heatsinks

cp642

Member
2001-08-17 6:00 pm
Q1,2 and 3 can be operated without heatsinks but it does not hurt to slot in one either. Any TO-220 heatsinks will be fine but pay close attention to the space on the PCB. It'll be a good idea to mount Q1 and 2 on the same heatsink for better thermal equillibrium as it'll affect the final DC offset.Then again it can be something that you can live without. For the output go for IRFP540 or better yet IRFP240
 
I show about .3W dissipation on the differential transistors, but bear in mind that the current source (Q3) will be running twice the current, hence about .6W. Speaking purely for myself, I prefer not to run a TO-220 case over about a half watt without a heatsink, so I'd stick one on the current source. One of those little three, four, or five-fin heatsinks will be more than enough, and they're cheap. Whether you run one on the gain transistors is up to you. They'll be warm to the touch, but not hot.
As for the outputs, of those two I'd say go with the IRF640, purely because it gives you a little better SOA. The IRF540 would work fine, and might arguably sound a little better, but you'd be running closer to its limits, heat-wise.
I used IRF644s in my Aleph 2s--also a TO-220 case--but once I got the water-cooled dingus going, heat wasn't a problem for me. You'll have to judge these matters according to what heatsinks you can find.
cp642's point about the TO-247 case devices is valid. They give better heat transfer, but are more expensive. Your choice.

Grey
 

janey

Disabled Account
2001-01-06 1:32 pm
i woud like to run my aleph 60 on 25V rails, so the heat
disipation is not so big problem. i have two 30cm x 20cm x 4cm
heatsinks per monoblock.
do i need any other modification on the circuit becouse i use 25V
rails.
i want to build cheap amp posible.
thanks
 

janey

Disabled Account
2001-01-06 1:32 pm
here in my country i cant get irf240 or somethink like this for
reasonable money. one irf240 cost like 15 or more irf540 or irf640
if i build aleph 30 the irf640 is not good enough.
so i want aleph 60 on 25 rails with around 2.5 A bias curent.
the final disipation will be around 125W or 62.5W per heatsink.
this is only 10W per output device.
how much power i will get from output?
i would realy like to know if i need any other modification in the
circuit (gain, bias ...)

janey
 

cp642

Member
2001-08-17 6:00 pm
Since you'll be running the amp with a 25V rail, the advice is build an Aleph 30 front end and match it with an Aleph 60 output stage ( 12 MOSFETS per ch ). Run it at a 2.5A bias. The power output you might expect is equivalent to the power output of an Aleph 30, 30 Watts > 8 ohms and more into lower impedence loads. Be sure your power supply and heatsinking is brute enough to take this punishment if loads of less than 8 ohms is expected.
 
aleph project

Janey,

I understand your situation.

But my suggestion would be to try and source the prefered output devices and not rush into it or compromise the design in the short term. Have your thought of Mouser, digi key from the USA for Farnell, they operate out of Europe.

At home I found a small company that was willing to import 20 IRFP240 for about A$4.00.

You may even be able to buddy up with some of the other forum members and buy jointly from a prefered location.

If you buy the IRF240/244s for say the Aleph 30 (6 per side with 2 heatsinks per mono block), then later on you can upgrade to the Aleph 60 by doubling the hardware.

This is how I got going and I learnt a lot with the Aleph 3 and now have an Aleph 5.

regards

Ian
 

cp642

Member
2001-08-17 6:00 pm
My calculations come in close at 175W at 2.5A of bias current.This is only the dissipation figure of the amp not accounting for the dissipation in the power supply. I must admit that I'm not good at this :p Perhaps someone can lend me a hand in the power calculation part? Grey any comments?