I recently bought a non-working Krell KMA-100 MKII for $180. The main fuse breaks when powered on. Since I'm unlikely to find (or afford) a second unit, I'm researching whether converting it to a KSA-50MKII is reasonable. I'd like to use the chassis, transformer, large filter caps, heat sink assembly, etc, but set aside the original driver and output boards, and replace them with entirely new PCBs and components. Some things I need to determine:
Power transformer: I read in another thread that a stereo to mono Krell conversion does not require a transformer swap. Assuming mono to stereo might also be possible, what would be the best way to configure the secondaries for two 50w stereo channels? Is there a way to supply each channel with 1/2 the original rail voltage?
50W or 100W: Given the PSU question above, if I need to live with the original KSA-100 rail voltage, can I still safely make this a KSA-50 re-build, or would there be a large amount of re-work to the KSA-50 circuit required?
If it helps with current/ heat issues, I'm ok with biasing the amp back to 10-20w of class A operation. My goal is just to retain as much of the original Krell sound as I can. Appreciate any advice on whether this might work or not.
Jason
Power transformer: I read in another thread that a stereo to mono Krell conversion does not require a transformer swap. Assuming mono to stereo might also be possible, what would be the best way to configure the secondaries for two 50w stereo channels? Is there a way to supply each channel with 1/2 the original rail voltage?
50W or 100W: Given the PSU question above, if I need to live with the original KSA-100 rail voltage, can I still safely make this a KSA-50 re-build, or would there be a large amount of re-work to the KSA-50 circuit required?
If it helps with current/ heat issues, I'm ok with biasing the amp back to 10-20w of class A operation. My goal is just to retain as much of the original Krell sound as I can. Appreciate any advice on whether this might work or not.
Jason
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I'd personally rather get the original amp working, if it wasn't too expensive. That's just me though... it's your amp! A steal at 180. heh - that would make one he// of a KSA50 clone.
I'd start by disconnecting the amp from the PSU, and measure + / - from the PSU. (you might find that the fuse still blows with the amp disconnected from the PSU, which could send you in different direction) This and the VA rating of the transformer will dictate what you can make it into without swapping the transformer. Report back what you find.
Sure, you can make a stereo amp from the single PSU, as long as the I and V work for the goal.
I'd start by disconnecting the amp from the PSU, and measure + / - from the PSU. (you might find that the fuse still blows with the amp disconnected from the PSU, which could send you in different direction) This and the VA rating of the transformer will dictate what you can make it into without swapping the transformer. Report back what you find.
Sure, you can make a stereo amp from the single PSU, as long as the I and V work for the goal.
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I would, too, but only having one would limit it to something like subwoofer use. The gentleman I bought it from and I both agreed that if he had two broken units instead of one, the pair would probably sell for over $1500. That's why I plan to store the original driver and output boards. If I find a second amplifier, I'll return this one to original condition.
In the mean time I'll start to disassemble, clean it, and test the power supply. According to this post the KSA-100 rails are +/- 53.5vdc. So, I'm wondering if I could rewire the secondaries for lower voltage (which I don't think is possible), or would the KSA-50 MKII boards run safely at 53vdc?
If not, my thought is that I'll need to run KSA-100 boards in it, while keeping the bias low enough to make sure current draw stays ok.
In the mean time I'll start to disassemble, clean it, and test the power supply. According to this post the KSA-100 rails are +/- 53.5vdc. So, I'm wondering if I could rewire the secondaries for lower voltage (which I don't think is possible), or would the KSA-50 MKII boards run safely at 53vdc?
If not, my thought is that I'll need to run KSA-100 boards in it, while keeping the bias low enough to make sure current draw stays ok.
Things might have just gotten a little easier. I ordered two sets of KSA-50 MKII PCBs from Jim's Audio, and as part of the order, they emailed schematics very similar to the ones attached to my earlier post. The schematic calls for +/- 48vdc rails. This is much closer to what I've read is the stock rail voltage (53vdc) in the KMA100 MKII. I expected something more around +/- 30vdc.
I'll need to verify, but assuming 53vdc is correct, I don't think I'll have to modify the supply, or change anything in the KSA50 circuit at all. I'll let each channel pull from a single set of rails, and while not quite as nice as dual mono, this might eliminate a lot of work. It would also make it simple to revert the amplifier back to original.
I'll try to get the PSU up and running, and see if I can post actual measurements in the next day or so.
I'll need to verify, but assuming 53vdc is correct, I don't think I'll have to modify the supply, or change anything in the KSA50 circuit at all. I'll let each channel pull from a single set of rails, and while not quite as nice as dual mono, this might eliminate a lot of work. It would also make it simple to revert the amplifier back to original.
I'll try to get the PSU up and running, and see if I can post actual measurements in the next day or so.
The small blower cooled heatsink in the corner tells me that this amplifier cannot be converted to ClassA operation.
Consider a ClassAB stage.
If you want lots of ClassA, then increase the number of paralleled output devices. Don't just bias it up to become a high bias ClassAB.
If you convert to a mosFET (either Vertical or Lateral) output stage you can increase bias as high as the heatsink will allow. Aim for at least 500mA for each channel.
The KSA50 ran on ~ +-38Vdc and was biased to ~1.9A That's a dissipation of 144W per channel. add on some for the drivers and double for stereo and you have ~300W of heat to get rid of.
Changing to +-53Vdc and ClassA is not possible for that small heatsink.
The KSA100 ran ~2.6A bias and the output dissipation was~ 275W per channel.
Consider a ClassAB stage.
If you want lots of ClassA, then increase the number of paralleled output devices. Don't just bias it up to become a high bias ClassAB.
If you convert to a mosFET (either Vertical or Lateral) output stage you can increase bias as high as the heatsink will allow. Aim for at least 500mA for each channel.
The KSA50 ran on ~ +-38Vdc and was biased to ~1.9A That's a dissipation of 144W per channel. add on some for the drivers and double for stereo and you have ~300W of heat to get rid of.
Changing to +-53Vdc and ClassA is not possible for that small heatsink.
The KSA100 ran ~2.6A bias and the output dissipation was~ 275W per channel.
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I don't know the source of the two .pdfs, but the numbers in there concern me.
I think they are not KSA50 values from Krell.
Take the Vbe multiplier using 4k7 and 5kVr. That just looks wrong.
The red numbers added to the output stage also look wrong.
Thank you for the details. Really helpful. The amplifier in the picture is a Krell KMA-100MKII, which should be 100w mono, Class A. My thought was to convert it to a 50W stereo KSA-50MKII, with the hope that would provide a similar load to the PSU and dissipation in the heatsinks. Being willing to back down on the bias seemed like a reasonable way to rein heat and power consumption in if 50w stereo was still too much. I use horn-loaded Altecs and JBLs, so I really only need a few watts per channel anyway.
If the KSA50 ran on +-38Vdc, that actually makes a lot of sense, with the KSA/KMA100 at 53Vdc. I'm not sure why the Jim's Audio schematic calls for +/-48Vdc, but I'm now wondering if that's a typo, as it would be an easy enough mistake to make.
The KSA50 MKII schematics I attached earlier were the clearest, most original looking schematics I could find. I've just received Jim's KSA50MKII schematics (attached) which is what I'd actually be using, unless they have some sort of major issue. I've rebuilt/ restored a number of vintage amplifiers, but am still a journeyman when it comes to circuit design so I can only really trust that they're correct.
If the KSA50 ran on +-38Vdc, that actually makes a lot of sense, with the KSA/KMA100 at 53Vdc. I'm not sure why the Jim's Audio schematic calls for +/-48Vdc, but I'm now wondering if that's a typo, as it would be an easy enough mistake to make.
The KSA50 MKII schematics I attached earlier were the clearest, most original looking schematics I could find. I've just received Jim's KSA50MKII schematics (attached) which is what I'd actually be using, unless they have some sort of major issue. I've rebuilt/ restored a number of vintage amplifiers, but am still a journeyman when it comes to circuit design so I can only really trust that they're correct.
I have to agree on keeping it original. Just fix it and resell it or wait for another mono.
It would be a shame to gut it and do your own thing and loose its intrinsic value.
What you will wind up doing is gutting everything for the shell to install 2 blower/ heatsink column,s that will take valuable real estate along with a new transformer that will fit those needs and enough room for everything else involved.
Then you will have to drill the back chassis to fit another dual binding post, rca input. etc.
There goes its value to anyone else when you might sell it someday.
If the transformer is dead , Maybe a remake would be in order...
Regards
David
It would be a shame to gut it and do your own thing and loose its intrinsic value.
What you will wind up doing is gutting everything for the shell to install 2 blower/ heatsink column,s that will take valuable real estate along with a new transformer that will fit those needs and enough room for everything else involved.
Then you will have to drill the back chassis to fit another dual binding post, rca input. etc.
There goes its value to anyone else when you might sell it someday.
If the transformer is dead , Maybe a remake would be in order...
Regards
David
+1 Repair the original KSA100
--This is a popular amp with extensive internet information on test and repair
--You can set a lower output Class-A bias current so it runs cooler, and keeps your electric bill low.
--You will get a good education in Krell ckts, and experience testing components like each output transistor.
--an incandescent light bulb is sometimes put in series with the AC plug to limit fuse current for initial testing
--you can remove power wires to run/test just one channel at a time, and do A vs. B measurements
--study how to measure the big power capacitors, and estimate their wear
I owned a KSA100, upgraded to KMA100 monos which I gave to my son, now own a couple KSA250s which I rebuilt with new caps.
--This is a popular amp with extensive internet information on test and repair
--You can set a lower output Class-A bias current so it runs cooler, and keeps your electric bill low.
--You will get a good education in Krell ckts, and experience testing components like each output transistor.
--an incandescent light bulb is sometimes put in series with the AC plug to limit fuse current for initial testing
--you can remove power wires to run/test just one channel at a time, and do A vs. B measurements
--study how to measure the big power capacitors, and estimate their wear
I owned a KSA100, upgraded to KMA100 monos which I gave to my son, now own a couple KSA250s which I rebuilt with new caps.
Well, I'm in on the keep it original! I just was given a ksa50 mk2 in good working order. The output transistor had been changed out to the (ON) from Motorola mj15003 and mj15004. Do these really sound different or is it because everyone re-bias a little lower?
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Hello,
My name is Peter from Germany and some time ago (to long in fact) a corner of the left board of my KSA 50 MKII smoked away, because the banana plugs at the left speaker hat contact after some spring-cleaning.
The epoxy of 1-2 square inches is blown away, minimum the 2 (pairs of) power transistors and the parts around related to the power ciruit path must be changed.
Now, as I have again more time to listen to my Infinity RS II, I want to get the Krell back to work, to run the Infinity in bi-wiring mode again.
A skilled friend helps me to check the mess.
We learned that there are many China boards - somehow similar - but not the same.
Then we heared that there are currently many low quality China fakes of the power transistors on the market.
As the damage on the board is not yet totally clear I would like to know if there are boards I can buy whith the same design / interface, so that it would be possible to exchange the full boards?
I am a little bit afraid of all the chances to learn, if we are going to repair it, so I want to see if there is an alternative.
Thanks in advance, as there are thousands of lines to search for, just for the KSA50
Peter
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