How can i best limit the heat disspassion in an Aleph based design

hifi

Member
2001-03-06 3:07 pm
sweden
I have an old amp chassis with a rather nice Powersupply and with not so massive heatsink it gets to hot if it is asked to dissipate more than 60W per chanel...so i wonder if anybody have some nice advice how to moddify the Aleph 5 preferarably since i already maked the circuit bords (but i can redesign and build them) or should i just go for a ZEN amp...maybe better than a crippled aleph?


/micke
 
micke,
Your two best options are to:
1) Choose a small Aleph, like the 30W, then perhaps use a fan to help your heat sinks along.
2) Reduce the bias to the output section. This will reduce the heat dissipation, but will also reduce current output, hence wattage output. This isn't hard to do, but is not as good a solution as option 1.
3) Take a look at various heat sink threads here and surplus sites, with an eye towards getting your hands on larger heatsinks.

Grey
 

hifi

Member
2001-03-06 3:07 pm
sweden
I have reduced the number of output devices to 4 (instead of 6) and lowered the voltage to +-25v that helpt a bit but im not home-free jet...witch resistor was it that i needed to recalculate? cant fint the thread...

Should i also try to replace the 1ohm bias resistors with say 1.5 or 2?

but how dangerous would it be to runt the mosfets att 60c?..


yes i will build a more "serious" aleph amp later on when i can get a hold of some better heat sinking..

/micke
 
micke,
Ouch! You're not making this easy...
First, you might want to rename this as an Aleph 3, instead of an Aleph 5.
An Aleph 3 has 2 pairs of output devices, which you've already done. It has a rail of +-25V, which you've already done. What remains is to change some part values. Fetch the Aleph 3 schematics, and we'll use those part numbers.
-First compare parts values here and there. Change to Aleph 3 values in all cases *except* R113.
-R113 will set your bias current. Use the Aleph 3 value of .47 ohms for R120-123, but *decrease* the value of R113. The value shown on the Aleph 3 schematic is 47k. Start coming down off of that value until you reach a measured voltage across R120-123 of approximately .25-.3V. This will put you in the vicinity of 60W dissipation per channel. We are assuming that you are confident that your heatsinks will safely dissipate 60W/channel (and assuming that all output devices per channel are mounted on the same heatsink) as you said above. You may need to adjust R113 further, depending on how much heat you can deal with.
-Assuming that I'm scratching this out correctly (this is just me and my trusty pocket calculator--no warranty expressed or implied--your mileage may vary), you'll end up with about 11-12W output into an 8ohm load, or about the same as a Zen, albeit with a more sophisticated circuit. The amp will swing approximately 15-16V, but will current limit before it gets to express its full power.
Good luck.

Grey