Reverse engineering Krell KMA 160

[jwb]

Originally posted by Sze
nice job JWB

What will you use for the power Fets, Q1,2,3,4, in the output stage? Are the VN and VP0210N5 available anywhere?

I haven't looked. Supertex makes a VN2210 which might be a successor to 0210. I'll look around. Surely there's something more modern that can be used there.

YOu didn't specify the voltage of the Bulk supply for the output stage. Do you happen to know what it is.

I have no idea. Haven't turned on the main supply yet.

[jwb]

It seems the VN2210 is a much bigger device. Much lower Rds(on), higher Vgs(th), much higher capacitance, slower. Maybe not the perfect substitute.

[Sze]

JWB, Please keep us abreast of your progress during construction. I am intested to see how this project of yours turns out. Please post any pertinent information or problems that you encounter while you build this amp.

Thanks
Dan.

[K-amps]

Originally posted by Zero Cool
I am VERY interested in seeing the final schematic and any photos you take.

Yes these early Krells like the 80's the 100's and 160's were all hand built if i am not mistaken. Krell was very new at that time. 1984 i think. but then again, look at the very early Levinson stuff as well, they had MFG problems as well.

BUT, Dan D Agostino is still the king of SS amps in my book! I dream of owning a KSA-250 some daY! Droool!!!


Zero

Zerocool,

Just my 2 cents, I own both the KSA-250 and a KAV-250... at the risk of being bashed around, if I were not consider the looks of the amps... the KAV sounds better. ;-). Both clip at around 330w at 8E.

The KSA-250 is supposed to be pure class A to full power but it is not, it is fixed biased class A to about 28 watts, from then on the auto-bias circuit kicks in. This is the same as all the KSA-300S and fpb series except that is NOT a sustained bias implementation. It is dynamic. The KSA-250 has 24 MJ15024/25 devices per channel and a 4.5kVA toroid.

cheers!

[Zero Cool]

Originally posted by K-amps


Zerocool,

Just my 2 cents, I own both the KSA-250 and a KAV-250... at the risk of being bashed around, if I were not consider the looks of the amps... the KAV sounds better. ;-). Both clip at around 330w at 8E.

The KSA-250 is supposed to be pure class A to full power but it is not, it is fixed biased class A to about 28 watts, from then on the auto-bias circuit kicks in. This is the same as all the KSA-300S and fpb series except that is NOT a sustained bias implementation. It is dynamic. The KSA-250 has 24 MJ15024/25 devices per channel and a 4.5kVA toroid.

cheers!

My understanding is that the KSA-250 can be upgraded with the Sustained Plateau Bias like the 300 and FPB models have.
A good freind of mine has been following my audio dream and purchased a KSA-250 several years ago. it was just sent in for updates/upgrades at Krell and he had the SPB upgrade added.

Im sure this big amp is not full class A to full power. i never considered that to be a fact. But, many many audio reviews in many magazine, even today still make reference to this amp. I mean were talking about an amp that can deliver 238 Amps peak per channel! can develop 6hp, has 88 joules of storage capacity, can literally deliver more pounds per stoke then a jack hammer!!!
AND, while specs are fun. this has been the amp that all others have been judged by for many years!

Is it a perfect amp? No im sure its not. But it is one of the best amps Dan D'Agostino has ever built. and, let me quailify that a bit more. At just under $3000.00 used. it is the best amplifier that is close to being within my finacial reach as well.

I would love to own a pair of audio standards, but that doubles the price and then some and that is definitly out of my price range.

I was very excited about the KAV-250 when they first came out. But, correct me if i am wrong. But my understanding was the KAV line was produced as a lower cost alternative for the Home theater crowd. and they run full class AB. Isnt that like buying a budget Ferarri??? Somehow that just seems wrong.

I dream of owning a KSA-250 for many reasons. Of all the amps ever built, this one has recieved the most ink as far as i have seen. I would like to own one to uderstand what all the hub bub is about. But, i have to get to that point first, before i can move forward on my audio nirvana path. And at the moment, i cant even afford 2 of the output transistors used in one of those amps, let alone 24, and i cant even think about affording a whole amp.....so for now. its still just a dream.

Thanks for your .02C worth however. i am now that much richer!



Zero

[K-amps]

Originally posted by Zero Cool

My understanding is that the KSA-250 can be upgraded with the
Zero

Zerocool,

I am a power freak... hence when I decided on buying amps in 1997, the Sunfires were pretty much hyped, so I got the Sunfire Signature (600w RMS) with B&W 801's.... I was ... disappointed. To me the sound was screechy, thin and fatiguing (no disrespect to Mr. Carver.. but it was not for me.)

I took it back to the dealer who had the FPB-300's, I tried those and liked them a lot, however $6500 was not in my budget, so he said for 1/3rd the price why don't I try the KAV-250? I took one look at it and said... nah, thats not a Krell . However we decided to give it a try.... within 20 minutes I was home with my KAV-250.

I still wanted to get a KSA-250 because of the ads in the early 90's with the amp sitting next to a jack Hammer etc... I called Krell up and asked them about the KAV, they told me it was Class-AB (just like the Classe/ Halcro and some other very nice amps) I was a little put off but then asked him which amp would sound better, the KAV or the KSA... He said the KAV. I got a little suspect of his recommendations.. so he added, most of the sonic signature is determined in the Input and VAS stages, those all run in class A in both amps. He added only the OP stage of the KAV is class-AB rest is class-A. He said because the KAV was built with newer topologies and components it sounded better.

In my tests, the KSa sounds a litle dark, the KAV more detailed. Both sound relatively warm/ soft and pack a killer bass punch.

My tests were done with the 801's.

I suspect at lower than 4 ohm loads, the KSa will come out as the winner.

Also sheers looks, the KAV does not come close to the KSA.

Both are within the $1500 to $1900 range on ebay today.

Yes the KAV-250 does not look like a Krell but it sure as heck sounds like one.




cheers!

[jwb]

K-amps,

What does the input stage of the KAV look like? What transistors are used? Perhaps I can adapt any improvements into my KMA-to-KSA project.

[K-amps]

Never took it apart, just the top cover. These suckers are not user friendly to dis-assemble, additionally I do not have the right tools. ;-)

The only thing I remember is that it uses 4 paralelled drivers to drive the OP stage (or something.)

Toroid is a custom order from Plitron, 4 pairs of Mj21195/96 class OP devices (actually custom Krell parts but MOT came out with the MJ2119X series shortly after they fulfilled Dan's order for 1 million OP devices. ;-)

8X 4700uF caps.

[jwb]

I'm probably going to AC-couple the input of this amp, and add zener protection on the gates of the input follower. I really don't see why they might have ommitted them in the original design: while I understand the desire to minimize the parts count, this board is not designed under that philosophy. There's already 72 transistors per channel! I don't think zeners are going to harm anything at all.

[jwb]

So what is a decent blocking capacitor, anyway? I need something that's not just huge. Something like the MMKP 383 seems decent. 250Vdc/125Vac limits seem, well, quite overspecified.

Anything wrong with these metallized polypropylene boxes? 11x31mm is still a big footprint but these are the most reasonably-sized ones I can find.