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
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.
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
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
version 2 and LspCAD SPL and phase prediction.
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.
components available from Jantzen Audio.
of C20 "as-is" (red) and one 90 mm magnet added
Reading merged with nearfield response at 350 Hz.
SPL from drivers without crossover attached. Right: SPL
from drivers with crossover and summed response from
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
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
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
comments from Volker/Germany.