An ATC SCM100A clone with a little twist

This project originally started in September 2011. It was left around 25% complete for almost two years before i resumed with the same passion as ever – to get the ATC sound.

I had been collecting the required drivers for quite a while, and managed to get hold of a pair of everything i wanted (and more), including:

1) A couple pairs of SM75-150S “Super” domes 16 ohm
2) A pair of SB75-234 9″ Bass drivers
3) A pair of SB75-314 12″ Bass drivers
4) A number of odds and ends from some deconstructed ATC SCM50 speakers, god knows what happened to them but the haul included a par of original baffles, the straight horn profiles for the dome, some domes without the magnets, various small wires, caps etc.

I had been inspired by a design by Lukas Fikus (aka Lampizator) which was a 4-way using the 234 open baffle and a ribbon top end, see Speaker Project 10 (unfortunately when i last checked most of the pictures are missing – hope they come back soon!)

I built a single prototype having got hold of some classy ribbons, namely the Aurum Cantus G1.

I found that speaker sounded amazing, but i used the 314 bass in a 40L sealed enclosure (if i remember correctly) with the aim of applying Linkwitz equalisation. I never put the required effort to get the bass sorted out, and i felt the bass driver could offer more. In any case, i gradually gravitated to the idea of building a straight up ATC clone (i.e. ported enclosure) and the second prototype speaker of this design was never built. (Affectionately became known as “the chair” it hung around for some years).

But no way was i going to forego the ribbon tweeter! – it is amazing in the sweet spot and well worth the downside of limited vertical dispersion.

And so we come to the “twist” in this ATC clone, the usual scanspeak soft dome has been replaced with a first class ribbon.

Of course that meant i wasn’t able to use a pre-made kit a la Wilslow Audio K50 or K100 because that is cut out for a plain old round dome tweeter.
Never mind, i prefer to do it my way anyway.

THE DESIGN:

The original plan was to integrate amplifier into the cabinet. To cut a long story short, while more than viable especially using the Class-D amplifiers available, the added complexity to the project steered me away.
I decided the make this a simple “Driver inside a box” design with external amplification and crossovers. Total flexibility.

The crossover of choice is the miniDSP 8×8, previous detailed here.

The enclosure i comparable to the Wilslow Audio design, around 105 cm high, 43.5 cm deep, 38 cm Wide. Using 25mm MDF my calks gave an internal volume of around 115 litres after deductions (give or take).

Running this though winISD gave a port length of 10cm using standard soil pipe (110mm external, 103mm internal diameter). I was using a gently flared port (1/2″ round over), so i made the port slightly shorter and attributed half the flare length to the port (6mm or so). Quite short!

Box lining is nothing fancy (upholstery wadding 250 gsm), but decent use of Monacor bituminous self adhesive pads was made (MDM-830)
Having inspected ATC enclosures i have noticed they have a no nonsense approach. Nothing too fancy, and it has made me less fanatical about getting the best “Audiophile” filling unless really warranted.

Still playing with the crossover (love miniDSP) but basically LR 48dB at 380Hz for the bass/mid, 3.8KHz for the mid/tweeter.

The tweeter has a passive high pass (for essential protection with a ribbon) at around 1KHz, so by 3.8KHz the assumption is that we are well into the passband.

Future work will be to measure and equalise as necessary, but having tuned by ear its already a most enjoyable speaker – if you can find the space for it.

THE BUILD

Onto the build process. Here, as usual, i have put a large number of photos documenting the process. I hope it is useful to someone.

I am most happy with the end result. These speakers kick butt, they sound how they look. The tweeter adds finesse missing from the commercial design.

Glueing up the main panels with rabbeted joints:

Bracing for the cabinet, two per box and shaped to allow for the rear of the bass driver. Original plan was to put amps in the back hence why one brace has open space to the rear.

Rear panel with holes drilled for three pairs of binding posts:

The G1 ribbon, protected with paper as i needed to make a routing template with it:

Routed around the tweeter to first produce this rather large template:

Routed inside the larger template to product this, the final template for use with a template guide on the router:

Like this:

Usingn the Jasper Jig to make the bass cutouts and rebates (and the Mid Dome cutout, which doesn’t require any rebate due to rear fixing design)

The rebate depth for the 314 bass driver is 14mm, and 16mm wide. There used to be full specs of all drivers on the ATC website, but since a recent website update that very useful file seems to have vanished (i have it somewhere). Anyway, the required data is still thankfully available on the Solen website:

Front panels almost finished (oversized, to be cut down and flush trimmmed later):

Whoops! A minor roundover error put right with some “High Performance Wood Filler” (2-part filler. that really saved my bacon – can you imagine redoing all those cutouts!). This stuff routes beautifully.

Rebating the rear of the front panel to allow for the Mid done to come up flush with the front surface. The horn on the ATC dome is actually around 19mm long so i had to take off something like 4 to 5mm from the rear to bring it flush (as done on the commercial speaker (very handy having those SCM50 front baffles hanging around):

Snug fit, as intended:

The Monacor self adhesive bituminous felt pads for panel damping. This adhesive is nice and tacky but i still (having read something on the web) decided to use some extra 3M mounting spray which apparently bonds nicely with the adhesive already there – i don’t want these things falling off or not coupling with the panel.

A few staples give piece of mind:

Panel lining with 250 gsm upholstery wadding. Adhered using a standard upholstery adhesive in a spray can. Sticks very well:

Glueing the front panels on:

The protective passive high-pass filter for the ribbon tweeter. Second order with 13 mfd 400V cap and 1.6 mH air core inductor, purchased from Solen Electronique.

Mounted to a small board, this allow the crossover to be removed (it is attached using insert nuts and machine screws), should it need to be eliminated or changed at a later date (removable through the bass driver cutout):

Glued to the inside of the cab:

Cutting the 110mm bass port pipe using a home made jig to keep things parallel (enough):

These circular cutouts are what hold the port to the inside of the cabinet, allowing the port surface to stay flush with the flared hole on the front baffle, if that makes sense:

Like this, the ports inner surface is rebated:

Hot glue works perfectly for this:

And then glueing to the inside of the front baffle:

Insert nuts inserted for the drivers (tweeter and bass) M4 for the tweeter. Now the bass is apparently (according to ATC) suppose to use M6 bolts but i found the heads too big to sit inside the foam gasket that goes round the front of the driver. (If anyone knows what that gasket is really for please let me know, because i have never seen this driver rear mounted and the foam is very hard and would make a terrible air seal anyway). I found M5 bolts much better and fit snug, but used washers to increase the surface area in contact with the driver. Last of the wadding applied:

Glueing up the front baffles no going back:

Onto the grilles. I used 9mm MDF because most of it was being cut out anyway and i wanted the grille to be substantial enough not to vibrate. Marked out the cut outs:

Checking the fit:

Drilled holes for the speaker grille fixings:

Drilling corresponding holes and rebates for the front baffle fixings:

Follwing gluing, flush trimming of the front and rear baffles and a 1/4″ roundover on all edges:

To ensure a better finish the MDF edges were covered with the High performance Wood Filler 2-part and sanded smooth. In actual fact i should have put more on but working time is short:

Following two coats of Zinsser 123 primer/selaer ) which dries fast and sands well. This is shown after sanding ready for topcoat:

Now we’re getting somewhere. This is after three topcoats roller applied (Dulux Satinwood) in a subtle light grey. here you see castors attached into the cabinet, an idea that was quickly abandoned. The weight of the cabinets is no joke and the MDF was not suitable for holding the M10 insert nuts. I decided on a totally different approach which was far more satisfactory and robust:

Speaker grille fixing inserted and hit home, flush with the front baffle:

Test fitting the grille having inserted the fixings, fits perfectly. I love it when a plan comes together:

Onto the Grille Cloth, the best i found is this quality stuff from AS Trim. It is thing (acoustically transparent) and good the work with, good stretch and number of colours available. I chose this light grey to contrast slightly with the cabinets:

Some help from an expert seamstress:

Castors removed i cut discs of rubber for the feet, I wanted a quality rubber with good grip, this stuff manufactured by SVIG is designed for the soles of shoes but has a low shore hardness of 50 so it has more grip than a usual sole. It also cut well with the hole saw:

Adhered to the underside of the cabinets. The corner protectors add a bit of style but also serve their function in protection the speaker corners during transit, The bottom edges are particularly vulnerable and i have seen commercial ATC speakers damaged on the corners in many photos. They requires some work to create a quarter round cutout allowing the rubber feet to come close to the speaker edge, maximising speaker stability:

The end look. Here you see the alternative mobility arrangement. Castors aren;t essential but boy, once you feel how heavy these speakers are you realise how convenient it would be to be able to wheel the around (for vacuum cleaning etc). Plinths of oak are used with castors fitted, speaker sits on top with rubber feet. This gives all options for the future (i.e. to replace the plinth with one with spikes, to change castors, or to place the speaker directly on the floor – more flexible approach):

And the end result!

MiniDSP crossover and Denon AVC-A11SR for amplification (6 channels used). This works really well, it sounds great: