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Krell KSA-150 Class-A Power Amplifier Restoration

A couple of years back I performed a comprehensive tear-down, overhaul and upgrade of my gorgeous Krell KSA-150 power amplifier. I made a video of it, which you are most welcome to check out.

There’s not much you can buy that can touch the performance of a class A amplifier of this quality – let’s face it – the Krell KSA-150 was ten grand – in the early 90’s! That’s a hell of a lot in today’s money and it would have bought you a nice new car back then – think about that for a minute and you get a better idea of the kind of amplifier we are talking about here.

The overhaul on this particular Krell KSA-150 was necessary for several reasons, most notably because of the age of this hot-running class A power amplifier, and also because it was ‘repaired’ very poorly by Trevor Lees Audio, trading as Christophersaudiophilestore, on eBay. Trevor Lees is a hifi dealer in Melbourne and well-known for being a bit dodgy. Trevor misrepresented the condition and history of this amplifier in the eBay listing and in conversations with me. He was ever so friendly, until I discovered the botched repair job. I was assured that no repair work had been carried out on the amp and that it was in “like new” condition. Trevor also said he had sold the amp new to its previous owner – I discovered both statements were lies, shortly after having the amp freighted to Perth.

Now, you might be wondering how I can get away with saying openly on my website that Trevor Lees lied about this amplifier. Simple – it is not libellous to state something that is factually true, and this is! This is precisely why Trevor has not been able to do anything about this article, or my YouTube video, which has now received over 30 000 views! He has threatened me, told me lawyers will be in touch and so on, but not been able to stop me sharing my video or discussing my experience. I have some very amusing emails where he has threatened me with various types of legal action, but interestingly, nothing has ever materialized…

The other consequence of age is that heat will eventually cause certain parts in all amplifiers, especially hot-running amps like this especially, to fail. Electrolytic capacitors are a type of electronic component that store charge. They are constructed most often from layers of aluminium foil and paper that has been soaked in a special electrolyte. This conductive electrolyte will inevitably lose moisture to the atmosphere over time, proportional to the temperature of the operating environment and the amount of current flowing through the part of the circuit associated with the capacitors. Without wanting to get into a debate about whether current really does flow though capacitors, let me suggest you watch this for some explanation, if you care.

“So what?” you say… Well, as the capacitors dry out, they become more like a resistor and less like a capacitor – ie they store less charge, conduct more current and get hotter and hotter until they fail, burst, split, leak and generally cause all sorts of problems. These need to be replaced in order to restore circuit functionality. Amplifiers tend to have many and hot amplifiers like Krell and others will have problems after a number of years. This is not a possibility, it is a certainty.

E1mVy

In the image above, you can see the typical appearance of capacitors that have swollen and leaked, due to their internal resistance increasing, causing them to overheat and vent electrolyte through the pressure release vents on the top of the can. This stuff is corrosive and of course at this stage the capacitor is totally stuffed.

This service entailed dozens of new, military or industrial spec parts, rated to work at higher temperatures than many of the originals. At the same time I took the opportunity to check, clean and test many other parts, remove all the output transistors, replace all the individual transistor heat sink washers with new silicone types with improved performance, and just generally tidy up solder joints and replace damaged parts that the service buffoon before me had left in place. You will have seen all these if you watched the video.

The result of all of this hard work on the Krell KSA-150 was dramatically improved sonic performance, which is easy to explain when you consider how capacitors fail and how their performance in the circuit in their intended role will drop off as they age. I strongly recommend owners of amplifiers like this to have them serviced before parts start to fail. Other minor work can be completed at the same time and this sort of restoration really does give equipment like this a new lease on life.

6 thoughts on “Krell KSA-150 Class-A Power Amplifier Restoration”

  1. looks like a fantastic restoration,shame there are not more people who do such a beautiful job.i myself have a very nice ksa 150s I need to restore.sorry to hear you got stung by that seller.
    happy listening.

    1. Thanks for that +ve feedback Wayne, I have found as you suggest that there are very few of us doing this type of work. Trevor Lees is known in Australia for being a shonky operator – I just wish I had known that before i bought the amp!

  2. Hi, my name David mozingo from Greenville nc, 27834. You are a dyeing breed sir. I to was taken for a ride a pioneer SX1010 …I found sansui au 717 amp. For $250 dollars I wanted her to play unit, It some issues playing, by the way it was in a pawnshop, manger said they had a tech. Guy..but I will go back and get that same price in writing to. I enjoy my tube amps. But most of these amps. Lack in bass vs trans. Amps.
    My question sir is does this sound like a good deal?. And your location please if tech guy does bad job..my phone # 252-814-9071…I have another idea to offer $125.00 and tell them I would get repaired..thank you. David Mozingo. USA

    1. Hi David, thanks for your kind words and yes, for around $250, a Sansui AU-717 is a good deal, as long as it is cosmetically good and working well. If they can get it working nicely for that price, I say go for it. These are superbly well-engineered amps compared to mass-market gear now. I am located in Perth, Western Australia, so I might be a bit too far away. I love the USA by the way, I’ve had two great trips to the States now and hope to make another one soon! Regards, Mike.

  3. Glad to have found this site, this man knows about vintage audio. And quality repairs. David Mozingo North Carolina 27834. USA

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