Vifa C17WH-69-08 + SEAS 27TFFC
Copyright 2006 Troels Gravesen


2-way d'Appolito
Get 95 dB/2.8 volts sensitivity for your low-wattage amp - in case it runs 4 ohms speakers.

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Workshop set-up with a d'Appolito arrangement in 39 litres vented cabs.
My Goodness, these speakers are sensitive! Watch that volume knob before you turn on the music.

The Vifa C17WH-69-08 is a "special" driver. Special because it's so ordinary - probably made in tenth of thousands - and special because a lot of things went right with this driver. Sometimes a driver turns out to have the right cone material and coating for a given size, thickness and weight; the right voice coil diameter; the right surround providing the right damping to cone motion; the right magnet size to provide useful TS-data, etc. When all the individual parts are made right, the result may be better than what we might expect from the sum of the parts.
A curvilinear cone like this will mostly have a smooth frequency response compared to a straight-sided and deep cone like the Philips 9710 and this Vifa C17WH-69-08 is no exception. It's basically flat up to 2-3 kHz and what happens above, is a series of modest resonances that are easily taken care of in the crossover. A straight sided cone, like the Philips 9710, has great bass performance, but above 2-3 kHz the response rises some 10 dB and sharp filters are needed.

So here we have a stamped metal chassis, thin paper cone and a foam surround. Some 15 years ago we might have rejected this driver as a useless old-timer. Yet it must be one the most often used drivers. Basically it costs nothing and some may reject this driver due to the fact that it carries a foam surround that will rot in 15-20 years time. Applying some thin silicone liquid to the foam may extent its lifetime but don't use silicone spray due to the solvent used. It must be pure silicone with no additives. The type of silicone that comes in a bottle with a small brush is usually good. A foam surround may be one of the tools to make a better than average midrange presentation. Foam surrounds are still around - and for good reasons. G. A. Briggs found the foam surround superior to corrugated paper and cloth some 45 years ago - but unfortunately didn't compare with rubber. Very little can be found in the books of Colloms and Dickason on the impact on sound and overall performance of drivers due to various outer surround materials.

As long as the C17WH is not forced to play excessively loud, it can deliver some of the best midrange performances available . If driven too loud, it starts sounding compressed, but which driver doesn't? Well, the Audio Technology 15H52 is one, but that's another story for the wide baffle section. The C17 is found in numerous versions, most often the "WG", fitted with a smaller magnet and allowing decent bass from a small cabinet. The "WH" has a larger magnet and sensitivity around 90-91 dB/2.8 volts and from two in parallel we may expect minimum 95 dB/2.8V. However, this will result in an impedance of 4 ohms, or rather 3.5 ohms. So possibly not the ideal partner for single ended triodes. But push-pull triodes and small class A amps may like it a lot. We'll see. (I've already had the question if the "WG" can substitute the "WH" in this construction and the answer is no. Different driver = different crossover and box tuning).

The C17WH-69-08 is not an easy driver to find and it is no longer in the Vifa catalogue. I found a local dealer having some boxes of C17 drivers and I bought four. Try a Goggle search for "Vifa C17WH" and you will find a few dealers still having these drivers.
Make sure the foam surround is looking reasonably fresh and do the nail test before buying. Squeeze a nail into the foam close to the perimeter of the surround and if you are left with a mark and the foam doesn't rise again, we have an old surround that may only last a few years.
This driver came to my attention from visiting a guy having a speaker system from a single C17WH-09-04 in a rather special box. For mid the Vifa was supplemented by an excellent 3" Brazilian pp upper midrange (Bravox) with foam surround, and for treble a 25-something SEAS tweeter. The C17 was housed in an 18 litres vented enclosure where a square, wooden "vent" was loaded by a short exponential horn. I haven't got the drawings but I'll try to push him hard to reveal some more details.

The C17 is also very well know from the Living Voice d'Appolito and a low-cost version is some 1500.
Now, Living Voice may possibly use 10 ohms (DC resistance) drivers to make an overall claimed impedance of 6 ohms. I haven't seen any reviews including measurements on this speaker. The Living Voice is using a C17 version with a larger magnet compared to this C17WH and the crossover appear to be 6/12 dB and an LCR circuit is keeping the impedance flat in the midrange. To have the C17WH contributing in the treble area may be appealing to some, but to me it isn't. Having the treble beaming at you due to a large cone is not nice, neither from the C17, nor from any other driver.

The C17WH is very much in family with the old SEAS CA17RCY, which Wilson Audio used in the first WATT and the first WATT/PUPPY. The SEAS CB17RCY is heavily coated and has a rubber surround with some trouble around 1 kHz due to a non-optimised outer suspension. The new SEAS CA18RNX has a really strange frequency response profile and some 2-3 dB lower sensitivity compared to the old drivers. I really wonder if it is as bad as it looks from the brochure.

Vintage SEAS CA17RCY.

SEAS 27TFFC tweeter having just enough sensitivity to cope with 2 x C17WH-69-08.

Workshop C17WH set-up with an AudioTechnology middriver following the event.


Crossover is quite simple. Basically 3rd order topology, where 12 dB electrically to the midbass does it. The notch filter is optional, but I suggest you buy the few components needed to try this. The 3.3 mH should be cored and may have high resistance. 0.8 mm wire is fine.

TS data
Measured with the CLIO and added mass method used (10 grams).

These drivers are remarkably alike and no need to make pairs here. The Qt = 0.34 is perfect for modelling. With an 18 litres cab (1 driver) the F3 is 62 Hz, which may seem high, but the d'Appolito arrangement delivers a very good bass performance. It may have an early roll-off, but the slope is shallow.


Not much to be said about the cabinet. 19 mm MDF. I strongly suggest some bracing at tweeter height, below lower midbass and below vent. I have used an extended (depth) 2.5 clone test cabinet, thus the 225 mm width. Place the vent to the rear to eliminate any midrange emission. Make sure the front panel and driver position is as on the drawing to make the crossover work properly.


Individual response of drivers. The response from the C17 drivers is not the same due to different placement on the front panel. For the LspCAD simulation both drivers in parallel were measured at 1 metre distance.
Red = upper C17, blue = lower C17. Green = SEAS 27TFFC tweeter.

Connecting the C17 in parallel reveals this picture. Note the 96-97 dB sensitivity in the midrange. No serious break-ups from the paper cones and a smooth roll-off above 7 kHz. The C17 response is merged with nearfield measurement at 350 Hz.

Here's the response of the finished speaker including the midrange notch filter.

The impact of the midrange notch filter is seen here. This may seem a small difference, but try it and tell me what you think. It makes quite a difference in terms of midrange projection and adding the LCR circuit may leave you listening to a wider range of your recordings. 1-1 dB in the 1 kHz area is clearly noticeable. I'm still in doubt which I like the most.

Impedance of final construction. The notch at 170 Hz tells that some work needs to be done on placement of the damping material. Cabinet height is around 1 metre, which equals half the wavelength of 344 Hz = 172 Hz.
If you want a flat impedance profile in the upper midrange, ad an LCR circuit from 22 uF + 0.56 mH + 6R8 at crossover input.

Port response. Quite broad, actually. I still have to experiment with other types of vents. I have some 80 mm vents I have to try. Don't expect a lot of things going on at 30 Hz.

Left and right speaker. Quite similar where it counts. The response here is normalised for 2.8 volts signal at 1 metre distance and reveals a sensitivity of around 94-95 dB in the midrange. This should equal an efficiency of approx. 92 dB for 1 watt.

PS: While writing the draft of this article I had the offer of an original pair of C17WH-09-08 drivers. These are from 1982 and are still going strong. I'll add the measurements asap and we'll see how these drivers did some 25 years ago.

Here's the Vifa codes on drivers.


C = steel basket
17 = diameter
W = woofer
H = 90 mm magnet
69 = custom, coated
08 = impedance.

The really old drivers:

Left: Current C17WG-69-08. Right: The 25 year old C17WH-09-08. The foam surround has never been changed,
but my guess is that these surrounds will not last another 10 years unless the drivers are kept away from any UV light.

A few changes had taken place since the early Eighties. The driver has gotten a new chassis allowing a wider spider with a possibly enhanced bass performance. And the dust cap has been changed from a wide and shallow paper dome to a small fabric dome. The paper dome makes an overall flat cone geometry and this may possibly have an impact on dispersion. Whether there are any sonic differencess between the old and the new drivers is difficult to tell. Setting up the two 8 ohms drivers in the test cab with the current crossover produced the below seen frequency response and it sounded good.

Lower speaker in both cases: Red = old driver. Blue = new driver.

Frequency response from the old drivers and current crossover.

The C17WH-09-08 paper dust cap is used by Living Voice in their more expensive variants of the speaker and having some spare paper dust caps I might try replacing the current fabric types. At the same time I might dampen the pole piece as I'm sure this has not been done. Despite the centre polepiece being ventilated (by a very small hole), I'm sure there are resonances between the dust cap and pole piece.