Andrews  Tannoy  GRF-R  story

Tannoy GRF Rectangular Horn cabinet

This is the second GRF I have built, having completed the massive GRF Corner Horn last year.

The approach I use is consistent with all my speakers – and relies on CNC’ing a series of identical ‘slices’ to form the complete horn block.
In some models, built-in braces are included in the slices. For the GRF-R, two such brace slices were included.

Each slice also has computer-cut dowel holes to ensure 100% alignment of slices.

10mm Oak dowels are used, along with standard PVA glue. This sort of build is a great DIY’ers project.

For this project, I used the various drawings from the net of the GRF-R. The final measurements were 1070mm tall x 500mm wide x 550mm deep.
Each cabinet weighs close to 100Kgs. (I have offset a little of width and added this to the depth, aiming to retain the same horn volume and mouth size.)

14 individual 32mm MDF slices make up the internal slices. Outer panels are 19mm MDF. The front baffle is 40mm thick.
Wool sound deadening material is used in all the cavities.

I have added a thin layer of felt to the cut-out of the second cut-out to seal the back of the driver.
The fit is very snug…and the driver can sit in place even without mounting screws.

I have CNC’ed a front speaker cover, which can be veneered or covered in audio cloth.
I’m not sure what I’ll do to finish this aspect off…preferring to keep the driver exposed. I would also like to keep the horn mouth exposed.
I will add a piece of 2mm veneer to the inside of the horn mouth to tidy things up there.

I have used spiked audio foam to the top and upper sides of the comp chamber.

Using the design from Laurents’s cabinets (see here ) I hand shaped the ‘tear drops’ out of
hardwood. This was a tricky process…using a circular saw and rasp to obtain the desired shape. The finished measurements were approx
50mm x 100mm. The ‘blocks’ were then cut in half to allow fitting. I also had to carve out some material so the blocks could sit on top of the
internal brace supports in the cabinet. I have temporarily nailed these blocks in place while I undertake further listening tests….but I am
VERY happy with the immediate improvement these made. The sound stage moved way back; and the bass became considerably
tighter/punchier, while loosing nothing to the incredible depth already there.

I am presently using HPD Royals (Canadian special edition), with the original Tannoy cross-overs. These seem to work very well.
I have the settings on middle for both roll-off and energy. I may hard-wire these later. The cross-over housing is temporarily sitting
on one of the internal brace ‘ledges.’ Internal cabling has been sealed using Blue-Tak until I complete the cabinets fully.

On the assembly side, I simply glued up the 14 slices in one sitting, using a consistent bead of PVA between slices. I use 7 or 8 “F” clamps,
plus 2 or 3 larger pipe clamps to ensure the slices are pulled together nicely. I allow a min of 24 hours before the clamps are removed.

The outer panels are all computer cut to the exact size. These are glued in place, using thin tack nails to align the panels as the glue dries.
The nails are then tapped in and puttied over.

When preparing the driver cut-out, I use a standard jig-saw…but the shape of the front section, makes the use of the jig-saw tricky.
For future builds, I will incorporate a front baffle made from 25mm ply, with a computer-cut hole already prepared.
This panel will then be glued/clamped into place. The same approach will be used for the cut-out behind the front panel…
where the back of the driver resides.

I use standard bolts and T-nuts to house the driver securely.

I have also fitted small trolley wheels to the underside of the cabinets. These have been invaluable…especially when finishing the cabinets
and for room placement when listening to music.

Braided solid-core Cat-5 cable is used.

Coming from single-driver speakers (Lowther, Carfrae, Fostex etc), I must say I’m enjoying the sound quality of the big Tannoys.
The low and powerful bass response is very beguiling. The imaging is very good, though not as sharp as the Lowthers, obviously.
I tend to listen to easy blues, vocals, instrumentals, and the Tannoys produce a lovely open sound. It can certainly fill the house
with great volume when required, with little distortion. All this from a lovely 300B SET amp with a meagre 8 watts/RMS.

I’m happy…!

Andrew Tilsley.

Sydney, Australia.



© PE1MMK Hans Hilberink, Andrew Tilsley, last update:  21-05-2013.