aleph 3 bias and heating problem

Dear fellow DIYers, my name is panos from Athens, Greece and i am the happy owner of an aleph 3 amp based on a copy of the schematic(aleph 5) Mr Pass sent me 2 years ago. The sound is objectively very natural and sounds like a huge SET amp with better dynamics and attack. I also designed a nice pcb for it, able to accomodate 3 pairs of IRFP240s biased at 1amp total adjustable by a precision trimmer of 220k in parallel with R19.
The power supply consists of a 680w toroid and 2X 100.000ìf comp grade caps. The rail voltage is +/-27V after 1 hour warm up of the amp. My problem is heat generated reaching 60C with ampient temp 24C. The heatsings are massive(thought so) but cannot raise the bias beyond 1 amp per channel. Dimensions each of the 2 HSs are:L42cm x W18cm x H3.5cm , each having base plate of 1cm and fins 2.5cm, number of fins 22 each HS.
I was wondering if someone could come up with any kind of proposal concerning the increase of bias curent to 1.5 or 2 amps per channel, i can accept also any proposal concerning the estimated thermal resistance of these HSs, and any proposal conserning changes in the active current source so that more curent will be available to demanding music passages as i thing distortion comes into play a bit early for the supposed wattage. Any help would be more than welcome, i am able to provide also pics of the amp itself.
I would also like to thank once more Mr Pass for his precious articles as he helpmed me as others too learning the basics on amp design and construction, THANKS AGAIN Mr PASS!
 
Panos,
There's nothing for it...Alephs run hot. I was unable to find heatsinks large enough to do the job for any reasonable sum of money, so I went for a water-cooled strategy to get rid of the heat.
The distortion could very well be due to the low bias current. When you hit a peak, the amp runs out of steam--clips. The cure here will be to get the bias up.

Grey
 
Dear friend , I thought so but since i found this site with other pass diy freaks as me, i hopped that maybe some body would have more info than i have right now, maybe i will rethink the option of installing a blower to the heatsinks which are really massive if u checked the dimensions. Do u have any idea about their thermal rating as my apms equilibrium is about 54W each?
Thank u in advance for your attention :)
Panos

PS. wowow!!! i think that diyaudio.com is my dream site !!! Never thought tha so many people around the globe are funs of Pass designs and more over DIYers!!!
 
yep u are correct! I actuall started my interest on the aleph topology when i had an aleph 5 at home for some listening and i was amazed, but the heat build up on the 5 was semi comfortable to touch, moreover my heatsink area seems equal or a bit less than the total of aleph while aleph5 idling at double wattage than my clone. then i decided to built my aleph since i had the sinks ..but when i raise idle curent to 2 amps(114w of heat) my sinks go to 72C is this a temp that the output mosfets can withstand for long?

Thanks again :)
 
Dear Mr Pass! I really feel very honoured to have an answer from u! I really admire your work! Ever since the first time I listened to music from an aleph5 it was a unique experience! Since I built my aleph clone I experience that feeling everyday! Thank u for sharing with all of us your knowledge and skills, something very unusual among other commercial equipment builders! I would like to ask what is the maximum acceptable heat that I may have on the heat sinks so that I can adjust the current accordingly?

Thanks once more for sharing your knowledge with fellow DIYers.

Sincerely yours

Panos Tsitimakis
 

cp642

Member
2001-08-17 6:00 pm
Panos,
For reliable operation, the MOSFETS junction temp should be kept at 100 degrees C or so, in your case the heatsink temp should be in the region of 50 to a max of 60. Exceed that you will reduce the reliablity of the MOSFETS. To echo Nelson's advice...double your heatsinks if you intend to increase the bias.
 

hifi

Member
2001-03-06 3:07 pm
sweden
this might not be my place but i believe NP have ansvered that question a number of times and the answer have always been 55c for the mosfets. (ie heatsinks the mosfets will run hotter)

btw u can use this formula

Temp = Peffekt(lamdacase + lamdajunction + lamda heatsinktoair)

and se the different temps the junction temp shouldent go over 100c and the limit for the mosfets is 150c then they die....a crude and violent death...or something ;-)

there is a older thread where this is better explained!

/micke
 
you have to be very careful in that last formula though to understand the difference between one, say, 200W point source of heat on a 20" length of heatsink and 10 20W heat sources spread out with 2" spacing on the 20" length of heatsink. These yield very different results so make sure you are actually calculating the correct data here.

Also make sure that you calculate power dissipation in each source (mosfet) correctly.