Repairing the Krell KSA150-auto shut off

WARNING: this is for your reading pleasure, or for experienced DIY'er who has enugh electronic knowledge background.risk of electric shock or severe equipment damage / personal injuries may happened if you not sure what you are doing !!!

Hi all, just to share out how I bring up a dead KSA150..

What : Krell KSA 150.
Why : Auto power off.
When : Upon power up for less than a second.
Method: Suspect auto protection circuitry being kick in due to DC swing at output. Upon stage by stage check, found right channel faulty. There is inrush current appear at output stage.



Open the KSA150 cover. What a Krell, well built and premium parts here n there.


RIP the Krell


Problematic Section


Protection circuitry functioning normal.. just replace the Zener that burst!


Take off 1st driver board-Nice layout,symetric design,real balance topology-BRAVO Krell engineer !!!


The 2nd driver PCB


The power horse, 20 Motorola BJT in one channel.Mind u!


The damaged area-Burned trace


Again burned trace.


Repaired the damaged area, replace some component and re-bias again.I use 1.28Amps.


YAHOO !! the Krell sing like having a new life. getting warm after 20 minutes..
Again, although being scold by Mugenfoo also I have to say.. better to use Krell in aircon room.. haha ..happy DIY

Impressive Mr.Wabun but you said to RIP a Krell KSA150 which means to pronounce it dead and bury it. Instead you went and brought it back to life so I think you should say ressurect it or revive or even repair it instead of rest in peace.

Hey but thanks for sharing and great job and great show of skills!

Wabun, yes congratulations on a good job. But I need to highlight to you, the damage shown by the unit you repaired suggests more a power supply problem than simply overheating. I know you have an opinion about using high powered amps but it would be good if you can be more open minded about it.

Krell is a good amp actually.

old Krell KSA-150, Bias was reported to be 1.5amps.

If the amp still blows at 1.5Amp bias at room temp ( without aircon), then the root cause of the problem is still not fixed yet.

Well done. Look simple to someone trained in this field!

Actually my opinion is all amps are good in their own respect. A high powered beast like Krell or Mark Levinson have their own unique strengths (and constraints). An SET amp has its own unique magic (and constraints), similar a regular valve amp like Audio Research, Jadis, etc too have their own unique magic and constraints.

Within my circle of friends and acquaintances who play high powered amps, Krell, Soulution, FM Acoustic, etc, it is actually vritually a given that they have an air-cond room (physical property of such amps more or less mandates an air-cond room for maximum comfort, eventhough they can be played in a well ventilated room with fan only), and also almost always some sort of power conditioner. Such amps can be quite sensitive to power spikes and other such nonsense Tenaga sometimes like to subject us to.

I salute you Wabun because repairing an amp like Krell is not as straight forward as other more normal amps. A Krell tends to be very packed inside, as you have found out, and can be quite complex. So it shows your level of skills. Have you tried it out in your system yet ? I hope you have a chance and if you play some big scale classical symphony orchestra music, perhaps you might develop an appreciation for why many people wants high power amps, because of the dynamic and control on the music they give. You will also understand too why valves have many followers as the Krell of the vintage like the one you have, tends to be a bit 'hifi-ish' more than usual. Later generations, especially the KAS and MRA reference amps are extremely musical.

hifi-ish = the dreaded v-shaped equalisation? ala michael jackson's "jam" or britney spears recording?

hehe. i'd love to have a KSA 150 working under a superlush tube preamp.

gj wabun. good to see you back. the power to give (and so is the willingness to share) is the greatest power of all. respect.

Hi WongKN, honestly after spending time listen to it, Krell has its own character. Matched correctly, it really sings very well. Previously I have prejudise on high power amp because have't "exposed" to it. Heard some bad setup then made me conclude they are bad. After exposed more, my mindset changed. Every brand has its own strengths and weak point. We can't say a particular brand gud or bad...

WongKN,
>>>>>Krell of the vintage like the one you have, tends to be a bit 'hifi-ish' more than usual. Later generations, especially the KAS and MRA reference amps are extremely musical<<<<<

How does the FPB series compare with the KAS and MRA ?

Also someone told me that really good Krells starts at 300 W/ch and above, and it is way above those of lower wattage. Is this true ?

My use of the term 'Hifi-ish' is probably a bit unusual for most. I use this term to mean an equipment which sounds very right from every aspect of hifi, bass, midrange, treble, etc but when listened to, somehow lacks that subtle 'musicality'. My use of the term 'musicality' is meant to say a rendition of music that makes one unable to resist but to listen to, enjoy the music, with feet tapping uncontrollably and head nodding in agreement. In some very small way, I am a trained musician (with certs from the royal academy of music as well as the NATD) and being able to 'get in' to the music is something very important to me (no, I don't mean participating in jamming sessions though ).

To be honest, a number of Krell stereo amps do not have this quality (musicality) in abundance. They sound very right, exemplary, cannot fault from the standard typical hifi parameters, even measure extremely well BUT somehow one is detached from the music that is coming from it. In contrast to this is the older Mark Levinsons, which has some glaring hifi-type fault, like a dark sounding sound with lack of high frequency extension for e.g. But a pair of Mark Levinson No. 26 and 23.5 will sound very musical. When Krell came out with the sustained plateau bias system, they finally managed to deliver true musicality from their amps. The original KAS and KAS-2, the KAS-2 especially were supremely musical, rated as the best amps by many who heard them. The MRA was another step upwards. The FBP series of amps, the stereo and especially the monoblocks continues along this tradition though they seem to have lost something along the way especially when compared to the KAS, KAS-2, MRA. But to be fair to the FBPs; the KAS, KAS-2, MRA were top of the range, reference level amps in their time.

Valves and esp SET have their own magic. But I have explained before, especially on a side by side comparison, they are ultimately lacking when compared to a top grade SS like Krell, ML, FM Acoustics, etc. It is difficult to know when one have not really listened to two side by side. However, I must also be fair to point out that the top rated valve amp I have listened to are only the AN Ongaku and Gaku-On, as wel as the Jadis (JA500 I think) monoblocks so I might not have listened to the best valve or SET amp available.

I have listened to a tube-SS pre-power combo. It is true to say they offer unique strengths of their own, i.e. in combining the best of both worlds. But to be honest, the best tube-SS pre-power I have heard - CAT SL1 with Krell MDA monoblocks, somehow could not deliver the same result as the best all tube or all SS pre-power combo I have heard. Again it is important to highlight that the best pre-power I have heard are Jadis pre-monobloc for tube and Krell KCT with MRA for SS, so they outclass the CAT-MDA by a lot.

I would hope wabun won't let this opportunity go to waste and at least try out the Krell KSA-150 in his system. But the one he worked on is one of the much older models, before the sustained plateau biasing technology came out so I must also advice him to be prepared to very good hifi performance but maybe lacking in the musicality department. However, a lot depends on the rest of his system also. This KSA-150 can work with and do require quite high quality component to show its best eventhough it will deliver very good results with lower grade equipment.

Oops, after typing the long post, I was notified and read the post from wabun and carz. But as I spent so much time typing, I decided not to let my post go to waste.

I am happy to read that wabun indeed did try out the KSA-150 and he has developed an appreciation for its unique strength. With this new experience, I would think it can only help him when it comes time for him to upgrade his JunPan pre-amp. So many local designers have managed to come out with very good hifi components, it is very good to know that one of our own forumers is capable of doing that as well. A no-holds barred JunPan with top grade specially chosen components matched to something like a Krell KSA-150 would make a very nice pre-power combo.

Carz, I count myself very fortunate to be invited by this guy whom I really do not know well but since I am very close to this group whose opinions he values, he invited the group to listen to his brand new (at that time) MRA amp. And I, thick-skinnedly, asked if I can join. He said 'OK, please come as well'. I visited him a few times but as he has since joined his own select group of hifi kaki's, I have not visited him the past few years.

I still miss the MRAs, but I know I will never be able to justify buying a pair. Even the Soulution stereo amp in Audio Image cannot elicit the same level of sonic excellence as what I remember was possible from the KCT-MRA. Basically when listening to the KCT-MRA, driven by Clearaudio Master Reference, AirTangent Reference and VDH Grasshopper + Krell KPE Reference and then driving JMLabs Utopia, the most lasting impression I got was I forgot and did not bother to think about the sound during all those listening sessions. The system really transformed the atmosphere of the listening room. All I was interested in is to let myself be (uncontrolledly) carried along by the music. Regular pop records turned the room into a recording studio. Jazz at the pawnshop turned it into a cafe. And so forth. The other striking thing was the sheer ease which the music flowed. In some great music pieces, the musician is able to draw us in and carry us along with the music, slowly building up the excitement until the peak crescendo. We know, instinctively that the highlight is coming, and when it comes, I was not dissapointed. It's very difficult to put into words (one of the reason why I will never be a good hifi reviewer !! ). The thing is that its only when one hears it then one knows inside our heart that 'yup, THIS is the way the real music is supposed to be'.

The Krell KSA150 won't be my final set down, I have a new toy..
a Counterpoint SA20 which is a hybrid..it still in my "bengkel"
I will post out some photos once complete restored. Honestly
it does sounds more engaging than Krell when match with my JBL
very interesting amp by its own..

Ah, the Counterpoint SA20. I almost bought one last time but the dealer pissed me off. Too bad for them. Was good for me because I went on to much much better things. Yes, the SA20 will potentially sound more 'musical' than the KSA150. Hybrids was quite the rave at one time because they offer the potential to combine the power of a SS with the lushness of valves.

Looks like tinkering with Krells has made our wabun more famous here than all the other DIY stuff....

wabun wrote:The Krell KSA150 won't be my final set down, I have a new toy..
a Counterpoint SA20 which is a hybrid..it still in my "bengkel"
I will post out some photos once complete restored. Honestly
it does sounds more engaging than Krell when match with my JBL
very interesting amp by its own..

Hi Wabun,

Please let me know the outcome of your Counterpoint SA20 journey. I own a few Counterpoint amps and I have one NPS200 that has a faulty standby button. Sent to two guys to sort things out but the problem still persist. Hope that you can be of assistance.

HI Wabun, how did you remove the first driver board? Looks like the BNC socket is stuck on the aluminum plate. Also one of the heat sinks is screwed in from the driver board side. I am trying to remove my driver board to disassemble the unit. Thanks!

HI Wabun, how did you remove the first driver board? Looks like the BNC socket is stuck on the aluminum plate. Also one of the heat sinks is screwed in from the driver board side. I am trying to remove my driver board to disassemble the unit. Thanks!

you need to take out the nut & washer from the BNC, bent the cooper rail a bit ( not too much ) and 'slot' out the PCB from the aluminium chassis, the heatsink is a neat job for Krell here wherby the output transistor are screwed to one big pcs of aluminium and the aluminum block is attached to the outer heatsink with Torx screw. The whole amplifier is disemable, so with more patience & common sense, you will able to figure out how, just dun use force, you dun need so , the Krell layout is very well designed and modular.
you can PM me with photo for section you need more info. Hope this help :-)

Hi Wabun,

Please let me know the outcome of your Counterpoint SA20 journey. I own a few Counterpoint amps and I have one NPS200 that has a faulty standby button. Sent to two guys to sort things out but the problem still persist. Hope that you can be of assistance.

My Counterpoint SA20 output MOSFET 'kong' , this type of lateral MOSFET is unable anymore, so I use SK1058 for instead (with current share resistor -for sure) quite expensive to replace all but hopefully can bring it up lah..will keep you posted once I complete the bias & Vgs adjustment, I stil facing some oscillation issue n a bit of thermal runaway..

Dear Wabun
I have a krell KSA150 that i have opened and i think the protection circuit board have been adulterated. Besides mine have the top relay caps inverted with the picture you´ve shown it has diodes at the back of the relays (on the opposite side of the board). My question is : are this diodes originally there originaly ?  Do you have better pictures of the 2 sides  of this board that you can show me?
Regards Jonny-