A while back I bought a Wiim Amp Pro (WAP) to replace the electronics in my bedroom stereo system. After I realized the bass management can be used as a crossover, I decided to try it in my living room system, driving the Quad ESL-63s that I recently rebuilt, and an SVS 3000 Micro subwoofer. I was impressed with the Wiim amp's performance and decided to put the WAP back in the bedroom and I bought a Wiim Amp Ultra (WAU) for the Quads/sub (20% off on Black Friday!). The WAU is similar to the WAP, but uses a different DAC and has a higher power output- 100W/ch at 8 Ohms and 200W/ch at 4 Ohms. It also has a touch screen I don't really have a use for.
Wiim Amp Ultra sitting on top of SVS 3000 Micro subwoofer.
The Quads, as wonderful as they are, are not perfect. The upper end of the audio spectrum (beyond my ability to hear it) is rolled off a little (easily corrected with an equalizer in the WAP or WAU), and they are incapable of producing low bass. Some people also complain of limited "dynamics" and maximum SPL - to me those are the same thing- if maximum SPL is limited, of course, "dynamics" will be limited.
The Quad's poor bass performance can be blamed on two things. First, they are bipolar radiators which means they emit sound from both the front and back sides. The front side and back side radiation are 180 degrees out of phase, so when the sound wavelength they produce is long compared to the size of the speaker, the front and back radiation tend to cancel and the result is weak bass. Second, the spacing between the diaphragms and stators in the drivers limits the maximum diaphragm excursion, and thus maximum sound pressure level (volume). The greatest excursion is demanded when reproducing low frequency sounds, so if you can prevent those low frequencies from going to the Quads, they can play everything else louder. This also addresses the "limited dynamics" some people complain of.
The bass management in the WAP/WAU can be configured to keep the lows out of the speakers and send them directly to the subwoofer. The crossover is a Linkwitz-Riley 4th order type which means the low frequency roll-off in the Wiim amp will be 24 dB per octave below the crossover frequency. One octave is a doubling (or halving) of frequency, so one octave below 90 Hz is 45 Hz. So at 45 Hz, the signal going to the Quads from the amp will be 24 dB below what it would have been without the crossover being turned on. That means the Quads will be able to play much louder than they would if the full range signal were being sent to them.
Linkwitz-Riley crossover response curves for 2nd (12 dB/oct), 3rd (18 dB/oct), and 4th order (24 dB/oct) implementations. The WAU/WAP use the 4th order curves. This image comes from the Rane site linked above. No, there won't be a suck-out at the crossover frequency. When the outputs of the drivers are summed acoustically, in your room, the response at the crossover frequency will be flat.
The SVS 3000 Micro sub has an 800 W amplifier built in and can produce lower bass at much higher output than the Quads ever could, so configuring the amp and speakers this way is a win-win situation. I get the lows that the Quads can't produce well, and the whole system can play louder. This is going to be true of any speakers you use that have poor low frequency output/response, including about 99% of all bookshelf speakers.
System Setup: The Subwoofer
The Wiim amp's bass management and the subwoofer can both be configured via my phone or tablet. It's a little bit of messing around, but once the configuration is done, it doesn't need to be done again.
Connect the sub output on the Wiim Amp Pro to the LFE input on the subwoofer. Open the SVS app on your phone and switch to LFE mode - that tells the sub's DSP that it will only receive low frequencies at its input so it doesn't need to run a low pass filter of its own. In my system I set the subwoofer output to -20 dB, but your system may need a different setting. This can be changed later, as needed.
Connect the speakers to the jacks on the back of the amp and the sub output to the subwoofer's LFE input. Open the Wiim Home app on your phone. Select the amplifier and open its settings menu. Select the "Sub Out" item. Turn Sub Output on, set the level (start at 0dB and increase later if needed), set the crossover frequency (the default is 80 Hz which is pretty good for most speakers, including the Quads), you'll set the phase later, so don't worry about it for now, switch "Subwoofer Bypass Mode" off, and "Main Speakers Output Bass" off.
Play some music that has some low bass. As you listen, flip the phase switch back and forth between 0 and 180 degrees. At one setting, the bass response will experience a dip and at the other setting, a peak. You can usually hear the difference pretty distinctly.
The final setting to make is to use the Wiim Amp's bass sync feature. The subwoofer and Wiim amp both have some delay resulting from the signal processing that goes on in their DSPs. There may also be a different "time of flight" between the main speakers and the sub and your listening position. Ideally, you want the subwoofer to be time aligned with the main speakers. The Wiim amps have the ability to perform the synchronization built in, but it's less than ideal. It uses a mic built into the amp to pick up test signals generated in the amp. It will then report the delay and adjust the signal(s?) to the main speakers and sub to be properly time-aligned. It would be better to use a mic at the listening position (as RoomFit uses), so right now, to do it right you have to put the amp at the listening position when you run the bass sync. Fortunately, using a mic at the listening position is possible in the latest beta release of the firmware for the amps, so in the near future you'll be able to use the same mic you use when you run RoomFit at the listening position.
I set up Roomfit in non-boost mode to adjust response from 20-200 Hz using a Harmon target curve with ERB smoothing and plugged a UMIK-1 calibrated mic into my tablet and here is the result:
Then I turned on the PEQ and boosted the response above 10 kHz by a few dB. Finally I ran the subwoofer and main speaker sync and it set an 8ms delay on the sub and it all sounds great.
Listening
When I want to play music, I select it via Tidal Connect or LMS and touch the play button on the screen. The Wiim Amp Pro wakes up and starts playing which triggers the sub to start playing within a few seconds. I can start, stop, select new music, skip ahead, skip back, and control volume, all from my phone, so there are no remote controls to hunt for or juggle and I don't have to go to the amp and sub to turn them on. They just start when I want to hear music, and when the music finishes, they both drop back into standby mode.
The amp can be placed out of sight, under furniture or in a cabinet, because there's no need to touch it. Do give it a little ventilation though because as efficient as class D amps are, they do generate a little heat:
WAU top side thermal image after playing for an hour at moderate volume, with low frequencies routed to the SVS 3000 Micro sub.
WAU bottom side thermal image after playing for about an hour at moderate volume.
This sort of operation won't satisfy you if you prefer to handle records or CDs and/or you like to twiddle knobs and flip switches, but for me it's ideal.
And in case you are wondering, yes, it sounds great! If you want more flowery superlatives, see some of the video reviews linked above. I don't have the vocabulary that those guys do.
I like to listen to music or recordings of tree frogs and crickets when I'm going to sleep so I've had a stereo system set up in the bedroom for years. The system consisted of an old (1983) Luxman RX-103 receiver, a SqueezeBox Touch to stream music from my server, and a pair of 20 YO Canton Ergo 22 DC speakers. The receiver was too big for the nightstands, so it sat on the dresser near the wall opposite the bed. Running speaker wires all over the bedroom wasn't an option, so the speakers were on the dresser, too. Unfortunately, that required turning the volume relatively high which might disturb my neighbors late at night because the walls/floors/ceilings are a little thin in this building.
An ideal system would fit on the nightstands near the bed so the volume could be kept to a reasonable, late-night level, but there was already too much junk on the nightstands, including table lamps. I started by getting rid of the lamps and replaced them with a couple PS 2014 hanging lamps from Ikea. It turns out that besides not taking up any space on the nightstands, they throw very nice patterns on the walls! They don't provide a whole lot of light, even when open, but that's OK for me.
The new system in place. You can just see the new amp in the left corner beside the speaker.
This is what it looks like with the lamps open.
A lot of the other stuff was disposed of or put in drawers and the result is that one nightstand has a speaker and my phone charger, and an alarm clock, and the other has a speaker, the new amplifier, and a CPAP machine. It looks a lot nicer, and is easier to keep dusted.
WAP!
When I was looking for amplifiers, I initially thought I'd continue using the SB Touch streamer. So I looked at the small class D amps by Fosi, Wiim, Marantz, Eversolo, and a few others. It occurred to me that the Wiim Pro Plus streamer in my living room system plays music from my Lyrion Music Server just fine, so maybe I could buy an amp with a built in streamer that would replace both the Luxman receiver and the SB Touch. Some of the amps I looked at had built in streaming with touchscreens on the front panel, but I decided I didn't need that as I'd be using my phone to control it anyway. I almost never used the touchscreen on the SB Touch.
After much digging through reviews and specs, I settled on the Wiim Amp Pro (WAP). It's small, has more than enough power for my purpose, and has a built in streamer that works with my server and Tidal Connect, just like the WPP in the living room. Both players can be synchronized so they play the same music at the same time (nice when I'm cleaning the condo).
The WAP has toslink, USB, HDMI, and line inputs. It also has BlueTooth in and out but doesn't support high quality codecs (yet). There is no phono preamp. It has built-in graphic and parametric EQ with room correction(!) called RoomFit, and real bass management that can apply high pass to the audio in the WAP, and low pass to the subwoofer output. The Wi-Fi antenna is built into the case so there's no ugly antenna sticking up from it.
Front panel- just a few LEDs and the volume knob with play/pause button.
Back side of the WAP. Yes, power supply is built in!
Speaker connections are solid! The speaker binding posts grip banana plugs securely.
The side holes easily allow 12 gauge wire, if you feel you must.
Bottom of the WAP. The holes along the sides and bottom are all they need to ventilate this thing thanks to the efficient class D amplifiers.
When I got the amp, I connected it to my Wi-Fi network via the Wiim Home app on my phone (already there for the WPP in the living room system), and it immediately started updating its firmware. After a few minutes it was ready to go. Lyrion Music Server saw it as a player without any messing around. Everything just worked exactly as it should, unlike so many other things these days. It's been in the system for about two weeks and I haven't had even the slightest trouble with it.
The amp can be controlled via Wi-Fi (phone or tablet) or its own Bluetooth remote control, so I don't need to access the front panel of the amp at all, and it may end up under the bed, out of sight, if it can get a decent Wi-Fi signal down there.
Technical stuff
The WAP uses TI TPA3255 class D amplifiers to drive the speakers. They are very efficient compared to class A or AB amplifiers. That efficiency is achieved by switching the output transistors on (very low resistance) and off (very high resistance) at hundreds of kHz, so they spend very little time between those states generating heat. That efficiency means they don't need large heatsinks which minimizes the size and cost of the amplifier. The PWM output from the transistors is run through a low pass filter to create a smooth analog output waveform that drives the speaker.
Unlike class A and AB amplifiers, class D amps don't normally provide a lot of dynamic headroom. This amp is rated for 60W at 8 Ohms (and 100W into 4), and that's all you'll get from it. That means if you're driving inefficient speakers and/or your room is large, it may not play as loudly as you'd like before it distorts audibly. Neither of these are problems in my situation.
I was curious about the switching frequency used so I connected the amp to an 8 Ohm dummy load and my Siglent digital oscilloscope to see what the output looked like when there was no music playing. Here's what I found:
This is the output of the amp with no music playing- about 600 mVpp at about 595 kHz. It's far beyond audible frequency range, even if you have golden ears, but could pose a problem if you're trying to listen to MW or SW radio anywhere near this amp. This signal could be the output switching clock leaking through, or it could be from the power supply that is also a switching circuit.
I used the scopes FFT to look at the spectrum of the output and found plenty of harmonics of the 595 kHz output.
That output looks ugly, but there's no audible component of it coming from my speakers, even with my ear close to the tweeter.
Here's noise in the audio band from 10 Hz to 20 kHz. It looks very quiet except for a little 60 Hz from the power line.
My scope's FFT is OK, but it's not an audio analyzer. I suggest that if you want to see a real detailed technical review, you check out Audio Science Review.
I have read online that some people complain of a whining sound coming from the amplifier. If I press my ear against the top cover of the amp I can hear a very faint whine, but there's nothing audible (to my 67 YO ears) otherwise.
Using it
It seems to pick up 5 GHz Wi-Fi very well (plays 24 bit, 48 ksps music via Tidal without buffering), so I haven't had to use the ethernet port to connect to my network like I did with the SB Touch. The front panel has only a status LED that changes colors to indicate different things, a few more LEDs to indicate the relative volume level, and a single encoder knob with a built in push button to control volume and play/pause music.
The volume control has an odd feel to it, almost like turning the knob in a soft piece of rubber. There are no detents. I have noticed that changing volume by any of the available means seems to react a little slowly- it takes a fraction of a second for the volume to change when I start turning the knob, so I tend to turn it a little further than needed for the change I want to make.
The unit has a BT remote control with a mic built in for doing the "hey Google" or "Alexa" thing. I don't use either, so I can't comment on the performance. You can put the amp out of sight - under the bed, in my case- or in a cabinet or drawer because it doesn't use IR for the remote control. The remote control's mic can't be used for RoomFit equalization tests.
The RoomFit function is a nice extra that you may or may not want to use depending on how picky you are about the sound. What it does is send a swept tone through the speakers, measuring the response in your listening position with a mic (usually in your phone or tablet running the Wiim Home app), and then it generates an equalization curve that alters the frequency response of the amp to achieve optimal sound quality. An uncalibrated mic's frequency response or directivity patterns are unknown so results of RoomFit using the mic in your phone or tablet will be questionable at best. For better results, you can get a calibrated mic (like my UMIK-1 that I used to check resonance when I was rebuilding the Quads) that plugs into a phone or tablet, and RoomFit can use the mic's calibration file when it does its magic.
The amp sounds great with the Cantons, as expected. I haven't tried driving the Quads with it yet to see if it sounds any different from the A12 amp that's in the living room system, but soon. I expect it will sound the same/fine until it starts to distort (if I can run it that loud without freaking out the neighbors).
The Quads can't play very loudly compared to "normal" speakers, mostly because the narrow spacing between the stators in the drivers limits the diaphragm excursion when playing low frequencies. The WAP's bass management could be used to set up a biamped system in which the low frequencies are sent to a subwoofer (keeping them out of the Quads) which would allow the Quads to play much louder than they could otherwise. More experiments to follow.
I use a diaphragm tensioning jig to set the resonance of the drivers to match the factory value of about 86 Hz. Even though the drivers play below their resonance in the speakers, there's no hump in the frequency response at resonance. So why does the resonance matter? Setting the resonance to match the factory value does two things. First, it ensures that there is sufficient tension on the diaphragm so that when the HV bias is applied, the diaphragm won't pull to one side and stick to one of the stators. Second, it ensures the sensitivity of the drivers will be uniform if the resonance of the drivers is uniform.
There are companies that sell kits to replace diaphragms on ESL-63s. They recommend tensioning techniques that are far from ideal. Most recommend taping the film down on a flat surface, pulling it tight as you go around the film. Some even provide a spring scale and recommend pulling the film to a specific value on the scale before taping it down. That's the sort of thing hobbyists were doing in the 80s, and isn't likely to result in matching driver resonances.
I rebuilt my ESL-63s using a diaphragm stretcher made from MDF and wood, that allowed the resonance to be set to a specific value. It worked well, but after multiple uses, adhesive from the tape used to secure the film built up on the wood and the tape started letting go before I could glue the diaphragm to the stator grid. I found it very difficult to clean the adhesive off the wood. Also, the opening in the center didn't match the size of the driver, so the resonance measured on the stretcher was different from the resonance measured once the diaphragm was glued to the driver. I'm rebuilding more drivers (going to turn my 63s in 989s) so I decided to try to make a better stretcher, one that would, I hope, provide the same resonance on the stretcher and driver, and wouldn't have the same problem with tape adhesive.
I found some promising aluminum extrusion in my materials pile collection. I tried sticking some of the double sided tape used to hold the film on the stretcher to the aluminum and pulling it off, multiple times and found it didn't leave residue behind like it did on the wood stretcher. The next thing to do was model the aluminum in CAD.
The aluminum I used has this profile. It's 56.8 mm high x 42.5 mm wide. Those wide, flat areas on the top, bottom, and sides are very useful for this application. I have no idea who makes this particular stuff, and neither does Google Lens. Square or rectangular aluminum tubing would work as well and would be it would be easier to design and print corner pieces.
I measured the drivers and found the opening to be exactly 583 x 175 mm, so I cut the aluminum a few mm longer than needed with a saw, then milled it square to exact, matching lengths. The axial holes in the aluminum are sized for a 1/4" tap, so I tapped them with 1/4-20 threads.
The next step was to design and 3D print corner pieces to hold the aluminum rails. I split each corner piece into two identical parts so I could print the part that inserts into the aluminum without using any support material. The screw holes were printed 4mm in diameter and drilled out with a 1/4" drill after printing.
This is what the corner pieces look like. Support material is used inside the screw insertion slot. The slot is about 12mm high to accommodate the button head cap screws that mount the corner pieces on the aluminum rails.
One of the corners holding two of the rails together. It's a tight fit!
The whole frame. The 1/4" hole in the green rail is for the tire tube valve stem. I had to get a 20 x 1 3/8 tube with a 60mm long stem and Presta valve.
Here's a driver sitting on the frame. The opening in the frame matches the driver (583 x 175 mm) within a fraction of a mm. The plastic corners fit so tightly I had to tap them in with a rubber mallet.
The next part of the design was to make a base that would allow the stretcher to be positioned vertically for resonance testing. In the previous design, when the stretcher tilted up, one edge hit the hinge support and stopped it from tilting further. That meant that during resonance testing, the tensioned diaphragm was in contact with the hinge support. I didn't like that. It doesn't take much to puncture and tear the tensioned film, so it's best to avoid physical contact with the film.
In the new design, I added "pins" to use for the hinge and an additional one for the support/tilt stop. It will allow the stretcher to tilt up to vertical, and I'll add a cord that will prevent it tilting so much that the edge hits the hinge support.
I installed three bolts for the hinge and support/stop.
The stretcher mounted on the stand. Yup, that's all there is to it. All that's left is to add a cord to the support/stop to prevent it from tilting too far, and some neoprene foam to the top surface of the stretcher frame.
First test with 6 um film. Worked well, but pointed out some minor issues. The pump hose has a pressure indicator that makes it hard to attach and detach the hose without accidentally closing or opening the valve. The film ultimately split, I think due to something sharp along the bottom edge of the frame.
After the film split during the first test I used a Scotchbrite pad and ran it over all the edges of the stretcher to smooth out anything sharp that might have caused the film to split. I had no further problems with film splitting.
The pump/hose I was using on the wood stretcher didn't work so well on this one. The pressure indicator on the hose took up too much space. I ordered another small tire pump that came with a hose and no pressure indicator. That solved the pump/valve problem.
There's one more problem I've been struggling with since I bought the roll of film back in the 80s. When the film is pulled from the roll, it generates a static charge that causes it to try to stick to anything and everything nearby (and pulls dust, cat hair, etc., from the air). When I put the film on the stretcher, it immediately curls under and tries to stick to the tape. Then I have to try to pull it free so I can position it where I want.
I decided to try to make a proper dispenser for the film that would kill the static charge generated when the film comes off the roll. I designed a couple conical end pieces with F608 skate wheel bearings and added two grounded steel tubes for the film to pass over as it comes off the roll. 8mm bolts go through the bearings and extend through the printed mounting brackets that are screwed to a piece of wood.
One end of the film dispenser. The other end is a mirror of this one. There are two conical plugs that fit the ends of the film roll, each of which has two F608 skate wheel bearings and an 8mm bolt. The ends of the bolts pass through the end supports that are screwed to a piece of wood. The film comes off the top of the roll, goes down under the bottom steel tube, wraps between the tubes, then pulls over the top of the upper steel tube so both sides of the film contact the steel.
The bad news is this method doesn't actually take the charge off the film. It simply provides a new means of charging the film by sliding the plastic over the steel tubes. The good news is that it doesn't matter. I placed the film dispenser on the work table just behind the stretcher and I worked out a technique for attaching the film to the stretcher that works fine even with the charge on the film. Getting rid of the static charge on the film would require some sort of ion generator that would spray the film with ions and neutralize the charge as it comes off the roll.
Now I pull the film off the roll right over, and attach it to the two front corners of the stretcher. Then I cut the film free of the roll and attach it to the stretcher at the back corners, pulling wrinkles out of the film as much as possible as I do it. Then I attach the film to the short sides of the stretcher and finally to the long sides. The new stretcher allows me to see the film as it attaches to the tape.
Film coming off the roll and getting attached to the stretcher. I've added neoprene foam (black) around the edge of the opening in the top of the stretcher.
Once the film is attached, I put some air in the tube and tilt the stretcher up to check the resonance and adjust the air pressure as needed to get to the target value (about 80-85 Hz).
Next I wipe the film and driver grid with IPA, then apply the 4693H contact cement to both, wait 15-20 minutes, check and adjust resonance again, and stick them together. I have found that with this stretcher, the final diaphragm resonance will be about 6 Hz higher on the driver grid than it is on the stretcher, so I set the resonance on the stretcher to about 80 Hz.
Resonance testing. I have added a couple marks on the stretcher frame (not visible here) indicating where the center of the diaphragm is to make positioning the microphone as accurate as possible. You can see the thin, black cord that is used to limit the tilt-up.
I pile some weight on top of the driver grid sitting on the stretcher and wait a few hours before letting the air out of the tube and cutting the driver free. Then I use a soldering iron set to about 260C to make the holes in the diaphragm around the center posts. When making the holes, I wear 5x loupes so I can see clearly, and keep the tip of the iron in contact with the post as I circle around it. Sometimes this creates fine plastic hairs that I remove using the soldering iron.
After letting the drivers sit for a couple days, I clean the excess glue off all the edges, tape off all the long edges, spray and wipe the diaphragms with IPA, then position the 3D printed masks in the center holes and on the ends, and spray with Licron Crystal. I make 4 passes, alternating L to R and R to L, then turn the driver around and do it again. I set the driver aside to dry while I spray the next one. I get reliable 10^8-10^9 Ohms/square resistivity using this technique.
Four drivers with newly recemented stators and new diaphragms, with long sides taped for spraying with conductive coating - Licron Crystal.
3D printed masks to block the conductive coating in selected areas of the driver.
I test the resonance after spraying on the Licron Crystal coating. I've been able to get pretty consistent results- here are resonance plots of 4 drivers I recently tested. Note- the lowest frequency peak is the one I use to set the resonance. The the drivers have multiple resonances and the mic picks up other sounds in the room (the AC running, garbage trucks outside, neighbors vacuuming, etc.), so it's safe to ignore the other stuff.
Final testing is done using electronics from an ESL-63 speaker- I have alligator clips on all the wires that go to the driver, and simply connect an amplifier and apply a signal. What I'm mostly looking for here is any hissing or whining noises coming from the driver when the 5.2 kV bias is applied, and any odd sound that a damaged driver might produce.
The two white boxes contain all the electronics from an ESL-63 speaker. I just connect the driver using the alligator clips, power it up and give it an audio signal from an amplifier.
Final testing is done with bias and audio signal applied:
The two halves of the early model drivers I have are held together by the three center screws and the four corner screws that hold the driver in the frame in the speaker. Newer ESL-63 drivers came with small metal clips to hold the two long sides of the drivers together. I think those clips are a good idea, so I ordered some 1/2" binder clips because they should fit in the 15 mm wide spaces in the driver grid. The 1/2" clips are actually 15mm wide (only in 'murica folks!) so I had to grind them all down to fit.
When I am ready to add more bass panels to the ESL-63s I'll write another blog post.
Update: I designed, built, and used an improved diaphragm stretcher that works better than the one in this post. You can see details here.
Now back to the original post:
I recently prepared a very long post on my efforts to restore a pair of Quad ESL-63 electrostatic loudspeakers made in 1983. That post was made almost daily as I did the work, and includes a lot of things I tried that didn't work. This post leaves out the stuff that didn't work and summarizes the stuff that did. I have omitted a lot of detail here, so check the other post if anything isn't clear.
Main parts of the rebuild:
1) remove tapes and their adhesive residues that were used in the assembly and replace them
2) Reglue stators
3) Replace diaphragms
4) Replace dust covers
5) Repair Electronics
It's best to do this work in a room that's free of dust, dirt, and pet hair. Keep a vacuum cleaner handy and stop to vacuum things off at each step in the process. It doesn't take much dirt to cause the drivers to hiss and whine.
I used lint free microfiber cloths whenever I wiped anything with either a solvent or water. Paper towels might leave lint behind that could turn into a source of noise.
I wore 5x and 2.5x loupes when doing detailed work- being able to see what you're doing is really helpful. Use plenty of light in the room, too!
The Tapes
For some reason these speakers had a bunch of different foam and other tapes used in their construction. After 40 years, some of the adhesive became brittle and the tape wasn't sticking well, and foam tapes were rotting away, usually leaving sticky residue behind.
Removing the old tape typically involved peeling it off, then scraping as much of the adhesive residue off as possible. Finally, solvents were used to remove the last of the residue. I used Goo Gone, an orange oil based solvent, and Goof Off, a very volatile solvent.
The purpose of some of the tapes used was questionable, but I replaced almost all of them anyway.
Note: some folks may get the idea that they can use double stick tape to mount the diaphragms on the driver grids. No, you can't. It doesn't work. The problem is that the tension on the film will pull the adhesive from the tape inward, toward the center of the speaker, more importantly, the tension on the diaphragm will be reduced when it does. That will lower resonance and may cause the diaphragm to stick to one of the stators whenever power is switched on.
Here are all the locations of the tapes that need to be replaced. A, C, and D are all inside the dust covers, so it's absolutely critical to clean the old stuff out and replace it with long lasting materials. Double sided tapes are used to mount the steel grids on the speakers and to mount the dust cover film. All others have adhesive on one side only.
Adhesive tapes for 2 speakers
location
total length
width
thickness
adhesive
type
A
driver mounting brackets
48"
3/8"
1/4"
single
foam
B
vertical frame supports
248"
3/8"
1/4"
single
foam
C
driver mounting brackets
120"
1/2-3/4"
3/4"
single
foam
D
top and bottom dust cover mounts
184"
3/8"
1/4"
single
foam
E
inside bottom panel
8"
3/8"
1/4"
single
foam
F
top and bottom of frame, holds grids
200"
5/16"
1/8"
double side
foam
G
top and bottom of frame, over grids
234"
1/2"
single
gaffer
H
vertical edges of grids
256"
1"
single
gaffer
J
dust cover frames
448"
3/4"
1/8"
double side
foam/other
Note: F tape seems redundant- the G gaffer tape wraps around the driver enclosure, preventing the grids from moving. Maybe I'll use some of the same tape used at B and D to prevent the grids from rattling...
I used EPDM rubber weather seal foam tape that should hopefully last longer than the original polyurethane foam tapes that the factory used. For the dust covers I used 3M VHB double stick tape. I didn't use double stick foam tape at the top and bottom edges of the metal grids, and I didn't apply gaffers tape around the top and bottom of the driver enclosure. I want it to be easy to go back in and fix things if it becomes necessary. If they're still working right in 6 months, maybe I'll put the tape around the top and bottom edges of the driver enclosures.
The Stators
Each speaker has four drivers. Each driver is made of two plastic grids that clamp together, one at the front side and one at the back side of the speaker. The front side grid has the diaphragm glued to it and the back side grid has conductive metal strips that contact the diaphragm. Both grids have stators glued to them. The back side stator has some fine nylon (?) cloth glued to it to dampen the resonance of the diaphragm.
The stators are thin, perforated PCB material that has copper on one side, covered with some sort of clear coating. The diaphragm sits about 2.5 mm away from the stators. When the stator glue bonds fail, the stators tend to curl inward toward the diaphragm, resulting in contact that makes noise even when the speaker isn't playing music, due to the 5.25 kV bias.
Once the stators start to let go, they need to be removed completely and reglued. If the stator glue bonds fail on the front side grid, the diaphragm has to be removed to reglue the stator, and then the diaphragm has to be replaced.
Usually, some of the stator glue bonds will have failed, so you break the stator free of the grid by pushing on the stator where the bonds are already broken and keep pushing until all the bonds are broken. You'll need to unsolder the electrical connection clips to remove the stator completely, and then clean flux off the solder pads using IPA.
Here's how you find broken stator glue bonds:
Original solder on one of the center drivers. Do you think they used enough solder? The black stuff on the edge is where one of the metal clips that hold the two sides of the driver together was positioned. All those clips had a little rust on them.
Aluminum clamped to the stator for desoldering. You have to remove the solder so the stator will sit down flat in the grid. The aluminum acts as a heatsink to protect the cloth that's glued to the bottom of the stator (back side stators, only).
Stator with flux from desoldering wick. That needs to be cleaned up.
This is what it should look like after you clean the flux off by wiping with a towel soaked in IPA.
Scraping the stator glue off one of the grids:
I also used the scraper to gently remove any excess glue from the stator. Don't get too aggressive- you don't want to cut into the plastic coating that covers the copper on the stator.
Once the grid and stator have been scraped and are ready to glue, vacuum them off to get rid of the glue dust. You don't want any foreign objects to get into the drivers or they will make noise.
This is the glue I used for the stators. One bottle is plenty to reglue all the stators for two speakers.
I applied the glue using a 5 ml syringe with an 18 gauge gluing needle. The syringe can be reused, but you'll need a new needle for each stator you glue, so buy a bunch of them. They're cheap.
Applying glue to the grid. I use clear polyurethane Gorilla Glue and wipe the grid with a cloth dampened with distilled water just prior to applying the glue. Then I wipe the stator with water before setting it down on the grid/glue. The glue needs the water to set properly, so don't skip this step. Glue only goes on the vertical ribs of the grid. It took me about 10 minutes to apply glue to all the ribs on one grid.
This is the stator clamping tool I made. The 1" thick foam strips are mounted on a piece of plywood about the same size as the driver. When gluing a stator, a couple kg of weight is placed on the plywood to ensure the entire stator is pressed against the grid.
This is how the clamping tool sits on the stator and grid. When you are actually gluing a stator, put a layer of saran wrap down before you put the clamping tool on the grid/stator. You don't want excess glue to stick to the foam.
I dampened and applied glue to the vertical ribs on the grid, placed the stator, covered with saran wrap, then placed the clamping tool and some weight on it to hold the stator flat against the grid.
The Diaphragms
The diaphragms are very thin (3 um) polyester film (Mylar is one of many brand names) stretched tight on a rigid frame, and they have resonances like a drum. I tested the factory diaphragms before I pulled them off their grids and found a consistent 86 Hz primary resonance. The test was performed by tilting the driver up vertically, then "thumping" the diaphragm close to its center, using a steel ball hanging from a thread. I placed a UMIK-1 measurement mic within 1 cm of the diaphragm and ran Room Equalization Wizard (REW) software on a laptop. I used the Real Time Analyzer (RTA) in REW to plot the spectrum of the sound captured by the mic. I used the same test method to tune the replacement diaphragms.
One of the drivers opened up. The piece on top is the back side driver grid, the bottom is the front side grid with the diaphragm attached. The gray stuff is the high resistivity coating on the diaphragm. No, it's not dirty, it's supposed to look like that. The metal tape at the edges of the top part makes contact with the coating on the diaphragm and allows charge to transfer to the diaphragm. You can see a thin cloth covering the stator. That's there to dampen diaphragm resonance. Don't remove it, and try not to damage it. Be careful when unsoldering the electrical connections to the stators.
Diaphragm coming loose in a driver. The problem is that the glue the factory used doesn't bond to the polyester film. This diaphragm has to be replaced, even if the driver plays OK.
This video shows how little effort it takes to peel the factory diaphragm off the grid. The glue they used didn't bond to the film.
I used a pneumatic stretcher to put tension on film and tuned the resonance to match the factory diaphragm resonance, before gluing it to a driver grid. The stretcher is a table with a hole in the center that matches the size of the driver grid, and has a wood lip. I put double stick tape on the lip on the underside of the table, neoprene foam on the top side of the table, and a bicycle tire tube is stretched over the lip of the table. The table is built so that it can tilt up for resonance testing/tuning while a diaphragm is being stretched.
Tilting stretcher table to allow easy and safe resonance testing of the diaphragm before gluing it to the driver grid.
The original diaphragms were a multilayer mystery, but comments by the engineer who designed the speakers indicate that they used a 3.5 um thick polyester film, and then applied multiple coatings to make the diaphragm slightly conductive, a necessary condition to apply charge to the diaphragm and have low distortion at large excursions (usually low frequency signals). I was unable to locate 3.5 um film, so I used 3 um film I was able to buy via ebay.
Replacing the diaphragms is a multistep process.
1) Remove the old diaphragm and glue that held them on their grids. I used a scraper and a solvent called Goof Off to remove the glue.
2) Lay the film on the top of the stretcher table, start at one edge and pull the wrinkles out of the film and stick it to the tape on the lip of the table. Then go to the opposite side of the table and do the same, then do the same at the other two edges.
Back in the 80s an engineer from 3M recommended 4693H contact cement for gluing the diaphragms and he was absolutely right about it. The stuff bonds to both the diaphragms (they way the original glued didn't) and to the grids. I used a silicone squeegee that I cut down to about 15 mm wide to spread the glue on the diaphragms and the grids.
How well does 4693H bond to the diaphragm? Here's me trying to peel off some film:
Rolling the film out on the stretcher table- film is stuck to the stretcher on the right end of the table to keep it from flying around. Note- my stretcher requires a significant margin of film around the driver to allow the film to attach to the stretcher. Be sure you order enough of the film!. I went through an entire 20m roll of film to replace the diaphragms on 8 drivers (two speakers).
This is what the diaphragm looks like on the stretcher before it gets stretched. Don't worry about the wrinkles, they will disappear as soon as the tube is inflated. The black stuff is neoprene foam that lets the slightly concave grid perimeter contact the glue everywhere.
3) Once the film is stuck to the table, I tilt it up vertically to inspect it and make sure the film is stuck down on the tape everywhere and make adjustments as needed. Once it looks good, I connect a tire pump and put a few strokes of air into the tube. That takes out all the wrinkles as it puts the film under tension.
This is what the film looks like on the underside of the stretcher before the tube is inflated. This is when you want to make any adjustments to the film position, before inflating the tube and tensioning the film.
The diaphragm thumper- just a ball bearing glued to a piece of thread. Simple but VERY effective!
The tight diaphragm being tested for resonance. The mic and thumper are positioned as close to the center of the diaphragm as possible. I made a couple 3D printed clamps and a short "boom" for the thumper. The RTA in REW displays the spectrum of the sound that the mic picks up.
The tight diaphragm glued to the grid (and stretched on the table) behaves like a drum and has a primary resonance and multiple other resonances based on the dimensions of the driver. The diaphragms that came from the factory had a consistent primary resonance of 86 Hz.
This is what the resonance typically looks like in the REW RTA. The primary resonance is the highest peak at the lowest frequency due to the longest dimension (the width) of the driver. The lower peaks at higher frequencies are due to the shorter height of the driver and maybe interactions/interference between the waves moving on the film. I set resonances of the new diaphragms slightly higher than the factory numbers, assuming that over 40 years the factory diaphragms may have "relaxed" a bit, and to allow for the extra mass that will be added by the conductive coating.
4) Tune the resonance of the film on the stretcher by pumping more air into the tube (or letting some air out, though I never had to do that). Once you have the resonance where you want it, disconnect the the air pump to prevent slow leaks through the hose or pump. Apply 4693H contact cement to both the grid and the film on the stretcher. I spread the glue with a silicone squeegee that I cut down to about 15 mm wide. Try to keep the glue on the film in the area of the neoprene foam. Let the two pieces sit for 20 minutes to allow the glue to set.
5) Once the glue is set, tilt up the stretcher and check the resonance one more time. Adjust the air pressure as needed, then carefully set the grid down on the tight film on the stretcher. The film and grid will bond instantly, so be very careful! I manually pressed the edges of the grid down on the film then put some weight on top of it and let it sit for a couple hours.
6) When you're ready to free the grid from the stretcher, let some air out of the tube and slice the film close to the tube, leaving a wide margin of film around the grid. Lift the grid up and turn it over to inspect it- there should be absolutely no wrinkles anywhere. If there are, you'll have to do it all again.
Grid stuck to the diaphragm on the stretcher.
After releasing air pressure, cut the grid free by cutting the film all around the grid near the tube, leaving excess film attached to the grid.
The wrinkle free diaphragm on the grid is ready to have the excess film trimmed off. Pull gently on the film and slice it away from the edge of the grid with the razor knife blade pressed against the edge of the grid, using it as a guide.
7) Now carefully use a fresh, sharp razor knife to trim the excess film from around the driver grid.
8) The next step is to make holes in the film around the the three center posts where screws pass through the driver. I use a soldering iron with a small tip and temperature set to 265C. Hold the soldering iron with one hand and use your other hand to steady it. Put the tip down on the film right next to the post and use the plastic post as a guide and just move the soldering tip around the post, keeping it in contact until you've made a complete circle around the post. Remove the soldering iron by lifting it straight up. Next do the same at the other two posts. Finally, use some tweezers to remove any hairs or discs of film that remain stuck to the posts.
Small tip on the soldering iron.
This is what the holes in the diaphragm should look like after removing film disc and any hairs that are produced. Use your other hand to steady the hand holding the soldering iron, and use the post as the guide for moving the soldering iron.
9) Now you can check the resonance of the diaphragm, you just installed. Tilt the driver up vertically and adjust the mic and thumper positions as close to the center of the diaphragm as possible. Thump the diaphragm and read the resonance in REW.
Checking resonance of a diaphragm after gluing it to the grid and making holes for the center screws.
In my setup, the driver resonances come out about 10Hz higher than the resonances I measure on the stretcher. I think this is because the open area in the stretcher is a little larger than the open area of the driver grid. When I stretch film I set the resonance on the stretcher about 10 Hz below the target resonance.
10) Apply the high resistivity coating to the diaphragm. The coating doesn't cover the entire diaphragm- some specific areas have to be masked off. I 3D printed a set of masks to make this job very easy.
This is the area (gray) of the diaphragm that needs to be coated. The left and right edges are left uncoated to reduce leakage current due to proximity with the metal frame that the drivers are mounted on. The center circles also need to be blocked out because of the metal screws that go through the holes.
Driver with masks and blue masking tape on long sides, just after spraying with Licron Crystal.
The masks are made based on the uncoated areas of the original factory diaphragms. The coating process is easy- put masking tape on the long sides of the driver grid, place the masks on the ends and on the center holes, shake up the can of Licron Crystal and spray. I spray in two passes, right to left, then left to right, then turn the whole grid 180 degrees and do two more passes. Then I take the masks off and set the grid on a shelf for a few hours to dry.
End mask to block conductive coating on new diaphragms. Dimensions in mm. If you make holes in the specified locations you can use screws to align the mask with the driver, otherwise, just put some masking tape on the edges to hold it in place while you spray with Licron.
One of the hole masks showing the M3 screw that fits into the holes in the grid supporting the diaphragm and ensures proper positioning when applying the Licron.
11) After the the Licron coating has dried, I test it with a resistivity meter to see that the coating is very weakly conductive.
Checking the resistivity of the coating, in this case 10^9 Ohms/square- great! The coating process is so reliable, I would consider the resistivity meter optional.
12) Once the diaphragms are all coated, reassemble the drivers by installing the metal clips at the edges and the 3 central screws and nuts. Then you can mount them in the speaker again and reconnect all the wires you unsoldered.
The Dust Covers
Some people claim the speakers sound better without the dust covers. Some go so far as to remove the socks, metal grids, and the dust covers, exposing the drivers to everything floating around in the air including dust, pollen, pet hair, and bugs, and running the risk of electric shock to anyone foolish enough to touch the drivers while the speakers are operating. I don't recommend operating without all those things in place.
There are two dust covers in each speaker, one each at the front and back of the driver enclosure. The dust cover frames snap onto the aluminum side pieces and a couple plastic pieces at the top and bottom of the driver enclosure. The frame is barely rigid enough to support itself when it isn't snapped onto the speaker. The dust cover is made from the same film that's used for the diaphragms, and like the diaphragms, it should be wrinkle free. The only way to achieve that is to put the film under some tension, then attach the frame pieces to it, then mount it in the speaker and use a heat gun to shrink the film and take out the wrinkles that will inevitably appear. This is by far the worst part of the design of these speakers.
Start by marking out the outline of the dust cover frame on the work surface using masking tape. Make sure you get it square by measuring diagonals! The inside edges of the tape are where the outside edges of the frame pieces will be placed.
Applying the VHB tape to the frame pieces. The tape is wide enough to cover two pieces at a time, so I stick it to the two pieces laying side by side, then cut them apart with a razor knife.
The frame pieces cut apart.
Roll out the film and tape opposite corners to the table, pulling and applying tension as you go.
Next tape opposite sides, pulling and applying tension as you go.
Now add more tape on opposite sides, pulling and applying tension as you go.
Place the metal bits into the ends of one short side of the frame, and peel the backing off the VHB tape. Use the edge of the frame piece and the two metal bits to align the piece with the tape on the table top, and stick it down- be careful not to let it touch in any wrong spot as the tape will not let go of the film!
First frame piece stuck down on the film. Now take the metal bits out of the ends and put them on one of the long frame pieces...
Note the orientation of the metal piece- they can fit two ways, but only one is correct! Peel a couple cm of the backing tape off.
Hook the metal piece into the short frame piece that's already stuck down, and start sticking the long frame piece down, peeling the backing away as you go. I flexed the frame piece a little to set the center of it about 1mm inside the blue tape outline- this will help tension the film when the frame gets installed in the speaker. Now stick down the other long frame piece following the same steps.
Second long frame piece stuck down. Now prep the other short frame piece...
Put the metal piece on the end of the short frame piece, peel some of the backing off the tape, and get the frame piece into position- don't set it down yet!
Holding the frame piece up, peel off the rest of the tape backing. Keep the center elevated so it doesn't stick to the film and then bring the end down and hook the metal piece into the long frame piece, and finally let the short frame piece down to stick to the film. The hard part is over!
Now cut the frame free of the film that's taped to the table using a fresh razor knife held at a very low angle.
Frame cut free of the film on the table. There are going to be wrinkles- don't worry about it. Take it to the speaker, slide it into position, and snap the frame down onto the mounts in the speaker. There will be wrinkles- don't worry about it! Start working on the wrinkles with a heat gun- use a low temperature setting and keep the thing moving- you don't want to burn a hole in the film!
Dust cover on the speaker. Notice how smooth the reflection looks in the surface of the dust cover!. But there are a few small wrinkles at two corners...
Small wrinkles in one corner that could not be removed with the heat gun.
Small wrinkles in another corner that couldn't be removed with the heat gun. The other two corners were wrinkle free.
That's it! Heat shrunk film relaxes over time, and eventually, wrinkles may reappear in the film. Also, the adhesive on the tape remains soft. Tension on the film will pull the adhesive toward the center of the dust cover, also releasing tension - this is why you don't use tape to hold the driver diaphragms! The ideal attachment would be to use the 4693H adhesive I used on the driver diaphragms- it will not move under tension.
When and if I get around to rebuilding these as 989s, I will design the dust covers to have rigid frames that will be much easier to install.
The Electronics
The schematic of the speakers changed a bit during the early years of manufacture, so your speakers may be a little different from mine. In my case, the electronics was all working fine so the only thing I had to do was replace the nonpolar electrolytic caps at the inputs of the speakers (based on their age, not performance). I ordered new 220 uF NP electrolytics and some 4.7 uF PP film caps to bypass them. I also replaced a 1000 uF 16V cap used in a power supply for the protection circuit in each speaker. This schematic appears to match my speakers, though the speaker were made in 1983 and the schematic is dated 1989.
An ESL-63 schematic from 1989. Earlier versions might be a little different. The red box is the input circuit, the green box is a HV clamp, the pink box has the delay line, the orange box has the LPF for the bass panels, the blue box is the HV bias circuit, and the yellow box is the arc prevention circuit.
A lot of the circuit is there to protect the speakers from misuse. Looking at the input circuit in the red box, there's a MOV (a type of self resettable fuse) that protects the audio transformers from too much current in the primary windings. You'll see the 220 uF cap in parallel with a 1.5 Ohm resistor connected to the low voltage side of the audio transformers. The transformers step up the voltage from the amplifier to a few kV (hence the warning stickers on the transformers). You'll also notice a triac that, when switched on, shorts out the primary side of the input transformers (and puts the 220 uF cap and 1.5 Ohm resistor across the amplifier output! That's probably the only condition when the MOV would open up, protecting both the amplifier - we hope - and the speaker from the large currents that would flow were the MOV not there).
That triac is driven by the arc preventer circuit (yellow box) which detects the ionization of the air that occurs just before an arc forms. So if the speaker is being badly overdriven, and the air starts to break down, the arc preventer will turn on the triac as described in the above paragraph. This only works if the speaker is powered on!
The blue box is the voltage multiplier that provides the 5.25 kVDC bias to the speakers. It's a standard Cockcroft-Walton voltage multiplier that charges the caps in parallel and discharges them in series. There's a small neon lamp and capacitor connected to the output as well as a 10 Mega Ohm resistor. The resistor limits current available to charge the diaphragms and helps keep them operating in constant charge mode for low distortion. The neon lamp only lights up and conducts when the voltage across it is about 90V. That happens at power up when there's little charge left on the diaphragms, and if the charge leaks away from the diaphragms indicating that there is some dirt or foreign object (like a dead bug or pet hair) trapped in the speaker between the diaphragm and the stator or frame of the speaker. There will always be a little charge leaking away from the diaphragms into the air, so the neon lamp will flash occasionally in a normally functioning speaker. The frequency of the neon lamp flashing is a good indicator of the condition of the speaker. That lamp should only flash briefly once a minute or so. Frequent flashing indicates that it would be a good idea to inspect the speaker and maybe do some cleaning. The neon lamp is inside the electronics enclosure where you can't see it unless you take off the bottom cover.
The green box is a high voltage clamp/limiter circuit. It sits across the HV output side of the audio transformers. If the voltage rises above about 7 kV, the zener diodes in the circuit will turn on and prevent the voltage from rising further. That protects the speaker from damage caused by arcing if the speakers are powered off (which prevents the arc detector from working) and a loud audio signal is applied (maybe some doofus turns on the stereo and doesn't know the speakers are powered off, doesn't hear any sound, and turns up the volume to try to get the speakers to play). There's an LED in series with the zener diodes that will light up when the clamp is active, but it's inside the electronics enclosure where you can't normally see it.
The pink box is the delay line that is responsible for driving the rings (segments) of the stators at slightly different times to simulate the pulsing of a spherical driver.
The stuff in the orange box is a LPF that drives the outermost ring and bass drivers (the top and bottom drivers in the speaker). The 360k Ohm resistors work with the capacitance of the panels to roll off the high frequencies sent to those drivers.
Bottom of one of the speakers with the cover removed. The input cap is the blue part located under the cement resistor, circled in green. There's plenty of room for the replacement caps I ordered. The only thing supporting the cap is the wire that wasn't properly soldered to the input connector and that thin yellow wire that's going to the PCB. Note- the empty space to the right of the green circle would allow for much larger film caps to be installed in place of the non polar electrolytic cap that both the factory and I used. The transformers at the top left and right corners are the audio transformers. The green PCBs at the bottom are the delay line. The board in the lower right corner is the HV clamp. The top center section has the power transformer, HV bias circuit, and arc preventer circuit.
After I removed the old input coupling cap and resistor you can see the 1000 uF cap in the arc preventer circuit power supply.
New input coupling caps installed. The new 1000 uF cap is under there, too. I was able to unsolder the old parts and solder in the new parts from the bottom side of the PCB without having to take the board out of the chassis.
I found another schematic, maybe the original version of the ESL-63:
Early (?) ESL-63 schematic. There's no clamp on the output side of the audio transformers, but there is some sort of clamp in the input side of the audio transformers. The arc preventer is there, but looks a little different.
