| Tannoy 10" Monitor Gold Copyright 2005 © Troels Gravesen DUAL CONCENTRIC LOUDSPEAKER, type LSU/HF/III.LZ/8
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| Tannoy 10" Monitor Gold Copyright 2005 © Troels Gravesen DUAL CONCENTRIC LOUDSPEAKER, type LSU/HF/III.LZ/8
|
To lower the resonance frequency,
the surround was treated with xylene and given some 40 Hz
sine waves for some hours. It's hard not to be caught by pure aesthetics when looking at this driver. Like some old cars, old houses, old furniture, a few designs turn out to be just right. We can look at these old objects and say: Yes, this is how it should look, this is how it should feel when you touch it, this is the colour it should have, this is exactly the proportions this product should have to render the beauty of its appearance in space. An add on shoes will mostly display size 38 (European), not the size 44 I use! Size 38 simply has the most appealing proportions and I think this 10" driver is the "38" of Tannoys.
The Original Crossover:
Not surprisingly, the crossover is different from the 15" MGs and the 25 uF capacitor suggests a lower DC-resistance of the LF units and so it is: 4.8 ohms. Usually an 8 ohm driver has a DC-resistance of 5.5-6 ohm. The HF unit has a rather high DC-resistance of 9 ohms. The tweeter section is basically a 2nd
order filter made from 8 uF and the tapped coil.
Frequency response "as is" after setting "level" and "roll-off" for flattest response in wide baffle cabinet. Frequency response of both
speakers compared to Rogers LS35A (green) for
same attenuation (this is not at 2.8 volts). Rodolphe from Australia has this
comment:
Left: Frequency response from crossover
version 1, shown later. 23-09-2005
Left: What is seen above are measurements of tweeter response without crossover attached at various distances. Red = 100 cm, blue = 50 cm, green = 25 cm, yellow = 12.5 cm, purple = 6 cm. Right: Here's the full picture of tweeter responses vs. distance to microphone. Black = nearfield, i.e. microphone as close as possible to the outlet of the phase plug. No phase shift when we measure nearfield! Now, that's something. Apparently the phase shift is generated in the horn in front of the phase plug. Yes, the dust cap has been removed and now the driver looks like this:
The MG IIILZ has a large voice coil of 2½" ~ 64 mm. The phase shift happens around 6300 Hz = 5.5 cm wave length. Not exactly the same as the diameter of the voice coil, rather the diameter of outlet of the horn inserted in front of the phase plug. There's a straight section of the voice coil in front of the horn and straight sided tubes usually mean trouble. Placing some Gaffa tape - loosely - at the plastic horn outlet to "smooth" the mouth opening to the paper cone this happens:
Red = tweeter response
with crossover. Blue = minimum phase. Now, this is good
news.. A few strips of tape in the horn mouth opening
completely eliminates the total phase shift at 6300 Hz.
The only problem is how this is implemented as a
permanent solution. A short piece of horn must be made to
fit into the
The simple tape experiment suggests an extension of the horn throat. Not an easy thing to make! Ideally it should look something like seen to the right. The insert could be glued to the original horn throat by a small amount of silicone glue. I have to talk to this guy at work who will make anything in glass, metal or plastic.....
Above is seen the impact of removing the dust cap. Bass and tweeter response with no crossover attached. Purple and green is without dust cap. This is a bit of a surprise. I had expected the heavy fabric dust cap to make quite a difference in tweeter response. Not so. Subjectively the removal of the dust cap makes a more "open" sound, but "open" may be a word I use because the speaker is more - well, open! I'm not sure a blind test would reveal any difference.
Impedance of both drivers. Very nice; Fs = 40 Hz. Now, it may very well be that these old corrugated paper surrounds are one of the things that makes a real Monitor Gold, but in this case most of the compliance was derived from the very first - and quite worn - "wave" on the surround where the rest of the surround was fairly rigid. So, the first "wave" was given a thin coat of acrylic lacquer to stabilise the paper, which increased the Fs to some 60 Hz! This was definitely not my intention, but only proved that the first "wave" was taking too much of the heat and that compliance should more or less evenly distributed over the surround. The surround is simply a corrugated extension of the cone material, apparently having the same thickness as the cone itself and it is coated with some slightly sticky damping material. To lower the Fs again the surround was treated several times with xylene solvent to solubilise the lacquer and some of the lacquer was gently removed by a sponge. The xylene treatment took the Fs down to 42-44 Hz again and settled here after some 40 Hz "massage" for some hours while drying after the xylene treatment. TS data
Left: Above data taken from: http://www.hilberink.nl/codehans/tannoy38.htm. Right: My measurements. The differences seen here between declared data and measured data are significant, to put it mildly! Efficiency is pretty close, but that's about it. Declared data would suggest a horn as a suitable compartment for this driver, where my own data points to a closed cabinet or stuffed transmissionline. Suspensions may stiffen over time and account for some of the differences (Cms = 0.43 compared to 1.1 mm/N). I don't think the alnico magnet has weekened over time due to efficiency staying the same despite huge difference in BL factor, 11.2 vs 7.6.
Left: Test cabs with version 1 crossover. Hmm... That Tannoy sound! Even before finishing the crossover they deliver a level of transparency and midrange coherency, a speed and transient attack that few drivers can match. Right: First time living room set-up. My wife had left a CD on the player: Værker af Hildegard von Bingen. Arvo Pärt: Te Deum. Akademisk Kor/Akademisk Orkester. Soprano:Marianne G Nielsen, conductor: Nenia Zenana. Why not? A quick look at the CD notes do not state where the recording was done...but obviously in a cathedral - and most likely here in Denmark. The recordings also reveal how difficult it is to get things quiet in a cathedral. You can hear cars passing by and doors being closed. Now, church recordings and female
vocals can be a tough test and the Tannoys
perform remarkably well. The solos and choruses are
portrayed with depth and a wide soundstage and all in all
not a problem for the Tannoys. Yes, they can be played so
loud that some midrange compression sets in, but you have
to play really loud before the Tannoys give up. So, what's playing? The
version 1 crossover follows the original quite closely.
Fortunately the bass section of both drivers appear to be
very consistent in amplitude and phase response and 1.35
mH and 25 uF makes a 2nd order low-pass section. Due to
the narrow (!) front baffle, only 38 cm, we have a bump
around 750-850 Hz and we need to get rid of this to make
an even and smooth midrange and if you think a notch
filter takes the life out of a driver, think twice. 10
ohms + 10 uF + 4.7 mH does the trick and we have a smooth
midrange.
Initially it was my intension to use the tapped coil for the tweeter section, but to be honest, it's much easier to leave it out. One reason for using a tapped coil is to omit the use of resistors in the signal path to the tweeter, but the original crossover has a number of resistors, so I not sure this tapped coil is such a big deal.
Left: Response of driver from V1
crossover. Tweaking of the driver itself is tempting:
The basket of the Tannoy is anything but a modern spider
chassis and damping of the inner surfaces might be tried.
Left: Step response. Not particularly smooth, but tweeter and midbass peaks are well integrated. Right: Tweeter response of both drivers. Quite good I should say, but this didn't mean that the final response from attaching the crossovers also made the same SPL. One tweeter had to be attenuated more than the other. The bump at 3 kHz is taken care of by the tweeter notch filter, almost a replica of the original notch filter. Crossover, version 3.
I'll skip crossover version 2
and go right to version 3. After a week or so you start
feeling at home with a speakers and can take a more
objective view of the whole thing. Numerous CDs and LPs
have been played and some were good, some were bad and a
few were very bad. Oh yes, and a very few were so good
that aural nirvana was accomplished. The latter may be a
problem because some recordings may be balanced in such a
way that an un-balanced speaker may go hand in hand with
these recordings and make things sound even better than
they were ever meant to do. The woofer's bump at 1500
Hz is a problem for making a perfect crossover, as the
bump is right in the middle of where the point of
crossover has to be. So, an additional notch filter was
introduced here and made a nicer phase tracking between
drivers, but this notch filter is optional and is
arguably audible. The V3 crossover has been running for some three weeks after removing the dust cap and smoothing the horn mouth with some thin plastic and tape, eliminating the 6.3 kHz dip and phase shift. So, it was time to move on. Coating of a cone is a serious matter and there's no go back. You may seriously destroy the driver from the wrong type of coating and produce a frequency response that's hard to correct in the crossover. Coating materials comes in all varieties and only a very few are available to the diy people. Often the coatings offered from speaker shops are simply PVA glue and take great care when using this material. Usually they will add considerable weight to the cone and reduce efficiency. Generally I recommend not to coat your drivers unless you are able to measure what is going on and are willing to sacrifice a driver when things go wrong. Very soft coatings may smooth
frequency response but also reduce the ability to reveal
detail where hard coatings may enhance resolution but
produce new and serious cone break-ups. So, the Tannoy cones were coated once with DAMAR and in the beginning produced a very nice and smooth frequency response in the 1-3 kHz region. However, when fully dry a dip at 1400 Hz appeared and getting rid of the break-up would take serious measures like making the cone into a sliced paper variant. The dip is hardly noticeable and I decided to leave it here because after further fine-tuning of the crossover, version 5, a nice overall frequency response was achieved and for the first time I could get a deep nulling effect from connecting the drivers with the same polarity: Green curve below = actual measurement. The sound absolutely changed for the better from coating of these 40 years old and soft paper cones. Enhanced transparency, subjectively a more precise bass.
The treble from the compression driver and the horn is special. Some like it, some don't. For some people it's too different from the treble coming from a conventional dome tweeter. The treble is incredible fast due to the 100 dB sensitivity of the driver and horn and I think that the best way I can describe it is that we here listen to "treble" rather that a "tweeter". The treble is more directional than what comes from a dome tweeter and toeing in of the cabinets was more important than it use to be. Normally I prefer the on-axis to cross behind my head, but with the Tannoys I sometimes toe them in to cross in front of the listening position. Tweaking the crossover of the Tannoys has certainly been one of the most difficult tasks ever. The bass driver's irregular response in the 1-2 kHz region for one thing. The compression drivers' broad bump at 2-4 kHz due to the horn loading. Had this bump only continued down to 1 kHz, things would have been easier. But this is probably as good as it gets. End. |
Frequency response from crossov
