Vifa C17 mkIV
Copyright 2006 Troels Gravesen

       
The C17mk4 on the terrace on a cold and cloudy December's day, 2006. The oak bars holding the spikes will be black - some day. Spikes are Jupiters from Soundcare/Oslo. Great when you have to move speakers a lot.

Qualifying for being part of my stock speakers, the C17s certainly needed some nicer cabs. Some 22 mm MDF sheets were bought and I took it to my local veneering company and had mahogany applied. Next to the veneering company is my local carpenter, having a table saw at the size of a tennis court, fitted with a huge saw-blade with uncountable teeth. Cuts MDF so sharp I usually cut my fingers bringing it come in my car.
There are pros and cons from this way of making cabs. The pros are obvious: Perfectly 45 deg. cut panels that fit exactly when glued together. The veneering is perfect too, and stays there for years and years even with the cabs left in the sun.

The cons are the fragility of pre-cut panels. From the second you leave the carpenter's workshop, every single physical object in the world will try to bump into the sharp edges and ruin your project. Getting the panels into the car, getting the panels from the car into your workshop, moving the panels a hundred times for routing, etc., makes you take a deep breath every time you grab a panel and it calls for utmost attention all the time. These razor-sharp edges can drive you nuts. I bumped a corner on one top panel and had to apply tiny bits of veneer to rework the corner. It won't be seen when finished, but knowing it is bad!


The cabinets


Bumped corner to the right repaired with a tiny piece of mahogany and some araldite glue.

Minor changes were made to cabs now having outer dimensions of 220 (W) x 300 (D) x 980 (H) mm. The reduced height is due to making a special vent towards the floor in order to enhance room-gain and get bit more bass extension. The cabs will have to be lifted some 40-60 mm from the floor on spikes. Thus, the depth was increased to 300 mm making a total net volume of 1.76 x 2.56 x 9.36 = 42.2 litres minus bitumen pads, braces and the vent. The exponentially shaped vent is inspired from reading old Karlson literature and a couple of not reported projects have been ahead of this one and I've found this an interesting way of enhancing bass performance from optimising room-gain.

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Left: Cutting plan for the carpenter and right: Vent with exponential mouth.


Cabinet dimensions

Cabinet assembly was not as easy as expected. The edges of the panels are very thin and will easily bend, so a huge amount of clamps is needed to make the edges as sharp as possible. Don't use too much glue and wipe off excessive glue as fast as possible, otherwise the glue will seal the veneer and ruin the oil finish.

Damping, Monacor MDM3


Two sheets of MDM3 were stuffed behind the vent at the bottom to reduce
standing waves between top and bottom.


3 pieces of 15 x 35 cm were stacked and attached to rear panel and top panel behind upper bass driver.
3 pieces of 15 x 25 cm were stacked and added to rear panel behind lower bas driver.
As can be seen, all internal walls were covered with 15 mm pure wool felt material.


The tweeters:


In order to dampen the waveguide, the cavities were filled with some acrylic filler. After a few days the waveguide now feels like piece of MDF.
Blocks of 22 mm MDF were routed and glued to the rear plastic chamber. In all, the tweeter now seems like rock solid block. Furthermore the waveguide will be glued to the front panel. I think all this will pay off. Routing for the two C17 drivers and the waveguide takes away a lot of the upper front panel and the damping of the tweeter + bracing right behind the tweeter will compensate for the loss of material. Another piece of MDF will be inserted between the round MDF block and the bracing behind.


Block of wood between tweeter and bracing.


The C17 drivers:


The C17 drivers were added an additional 90 mm magnet: Monacor SPM90, item 104260.


The crossover

Complete list of components for two crossovers:
2 x 1R0, 10 W MOX, I was short of 1R0 and used 2 x 2R2 in parallel.
2 x 8R2, 10 W MOX
2 x 5R6, 10 W MOX
2 x 1.0 mH, 0.18 ohm, cross-coil with wax impregnated paper insulator
2 x 0.33 mH, 0.19 ohm, cross-coil with wax impregnated paper insulator
4 x 33 uF pp capacitor
2 x 6.8 uF pp capacitor
2 x 3.9 uF pp capacitor, standard or supreme.

Connecting foil coils is a bit of trouble due to the flat and folded terminals, but this is how it looks. I also had some test-samples of supreme caps, here without the final red wrapping. Thus the huge 3.9 uF capacitor was used in series with the tweeter. I guess I can't spoil the C17 set-up much more than this - well, which should be silver coils, but the price on silver.....

The boards are 11 x 28 cm and will be placed on the side panel behind the vent.


Crossover attached to side panel behind bottom vent.


Measurements


SPL, 2.83 V, 1 m. Merged with nearfield reading @ 250 Hz. Vent output not included.


Left and right - doesn't get much better than this.


Impedance, left and right speaker.


Horizontal dispersion, 0-10-20-30-40 deg.


Vertical dispersion. Not surprisingly, a 1st order MTM doesn't excel here.
Red = tweeter height, blue = + 10, green = +20, yellow = 30.

ETC: Left = C17mk4, right = C20WH+DT300WG.
Where this construction excels is this ETC plot. The ETC (energy-time curve) is a tricky measure. It's calculated from the impulse response and according to d'Appolito, the ETC is a qualitative measure of time of arrival. Very useful for analysing room acoustics, but it also tells something about time-alignment of drivers and possible cabinet resonances. A sharp peak followed by a smooth decline may be an indication of time-aligned drivers, lack of serious resonances = stored energy and these two Vifa constructions appear to excel here. Higher order filters will store energy as can be seen below.

I've had a number of visitors recently listening to the current line of speakers and many have left pointing to this C20, saying: This is a strange fellow, it just has something.....


Left = 2.5 clone, right = Spendor BC1


Left = Rogers LS35A, right = JBL L100 century (not modified).

 

- the sound....

Well, something must have happened to the sound from all the changes that have taken place since my mkIII test-cab version. And so it has.
Initially the idea was to increase sensitivity by adding extra magnets to the C17 drivers and tuning the construction for another 1 dB. It didn't turn out well. Too much presence and a little too aggressive. Thus the 1 mH coil stayed in place - very much determining the overall level for the midband. In return the upper bass and lower mid came with enhanced robustness and solidity. Placing the vent towards the floor compensates for the slightly reduced bass due to extra magnet gap flux (= lower Qt).

So all in all a more firm presentation in lower registers. The rigid cabinet and heavy damping appear to pay off in an overall cleaner sound and the speaker can simply play a little louder compared to former mk3. Having a 3.9 uF supreme cap (test sample) I modified the tweeter crossover by lowering the 0.39 mH coil to 0.33 mH to get an overall balance in the 3-7 kHz region like before. The series resistor was reduced to 1.1 ohm, suggesting there is a small increase in overall sensitivity of -1 dB. Hard to measure with certainty. The waxed paper-wound coils appear to do excellent. Having made numerous changes, I cannot pinpoint the significance of individual changes. But I'm looking forward to more Jantzen supreme caps after this one.

The C17 work has been ongoing for almost half a year - and has been very rewarding. I dare say that for a modest price you can build a speaker that will compete with lots of high-end stuff. The overall level of transparency is extraordinary and for the size, the bass is deep and powerful. The C17 mk4 will easily be driven from 20 wpc PSE amps. Even 10 wpc will do if you do not play too loud.