SEAS A26: Improving the Stock Kit

The SEAS A26 is an amazingly musical kit of speakers as it is. But part of being an audiophile is the never-ending quest of taking something great and making it even better.

Earlier I wrote about two modifications I incorporated in my SEAS A26 build. First, I added bracing to stiffen the enclosure and support the magnet system. Second, I lined the interior with damping felt. These two improved the transient response and lowered resonance.

Since then my SEAS A26 build has undergone two more modifications. Let us get down to it!

2nd Order High Pass Filter

Over at the diyAudio forums, user Lozjek dropped a pretty masterful idea:

Quick improvement by adding 0.68 mH as a 2nd order to the tweeter — Significantly better tweeter power handling, on-axis response, driver phase still being ok. The XO point is pretty much defined by the woofer’s LP response, the only issue left being the tweeter’s HP and padding.


A26 2nd Order Filter

Remember, resistor R1 serves to attenuate the tweeter. By default this is 10 Ω. More on that later.

Wait, what — turning the beautifully simple first order high pass filter of the SEAS A26 into a second order one? Changing the very thing that makes it shine?

Well, not to worry. Be sure though to separate what a filter is electronically and what it is acoustically. There is no denying that this is a second order filter electronically. But as the following frequency response graph shows, acoustically it remains first order:

A26 Frequency Response Curve

Ingenious, no?

At this point you might be wondering what the fuss is all about. Why do this mod exactly? Well, as user Dissi points out:

The woofer of the A26 speaker actually is a bit too loud in the range from 600 to 1100 Hz. The tweeter is of opposite phase there and brings the level down by 2 dB. That’s how the original works.

But goes on to add:

With an additional inductor in the tweeter filter this doesn’t happen anymore and the hump at 900 Hz might become a problem.

All this piqued my interest.

Re-Engineering the Filter

I created a simulation of my own using Boxsim. I do like that software because of its optimization functionality, where it calculates different component values to meet a certain optimisation target such as phase or frequency.

Optimizing for overall balance and the default R1 resistor value of 10Ω, Boxsim calculated a 1 mH 0,41 Ω inductor. I ordered two 1 mH Jantzen Audio air coils 18AWG. The direct current resistance of these coils is 0,48 Ω. This is just north of 10% of the T35C002 tweeter’s resistance of 4,6 Ω, which is a good rule of thumb for component matching.

Jantzen Audio Air Coil

For a 12 Ω resistor R1, Boxsim calculates 0,97 mH with 0,42 Ohm DCR. Optimizing both R1 and L1, Boxsim actually calculates both this 12 Ohm resistor and 1 mH inductor. This lowers sensitivity from 10 kHz onwards somewhat. Psycho-acoustically, this this may sound warmer with less air. At that time, I preferred the flatter frequency response curve that the 10 Ω resistor provides.

Enough engineering, on with the results! I modeled the A26 again, this time in XSim. This allows to plot the stock A26 and modified A26 in one frequency response plot:

A26 Response Curve -- Stock vs. Modded

Contrary to earlier concerns, it seems that the coil actually decreases the hump around 900 Hz. Also it increases sensitivity from 2,2 up to 10 kHz. This matches with my subjective findings that the SEAS A26 now has more air, overall brightness while still retaining that warm 70’s bass. Well recommended.

SEAS A26 Loudspeaker Stands

The T35C002 tweeters should be at listening height. Like most tweeters they have a pretty steep the further off-axis you go:

T35C002 Frequency Response Curve

I never got around to putting my A26’s on good stands. But the new owner has.

Wait, what — again? You sold these beauties? Yessir I have. More on that later, in another post. For now I am real glad that they have found a new home with a standout audio enthusiast. And I am here to report on them.

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Here is what the new owner did:

I created stands, filled with sand, 35 kg a piece, spikes underneath, and put the speakers on them with theraflex putty. They even sound much better, the bass is a real experience, a feast to listen to. Now for a paint job!

More photos to follow!


A26: An Audiophile Loudspeaker Kit by SEAS

Building this SEAS A26 kit is a real trip down memory lane. My dad picked up a pair of Cabasse SAMPAN speakers in the 70s and so has literally played through them ever since I can remember. I just love the sound of them: warm and grand, sometimes bordering on bombastic, but ever so immersive.

I have longed for a pair of big-sounding vintage speakers ever since. But what to do when a set of floor-standers would not meet the Wife Acceptance Factor?

Introducing the SEAS A26

The SEAS A26 is a Do-It-Yourself kit for 28 liter monitors. The kit is a 21st century remake of the Dynaco A25, a legendary speaker in its time. More importantly, this set of largish monitors features an aperiodic port, that should make them sound larger than they are.

This was exactly the fun Do-It-Yourself project that I was looking for.

Woodwork for the SEAS A26

The front baffle calls for precision, so I had the baffles CNC’ed (computer-routed) by Kees of TechGraphix. An enormous audiophile himself, he even created his own CNC machine. Now that is DIY.

A26 EnclosureThe SEAS A26 plans do not specify how to attach the sides of the enclosure, so I mitered all edges 45 degrees for a seamless fit, as seen in Loudspeaker Magazine’s A26 build. My neighbor owns a high-quality table saw and was kind enough to help.

I glued the sides with wood glue, held them with a corner band strap, and filled any holes with MDF filler.

The cabinet is intended to remain closed, so I decided against drilling through holes to attach the drivers with nuts & bolts. Instead I pre-drilled 3 mm holes for use with 4 mm wood screws later on.

High Pass Filter for the SEAS A26

Although SEAS touts the crossover as a first order crossover, it really just is a high pass filter to the T35C002 tweeter. The A26RE4 woofer has such a smooth roll-off that it the amplifier drives it directly. Here, less really is more, and I admire its simplicity.

Tweaked SEAS A26 High Pass Filter

In the filter design above, I added a bypass capacitor as a small tweak. The components are as follows:

The main capacitor (C1) is a 3,3µF Mundorf MCap SUPREME (±2%, 600VDC). I also considered the Jantzen Audio Superior Z-Cap and Audyn Cap Tri-Reference. Of the three, the Mundorf is said to have that warmth that I am looking for, at a price point that is hard to beat.

The bypass capacitor (C2) is a 0,01µF Vishay Roederstein MKP1837 (±1%, 160VDC) made of polypropylene film. While not included in the original filter design, this is an inexpensive tweak to improve the transient response of the bigger main capacitor.

The resistor (R1) is a 10Ω Jantzen Audio Superes (±1%, 10W). In spite of being wire-wound, it behaves like a metal oxide film resistor, but without the added grain. That means it should handle the fast transients well that are so typical of treble sounds. The competing Mundorf MResist SUPREME (±2%, 20W) is four times as expensive for a 10W higher power rating that is not all that necessary for a tweeter.

Getting down to soldering, I always have a great time even though I do not do it much. I learned that my cheap 30W soldering iron lacks the energy to quickly melt 96/4 tin/silver solder. And so the result is not the prettiest, but does survive a good pulling and prodding. Here is a picture of a finished filter:

Soldered SEAS A26 High Pass Filter

I hot glued the resistor and two capacitors to a 12 mm MDF board, and tie-wrapped the main capacitor and resistor for extra strength. The open tie-wraps are for the internal speaker cables. They serve as strain relief for the solder joints.

Enclosure Bracing for the SEAS A26

SEAS recommends to brace the enclosure to enhance the performance. Research of enclosure materials supports that claim. For unbraced 18 mm MDF, his research shows the following resonant frequencies:

18 mm MDF Resonances

Now compare that to braced 18 mm MDF:

Much better! Gone is that first sweeping resonance. Others move up in frequency and are attenuated by as much as 14 dB. They extinguish faster too, except the one at 1200 Hz.

I settled on the bracing design of the World Designs A25 speaker kit, another remake of the Dynaco A25. That design supports the woofer’s magnet too, thereby improving its transient response.

A26 BracingThe bracing consists of a horizontal brace that intersects two vertical braces. Although the  dimensions of the World Designs A25 differ from those of the SEAS A26, the construction plans to the bracing were easy enough to modify.

The following drawings are for 18 mm MDF. The plans to the SEAS A26 call for 19 mm, but that size is not readily available where I live.

The two vertical braces:

A26 Vertical Braces

The diameter of the hole and corner radius allow for a few mm tolerance. I used a 57 mm hole saw, which still left a few mm for the “bump back” part of the magnet.

The horizontal brace:

A26 Horizontal Brace

The one critical dimension is the distance between the bottom vertical brace and the front baffle. This is 84,5 mm not accounting for any gaskets. I settled on 84,0 mm to account for my MONACOR MDM-5 foam sealing tape.

Enclosure Damping for the SEAS A26

The next question I pondered was how to damp the enclosure. I thought to ask SEAS themselves. Sure enough, none other than Håvard Sollien, R&D manager and designer of the A26 was kind enough to offer his advice:

It’s a good thing to line the interior with felt or wool. This is of course in addition to the polyfill damping that should be evenly distributed in the box.

Double-checking my bracing ideas:

I’m not sure how much sonic difference the extra bracing does in this cabinet, but it’s good to stiffen the cabinet and support the magnet system.

Awesome customer service. I really feel that Håvard went beyond the call of duty here. Much appreciated!

Following Håvard’s advice, I decided to line the sides with DAMPING10 felt. Although DAMPING30 would damp better, I could not justify its price.

It took real tinkering to get the protective sheet off of the self-adhesive layer. I thought that was pretty poor, especially considering the steep price for something made of recycled felt.

The picture below shows the bracing, damping and filter from the top:

A26 Damping

To maximize the damping on the back, I mounted the filter board on two risers and fitted felt between them. The risers are simple pieces of 18 mm MDF glued together.

Binding Posts & Cabling

The SEAS A26 plans for a terminal hole in the back, but who needs one when you can have binding posts? The gold-plated Jantzen Audio M6/27 binding posts are airtight without the plastic. Just drill two 6,5 mm holes, 19 mm apart, and tap the bindings posts into place.

The following picture, taken from the bottom, shows the protruding binding posts, internal cabling and the filter:

SEAS A26 Internal Cabling

The cables are Jantzen Audio twisted solid-core 6N cables, 6N meaning “six nines” or 99,9999% oxygen-free copper. The isolation is thick, the cables sturdy, and their purity should guarantee a long life, free of oxidation.

The thickness to the woofer is 1 mm² (17 AWG), to the filter and tweeter 0,5 mm² (20 AWG). That may sound small, but mind you: these cables are solid core, not stranded, so the actual amount of copper is relatively larger.

Finally, I used Intertechnik BS05G gold-plated banana plugs to connect the external speaker wire to the binding posts.

Enclosure Stuffing for the SEAS A26

I used black Sonofil as polyfill stuffing, 50 grams on the inside and 12 grams in the aperiodic port. Contrary to what is written in Loudspeaker Magazine, Sonofil sheets weigh 50 grams, not 62, so you need two bags.

With the bracing in place, dividing the Sonofil evenly is as easy. I calculated the amount of Sonofil per “quarter”, cut the sheets to size and weight, then glued them to the bracing. The 50 grams are divided as follows:

  • Bottom-back: 27 grams
  • Bottom-front: 12 grams
  • Top-back: 8 grams
  • Top-front: 3 grams

As for the aperiodic port damping, I did not fancy the looks of some sort of improvised grille. Instead, I cut a 23 cm piece of Sonofil to weight, rolled it up, and glued it in the port with transparent hobby glue.

A26 Stuffing

In the picture above, the bottom brace has stuffing attached on both sides. The back side is fully covered, the front has two strips from top to bottom. The top brace has a sheet on the back side, while the front stuffing is lying on the horizontal brace. I fixed all sheets in place with hobby glue.

Silicone Feet

These speakers are intended for living room use, on a Meranti hard wooden floor with kids crawling and running about. Not wanting to use spikes, I stumbled on a brilliant idea how to make silicone feet out of an ice cube tray.

DIY Silicone FeetThough the instructions are in Dutch, the idea is simple enough to grasp, but hard to get right. First, putting the silicone layer on layer is sure to leave unsightly air cavities. Keeping the kit gun center prevents that, but may not fill the entire cube.

Second, it can easily take two weeks for the cube to dry, and then some. I found that the bottom half would not dry without a hole in the bottom of the cube. I suggest that you drill holes beforehand.

I have subjectively listened to the speakers in three configurations: first directly on the hard wooden floor, then separated by a sheet of DAMPING10 and finally with the silicone feet. I can attest to a noticeably tighter bass when using the feet.

Measuring the SEAS A26

Loudspeaker Magazine posted the measurement diagrams with their review of their A26 build. Given that I have used bracing, damping and higher quality filter components, my build should sound even better.

Listening Setup

My amplifier is a Marantz NR1602. With seven discrete 50W channels, it is no power beast, but more than enough to fill my 26 m² living room. I play music with the following Audyssey settings:

And Blu-rays as follows:

  • MultEQ: Audyssey
  • Dynamic EQ: On
  • Reference Level Offset: 0 dB
  • Dynamic Volume: Light

I turned and pitched the speakers so that they directly face the primary listening position on the sofa.

Finally, I let the Mundorf capacitor break in for an hour or ten. I had not expected that to be necessary, but it was. Though the speakers sounded great on first listen, their midrange seemed to have collapsed the day after. Playing a variety of material opened them up again.

Listening Results

There is a great sense of musicality to these speakers. Treble is extended, with great clarity and air. I literally rediscovered some recordings that I thought I knew. Cymbals, snares and all sorts of other percussion sound snappy and precise. The T35C002 is the première driver on the A26, and it shows.

The bass response of the woofer is as good as my Klipsch Sub 8 was, and sits in well with the rest of the music. It is tight, deep and much more well-behaved and rounded than the Sub 8. Playing guitar virtuoso Steve Vai’s bellowing Warm Regards shows off the deep, round bass that this woofer is capable of.

The soundstage is much wider than the speakers are apart physically. This really shows in a good drum stereo mix, such as the opening to the 30-minute Stranger In Your Soul epic by progressive rock supergroup Transatlantic. The height is fairly level, as expected of monitors.

These speakers have a sweet spot for blues, country and jazz. Candy Dulfer’s saxophone sounds sweet and soothing. Eric Clapton’s “Chronicles”, B.B. King’s “Deuces Wild” and Willie Nelson’s “Milk Cow Blues” sound full and captivating. That is another characteristic of these speakers: a complete lack of listening fatigue.

The A26’s do need high quality recordings. The “Three Tenors in Concert” recording, capturing Carreras, Domingo and Pavarotti iconic open air opera performance in Rome, sounds distant and thin, as if it was captured by a mobile phone in the audience. This is not the fault of the A26’s, but a unforgivingly precise reproduction of a mediocre recording.


This speaker project turned out as a great success. The A26’s sure are the best speakers that I have ever owned. Compared to my dad’s SAMPAN’s, they sound tighter and more accurate, though not as majestic. These are high-end monitors, musical and spacious, hi-fi every way you look at it.

As definitive proof, even my wife admits to enjoy their musicality. And who would have thought: she even likes their looks without a grille. I dare say that I will never buy speakers again. DIY is here to stay!