Vifa C20WH

Looking at extinct Snell E/II and /III speakers, I came to think about the old Vifa C20, 8" driver. The C20 is found in numerous versions like the C17. The C20 has a lightweight paper cone, a narrow foam surround and the "H" provides a 90 mm magnet for a 1" voice coil, probably giving decent sensitivity. Googling the C20WH, I found a single shop where the C20 was available, probably some old stock and this is what I found in seller's storage.

Nice looking C20 drivers. Fresh foam surrounds and untinted cones. An additional 90 magnet was added to increase sensitivity. When you add an extra magnet it has be to pushed away from the speaker's magnet. If the extra magnet is sucked in, it will decrease magnetic flux in the gap. Strangely enough, if it's very easy to get the extra magnet in place, it usually only means a small increase in sensitivity.

If you really have to take special measures to keep the magnet in place, there's usually an significant increase in sensitity. I keep the extra magnet in place from tape after adding a small amount of silicon glue between the two magnets. A clamp is added overnight.

The usual frequency and phase measurements were done at 1 metre distance to front panel and files were imported into LspCAD and modelling could start. As can be seen below, the C20 is an unusual driver. It can almost be used without a crossover. Basically flat in all midrange, and had the smooth roll-off started at 1500-2000 Hz, it might have run from no crossover at all. But it doesn't... Any 8" driver mingling with treble is trouble due to beaming and the sound becomes honky. A very small inductor, 0.56 mH, bypassed by a 12 ohms resistor does the job. The 22 uF + 3.3 ohms resistor provides impedance correction - and a little more...

From recent C17 experience, the DT300WG crossover was almost given and a 1st order filter could be added and fortunately the phase tracking was nice too.
Piece of cake! A two-way system, impedance 8 ohms and sensitivity at 95 dB/2.8V. Due to the relatively high impedance, this speaker is close to having an efficiency of 96dB/1W/1m.
It's currently driven by a 6W 6C33 single ended triode amp and this modest amp has no problem playing really loud without clipping.

The Cabs: Follow link: http://www.troelsgravesen.dk/HES.htm

The Crossover

Crossover version 2 and LspCAD SPL and phase prediction.

The X-over

10 ohms resistor used for by-passing the 0.56 mH coil. 10R or 12R can be used.
I suggest using 22 uF pp in the bass section and 2 x 33 uF pp for the "68" cap, however electrolytic, as seen above, will do fine.

Crossover Kit

Crossover components available from Jantzen Audio.

Measurements:

SPL of C20 "as-is" (red) and one 90 mm magnet added (blue).
Reading merged with nearfield response at 350 Hz.

Left: SPL from drivers without crossover attached. Right: SPL from drivers with crossover and summed response from system.

Left: Impedance of final system. Smooth from 200-20,000 Hz and with the usual bumps from a vented system, tuned to 40 Hz (72 mm (ID) x 50 mm vent).
Right: SPL from system at 2.8V. Red = minimum phase.

Sound:

Compared to the SEAS CA21REX set-up, it is immediately felt that due to the lower Fs of the C20 drivers, 39 Hz, the bass is deeper. Maybe not as tight as the CA21, but overall a more balanced presentation.

The C20 provides an excellent midrange and due to the "waveguided" tweeter, a smooth integration is possible, something I wouldn't have thought possibly a few months ago.
From modest wattage amps high levels can be accomplished and the only question left is: What would happen if this set-up was placed in a heavy, well-braced and well-damped enclosure? It's obvious the AN cabinet adds colouration, although subjectively much lower than I had expected.

The CA21REX was really meant to be used in a coming 3-way construction and to make a long story short, the C20 + DT300 will stay in place as an example of a simple 2-way system of more than average sensitivity and how much fun can be had from a total of ~250 US $ - including cabs. I'm enjoying it a lot.

Builder's comments from Volker/Germany.