Recapping a "New" Stereo Amplifier: a 25 YO Krell KAV-300i

 I recently bought a "new" stereo amplifier via ebay. It's a Krell KAV-300i made in 1999, based on date codes on output transistors and capacitors. Back when it came out it was often referred to as "the baby Krell". It looks and sounds like new. Like other Krell amplifiers, it's appearance is "brutal" - gray and black aluminum and steel, with lots of sharp edges to remind you that you should tread carefully when you walk by. 

Why would one buy an old amplifier instead of a new one? There can be many reasons- for some it's a nostalgia thing, maybe you like the way it looks, it can be a good way to save some money, assuming one has the ability to fix or maintain an old amp. 

Most audio amps, old and new, are class AB designs, including this Krell. The technology hasn't changed (though some more modern amps are class D). A new class AB amp really shouldn't perform any better than an old class AB amp. The main difference between class AB amps is the quality of construction, ease of service including availability of parts, and the circuits immediately before the amp stage -preamp/input switching.

The Krell KAV-300i front panel. Watch out for the sharp corners and edges! Very simple controls and no pots to wear out or get scratchy. There are buttons for power on/standby, input selection/mute, tape monitor, and volume up and down. LEDs indicate power on (blue, of course!), standby, selected input, tape monitor status, balance status, volume level, and mute status.

The rear panel, also very simple. There's a line fuse in the IEC line connector, and two power supply rail fuses, one at each speaker output terminal. There are connectors for a tape loop, these days mostly useful for adding tone controls of some sort, a preamp output that can be used to drive a subwoofer, and three inputs, the first of which can be balanced or unbalanced. Back in the day, balanced inputs were mostly used when the signal source had to be located far from the amplifier, but these days people are using them for short cable runs because they sound "better".

Also, like other Krell amplifiers, the inside is a real work of engineering art. There are only a few wires inside the chassis and all connections and components are clearly marked on the two metal layer, glass-epoxy, plated-through holes, PCB's silk screen layers. I believe this may be one of their last amplifiers that were built with through-hole components, making them particularly easy to service. Their newer stuff uses surface-mount parts.

First opening of the amplifier shortly after unpacking it. This is what you pay for when you buy Krell!  I wanted to look for anything damaged by heat or age. When I separated the power amp and preamp boards I found some heat damage. To my pleasant surprise, there was almost no dust inside or outside the amp. There are four circuit boards- one on the far left with the transformer input voltage taps, the front panel board you can't see, the preamp board on top in the back, and the main amplifier under the preamp board. You might be able to tell that two of the main power supply filter capacitors (4 black cylinders) are bulging slightly on the tops of the cans. There were no signs of anyone ever having worked on the amp.

The connections between the preamp board and the power amp board under it are made using header pins, not wires, so you can't mess up the connections unless you bend the pins which is easy to do, when putting the two boards back together. Be careful!

For contrast, here's the inside of my Soundcraftsmen PM860 amplifier that I bought new in the mid 80s. This is more typical of consumer audio stuff: a rat's nest of unlabeled wires and hard to reach circuits on unmarked PCBs- as awesome as it was/is, it was clearly designed to be inexpensive (really, how much does it cost to put a silk screen layer on a PCB?) and not designed to be serviced.

80's vintage PM860 amplifier- 205 W/ch at 8 Ohms, 400W at 4 Ohms, and 600W at 2 Ohms. Look at the mess of wires, placement of PCBs, etc. Not pretty. Think about how much harder it is to service this compared to the Krell amp.

What the Krell?!

The KAV300i, first manufactured in 1996, is an integrated amplifier, meaning it has input switching, volume control, and power amplifier in one box. Volume control is managed digitally using buttons on the front panel or remote control. There are a series of LEDs on the front panel that vaguely indicate the volume setting. You can also adjust balance by 6 dB or so, but only from the remote control. There are no tone controls of any kind.

This amp sold for $2300 in 1999. Adjusted for inflation, that's equivalent of $4,337.35 in 2024 dollars. If you buy a $4k amp today is it any better than the old Krell? Is it built as well as the Krell? Maybe, but you can get the Krell for a lot less than $4k, if you're willing and able to change the caps.

Owner's manual here.

Note: Krell's use of the "i" appended to the model name predates Apple's use on the iPod that came out in 2001 (and almost every other low-end piece of electronic junk since). An example of Krell's advanced technology? Oh wait, Krell appended the "i", and Apple prepended it. Such innovation! If Apple had appended instead of prepended they would have had the Podi, Padi, and Phonei (pronounced like phoney). Kudos to the folks at Apple for preventing that marketing disaster!

Input selection is done using relays that produce a gentle "click" each time they are activated, controlled from front panel buttons and the remote control. There are four inputs- one balanced (with the right cables and signal source - my TEAC VRDS-20 CD player for example), and three unbalanced. If you touch a selected input button a second time, the amp mutes. Touching the same input button again unmutes the amp. There is also a tape loop (people still used tape in 1999), and a preamp output that's mostly useful for driving a subwoofer.

The power switch doesn't fully cut power- it puts the amp in standby mode which draws about 50W from the power line even when the amp is "off". True audiophiles expect their electric bills to go up when they buy Krell amps! I calculate it will add about $60 per year to my electric bill. 

The power amplifier is an all-discrete transistor, direct coupled, class AB design with apparently high bias as the amp sits about 10F above room temperature even in standby (in standby the bias is reduced). As far as I can tell, standby mode just shuts off the inputs, mutes the output, reduces the output stage bias, and turns off the input and volume LEDs. Standby mode keeps the heatsinks warm so the amp is ready to deliver full specced performance from the moment it is switched fully on.

Blue LEDs first hit the market in the late 90s, and were initially expensive, so naturally, all high-end audio gear of that era had to use blue LED power-on indicators. This amp is one of those. As time passed and the price of blue LEDs came down, manufacturers of cheaper audio stuff started using them, too (to make people feel like they were buying a piece of high-end stuff?). 

Basic specs of this amp: 150W/ch at 8 Ohms, 300 W/ch at 4 Ohms, -3 dB frequency response from 0.6 Hz to 90 kHz. Weight: 10.9 kg (that's 24 lbs outside the civilized world). Note the wide bandwidth- that's unusual, even compared to modern amps.

Here's a review from Stereophile from 1996.  

Here's a review of the amp from 2009 (?).  

More here.  

And here.

When the amp arrived, I checked for DC on the speaker outputs and measured -7 mV on the left channel and -12 mV on the right, both easily within +/20mV spec. Of course I hooked it up and it played perfectly. I tested all the inputs and outputs, and all the controls and found no issues.

Recap or leave it be?

If you buy any electronics that is 20 or more years old, including speakers, it's always a good idea to consider replacing ALL the electrolytic capacitors. Electrolytic caps have a liquid electrolyte that slowly dries out causing the capacitance to drop and ESR to rise. As ESR rises, the cap runs warm, speeding up the drying process. If it gets warm enough, some of the remaining electrolyte vaporizes and pressure builds inside the capacitor. Eventually it can cause the cap to burst (that's why the cans have grooves stamped into the tops- they weaken the cans so that's where the caps will open), sometimes splattering whatever is left of the caustic electrolyte all over the circuit board or interior of the chassis. Depending on the circuit, degrading caps may or may not noticeably affect the sound quality produced, but eventually, those caps may burst or short and destroy other components that might be a lot harder and more expensive to replace.

I opened the amp up and found almost no dust inside, despite ventilation holes in the cover, and slightly bulging power supply filter capacitors. The bulging caps indicate that they are on their last legs and really should be replaced before any of them blows.

One of the old main power supply filter caps with the bulging top. I peeled the plastic cover off so it would be easier to see. The bulge indicates that the cap was running hot and building up pressure in the aluminum can. This is a ticking time bomb, waiting for the most inappropriate moment to blow.

Speaking of ventilation holes... The heatsinks for the output transistors are bolted to the steel bottom cover of the amp which has no ventilation holes. The top cover has some small ventilation slots located directly over the heatsinks. Hmmm. If I were building an amp with internal heatsinks, I would probably put ventilation holes directly above and below the heatsinks to get some convection to cool the amp. Apparently Krell didn't think it was necessary. Back to the recap...

It was clear that the main power supply filter caps needed replacement, but should I bother with the others? Hmmm. The circuit is direct coupled from input to output, meaning that there are no capacitors in the signal path. That's why the low frequency response goes all the way down to 0.6 Hz. The audio performance of direct coupled amplifiers tends to be stable over time because there aren't any slowly degrading caps in the signal path. There are a pair of 3300 uF electrolytic caps connected back to back to act as a nonpolarized cap in each power amplifier channel (to roll off low frequency response?- not sure). 

If I just replace the main power supply filter caps and put the whole thing back together, how long will the other 25 year old caps last, and will one of them short and kill some hard to replace semiconductors in the process, or leak and corrode a circuit board, and how long will it be before the amp has to be repaired? Caps are cheap, so I decided the sensible thing to do is to replace ALL the electrolytic caps. Then, maybe, I can count on another 25 years of trouble-free service. More on capacitor lifetime below...

Alrighty then, recap it is!

I'm no stranger to recapping jobs, having restored a bunch of antique radios, and bringing my M Audio computer speakers back to life, and recapping the crossovers in my speakers, so I dove right in. First, I found the service manual for the amplifier online, then proceeded to mark out all the electrolytic caps on the schematic, then verified their locations on the PCBs, and parts list. I also measured the physical size of each cap used to ensure that the replacements would fit.

The original capacitors were all made by Nichicon, a Japanese company that makes caps specifically for audio use. The data sheet says they use some special electrolyte for the audio specific caps, and audiophiles worship Nichicon, so I used Nichicon's latest, high performance, audio-specific caps- the UKW series, most of which are stocked by Mouser Electronics. The main power supply filter caps are from the LLS series because the UKWs don't come in 60V power supply filter cap sizes.

The original main filter caps, 6800 uF @ 80V, were 35 mm dia x 40 mm tall. I checked the dimensions of the amplifier case and found that I could install caps up to 56 mm tall, still leaving 3mm of clearance with the top cover, so I ordered 8200 uF caps that are 45mm tall to replace the 40 mm tall original caps. It's about a 20% increase in capacitance that may help the amp deliver even more solid low frequency performance than it did with the original caps.

Note- there's a LOT of empty space at the front of the chassis that could be crammed full of power supply filter caps if one really wanted to increase the energy storage. The amp specs and sounds good with the caps that are on the PCB, so I decided to just place new caps on the board.

Note: the amp has a NTC thermistor that functions as a soft-start device preventing huge current surge from the power line when power is first applied to the amp (by plugging it in). If you were going to modify the amp for much larger power supply energy storage you might need to change that part.

There are a bunch of 10 uF, 50V capacitors scattered around the amp, used mainly for low voltage power supply bypass at different ICs on the circuit boards. For those replacements I chose some super high reliability, low ESR Kemet caps that will probably outlast all the others. 

Electrolytic capacitor specs include projected lifetime based on maximum ripple current at rated voltage and maximum temperature (usually 85 or 105C). As the operating current, voltage, and temperature are usually less than the rated values, projected operating life increases. Here's a detailed article on estimating capacitor operating life. The simple rule of thumb to take from the analysis is that if you keep voltage  and ripple current less than rated values, for every 10C drop in temperature, you'll see a doubling of capacitor's rated lifetime. So a cap rated for 85C that is operating at 55C (like maybe in this amp that tends to run warm), should last 8x its rated lifetime. 

Lets assume you listen to music two hours per day, every day. The UKW Nichicon caps I selected are rated at 2k hours at 85C, so we can expect them to last 16k hours. That's 8000 days, or 22 years. The Kemet ESL caps are rated for 8k hours at 105C. They should last an astounding 256k hours, or 350 years (ya, sure, youbetcha!). Finally, the Nichicon LLS main filter caps are rated for 3k hours at 85C, so expect 32 years from them. Since it is unlikely that I'll be listening to music two hours per day, every day for the next 22 years, the amp should be fine even longer than that, and will outlive me. I will put a note inside the amp that it was recapped in 2024 so that the next owner will know whether they should recap it. Even though the projected lifetimes are measured in decades, I'd still replace electrolytic caps after 20-25 years (as I have done with this amp), even if they test good.

I ordered the new parts from Mouser for about $72 including taxes and shipping:

Oops! I ordered 2 sets of the 3300 uF caps and missed a couple others. Always double check your order!

Second order that included some parts I missed in the first order and others that I discovered needed replacing after I took the amp apart. This brings the total parts cost to about $100.

Recapping the Front Panel Board

Removing the front panel is pretty easy- first take the top cover off the amp- that's 12 x T-10 screws, then remove three more screws from the bottom front and the whole front panel and its PCB will be free to move.

Red circles indicate the screws to remove to free up the faceplate PCB once the whole faceplate is free. 

The front side of the faceplate board. Note- the LEDs all stand off the board and have to fit into holes in the front panel of the amp, so be careful not to bend any of them! The new caps will be soldered on this side of the board. They must have used one of the front panels as a soldering jig in the factory.

Faceplate board caps to be replaced, five total. 1x 1000 uF @ 50V, and 4x 10 uF @ 50V.

I replaced the caps, bolted faceplate back together and tested to make sure all the buttons, LEDs, and the remote control were working. No problems!

Recapping the Preamp Board

Next it was time to replace caps on the preamp board. There are some tight fitting parts at the back panel that fit between the preamp and power amp boards, so the best way to remove and replace the preamp board is to remove the back panel first. 

Removing the back panel to get to the preamp and power amp boards. Take out all of these plus 3x T-10 screws on the bottom. The binding posts will still be soldered to the output wires on the power amp board. There is no need to desolder them.

You have to remove these four screws to separate the preamp board from the power amp board. They go into 1.5" standoffs that set the spacing between the two PCBs to 38.1 mm.

Top of the preamp board with caps to be replaced circled. There are 5x 10uF @ 50V caps and 1x 47 uF @10V. Note- the parts list says the 47 uF cap (the black one in the photo above) is a tantalum type, but there was an electrolytic cap on the board. I replaced it with an electrolytic cap since that was what was in there.

These long header pins connect the preamp board to the power amp. They make it a little tricky to get the two boards back together because it is very hard to see them when you're attempting to mate them with the header sockets on the power amp board. 

Once the preamp board was recapped, I decided to move on to the power amp board without testing because getting the two boards back together is tricky. The pins and their sockets are between the two boards and it's really hard to see them when you're trying to put them back together. The trick is to get the two horizontal groups of pins aligned with their sockets first then carefully check the vertical row of pins before pushing the board down to seat it.

Recapping the Power Amp Board

There are twelve screws on the bottom of the amp that bolt the two heatsinks to the chassis. There are also two bolts that pass through the bridge rectifiers that hold the board down, and finally another screw that goes into a spacer near the back of the amp. I found it easiest to work by also removing the screws holding the line voltage selector board so it could move around a bit. I did, but you don't really need to disconnect the wires from the transformer to the power amp board.

These are where the screws that hold the power amp board down are located. There are also 6 screws in each heatsink holding them to the bottom of the chassis. I also circled the place where the power transformer wires attach to the board.

These are all the caps on the top side of the board that need to be replaced. At the top of the picture, you see the four main power supply filter caps- 6800 uF at 80V. I replaced them with 8200 uF @80V parts. That row of 10 pieces near the top of the photo are all 330 uF @ 100V. I replaced them with 470 uF @ 100V parts. The group of four at the center are all 3300 uF @ 16V wired as two nonpolar caps. I replaced those with same spec parts.

The circles near the corners are four more caps that need to be replaced- 47uF @100V. I replaced them with parts rated for 125C. The central area that is circled is a place where the PCB has been slightly toasted by some parts operating at high temperatures.

Close up of the toasted area of the board on the bottom side. The darkened area is centered on the two power transistors, so I suspect they are the primary source of heat.

This is the toasted area on the top side of the PCB. Note the discoloration and cracking in the surface of the two circled resistors. There are two transistors right below them in the photo. I suspect the transistors are running hot. Those resistors had to be replaced. 6.19 kOhm 1% 1W metal film type.

New caps, Zener diodes, transistors, resistors, and heatsinks in place. The transistors, resistors and Zeners are stood up off the board for better air circulation around them. Heatsinks are made from perforated aluminum as I was unable to find commercial parts that would fit in the available space. Note- some folks who saw this photo on the DIY audio forums commented that the solder joints didn't look very good- I went through and cleaned them up after this photo was made.

The seven relays that switch inputs, etc. are all one type: Zettler AZ847-5. They are specced for minimum operating life of 1x10^8 operations, so I don't think there are any concerns about them. The Zetttler parts are no longer made, but small signal switching relays are used in a lot of equipment, and there are equivalent parts available with essentially the same specs and pinout- one is the Panasonic TQ2-L-5V. They cost <$3 each.

Note: The DC rails have 12A fast-blow fuses for each channel. The fuses that were in the amp were 32V rated parts. The DC rails sit at 75V, so using 32V fuses isn't really a good idea. I replaced all four with 250V rated parts. Some golden ear out there is going to tell me that Krell selected the 32V fuses for their "sound". Hah!

I replaced all the parts, checked and adjusted bias, then DC offset per the instructions in the service manual, closed it back up, and it's all working and sounding great and hopefully will be for another 25 years. Woohoo!