Krell KSA 100mkII Clone

[Alex helge]

Driver Board

[Alex helge]

The German Krell Distribution says:
The Bias across the 1 ohm emitter Dale resistors is 380mv – 430mv. The heat sink temperature is cirka 80°C.
The sound is much besser as with 620 or 690 mv.

The KSA 100 Mk 2 is better with the MJ15024/25 output transistors and has more resolution.
Best regards Alex

[Flodstroem]

there is no better way than have drivers and bias transistor placed on same heatsink along with outputs

I know that every design and/or designer often must make more than one compromise-but this thing is obviously verrrrrry important in long term,or is it important for designer's peace in mind.........judging by numerous pro amps I have opportunity to see or repair.....

This is also my experience after 25 year in audio service business (repair-man). Best solution would be if the driver-part of board could be cut of (eg. easily "snap-of") for those who wants to do so for to place the drivers on the main power heat sink.. I think Mark mention this in another post or thread.

Regards

[Mark A. Gulbrandsen]

Actually the 2SC/2SA devices are a bit better quality I think. I have always used them for the most part for input stages and they are pretty easy to get here in the States.

I can say for sure that mounting the bias device and the drivers on the main sink in the KSA-50 helped its long term stability. I have built it both ways... seperate sink for the three devices, and mounted to main sink. With the latter I have absolutely no dc balance drift or bias drift at all even after months of operation its exactly where I set it. With them mounted thiss way if you set it after everyting thermally stabilizes it plum just stays there. I also have no worries about thermally stressing the drivers over many cold-hot-cold cycles.

Mark

[PWatts]

Alex, you mention the Mk2 sounded better with MJ15024/25 transistors: was this a DIY upgrade or did the Mk2 come out with them at some stage?

I was actually hoping that we could avoid all the anguish of flying leads by doing the same as Krell did with everything on the same board, and just a larger heatsink as Mark is currently investigating. If I have to make provision for both a snap-off version as well as a bolt-down one, the board will get very cumbersome. BTW, buying on eBay is not as cheap if you have to figure in international shipping fees

Mark, if you want to mount the drivers on the main sink, what's the purpose of the extrusions? Ot are they for the pre-drivers?

I've already progressed quite a bit further with the design, streamlined a few things, routed all the grounds, fixed a few mistakes etc. As soon as the heatsink is decided on I can finish up. Unless something unforseen comes up I should have it done in a few days.

The cascode pre-drivers do not need a lot of heatsinking. Remember as I mentioned earlier, only two of these four really need sinking, and all four will dissipate about 2W in total. You don't need much to sink that. I'm thinking of a single large sink for the lot, it will be sturdier and look better than two or four separate ones. Isolaton is mandatory, unless you can find those MJF plastic tab version transistors.

I too think that TO220 is too big and overkill for the FET's. TO92 isn't necessarily better though, if you look at the Zetex datasheets you'll see a horribly high Rds of 10 and 20ohms respectively, and fairly high capacitance too. But, they take up space and that's one thing I don't really have to spare. The IRFD's are IMO a much better solution - why the hell do nobody take the time to check them out?!? They're cheap and easily available and a damn nice package. Added bonus is that they can be stacked on top of each other for extra power and lower Rds.

I should perhaps just mention that I'm unlikely to ever hear this clone in action since I have no intention of building one myself - I built two KSA50's for a friend and got intrigued by the design, but that's it. Maybe he'll decide to upgrade to a KSA100 though , but for the time being I'm doing this project out of goodwill and boring weekends at home - My day job research is on class-D and one has to remain loyal

[Zen Mod]

just to mention-if IRFD were good for Pass-Thagard A75........

[Mark A. Gulbrandsen]

Well with number 2 you only have .870 inches to work with to mount the device to... not much. With number 3 you have 1.1 inches which is almost 1/4 inch more space. Also with #2 how will you attach it to the board? Number 3 has a mounting foot to tap holes into and screw it to the board.

I agree on one heatsink strap piece like Krell did in the original for the cascodes.

I would prefer to go with the one piece board at this point. It has worked fine for many years in all the KSA-100's out there. I just want to know how you are going to make #2 sturdy so the devices are not supporting the weight of the sink. If there is no adaquate way to mount the sink to the board then a seperate driver board is fine and prefered from a mechanical standpoint. Even Krell went to a seperate driver board in the KSA-80.

I will try both types of devices ultimately and leave the ones in that sound the best... could be the Zetex's....could be the IRF's. Only listening will tell that story. Specs mean little to me... only the final sound characteristic. We already kow the Zetex's work well in a similar situation in the KSA-80..... but I will still try both types before deciding. Also the multiple emitter devices MJ21193/94 are what I settled on for my KSA-50 after trying many types both TO-3 and TO-247.... The Multiple emitter devices sound really really good but the specs are only mediocre!

Mark

[Zen Mod]

Originally posted by PWatts
.....................
I should perhaps just mention that I'm unlikely to ever hear this clone in action since I have no intention of building one myself - I built two KSA50's for a friend and got intrigued by the design, but that's it. .................

and what is possible intriguing amp for you this days ?

for me -Krell regarding ,maybe just Krell KSA25 ..... ,just for nostalgia reasons .........

[Alex helge]

Hello PWatts,

Is no directly Diy update, update of German Krell distribution.
The Motorola 15024/25 are output driver of the EUR ABS BEUR and "S" Serie.
Regards Alex

[PWatts]

Okay, I've been swayed by the lovely Zetex 2110G - the A can be used in the TO92 I provided for, and I'll try to make provision for the G on the bottom side - however there isn't room for lots of vias or copper pour to sink it so don't complain if it overheats!

I would say to go for #3 heatsink - it will work the easiest and resembles the original the closest. Any idea if it will be enough though?

So what is the opinion on the predriver sink? Each man for himself for whatever he can cobble together? In the straight line configuration like I have it now, I think a nice thick piece of 2mm aluminium plate, perhaps bent at the top for a bit more area, should be more than sufficient.

As to which MOSFET sounds best, I won't be surprised if there would be little or no difference. The only thing bugging me about the Zetex ones are the high Rds(on) compared to other FET's. Admittedly it's not as much a loading problem to the preceding stage as with capacitance, but it may cause the FET not to turn on as hard as with others. However, the margins are so large in this application that pretty much anything will work. Listen and decide, there are enough to choose from.