Upping the bias on Aleph 2

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Mark A. Gulbrandsen

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Joined 2001
#1
What is the proper way to up the bias on the Aleph 2? Should I replace the 56K resistor with a pot, or just say take half the value....27K and then make up the difference with a pot? Does this resistance have to go up or down to increase the bias?
My amp is running right at 3 amps idle but with all the heatsinking I have I could go higher and would like to try this.
Thanks in advance for any input on this.....
Mark
 
#2
Should I replace the 56K resistor with a pot, or just say take half the value....27K and then make up the difference with a pot? Does this resistance have to go up or down to increase the bias?
Click to expand...

If you increase the resistance the bias goes up. So I just put a 100k pot in series with the resistor (on my Aleph 30).

/Freddie
 
#3
Hi Mark,

Here are two threads that dealt with the bias setting. The first is for fcel's Aleph 2 and the second starts with the Aleph 5 and shows how close all the Aleph’s are as it William (wuffwaff) talks about adding output devices and turning a A5 into a A2. There is also some input from Mr. Pass.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=5007&highlight=Aleph+bias+increase
http://www.diyaudio.com/forums/showthread.php?s=&threadid=3589&highlight=Aleph+bias+increase

Rodd Yamashita
 
#5
Peter Daniel said:
Get the 200k trimpot, adjust it to 56K and put in your circuit. Then increase the value untill you get required bias, remove trimpot, measure and replace with fixed resistor.
Click to expand...

Yes, that would be a good way to do it. No need to try
calculating everything.

BTW, I'm thinking that for the Aleph-X, I may try to have
two bias settings controlled by a switch. Kinda a summer/
winter setting switch. 🙂
 
#9
I did the 200K trimpot install as Peter suggested and that is definately the way to do things. Will install the fixed resistor tommrrow. A bit more temperature rise but after two hours of operation I can still keep my hands on top of the sinks and its not uncomfortable at all. I think these heat sinks work really well!! I can forget the need for a Winter/Summer bias setting for sure.

I wonder what would have caused the bias to be a tenth of a volt low? anyone got any ideas.... I did use IRFP240's instead of the 244's in my amp. Could this be why?
Mark
 
#10
Another note: I raised the heatsinks up off the table(da). This has lowered the device case temp by almost 10 degrees to 57 deg.C after about a half hour or so of being elevated 3". Gee, now air is flowing through the sinks! I think I can lower the case temp even lower if I can find any mica insulators in TO-247 size. Am only using cheapo NTE pads for now till I find some.
Mark
 
#15
Mark A. Gulbrandsen said:
I donno....Nelson says that Micas with Thermalloy grease are the best there is right now. Which Bergquists did you order? I was considering them myself a few days ago as micas seem all but impossible to find.
Mark
Click to expand...

Sil-Pad isn't as good as mounting a device directly to a heatsink, but when you're using a packaging that has the drain on the case (like T0-247), you're better off with a Sil-Pad than Mica and grease. The heat transfer characteristics of Sil-Pad are excellent, and it leaves a lot less up to the builder. If you use too little grease, you won't get good contact, if you use too much, you'll have too much space between the device and the heatsink. You could argue that the PERFECT build using Mica and grease is better, but even the best DIYers are rarely perfect.

And then there is removing burnt up parts that are cracked apart and covered in thermal grease... 🙂
 
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