Vifa C17-III revisited, the C17 drivers
Copyright 2006 Troels Gravesen

Read Vifa C17-I and C17-II and C17-mkIV

23-09-2006:
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.

 

C17 drivers:

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 range.

 


These drivers are in mint condition and appear, as had the left the Vifa facility yesterday.

Impedance of drivers


Impedance of C17WH-16-08 drivers (left) compared to C17WH-69-08 (right). These drivers appear to be very much the same.

TS data

Compare to TS-data generated for Vifa C17-I.
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 C17/DT300 set-up.

To cut a long story short, the -016 drivers appear approx. 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 -16 drivers.


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:

C1031 is 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 performance.


C1031 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.