DIY: Tannoy GRF Enclosures

   Having finished upgrading my Tannoy Monitor Gold 15" drivers meant that I was ready to build Tannoy Monitor Gold In Test Enclosure (Thumbnail)them some enclosures. As an initial test, I put the drivers into non-optimised 110 litres bass reflex cabinets. I was very surprised when I played music for the first time. These drivers went really low and sounded extremely good. I was surprised because when I was simulating response using T&S parameters they didn't look like they were going to cause an earthquake.
   Following a rather promising initial tests, I have decided to build replicas of Tannoy GRF horn-loaded enclosures from 1960s. I always liked the look of them, especially with the covers on - they are actually not many speakers IMHO that look good with the grills on, but these do indeed. I knew that it will not be easy due to internal panels joining at 'funny' angles, but making GRF enclosures looked much simpler than making Westminsters ;).

   To save time and limit additional expenses I always try to follow the common proverb: "measure twice, cut once". With fairly complicated constructions, such as GRF, it is essential to plan everything and think about methods of doing things before attempting any work. I have started from doing research on the internet and downloading any available enclosure plans. I have found some drawings with measurements that looked like original Tannoy drawings. There were couple of measurements that were unclear, therefore, I have drawn the enclosure cross-section in 1:1 scale and calculated missing dimensions using commonly know mathematical formulas. Drawings were then re-scaled to allow for usage of 25mm plywood instead thinner ply used in original drawings (length, width, depth and expansion of the horn were kept as per original plans). Driver performance in GRF enclosure was then simulated using HornResp software. The results did not look too promising in terms of deep bass. However they did not look too promising when I simulated the Bass Reflex enclosure but sounded very nice, so I have decided to progress with the project.
   Methods of joining wood  may have impact on the size of the panels, so it is always important to consider these first. Different methods were considered, but too keep it simple and to avoid using screws, wooden dowels were chosen. All cuts were planned to ensure most efficient use of the material.

Wood Cutting
   Wood cutting proven to be a lot more difficult than I originally thought. As I am not a carpenter, I was intending to hire a professional to do the difficult angle cuts for me. I have approached many different carpenters and wood merchants in my town and none of them wanted to do it, notwithstanding that they had really good equipment! Fed up with that I have decided to do it myself. At the time I lived in a small flat, so I had to do all the wood cutting in my friend's back garden, using circular saw... Despite this inconvenience and lack of table saw, the results were surprisingly good. Router was used to cut flush mounts for the drivers and speaker terminals. It was also used to round all of the internal horn edges.


Enclosure Building
   Thick cardboard jig was used to align the dowel wholes on the enclosure sides with the dowel wholes on the horn panels. What seemed to be a good idea in theory, proven to be a little bit more tricky in practice (nothing that a bit of sanding would not fixed though). Small neodymium magnets were fitted into speaker front panels as well as the grills, to hold them in place. All gaps were filled with silver sand. Enclosures were glued together, leaving one side panel unglued, to allow for adding or removing damping material. Initially damping material was only placed in compression chamber and first part of the horn.
   Time to listen - very exciting stage. Well, I thought it would be... my first impression was disappointment. The upper bass was really boomy and there was very little of the really deep bass. The tonal balance was just strange, and not right to my ears. I have experimented with more damping, and ended up with the whole horn throat (except the exit) dampen with felt. This have greatly improved the total balance and speakers started sounding more acceptable, more balanced. Nevertheless, they were still lacking this very deep bass, which I knew well that these drivers were capable of reproducing. Most of my favourite recordings sounded ok but I was not blown away. The only time when the GRF enclosures impressed me, was when I listed to Carl Orff - Carmina Burana. The power and speed of the lower registers was just overwhelming - never heard anything like this before.
   However, I was not willing to keep these refrigerator size enclosures and be unhappy with lack of deep bass. For this reason I have decided to convert them into Bass Reflex enclosures. I have calculated port size based on drivers' T&S parameters, and converted the enclosures (Photos 16-30).
Time to listen again
... On this occasion, the first impression was totally opposite. Sound was nicely balanced, and the deep bass was there when it needed to be . Please, don't get me wrong, I can appreciate many more things than deep bass, but I have had certain expectation from these massive enclosures, and these expectations were met. Not only met, but exceeded. The bass was deep but well controlled. Meaty but not boomy. Precisely how I wanted it! Tracks like Dire Straits - Water of Love or Alice In Chains - No Excuses (Unplugged) were taken to the whole new level. Enough about the bass though, these speakers reproduce voices, saxophones and double bass like no other speakers (if you are interested in my views on the sound of these, please visit the Vintage Speakers section).
   To sum it up - one of the best speakers I have ever heard. Smooth and not fatiguing - well worth the investment and all the hard work.


List of Components & Prices
   Table below shows list of the parts together with prices, necessary to build the enclosures as described above.

Project Summary:
Total Time: 60h
Total Cost: 271.16


Miscellaneous: Item Specific Unit Quantity Price Per Unit Total Price
Plywood For Enclosures 2440x1220x25mm Board 3 49.15 149.15
MDF For Grills 1220x1218x15mm Board 1 11.10 11.10
Wooden Dowels M8 x 40mm Pack 3 2.28 6.84
Ronseal Wood Filler 500ml Container 1 3.66 3.66
Wood Adhesive 500ml Container 2 5.83 11.66
Van Dyck Crystals 500g Pack 1 6.95 6.95
Allen Bolts M6 x 50mm Pack 2 1.66 3.32
Captive Nuts M6 x 12.3mm Pack 2 1.23 2.46
Brass Flat Washers M6 Pack 1 2.20 2.20
Speaker Terminals Gold Plated Pack 1 7.57 7.57
Neodymium Magnets For Grills 6mm Pack 1 4.25 4.25
Vintage Speaker Cloth - Piece 2 18.00 36.00
Felt Sheet 2400x1220x10mm Piece 1 26.00 26.00




Copyright 2014 by Brast.