Krell KAV-300i Integrated Amplifier Repair & Restoration

I’ve just repaired and restored this lovely Krell KAV-300i integrated amplifier. Read on to find out how I did all that, and why they are such a superbly engineered amplifier.

Krell still proudly builds amplifiers and other high-end gear, from their base in Orange, Connecticut, USA. Established in 1980, they set about manufacturing a range of really different hi-fi equipment, aimed squarely at the top end of town. That’s where this KAV-300i was built.

All Krell gear that I’ve had the pleasure of working on has been superbly well-engineered, well-built, using the highest grade components that you’ll see in consumer-level equipment. This of course means it wasn’t (and still isn’t) cheap. And that’s where the KAV line came in…

The KAV-300i

Krell released the KAV-300i, as far as I can tell, in around 1995. It has the same distinctive build and materials as seen in amps like the legendary KSA-150, that I owned and restored. That means it’s carved out of blocks of gorgeous anodized aluminium and held together with stainless steel fasteners. I’m not kidding. You couldn’t build it better if you tried.

The difference here is that everything is downsized – capacitors, heat-sinks and ratings are smaller, as is the 2RU chassis. Compared to my old KSA-150, with roughly the same power output, the KAV-300i is diminutive.

This of course means that the KAV-300i cost less to make and was affordable for many more folks. There are still many of them around due to the superb build quality. They really don’t build much like this anymore.

This particular KAV-300i matches her power amplifier partner, the KAV-150, both owned by my customer and here for service and repair.

Specifications, as always, courtesy of the Hi-Fi Engine:

Power output: 150 watts per channel into 8Ω
Frequency response: 1Hz to 100kHz
Total harmonic distortion: 0.3% (trust me, wildly pessimistic!)
Signal to noise ratio: 95dB (line)
Dimensions: 475 x 387.5 x 92.5mm
Weight: 11.4kg
Accessories: remote control
Year: 1995 – 2000

Stereophile wrote a great review of the KAV-300i, back in the day, definitely also worth a read if you are looking to get one of these.

The Restoration

OK, let’s get down to it. This KAV-300i was in original condition which is great, but she was misbehaving. The main fault was distorted audio. I found a number of issues with this unit, mostly related to capacitor aging. The unit also exhibited issues with the board-to-board pins and pin header connectors. There were also a few dry joints for good measure.

Krell have been very kind over the years, assisting me with service documentation, where they have it and are happy to release it. A big thank you to Patrick @ Krell because, without that stuff, working on gear like this is much more difficult.

Stripping, Cleaning…

As always, I start with disassembly, inspection and cleaning.

Krell KAV-300i, as she arrived at the workshop.
Every control is made of metal. These pushers for example are spun out of stainless steel.
This is my first dusty look at the interior of the amplifier.
Preamplifier Board

I tend to work through equipment in a logical sequence, so it makes sense to start with the preamplifier board in this amplifier.

The preamp board performs preamplifier switching and line level buffering. All inputs are relay switched and the inputs are shorted when not in use. Nice work Krell.
And here it is, free of the chassis. I will rework this board as I consider necessary and replace the small electrolytic capacitors, because small caps always age fastest and the owner wants a full restoration.
Capacitors replaced with Nichicon FG (fine gold) in this case. I have a large range of premium capacitors in stock at all times. I choose what is most appropriate for the task at hand and based on what I have in stock. If I don’t have what I need, I order it in.

Power Amplifier Board

Now to get stuck into the board where all the heavy lifting is done. This amplifier and power supply board is a gorgeous one-piece fiberglass board. Typical of Krell is the absolute minimal use of wiring. The preamp board is connect to the main amp board by pins and pin headers. Power comes up through the board stand-offs, a really nice touch and highly reliable over the long term.

Check it out, there’s about five wires in the whole amp! Great work Krell.
Plan view of the main board. Note the two rows of original MJ15024 and MJ15025 output devices. Dual bridge rectifiers sit under the board, middle bottom. To either side, we have two filter capacitors per channel. These are modestly rated and certainly ready to be changed. up through the centre we have the 5 V and +/- 18 V regulated supplies and associated capacitors. Almost all of these were dead or dying, though they look OK from the top. Another reason why you HAVE TO TEST parts!
Apologies for this blurry naked shot…
Main-board out and ready for re-work.
This is the underside for those playing along at home.
Bad capacitor, very bad capacitor. A little on capacitor testing. Replacing every capacitor in anything, without having the ability to test them, is like waterproofing your cat – a complete waste of time! Now, if you can test the parts properly, with an ESR tester that also measures capacitance, you can make informed decisions about how to proceed. This cap was I think meant to be 470uF. It’s now a much smaller cap, with a big resistor inside it, passing DC through the circuit. This is never good.
Beautiful, all tested and all changed, replaced with premium laboratory-grade parts that will last a long, long time. Everything I’ve used here is super-low ESR, high temp rated, rated for voltage, high ripple current rated. I use premium parts in refurbs like this so that the unit will give its owner many more years trouble-free service. More pics below.
Power Supply and Underside

I replaced literally every electrolytic capacitor in this amplifier, simply because so many I tested were bad. As mentioned, I used only lab-grade  components from Nichicon, Panasonic, Nippon Chemicon, CDE, Kemet etc, to guarantee long service life for her owner.

Out with the old Panasonic filter capacitors, seen here, and in with some new Cornell Dubillier (CDE) parts.
Here are the new CDE filter caps installed, along with the rest on the top of the board. When replacing capacitors, it’s important not only to get the actual values (capacitance) and ratings (temp, voltage) right, but also to get the physical dimensions and package type (axial or radial) right. I always spend time making sure that I use parts that physically work on the board in question. My goal is for the finished product look as ‘factory’ as possible. If you look at these parts, and how I’ve installed them, you cannot tell that they are not factory-fitted.
Look great don’t they!

Very nice capacitors here. Note the parts dress – every capacitor is straight, mounted at the same height, with the same spacing, flush to the board. This really matters, to me anyway. I do things this way, because that I how I would want it done.
There are four capacitors on the underside. I replaced them with these axials, but despite ordering the slimmest parts I could find of decent quality, the new ones were just a shade too tight when I mounted the board back onto the chassis.
Temporary new replacement under-board axial. Too tight unfortunately, so the originals which measured perfectly stayed in this case.
Original skinny, but electrically perfect, axial caps!
Here I am preparing to replace the four small electrolytic caps on the front panel board.
Reassembly & De-Bugging

Time to put her back together. Thanks to Krell’s incredible tolerances, everything just fits, like it should.

After reassembly, I spend considerable time testing, adjusting and then even had to pull her apart again. I tested every single semiconductor on both boards, just to ease my curiosity, because of some occasional distortion that still seemed to be there.

I re-cleaned and prepped the board-to-board pins and did a little more re-working of joints and testing of resistors and transistors. Finally, she went back together again and tested perfectly, actually way better than spec.

Much cleaner now with re-worked boards, new parts and clean chassis.
A good view of the KAV-300i, reassembled and undergoing testing.
A better view of parts and trimmers.
Testing & Measurement

A word or two about testing. I like to be able to quantify things for myself and for my customers. It’s if I can send an amp home with a clean bill of health and some measurements to verify that. To that end, I’ve invested, over the years, in the test and measurement instruments needed to characterise this stuff properly.

Here’s a good example of this, in the final testing and debugging of the KAV-300i. I  feed in signals of known purity and amplitude, and measure them again at the output of the amplifier. In a crude sense, this proves that signals pass all the way through. But it tells us a lot more than that.

Measuring distortion of the waveform output at the speaker terminals, whilst the amp is driving a load or is open-circuit tells us a great deal about the health of the amplifier circuitry. In this case it helped me to diagnose a problem and fix it. It takes the guesswork out of debugging and troubleshooting, to be able to see with my own eyes that something I changed has dramatically reduced the distortion of a waveform passing through and amplifier.

The extraordinarily low measured distortion of this KAV-300i, is a testament to the design and the work I’ve done to restore this wonderful amp.

That’s a reading of 0.0023% on my distortion analyser. I feed the amp a very low distortion (0.001%) sine wave input while it drives an 8 Ohm speaker as a load. At the same time, I’m also monitoring the speaker terminals with my analyser. It’s a very precise way of seeing exactly whats going on with a piece of gear like this.
And that’s a reading of 0.0068% distortion, driving a 100Hz sine wave at about 10 watts continuous power. There are absolutely no distortion issues with this amplifier any more!

So that’s a look at the complete restoration of a Krell KAV-300i. I hope you’ve enjoyed this and found it informative. Don’t forget you can contact me (Mike), right here on my contact page, for assistance with your hi-fi gear.

Till next time!

2 thoughts on “Krell KAV-300i Integrated Amplifier Repair & Restoration”

  1. Fascinating to read your work Michael
    Always of interest to know about the vintage units and to see stage by stage pictures of what, what anf how you fix them
    Keep posting
    I am sure many others feel the same

Thanks for reading, leave a comment and let me know what you think!