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Building 120L cabs for 12" Tannoy Monitor Gold Rs.

Discussion in 'd.i.y.' started by Rockmeister, Feb 11, 2020.

  1. Rockmeister

    Rockmeister pfm Member

    This follows on from my 'help for beginners...' thread, where, most basics now sorted, I have moved to the actual build. I hope it'll be useful to keep this one more practical and I'll do my best to keep it updated with diagrams and photos as I move through the build.

    NONE of what follows later will be as long winded as this pre amble, but let's get the concept said and done before I get the saws and glue out...

    Why? because I want a project. At first glance, It doesn't seem to save huge amounts of cash as against just buying some battered production Tannoys, but in fact what I will end up with is, I hope, about as good as it gets for drivers of that period. Mine have been dismantled, checked, restored and cleaned, and the crossovers checked and returned to as new working spec. Once the cabinet is made to tannoy ++ spec, it is in fact a wise investment. I figure my total outlay will be near £1700.

    Of all the viewpoints in Tannoy 'restoration' I sit somewhere in the middle between total adherence to tradition (there are a lot of plans on the internet for this kind of approach) and modernising/'improving' on what was done in the 70's. Big Tannoys were the first eye opening speakers I ever heard. In 1972/3 I had extensive use of two pairs. 1 home made Tannoy kit (12's) and the other some proper tannoy 15" speakers in the university HiFi club. I loved both but now feel that although I do not want to alter the crossover,
    (there was the chance of 'up-speccing' the caps for example, but I was glad to find this not done. Just new old spec units used), I did feel that I could lose some of the warmth and looseness I remember in the lower octaves by using modern cabinet damping and anti resonance techniques. The old thread hints at these. I'll describe them as they turn up in the build.

    The cabinet size was chosen with the drivers to suit my room, but mostly to be sure of making the size and tuning that those speakers themselves would ask for for Christmas. Thus the choice of timbers, their thickness, the layers of damping and foam, and the port size and position all fell neatly into place later in the initial investigation.

    Aesthetics were going to be driven by a pragmatic mix of available timbers, my construction skills (ok but no joinery in sight) and by the finish. Mrs Rock and I decided early on to paint them, so MDF, filler and sandpaper will feature.

    [​IMG]Tannoy Monitor Giold 12" by John Dutfield, on Flickr

    These have just arrived.

    These next pics were from Cooky, who supplied the drivers. This is also all here. So is the wood.

    [​IMG]IMG_0523 by John Dutfield, on Flickr

    [​IMG]IMG_0514 by John Dutfield, on Flickr

    And so my next task is to clear the garage/workshop. This means ejecting car 2 (old classic ish) into the current storm so that's not happening. This isn't going to be a rushed job...maybe a month or so.
    Last edited: Feb 11, 2020
  2. Rockmeister

    Rockmeister pfm Member


    The main thing that matters for me is that the interior ends up as close to 120 L as I can manage.

    The timber must be hefty enough, the sheet material rigid, the contents of the cabinet when finished allowed for in that volume calculation.
    So, as follows.

    Original choice for the sheet material was 1" ply (I had a sheet in the garage), but having read about constrained layer damping, I wanted to use that idea without increasing the cab wall thickness...it would become enormously heavy and hard to work and possibly 'too' rigid.
    (Interjection. Someone on PFM will be able to test, measure and produce resonance figures, and since I'm confessing, later in the build, have reasons why certain construction materials and methods are preferred. That's not me. I have read all I can find, understood it and taken from it what I think is as good as I can do. Please don't ask me anything too technical. The groundwork and references are all in the previous thread. If you troll through that you can find what I found.)
    I therefore reverted to 18mm MDF, with an added 5.5mm layer of ply glued inside that. This is the damping layer I'll reference later. The front baffle however is to remain an Inch thick, and since i have some, Plywood.
    Most of the framing timber is to be 34mmx34mm, bar the brace that connects the front baffle and the rear panel. An 'H' shape of something around 70x50mm.

    So, that timber when inserted into the cab takes up 11.7 Litres; The driver itself occupies 4.3 Litres of space (front mounted on the baffle); The port and crossover use up around 1 litre. = 17 litres.
    I therefore need an interior space of 120+17=137 Litres.
    Aesthetically and to avoid obvious awkward resonances, I wanted to design something close to a golden ratio proportioned box. I never did it exactly, but the final size is close enough. Finally, the driver needed to be at around ear level. This will need a plinth, but a plinth is a common feature of Tannoys of that era.

    The interior of the final box therefore measures exactly 35" (889mm) H, by 20" (508mm) W, by 12" (305mm) D. For 137.6 L.
    I reckon the .6 doesn't matter too much. Maybe I'll add a heftier cross brace.

    The exterior will therefore be roughly 37 x 22 x 14.
    I envisage a plinth of about 4" height if that works out as planned.
    chiily, Fatmarley and cooky1257 like this.
  3. jtgofish

    jtgofish pfm Member

    A good project.I built a 150 litre ply box for the Gold 12s which worked nicely.I just used 19mm hardwood ply .
    I look forward to seeing the project progress.In all metric measurements only hopefully!The neanderthal system of weights and measures is surely redundant by now?Unless you are in the US of course.
  4. Rockmeister

    Rockmeister pfm Member

    I use both. Some readers might have preferences each way and I hope to provided measurements accurate enough to build from. Measurements that need to be precise will be in mm.
    booja30 likes this.
  5. Pete MB&D

    Pete MB&D Pete Maddex, the one and only!

    Thank god you aren't using centimeters!

  6. Rockmeister

    Rockmeister pfm Member

    I've got 90% of my supples now, but think I may need spikes, or at least want the option, so I intend to drive and glue some threaded inserts into the timber 'legs' of the plinth. Given the likely weight of the cab I think M8 or M10 spikes will be in use. Anyone please know a good UK supplier of spikes and threaded inserts? Thanks.
  7. eastone

    eastone pfm Member

  8. Rockmeister

    Rockmeister pfm Member

    excellent, thanks east.
  9. Rockmeister

    Rockmeister pfm Member

    half way through the first cabinet. A garage build and too cold for some of the glues, so running back and forth to the house :)
    [​IMG]IMG_0124 by John Dutfield, on Flickr
    Two side panels, showing the 34mm x 34mm that will form the inner frame.
    The sides top and back are 18mm MDF. The base and baffle are 25mm ply.
  10. Rockmeister

    Rockmeister pfm Member

    Constrained layer damping.
    The idea is to add a thin rigid layer inside the wall of the speaker, to make a sandwich.
    Outer layer of 18mm MDF (classed as a 'soft' material for this exercise)...

    [​IMG]IMG_0125 by John Dutfield, on Flickr
    I'm using 5.5mm ply though there are 'better materials but very expensive. We need non resonant, thin, hard and rigid (for it's size). The ply is cut to fit loosely inside the timber frame. As long as most of the panel is damped, it will resist 'ringing'.

    [​IMG]IMG_0126 by John Dutfield, on Flickr
    Key is the 'glue'. This has to be an adhesive which not only has a high impact adhesion, but also dries soft and stays flexible. I used PU18. This is key to the damping. Any rigid layer would prevent the effect. This layer is not as thick as I ended up using...I about halved that again to make sure no plywood directly touched the MDF.

    It's important to allow for this extra thickness when you are calculating the internal volume of your cabs.
  11. Rockmeister

    Rockmeister pfm Member

    Driver and Port cut outs now done.

    [​IMG]IMG_0128 by John Dutfield, on Flickr

    I erred on the loose side for both the driver hole and the rebate, but over did it by about 1.5mm. Given my skills I'm happy enough with the result.

    [​IMG]IMG_0135 by John Dutfield, on Flickr
    You can see the clearance at the front here.

    [​IMG]IMG_0134 by John Dutfield, on Flickr

    On the inside also some spare room but that proves to be helpful with this problem...

    [​IMG]IMG_0140 by John Dutfield, on Flickr

    That's one of the driver mounting bolt holes marked. It's just 5mm from the cut out edge. Far too close for comfort, hence the tannoy 'inner baffle'. A rough calculation means I can find another 3/4 cm of 1" ply to mount the bolt into. I'll sink a T Nut up into the inner surface to receive the bolt, and these wedge brackets will be glued and screwed into place.
    [​IMG]Scan by John Dutfield, on Flickr

    bad drawing but you get the idea. It could be a whole baffle all round but wedges will take less volume and be easily strong enough. The driver mounts from the front so I can fit these before the driver goes in at the end.

    The port cut out was 136mm for a gentle fit, allowing the port to be taken in and out for length adjustment whilst tuning the cab later. It'll be firmly glued into place once all is decided.
    Last edited: Feb 19, 2020 at 10:00 AM
    Mike P and cooky1257 like this.
  12. cooky1257

    cooky1257 pfm Member

    Looking good John. Don't throw the cutouts away, stick em to rear panel.
    Rockmeister likes this.
  13. Mike P

    Mike P pfm Member

    Nice job, I'm following this with great interest.

    Have you decided what material(s) you're going to line the cabinets with yet?
  14. Rockmeister

    Rockmeister pfm Member

    yup...already purchased a box full of this stuff.
    AFRP "ROOM PACK" Pro Acoustic Foam Treatment Tiles 12x AFW305 & 12x AFF305
    find it on ebay.

    although I read this and am yet to think that bit through fully. i.e. where to place it, and how deep etc etc.

    And for info, these are the ports. Really excellent service from this company. HP100's. I bought 4 so I could do some experimenting, and get things wrong! :)
  15. Mike P

    Mike P pfm Member

  16. Rockmeister

    Rockmeister pfm Member

    See #10 Mike. Just the C.L.D and the foam is the plan
  17. Rockmeister

    Rockmeister pfm Member

    Invented and fitted the required inner baffle for the driver mounting bolts today. A tad more complex than I thought, since the 4pin connector is blocked by the new baffle, meaning I had to get the file out. I used 25..ply so 18 may not have been a problem?

    [​IMG]IMG_0141 by John Dutfield, on Flickr

    Here's my solution :)
    [​IMG]IMG_0142 by John Dutfield, on Flickr
    who said craftsmanship is dead?:)

    And a close up showing the clearance between the speaker chassis and the new baffle. 45 degrees is about right.

    [​IMG]IMG_0143 by John Dutfield, on Flickr
  18. eastone

    eastone pfm Member

    Frank, is there some sort of technical rationale for using the cutout in this way (cancellation, perhaps?) - or is it just a nifty two birds / one stone thing...
  19. cooky1257

    cooky1257 pfm Member

    It just allows you the thicken up/ brace the middle of a panel- ideally the one behind the driver.
    eastone likes this.
  20. Fatmarley

    Fatmarley pfm Member

    The only thing I would say is that I can't stand T-Nuts. They just seem like a big faff to fit and if you get one that spins it can be a real pain in the arse. I only ever use wood screws now, and they work just fine - YMMV.

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