580mV and 0 offset. The bad channel still plays but with audible distortion.
As I get ready to open it up and troubleshoot, can someone give me some quick pointers to which component may have likely failed causing this? I am sure this information can be gleaned from somewhere in this long thread, but my searches so far has not been able to get at it specifically.

thanks
uv
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Old 19th September 2011, 03:26 PM   #11035
Zen Mod is offline Zen Mod  Serbia
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pull out jfet an mosfet on faulty side and check them
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Old 19th September 2011, 04:53 PM   #11036
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Pull r11 and r12 and check to see they are soil 0.47r - most likely one of these smoked and its resistance is much higher.


Fran
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Old 20th September 2011, 02:25 PM   #11037
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Thanks. Will give these a try.
Could be the input devices. I had accidentally cranked up the preamp to max once when I turned it on recently. It's a tube preamp with a gain of 10 or 11db and it was hooked up to a cd player. So it must've sent quite a few volts through. Incidentally what is the input max voltage for the F5 using 2sk170/2sj74 on the input?
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Old 3rd October 2011, 06:41 AM   #11038
j.leger is offline j.leger  Canada
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Hi, is there any issue with running a 2sk170 with 2sj108? I can't seem to find a j74.
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Old 3rd October 2011, 08:01 AM   #11039
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just go for it
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Old 3rd October 2011, 02:47 PM   #11040
dnewman is offline dnewman  United States
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Quote:
Originally Posted by j.leger View Post
Hi, is there any issue with running a 2sk170 with 2sj108? I can't seem to find a j74.
As of this morning, B&D Enterprises here in the US still had 2SJ74BL as well as matched pairs of 2SJ74BL + 2SK170BL.
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