Supravox 215GMF
Copyright 2007 Troels Gravesen
       

Tweaking the 215GMF for better performance


Supravox 215 GMF drivers.
Supravox website.

France is a wonderful country of specialty products; from wines and cheeses to not least strange dynamic drivers not found anywhere else on this planet. Take at look at PHY-HP, Fertin and Supravox and you will find drivers designed for open baffles, horns or large vented Onken-type cabinets.
Supravox has an extensive range of paper cone drivers, from 5" to 15". All made in the classical tradition with lightweight paper cones, voice coils wound on paper formers and corrugated, coated paper surrounds.
I've had the opportunity to play around with the 215GMF and the 215RTF, the latter with a 25 mm voice coil and a somewhat smoother frequency response compared to the present 215GMT.

First time you have the 215GMF at hand and look and the TS data, you wonder what a ~12 grams paper cone with a suspension providing an Fs of 65 Hz can do. Will this driver really play any bass? One of the suggested applications of this driver is a two-way construction with a large horn taking over from around 500 Hz. The 215GMF is sitting in a large Onken-type cabinet and I have carefully tried to calculate the volume and vent dimensions of this cabinet and Mr. Thiele and Mr. Small may think that some people really don't give a damn about what they have done. I calculate a net volume of 104 liter and a port are of 312 cm^2 and a length of 300 mm, thus a vent tuning of 38 Hz. This provides a frequency response as seen in the graph, green curve. Well, if a constructor of modern hifi loudspeakers would present this at an AES meeting, I guess he'd be ridiculed. But this kind of drivers are living their own life and mounting the 215GMF in my 95 liter Tannoy test cabs, tuning the ports to around 40 Hz, I got a bass response much better than expected.
Setting up a two-way with a point of crossover around 1200 Hz wasn't that difficult, but the sound just wasn't right. The dip between 1 and 2 kHz simply ruined vocal presentation and the driver's been on the shelf for quite some time until I decided to take a second look at the dust cap. Read below.


One of the cabs suggested for the 215GMF driver. I'm not sure where the "Jensen" comes from. This is a
very Danish name and maybe someone can tell me what Jensen has to do with this. Jensen drivers/US?

- thanks to Teit/DK for mailing me this link:
http://www.pmillett.com
http://www.pmillett.com/vintage_data.htm
-from the 2nd link you can download a Jensen catalog (12MB) and
an Altec home-hifi catalog too (4 MB)
- great stuff!


215GMF before and after removal of dust cap. Note paper used for voice coil former.


Polyester foam plugs = 34 mm diameter, 20 mm height. Actually I made 20 plugs before I had two being straight.
I suggest making plugs from 10 mm polyester foam and gluing them together. Much easier.


New appearance of 215 GMF


Blue = driver as-is. Red = after removal of dust cap.
Yellow and green = minimum phase.
Measurements merged with nearfield response at 450 Hz. Driver in 95 litres vented cab.
Measurements taken at 1 m distance, 2.83 volts.


New TS data generated and checked in LspCAD. All data seems to fit well with cross calculation.


Supravox 215 GMF + Fountek NeoPro5i
2006

I had the opportunity to borrow two Fountek NeoPro5i ribbons, thanks to Gorm at GMSound, and tried setting up a two-way system from this ribbon and the 215GMF driver. This was before the 215GMF tweaks. I used my Tannoy test cabs, 95 liter and routed some new front panels to accommodate the drivers. The NeoPro5i is a huge ribbon and really goes low making points of crossover as low as 1 kHz possible. And the sensitivity is extreme, 100 dB/2.8V. If you need a versatile tweeter for your high efficiency dynamic drivers, go for this one.
As seen below, the drivers were crossed around 1200 Hz in order to try getting rid of the terrible 215GMF performance between 1 and 2 kHz. A rather rugged decline is seen for the Supravox and somehow this still appeared to colour the overall presentation. Vocals, again, were the problem, so the project was ditched.

I don't have the Neos any more and will try to mate the tweaked Supra with a tweeter fitted with a waveguide, providing sufficient sensitivity and allowing a reasonably low point of crossover. You don't find many domes with 95-96 dB sensitivity, if any, of reasonable quality. We have to use acoustic amplification, ribbons or compression drivers fitted with horns.


Left: Individual and summed response of drivers from crossover.
Right: Fountek NeoPro5i on the same baffle without crossover. Excellent performance.


Supravox 215GMF + JBL LE26-waveguide

Having done the 215GMF mods, I tried an old JBL LE26 tweeter, which I've found to be an excellent treble driver, having an unusual 94-95 dB sensitivity. JBL seems to have solved the 13 kHz peak problem from the former LE25. The four LE26s I have for the moment, do not exhibit this problem and fitted with a modified Monacor waveguide, we gain an extra 5-6 dB sensitivity around the point of crossover, reducing distortion significantly. The best L26 I have go straight to 17 kHz. Not bad for an old conical paper coned tweeter. I quickly made a plywood faceplate for the LE26, not particular nice, but it works and fits in on top of the 215GMF in my Tannoy cabs as seen below.
The usual measurements were made, and LspCAD helped in getting a crossover working with a point of crossover around 2.3 kHz. There's still some dip from the Supra right above 1 kHz, but nothing that could disturb my peace from this arrangement. The LE26 fitted with a waveguide is so good I wish I could find a similar tweeter for my other high-efficiency projects. But this driver is only to be found on eBay from time to time.


Simulated - and fine-tuned - crossover for the two-way set-up.
An impressive 96-97 dB/2.8V seems to the results - and so it feels when connecting to
the amplifier. This speaker really doesn't need a lot of power. 5-10 watts will do.

Left: Two of my best LE26 drivers with flat faceplate. The LE26 comes with a slightly convecs faceplate, not as good as a flat faceplate. Simply turning the faceplate upside down, provides a better response.
Right: My first two LE26s fitted with the modified Monacor waveguide. At 2-3 kHz we reach a 100 dB sensitivity. The impedance of the LE26 is on the low side, 4-5 ohms.

Left: Impedance of LE26 and 215GMF in Tannoy test cab. Right: SPL from drivers in cabinet.


Individual and summed response of drivers driven from crossover network.

I played this set-up for quite some time and there are pros and cons. It can play enormously loud - and it hard not to do so. Acoustic Live from Nils Lofgren was on the CD-player many times and impressed quite a few visitors. The speed, the transient attack is phenomenal. Even vocals are handled reasonably well. The treble is so good, it's become my first choice in all my high-efficiency experiments. The 215GMF is not un-coloured and I guess an optimised cabinet could lift its performance even further. I'm thinking of the designs from Decware: http://www.decware.com/newsite/mainmenu.htm. The NFX design might be an option for the 8" Supravox drivers combined with the LE26-waveguide.