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Stunning Krell KSA-100S Amplifier Repair, Restoration & Review

Good news, we’ve saved another stunning, irreplaceable amplifier, this time a Krell KSA-100S power amplifier from Krell’s golden era.

Welcome back classic hi-fi lovers! If you like amplifiers you’ll enjoy this one. Almost nothing is built like this beautiful amplifier now or even back then. In fact, gear like this stunning Krell KSA-100S is so hard to find and so horrendously expensive to replace that fixing it is almost always worth doing.

Short of catching fire or being dropped through the roof after being stored in the loft (true story for another time), there’s almost nothing that would prevent an amplifier like this KSA-100S from being repaired, other than finding someone capable of repairing it.

Krell KSA-S
Now THAT’S an amplifier!

Originality

Documenting and writing about the repair and restoration of one of these beasts, that’s another story. The diagnostic, repair and restoration work involved in fixing one of these is one very big thing. Documenting all that and crafting it into an article is another huge undertaking. It takes many hours, which is why I don’t do it very often. Actually, I don’t believe there is another commercial repairer documenting such rebuilds, making us somewhat unique.

There are no manuals for how to approach work like this either and professionals rarely put pen to paper about it. As a result, I’ve had a few repairers copy my approach and methods over the years, even down to the content and visual style of the website. I recently even had a guy try to leverage his own business off of this article, posting his business details as a comment under my Facebook post about this job and trying to steal potential customers! Scummy move, I really don’t understand what drives some people. Ethically, I’d say not much…

Anyway, I think it’s important that people see what’s involved in a job like this. It’s a serious technical endeavour. There are dozens and dozens of big projects just like this waiting for me to turn into stories. I don’t have the time to write them, we’re doing it this time though. Let’s go!

Specifications, Courtesy of Stereophile

Power output: 100W/Ch into 8 Ohms (20dBW), doubling with each halving of load down to 800W into 1 Ohm
Frequency response: 20Hz–20kHz, +0.0/– .1dB; 1Hz–150kHz, +0.0/–3dB
Distortion: <0.1% at 1kHz; <0.5%, 20kHz, full power
Slew rate: 100V/µs.
Input sensitivity: 1.4V RMS
Gain: 26dB
Damping factor: >60
Input impedance: 47kOhms
Power consumption: 100W, idle; 1400W, full power (oh yeah!)
Dimensions: 19″ W by 8.5″ H by 19.25″ D.
Weight: 80 lbs / 36kg
Price: $5500USD (1994); no longer available (2007)

Background

You can read more about this amplifier in a great Stereophile review from the time. The S Series, of which the KSA-100S is a part,  came after the non-S series, of which my old KSA-150 was a part. Now, these non-S amplifiers were a true class-A design, (well, up to a point) with no sliding bias and dissipated much more power at idle as a result. I would argue they sound better too, but the S series models like this KSA-100S are easier to live with day-to-day due to their more modest power consumption.

Krell KSA-100S
Eagle-eyed viewers will spot the Jamicon capacitors that someone misguidedly installed at some point. Jamicon? Why would you do it? Other than that, she is mostly original, but has been repaired before. Note the two massive 68,000uF caps, these are perfect. We could replace them, but there’s no NEED to replace them and they are not exactly low hanging fruit in that regard. Most of the smaller caps are ready for replacement. Note also the browned soft-start relays on the front panel. I replaced these with brand new OMRON relays of the exact same brand and spec as factory.

Released around 1994, there were various ‘S’ series models including the KSA-100S, KSA-200S (mega rebuild article coming soon) and KSA-300S. This is the daddy and a stupid-heavy monolith that impresses the hell out of me whenever I see one.

These amplifiers are very impressive, featuring all of the Krell hallmarks of the time: next-to-no wiring, crazy good build quality, amazing performance, of course, and stunning looks.

Problems

This Krell KSA-100S came to me at the same time as the matching Krell KRC-HR preamp which I also repaired and overhauled, more on that in another article. I did overhaul this Krell KRC-HR though for another customer, so take a look at that for background.

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This is the owner’s Krell KRC-HR, repaired before repairing this KSA-100S

This Krell KSA-100S had released a large puff of magic smoke in a moment that startled her owner and signalled the end of normal operations, for a time. She’d blown various hard to access parts, as well as harbouring a dirty little secret in her power supply that kept her from powering on after the initial and extensive repair and restoration work.

My apologies again to her lovely owner for that period being so long. Unfortunately, this amplifier came to me at a busy time and had several different and unrelated issues, all of which had to be resolved before I could give her back. On an amplifier like this, that needs time, space and return time in between other pressing jobs. Rarely is this all available at once.

So what are those issues?

  • Magic smoke from output device failure in the right channel
  • Front control module relay and board heat damage
  • A high current power supply rail was absent
  • The unit had suffered shipping damage at some point

This Krell KSA-100S had been previously repaired, though not in the way I would do it, likely leading to this failure now. Without further ado, let’s get straight into the repair and restoration process.

Repair and Restoration

There were two goals with this Krell KSA-100S:

  • Repair all faults
  • Restore her to allow for years more trouble-free enjoyment

Note: This is commercial work, there are budgetary considerations. This is not a “spare no expense” undertaking, commercial work rarely takes this form. Replacing everything would be preposterously expensive, wasteful and stupid! 

Initial work involved a thorough visual inspection and then disassembly, separating the amplifier into its modules so that each could be repaired and restored in turn. I’ll take you through that process by looking at the work we did on each of the amplifiers modules: the control board, left and right amplifier modules and power supply module.

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Krell KSA-100S
With the whole front assembly removed, we can get stuck into the amplifiers. Note that this model came with either a toroidal or an EI transformer as this model has. It depends on the revision and market it was intended for.
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Working on an amplifier like this takes up a lot of space, seriously.

Amplifier Modules

There are two of these of course so we start by removing the front panel and then the two amplifier modules. I’ll provide the most detail on the damaged channel.

Krell KSA-100S
Here we see the right channel amplifier module released from the chassis.
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And here is that module. Note the heavy bus bars and almost total lack of wiring. Krell always made/make extensive use of board-to-board pin-connectors and even standoff that carry power. This simplifies service and improves reliability.
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Here we get a better look at a KSA-100S module. This top layer is the sliding bias control and feedback for protection. Note the superb layout, lack of any kludges, premium parts, except those Jamicon caps…
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Whilst underneath we have the input, driver layer, some power supplies and the critically important and often overlooked emitter resistors.
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On the flip-side and after removing the heatsinks, we have the output devices, still mounted to their original thermal pads, complete with writing denoting the output device arrangement. This has been apart before as I mentioned.
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Here we see overheated emitter resistors indicating a high-current event in this channel. Note also the messy soldering showing previous rework of this board. It failed again because the work wasn’t done properly. That’s because whoever did it didn’t know any better or was under instruction to save money.
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Detail showing what I just mentioned more clearly.
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To strip the module we must separate the layers and desolder the output devices before unscrewing them for testing. Having the very best soldering tools and test equipment is paramount here.
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I test every single device and mark the dead ones. In this channel, two devices had failed, all were therefore replaced.
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With output devices removed I cleaned the board and replaced all electrolytic caps with super long life, low ESR, high-temp, high-ripple current parts. Note the compromised emitter resistors.
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As always, I use premium parts and fit them with as much care and attention to detail as is actually possible. Note component fit, alignment and general dress. Do these details matter to you? They certainly do to me and this is just how I work, though sadly not many others. Note the awesome WAGO connectors, best connectors ever!

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This is just the start, my friends, a growing pile of Daewoo and Jamicon caps that really should never have been in this amplifier.
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Next, we remove all the emitter resistors. Why? The over-current event that stressed a few affects all of them. Is it worth saving a few dollars when we’ve already gone to this much time and effort? Of course not, yet so many try to. All resistors must be tested, looking for signs of high or low R. A sensitive low-Ohms meter is useful for this.
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Lovely new Dale parts. These only come from large commercial vendors, nothing like this is available locally, especially not in places like Jaycar.
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Forming the leads is tedious. This is where the money goes, this level of detail means the job looks letter than factory when finished though and it won’t fail again for a very long time. I also space the resistors up off the board just a little further than factory, helping protect the board in the event of a disaster.
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Painful, but this part lead-forming process must be done exactly like this to allow for the correct spacing above the board. This reduces the chance of board damage in the event of overcurrent failure.
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New emitter resistors installed

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Super-premium RIFA axial electrolytic capacitor in the foreground. This is literally the very best capacitor you can put here. No point messing around with bias circuitry. Likewise, the other replacement parts I’ve used are some of the best industrial-grade long-life parts available.
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Finished boards in the right channel, ready for reassembly.
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We mate the output device heat spreader assembly to the mainboard, then install all the plastic insulator tubes…
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Followed by new ultra-premium silicone thermal pads…
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And a new set of hand-matched output devices from the same batch. Some may be wondering why I replace all the output devices in a failed channel. It’s for the same reason I always replace all the emitter resistors. If one device has been stressed and failed, the others have been stressed and are more likely to fail. Why risk it? Does it cost more to work this way? Of course. Is that cheaper than cutting corners and having to do it all again later? You bet!
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New output devices are in place, ready for the next stage.
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Main/driver board and output devices soldered in place and bolted together. Even the sequencing of that is critical. Everything should be bolted together before soldering the devices in place. This ensures there are no stresses on PCB pads.
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I even clean and lubricate fasteners on jobs like this, everything matters with this type of rebuild.
Krell KSA-100S
And here she is, the damaged channel, now fully repaired, refurbished and reassembled.
Krell KSA-100S
A few glamour shots, just for the heck of it.
Krell KSA-100S
Such a nice result!
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Eagle-eyed viewers may note the slightly discoloured resistor, front right. I checked it and the surrounding parts carefully, it was perfect.
Krell KSA-100S
Right channel: DONE!

OK, so let’s really quickly run through the left channel.

Before:

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During:

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And after:

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Krell KSA-100S
And here’s the left-hand channel, also done!

It’s worth mentioning that the left channel had not failed and exhibited no signs of over-current or thermal stress. I retained and re-used the original output devices and emitter resistors as these tested and measured perfectly and this saved considerably on the total bill, with no performance or reliability penalty. Naturally, new thermal pads were installed and all the same restorative work was done on the channel.

Control Module

The control module, situated behind the front panel, controls the operation of the amplifier including soft-start, master/slave mode and some other stuff we won’t go into here. These control boards suffer over time from heat damage, especially around the inrush limit resistors and relays. In this case, the relays had really suffered and the owner had even attempted an ill-advised repair in this area. Time for new ones, and a total overhaul.

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Plan view of the control board. Small power supply bottom left, relays to the right. Super eagle-eyed viewers may note that the last pin in that orange header connector is oxidised and needs re-work. I did that re-work.
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Close up of the heat damaged relays. Note the uppermost relay plastic cover has deteriorated to the point where it is barely retained. Note also the thermally damaged capacitor.
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Commencing the relay removal process. This is not easy due to the large lug size and high thermal mass of through-hole pads and traces. After desoldering, the relays have to be cut apart with a Dremel tool. Inrush current limit resistors are visible here.
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There’s not much left after the fully destructive removal process.
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The new OMRON relays are identical to the original parts. Capacitors have also been replaced with premium new parts.
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This is how it should all look with all the important parts replaced and everything else cleaned, tested and signed off.

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Power Supply Module + Further Diagnostics

I actually reassembled this Krell KSA-100S twice. That’s because, after the first reassembly, testing revealed that after power-on, the amplifier only ran for around 30 seconds before shutting off again. There were some positive signs though: there appeared to be no high current faults and no she actually powered on, with no smoke and no signs of distress. So far so good, kinda…

Further testing during the brief period she would run for revealed a DC offset at both outputs, before the protection relays. The amplifier was not coming out of protection, so this was the only way to measure that very important metric. Why is that important I hear you ask? Well, the amplifier protection circuitry will shut the unit down if it senses significant DC at the outputs, that’s what protection circuitry is for. If there’s DC, there’s a problem.

This was upsetting after all the hard work but it just meant more work was needed. Once I’d settled down, I tried to understand why the DC offset might be there, looking at the schems etc. Further testing revealed that one of the high current rails appeared to be low, actually non-existent. That would certainly account for the weird DC asymmetry.

I discussed the fault with a colleague and with Krell before going further. My feeling was a blown trace or maybe an opened diode bridge and both my colleague and the guys @ Krell agreed. I decided to tear it apart again and strip the power supply, thanks to Jason and Raymond and the Krell team who encouraged me to stick at this repair, because it certainly grew tedious at one point!

Krell KSA-100S
Please put me back together Mike!
Krell KSA-100S
As you can see, work like this is not trivial, taking up a good chunk of the work area and requiring serious concentration and attention to detail, so as not to miss one of the hundreds of different checkpoints involved in rebuilding one of these magnificent amplifiers.
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So, sometime later, with a new workshop benchtop setup, here we go… I washed the chassis after stripping the power supply.
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All transformer leads must be de-soldered. Then we can unscrew the filters and release the board.
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And look at what we have here… Yep, that will do it, that will absolutely prevent the output of that diode bridge, which is a rectified and filtered -50VDC rail from ever getting where it needs to go.

So, let’s comment here a little about what we see in this photo. This joint might look like it was never soldered, but of course, it was. It may have passed through a wave soldering machine, I don’t know if these joints were hand soldered or not but I’d suggest wave soldered. These are heavy through-hole vias and pads, they wick away a lot of thermal energy and that can mean the initial soldered joint is marginal. I’d say that’s what happened here.

Over time, the joint heats and cools, and passes a ton of current. This creates more heat and gradually the joint weakens and cracks. Once that happens, joint resistance increases and flowing the relatively high currents seen here creates quite a bit of heat, creating localised vaporisation of the solder, until there is simply none left in the joint. I’ve seen this failure mode before. It truly looks like there was never solder here.

Repair involves thorough desoldering, cleaning and defluxing, prepping the material with the glass fibre pen, re-cleaning and then a high temp hand-soldered joint with powerful soldering iron and premium leaded solder. The joint will be good again for many years after that.

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This is how it should look after rework
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There are four bridge rectifiers here, I reworked all four to be safe, even though only one joint on one bridge was bad.
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One of these caps had leaked, indicated by this cloudy, matte appearance underneath and a characteristic smell of electrolyte. We repaired the board here and replaced parts as needed.
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Much better, plus the washed boards always look good.
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The high-voltage driver supply filter capacitors are arguably the most important so I replaced those with premium Nichicon audio-grade parts of a higher rating and spec.
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Nichicon Super Through – super good!
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A detail shot of Nichicon HV filters and of just how clean the chassis now is. I straightened the transformer mounting flanges as well, these had been damaged in a previous shipping event.

Miscellaneous

I always forget to add the details about the many hours spent cleaning connectors, trimming wires, testing components, neatening components dress, cleaning boards, washing the chassis using my proprietary process. It’s partly because it’s so embedded in the way that I work, that it’s process work I don’t think about, I just do it.

That’s a mistake because I know from experience that it’s all the little stuff that comes together to make a really significant difference. That big orange front panel connector for example had a heavily oxidised main power pin. I measured it for resistance and it was bad, no doubt causing local heating and further degradation. I spent some time just on that pin, and on the connector in general.

The details matter, that’s the take-away.

Results

After finding and resolving the hidden power supply problem I re-assembled the amplifier for the second time and briefly admired the final result. Hitting the power switch, I was thankfully greeted with the customary full LED ladder telling me she was running at full power, followed by the LEDs gradually extinguishing as power consumption dropped down to idle. It’s a curious quirk of these amps that they slam on to full bias from dead cold and it’s always a bit scary because you really need to let them do that to know if they will run properly or not. They won’t power up current-limited.

Krell KSA-100S
Finally, all back together, spotlessly clean and sounding and running like new, or even better, given the attention to detail and quality of parts.

Her owner was extremely pleased we were able to resurrect this classic beast, with good reason. I was pleased to work on it but importantly, there aren’t too many places in the southern hemisphere offering this sort of service with this attention to detail and ability to repair these sorts of amplifiers, with tricky faults. Many people send their amplifiers back to Krell but you don’t have to. Best of all, Krell is kind enough to support the work I do here.

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I’ve yet to hear back from the owner in terms of how the Krell KSA-100S sounds in his system, but it’s either incredibly good, or perhaps doesn’t receive the requisite spousal approval. Maybe it’s both?! Both this and the KRC-HR I worked on for him sounded fabulous after restoration and overhaul. Sometimes people get other gear whilst waiting for repairs like this though and that’s completely understandable.

Performance

Sonically, I’ve heard and worked on quite a few amplifiers from this series over the years and they all sound fantastic. Very strong, deep, taught bass, clean clear mids, airy treble, never fatiguing and always able to command control of the speakers they drive. This is only a 100 Watt per channel amplifier into 8 Ohms, so in terms of sound pressure levels, this must be kept in mind. But if you have power-hungry speakers that need a good ‘hand on the collar’ to control them, this is what you need, or something like it.

Compared to ‘affordable’ modern-day gear, this is in a different universe. Anyone who tells you otherwise simply doesn’t know what they are talking about and that wouldn’t be uncommon! Compared to really expensive modern gear, this is better made than almost all of it. It kills Krell’s modern gear.

Bang-Per-Buck

Let’s chat briefly about bang-per-buck owning and restoring an amp like this. I often tell people how undervalued equipment like this is, even now. To replace this with new, you’d need to spend maybe $25K to $50K AUD, to get like for like.

Of course, you can buy a 100 W per channel amplifier for a couple of grand, but this ain’t no regular 100 Watt amp, friends, nor does it sound anything like what a few grand will buy you, new. As soon as I plugged her in and listened to a few tracks, even in the sub-optimal workshop environment with the Yamaha NS-10Ms, I could hear the difference between an amplifier like this and a regular amp. Do you see where am I coming from with this? You don’t want Bluetooth in your amplifier, or a DAC. Trust me on this and ask me why in the comments if you don’t know.

Generally speaking, you don’t do the sort of work we’ve just done to this beauty to try to make the money back in a quick sale. You might do a minimal repair in a case like that, this clearly wasn’t that sort of job. Work of the type we’ve done here is typically reserved for those wanting to keep their equipment long term and get the most out of it. Think of it as an investment in your cherished hi-fi equipment.

It’s like rebuilding a car engine. It may not add significantly to the sale price because people expect it to run. It will significantly improve the sell price though if the vehicle or amplifier doesn’t run. A repair and rebuild is a no brainer in cases like this. Either way, what it will do is make your car/amplifier/whatever perform better than it perhaps ever has and you really can’t put a price on that.

Amplifiers like this are barely made now, so for the few thousand dollars you might pay to obtain one, understand that it’s one of the all-time great values in hi-fi. This is indisputable, whether people understand it is another thing, but the value is unquestionable.

I mean, it’s either a Krell KSA-100S like this or a NAD C298 for example… They are simply not from the same universe. One is a memorable, legendary piece of gear still highly sough after, the other isn’t. You pay your money…

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This stunning Krell KSA-100S cleaned up beautifully and, repaired and restored to her full potential, she represents a statement peice for those who really value their equipment and amplifier performance.

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Krell KSA-100S
Wow, what an amplifier!

On that note, as always, thanks for reading and you are welcome to get in touch if you have a Krell amplifier you’d like me to take care of for you.

Stunning Krell KSA-100S Amplifier Repair, Restoration & Review

$2500 - $5000 AUD
9.7

Build Quality

10.0/10

Features

9.0/10

Sound Quality

9.5/10

Serviceability

10.0/10

Bang-Per-Buck

10.0/10

Pros

  • Unmatched build quality
  • Stunning performance
  • Unmatched ability with low impedance loads
  • Superb serviceability

Cons

  • Heavy, who cares
  • Big, who cares
  • Complex

8 thoughts on “Stunning Krell KSA-100S Amplifier Repair, Restoration & Review”

  1. Fantastic job again Mike. Your write ups are confirming that sharing your repair philosophy is the right thing. We might scare a few clients off when we tell them how much time and effort is put into bringing their “loved ones” back to life, but the pure joy of the clients getting their units back better than factory is worth it! Now I can start my day (classic Dual day today) with a smile on my face.

  2. Lovely to see a beautiful beast restored and reinvigorated… this era of Krell is sublime…

    She is a beauty.

    Most modern amps under 20k won’t get close to the sound of this.

    1. Thank you, much appreciated! I’ve developed a proprietary cleaning process but, for commercial reasons including other businesses who visit and copy ideas, techniques etc, I’ve become a bit guarded about some of the details of my work.

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