Krill construction thread - 100W version

[Andypairo]

Hi all,
since the original Krill thread has become way too big I think that is better to start a separated thread dedicated to building questions and experiences.

This way the information for building a Krill amp will be easily available.

Attached the schematic for the 100W amp.

Some notes:
-R5 is actually a 100 Ohm fixed resistor paralleled with a 100 Ohm trimpot
-I omitted the 100Ohm//1K trimmers between the emitters of Q7/Q10 and the collectors of Q8/Q11 since these are not present on the boards.
-R27 is the bias trimmer

Ciao

Andrea

[c2cthomas]

All Right Andrea!!

How is your construction going?

[Andypairo]

Hi Thomas,
I've completed one channel but I'm experiencing biasing troubles.
It looks like Q7 and Q10 get too hot and "cut" the bias. I'm not able to set the bias higher than 20mA/device at the moment without cooling them.

I informed Steve and will post progress here.

Ciao

Andrea

[AKSA]

Andrea,

they each dissipate around 600mW, so maybe good heatsinking is required..... I guess the low Vbe at high temps cuts off the bias string completely. It is surely just a temperature effect.

Hugh

[Andypairo]

Hugh,
I agree that the cause is most likely a temperature effect, but I'd like to hear also Steve's opinion about it, since he never mentioned the requirement for heatsinking (he stated that the BJTs run hot on purpose) and also because those boards were used for a commercial 100W monoblock amp.

Ciao

Andrea

[boraomega]

Originally posted by Steve Dunlap:
The rest of the TO220 devices will dissipate about 0.35W each and should not need heat sinking. I run them warm on purpose.

I asked: Which purpose?

and Steve answered:
I have been waiting for someone to ask that question. At 10mA of current the transistors run at an elevated temperature for two reasons. One reason is the increased gain and linearity. The other is that this pretty much swamps any change in die temperature caused by changing output current in these devices.

[Steve Dunlap]

Hi Andrea,

Q9 and Q12 should be running at about the same temp as Q7 and Q10. Is this the case? You should just be able to keep a finger on the transistors. If they are too hot for that, check to see that you have no more than 10mA through R16 and R17. I have not encountered this problem before. If the transistors are getting too hot and cutting bias, and all else checks out OK, you can either use a small heat sink on the transistors or increase the value of R16 and R17 to reduce the current.

Q4 and Q6 should also run at the same temp, but they will not effect the bias.

[Steve Dunlap]

boraomega,

Andrea is building the 100W version. The transistors in it will dissipate about 0.56W. Still well below the 2W rating.

[PH104]

I'm putting together a BOM for parts for the 100W version as posted on thread 1013 on the original Krill thread. It's attached here for reference. As a rough starting point, the parts to build the amp according to that schematic are about $90 per channel, exclusive of transformer, chassis, hardware, heatsinks, etc.

I'll post the BOM once I check it against the boards, especially as Andrea mentions that there are no provisions on the boards for some of the potentiometers.

Also, the schematic shows R21 and R22 as being 10K resistors. These should be 10R.

Phil

[boraomega]

Originally posted by Steve Dunlap
boraomega,

Andrea is building the 100W version. The transistors in it will dissipate about 0.56W. Still well below the 2W rating.

That is OK with me Steve. Don't get me wrong, I just tried to help Andypairo with his question: "...but I'd like to hear also Steve's opinion about it..". That's all!