Vifa C17-III revisited, the C17
2006 © Troels Gravesen
Read Vifa C17-I and C17-II and C17-mkIV
This file is replacing former C17-III
file, as a minor - but important -
change has been made. Aake
in Sweden stayed
home one day - alone - and played rather loud -
and send me a note telling the tweeter appeared
rather grainy and distorted at times and some
mails back and forth made it clear there was
something to it. I hadn't run the C17
particularly loud and doing so, I had to admit
the tweeter took some serious cone excursion on
certain recordings. Taking the speaker to the
workshop and connecting a sine wave generator
made it clear that something around 500 Hz wasn't
as it should be. First I thought the front
panel/waveguide really wasn't airtight and the
C17 drivers would introduce some pressure into
the unit, but disconnecting the C17 drivers made
it clear this wasn't the case either.
Well, 1st order filters
are tricky and looking at the impedance
profile it appeared I'd be too optimistic on the
impact on tweeter performance from the V6 tweeter
LCR-circuit, basically used to smooth the
roll-off characteristic of the tweeter and not
really flattening the overall impedance. The SPL
from the tweeter at 500 Hz was some 25 dB down
and I had expected this to be sufficient - and it
is, as long as you don't play excessively loud.
So, time to revise the crossover once again. The
LCR circuit was changed to make an overall flat
impedance profile and even lower the impedance a
little above 2 kHz to make possible some
amplitude adjustments in the lower treble. Having
only a single cap to operate with leaves few
tools for making the tweeter behave the way you
want. At 500 Hz the SPL level from the tweeter
was lowered further 10 dB down to 35 dB below
average summed SPL. What also appeared during
this exercise was that the C17WH-16-08 drivers
had been burned in and the minor notch-filter in
the high-pass section could be removed. All in
all a simpler crossover and better performance.
More details below. Thanks to Aake for his
feed-back. Only tells how important this is.
I have no overview
of how many variants Vifa has produced of the
C17, but a great many of these drivers appear to
be identical despite being labelled differently
for various customers.
The C17WH-69-08 driver used in
the C17-I and C17-II has a brother in the C17WH-16-08.
Or at least it appears so. Visiting a local
dealer, some 150 drivers were in stock and it was
obvious to make a thorough study and see if they
would be fully compatible with the
"-69" drivers. I've had one of these
"-16" drivers before, apparently
similar to "-69" but you never feel
sure about the consistency of drivers from this
These drivers are in mint condition and appear, as had
the left the Vifa facility yesterday.
Impedance of drivers
of C17WH-16-08 drivers (left) compared to C17WH-69-08
(right). These drivers appear to be very much the same.
Compare to TS-data
generated for Vifa
Two differences here: The -69 data was
generated with added mass method, where for the
-16 drivers I used added volume. These two
methods usually do not give the same results. As
a result of this the moving mass of the -16
drivers appear 3 grams higher and calculated SPL
is slightly lower, where the actual SPL
measurements for 2.8 V gave slightly higher
results. Later I'll measure the -69 drivers with
added volume and compare data again.
Frequency response of
"-16" compared to "-69" in the
To cut a long story
short, the -016 drivers appear approx. ½ dB more
sensitive from an average of 4 samples of each.
There are variations in SPL from these drivers as
seen on the graph below. Ideally we should buy a
box of 12 and pick the four best. Obviously not
possible. However, the variation is manageable
and the differences will even out from having two
drivers sharing the same frequency range. If
difference between left and right speaker seems
noticeable, I suggest swapping the upper (or
lower) drivers to average the response better.
Below SPL of 4 x -69 and 4 x -16 drivers
SPL from -16 drivers.
SPL from -69 drivers.
The V8 crossover:
V8 crossover: SPL from left and right speaker with the
Impact on tweeter impedance from V6 and V8 crossover.
Impact on tweeter SPL from V6 and V8 crossover.
V8 crossover (red) and SPL from driver without crossover.
Note the smooth roll-off from the C17 drivers.
SPL from tweeter before (blue) and after (green). Some
9-10 dB lower
response at 400-600 Hz.
The V8 crossover:
an electrolytic capacitor - no problem here in a
parallel circuit. You may bypass the electrolytic
cap with a 1.5-2.2 uF pp to possibly improve
can be an electrolytic capacitor bypassed by a 1 uF pp.
Here I have chosen to use 2 x 33 uF pp.
More pics of the C17WH-16-08 drivers:
These drivers are in perfect shape and properly protected
from UV light, they will last for many years to come.