Sonolok Acoustic Panels for the Living Room Wall

Sonolok acoustic panels come in boxes of four. 

The TV in my living room hangs on a wall that's about 3.3m (11') high x 6.7m (22') wide. That wall has had TVs and other things hung on it by previous owners of my condo, all of whom did less than professional repairs to the drywall. It looks awful. It also happens that the opposite side of that wall is my neighbor's bedroom, and it happens that my stereo speakers are positioned along that wall on either side of the TV with their rear radiation bouncing off and going through the wall.

 

This is what I started with. Is it really that hard to patch a hole in the drywall? And whose idea was it to just paint over it?

I've have been thinking about what to do about it for a couple years. I wanted something that would look nice and not seem out of place in the room/building. I considered just repairing the wall and painting it, or hanging wall paper or a wall paper mural. 

On my recent semiannual trip to Costco I saw something called Sonolok acoustic panels on sale for $30 for a box of four panels (they go for $90 per box on the manufacturer's web site and $40 per box at Wayfair). I looked them up and watched some YouTube videos about installing them and decided this was the way to go. The panels will theoretically reduce sound transmission through the wall, though the manufacturer provides no specs to that effect, and will provide some absorption and diffusion of the rear radiation from my speakers, all while looking pretty good and fitting with the look of the room/building. The panels get good reviews of their acoustic properties on amateur audio forums.

Each Sonolok panel consists of a dense synthetic felt pad 600 mm square x 10 mm thick with 27 mm wide x 13mm thick walnut veneered (really?) MDF strips stapled to it. The strips are spaced 13 mm apart. The wood strips are offset on the felt so that two edges of each panel will neatly overlap adjacent panels. Each box of four panels comes with two right side and two left side end-pieces to neatly finish the look of the wall. There are also black drywall screws and a bunch of round, black, felt stickers to hide the heads of the screws. 

My building was converted to condos about 20 years ago and uses steel studs, spaced 24" apart, covered with 5/8" drywall on both sides. I checked building codes to see what the allowable lateral loading on the studs would be (you can't just start putting heavy stuff on the walls without regard to the loading capacity!). This wall, like all the walls in my condo, are non load bearing (the rafters, posts, and beams carry all the load). I found that acceptable loading on non load bearing light gauge steel studs used in my walls was 10lbs/sqft. The wall is approximately 12' high x 22' wide, so 264 sqft. The 5/8" dry wall on both sides of the studs loads it to 4.4 lbs/sqft. The Sonolok panels add about 1.65 lbs/sqft, so the load on the studs will be a bit over 6 lbs/sqft, well within the allowable 10 lbs/sqft limit.  

Front side of one of a Sonolok panel. The felt backing is 600mm x 600mm (NOT 2'x2' as at least one Youtuber would have you believe). The wood slats are 587mm long x 27mm wide x 12mm thick, and are spaced 13mm apart. Don't worry, the dark area is the shadow of my head when I took the picture. The color of the wood slats is perfectly uniform.

The back of a Sonolok panel. The felt pad is 600 x 600 x about 10mm thick. The wood slats are stapled to the felt. Note: at least one YouTube reviewers says the panels are 2' square. NO they are NOT! 600mm is not 2'.

One of the panels with the right and left end pieces. 

When you put the end pieces in place, their slats are spaced the same as the others for a very neat look.

When the panels are installed, there will be neat, uniform horizontal gaps between the wood slats. This eliminates unsightly mismatches in the slat positions if the panels/slats aren't placed perfectly. One YouTube installer actually cut the felt at the top of the panels off so the slats would contact each other. Then he suggested hiding the resulting mismatches by using wood putty and sanding. That's insane!

The recommended mounting method is to put construction adhesive on the back sides of the panels then screw them to the wall to hold them while the glue sets up. I had a different idea. I decided to mount the panels on furring strips that would hold them away from the wall, allowing things like network, power, and speaker cables to be hidden behind the panels.

This is the wall that will be covered with the Sonolok panels. The plan is to start the panels at the top of the baseboard and go up from there. There are a couple electrical outlets and a TV mount that will have to be accommodated. Note: the light spots are reflections of the sun from the disco mushroom sitting in the window.

At the top of the wall there are unevenly positioned rafters. The top edge of the Sonolok panels  get close but won't require any trimming around the rafters.

As usual, I started the project by making a CAD model of the wall and the Sonolok panels so I could figure out the layout before starting the installation. 

This is what the wall will look like when it's done. The green thing is the wood structural post that stands forward of the wall by about 200mm. This image doesn't show the TV wall mount that I'll have to make cutouts for. There are 5 vertical x 11 horizontal Sonolok panels here, with right and left end pieces. The wall is a a few mm wider than the panels and end-pieces so I'll have to finish the right side with a custom end pieces to hide the furring strips.

Tools

 

Besides the usual electric drill/screwdriver and a tape measure, I used a couple special tools for this job:

Laser "tape" measure. IRIC it was about $30 a few years ago when I bought it. I have checked it against a couple different tape measures and found it to be very accurate.

3 axis laser level I bought from Temu for about $40. It projects horizontal and vertical lines so it can be used to check the squareness of walls, the level of floors, and to follow studs in the wall once they have been located using a magnet. I verified it's accuracy by marking a level line (as reported the this level) on the wall then rotating the laser level 180 degrees. The line remained in the same position, so I could trust the accuracy of this unit.

A razor knife with a fresh blade cuts the felt very neatly and I used a pull-saw to cut furring strips and most of the Sonolok wood slats. 

I added additional staples to the Sonolok slats to ensure that every piece would have at least two staples holding it to the felt backing before I sawed through the slats. 

Special considerations

I was planning to use the baseboard as the guide for placing the panels, but the laser level revealed that the baseboard that follows the floor is neither flat nor level. 

The green laser line is aligned to the baseboard at about the middle of the wall...

Left end of the wall at floor level. 

Level at the right end of the wall. The floor at the left side of the wall is about 30mm higher than the floor at the right side of the wall.

The left end-piece and panel had to be cut to fit around the bricks in the upper left corner of the wall.

There are three outlet boxes that had to be allowed for and a wall-mount for the TV. 

Decisions, decisions...

The original plan was to leave the baseboard in place and follow its top edge with the bottom of the first row of Sonolok panels. That wouldn't work because, like the floor, the baseboard's top surface is neither flat nor level. This install was over a wood floor that occasionally gets vacuumed and wet-mopped, so I couldn't just remove the baseboard and run the panels down to the floor because the finish on the MDF slats would not hold up against the vacuum cleaner nozzle or a wet mop. The baseboard hides gaps between the drywall and the floor, and protects the soft drywall from things like vacuum cleaner nozzles and wet mops, so it needed to be left in place.

I settled on mounting the Sonolok panels on furring strips. The furring strips are thicker than the baseboard, so the Sonolok panels overlap the baseboard. I just installed the bottom edge of the Sonolok panels along a level line overlapping the baseboard a little. That allows the panels to be high enough to protect them from the vacuum cleaner and mop. I didn't have to trim the bottom edge of the panels to fit the unflat floor or the baseboard.

This is how the Sonolok panels, mounted on the furring strips, overhang the baseboard. The gap between the Sonolok panel and the baseboard, and a few horizontal gaps between the furring strips allows me to bring speaker cables out from behind the panels. 

Preparations

I put masking tape on the floor at the baseboard and the right corner of the wall and painted the baseboard and right edge of the wall black. I also painted the left edge of the wall black right up to the bricks. Then the power outlet plates and TV mount were removed and the areas around them painted black, as well as the left and right edges of the wall.

Black paint on baseboard, surrounding power outlet, and on the right edge of the wall.

Black paint around power and CATV outlet boxes, TV mount, and baseboard. Note the vertical lines marking locations of studs. I replaced the original white outlets and cover plates with black ones.

I used a magnet to locate the studs (it sticks where there are screw heads holding the drywall to the studs) and drew vertical lines on the wall over the studs, using the laser level as a guide. I added horizontal lines on the wall to indicate where to mount the furring strips. The laser level makes all this very easy to do.

Installation

The next step was to mount the furring strips. I used 1x2" (actual measure 0.75 x 1.5"- why do they do that?) and 1 x 4" (actual measure 0.75 x 3.5") strips that I screwed to the studs using drywall screws intended for use with thin gauge steel studs. I left some gaps between the ends of the furring strips to allow for vertical cable runs in strategic places (close to the speaker locations and power outlets). Blue painter's tape on the baseboard marks the locations of the gaps in the furring strips so speaker cables can be brought out from the wall.

Wherever the ends of the furring strips were going to be visible I painted them black.

This is the layout of the furring strips with some of the Sonolok panels shown so you can see how they attach. The Sonolok panels are screwed to the furring strips without any glue so they can be removed to run cables as needed. The left end-pieces are glued to the furring strips and the ones on the right are screwed down.

I determined where the left edge of the panels would be, and placed the left edge piece and 11 of the panels and a right side end piece along the wall to precisely locate the right edge of the installation about 30mm from the edge of the wall on the right side. 

With all the prep and painting, it took two days work to get the first two rows of panels installed. They were the worst part of the job because they were closest to the floor and required a lot of bending over and sitting on the floor to work. They also involved making cut-outs for the power outlets and TV mount and that took extra time.

Installing the panels. I left horizontal gaps between the furring strips to allow power and CATV cables to go to the TV behind the panels. I considered lowering the TV enough to cover the outlets but decided against it as I need access to the power outlet for vacuuming, etc.

The first two rows almost complete. The left end-pieces are glued and the right end-pieces are screwed to the furring strips.

I used the green 09laser level to ensure the panels were aligned properly before screwing them down on the furring strips. 

I installed the panels using seven screws each- 3 at the top and bottom edges and one in the center. 

Cutouts

The positions of the cutouts were determined as I installed the panels. I started on the bottom row, placing the panels from the left edge and working toward the right edge. When I encountered a power outlet, I laid out its location on the panels using blue painter's tape, then used a framer's square to draw the cutting lines on the tape (sorry, I didn't take any pictures). In places where cutting the slat would result in a piece of the slat being held by a single staple, I added staples to stabilize the positions of those slats (example- the short slats above the TV cutout) before cutting the slats.

At some cutout locations, and at the far right edge of the installation, when viewed from the side, the ends of the furring strips were visible. I painted those black before installing the panels.

Problems

When I bought the furring strips I selected straight, untwisted boards. By the time I got to putting up the third row of panels, the 1x2s that were sitting on the floor in my living room for two days warped so badly I couldn't use them. A couple of the 1x4s also warped to unusable condition. I was able to cut them into shorter pieces, leaving out the twisted parts, and got them to work OK, but next time (hah!) I'll invest in better quality wood. For this application, it would actually be better to use strips of MDF or even plastic because it won't warp. 

The top left side panel had to be trimmed to fit around the bricks up near the ceiling. The slats are held on the felt with staples, and are made of MDF, so it's impractical to try to trim the panel at the shallow angle that would be required to follow the outline of the bricks exactly. I painted 50mm or so of the wall black, up to the bricks, and cut the left end-piece and the panel squarely to fit. Then I marked and cut the two left-most slats and felt on the panel to lengths that would fit under the overhanging bricks. I think it came out pretty good. 

The upper left corner panel was cut to fit around the bricks. First I measured and cut the left end piece, the I measured from the wood on the lower panel up to the bricks following the edge of the wood on the lower panel and trimmed the two left-most slats and felt on the upper panel to fit.

I wasn't sure how I was going to close the edge of the panels at the right side of the wall. Then I looked at the many extra left end-pieces I had and found that the wood strips are just the right width to close off the opening created by the furring strips. I removed the wood slats from the felt and glued them to the right side end-pieces and it looks very professionally finished. If I ever need to run cables through the right side I can remove one of the right end-piece plus slat, cut it open enough to run cables and then put it back. Or just make a new one with some of the many extra end-pieces that were left over from this installation.

Detail of one of the right edge finishing pieces. I removed the slat from a left end-piece and hot melt glued it to the felt on a right end-piece using a simple jig to keep them aligned. The right end-piece with the left end-piece slat attached is just held in place with screws so I can remove it easily to run more cables if I ever need to. Note: I painted the wall and the end of the furring strip black, but didn't really need to.

Jig used to hot-melt glue the left end-piece slat to the right end-piece felt. I used spring clamps to hold the right end-piece against a board, taped off the slat, then applied glue to the felt. Masking tape prevented the glue from going to the wrong places. I set the slat (removed from its felt) from a left end-piece down on the glue.

Installing cables

I prepped a set of 12 gauge speaker cables (I bought a 250 ft spool of 12 gauge speaker cable via Craig's List for $15!) for this installation. I hung the cables using some thin plastic held in place with thumb tacks. The hangers are only there to keep the cables from getting in the way when installing the acoustic panels. I also ran a piece of black network cable to the left side of the wall where the WiFi router will be placed.

Cable hangers made from thin plastic strips tacked to the wall.

I bought a power-strip/surge protector with a flat, rotating plug so that I could route the power cable up the wall under the Sonolok panels. The end with the sockets will be installed on the wall behind the TV. I will use it to power the TV, Shield Pro, and Bluetooth transmitter.

Power outlet under the TV with flat plug that rotates so the cable can go straight up under the Sonolok panels. The other cables are HDMI and optical fiber that go from the TV to the amplifier.

 

Finally

 

It took about 4 days to hang all the furring strips, run the cables, and trim and mount the Sonolok panels. I used 14 boxes of the panels at $30 each, and spent $40 on the laser level, and another $50 or so on furring strips and screws, and maybe $15 for replacement power outlets and face plates, for a total cost of about $500 to cover this 3m x 6.6m (~10'x22') wall.

I don't yet know if the Sonolok panels reduce the sound transmission through the wall a meaningful amount. They will certainly help at least a little. I'll have to ask my neighbor if he notices any difference.

The newly paneled wall is on the west side of the room. The east side wall is mostly very large windows, with heavy velvet curtains that can open and close, and is very reflective for sound with the curtains open and very dead when the curtains are closed. With the curtains open, the stereo image from my speakers seems to be a bit more sharply focused than before the wall was paneled, but that may be because I'm paying more attention to it. With the curtains closed the sound seems less satisfactory, as if the room is just too "dead". 

Living room with Sonolok paneled wall. Windows on the left are the east side of the room, Sonolok wall is on the west side.

This is what it looks like with the curtains open during the day.

 

 

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!