Originally Posted by
Ian Mackenzie
As Rob mentioned I would start out with a 2425 so you have a baseline and consider purchasing a v.good measurement kit if you intend to explore using other compression drivers.
An acquaplas treated titanium diaphagm is a significant advance on the stock titanium diaphagm and requires no further modification of the network other than increasing the level by 1 db as there is a loss of about 1 db after acquaplas treatment.
One of the key differences in using an acquaplas coated diaphragm is a much broader range of useable adjustment (sweet spot)for the horn level over the untreated diaphragm. This is not so much to do with a smoother amplitude frequency response but a much improved (damped) time domain response above the mass rolloff frequency of the diaphragm. This is quite audible. The timbre tonal transition from the 2122H to the horn is very close with the acquaplas treatment.
The stock crossover was designed around the 2421/2425 driver and other drivers would require re-optimisation of the network. (the impediance and FR of the drivers/horn and lense assembly mounted on the 4345baffle will vary from one driver to another).
Adjustment of the L pads may compensate for some differences but will not necessarily give a true indication of how the system is performing with an alternative driver.
Some work is currently in progress on using other drivers with this system but I would recommend the acquaplas treatment of a stock driver as the known best alternative at the moment.
Ian
The images are the acquplas treated diaphragm appearance, the response curves are live in room tests on an actual 4345 baffle and are un smoothed, full raw response of the acquaplas diaghrapm, original equivalent HF filter response (acquaplas treated), full range response with HF Lpad adjusted +1 db (acquaplas treated) and the full range response with the stock titanium diaphragm). Note there is a 2 db rise in the microphone above 10 khertz. Small variations in response are due to room interference but I think there is a little less noise in the treated diaphragm. As can be seen in both cases the overall response is remarkable smooth but to the ear there is a tonal difference.
These tests were done some 12 months ago with a fairly basic measurement setup but you get the idea. In the end it ain't what you can see but how it sounds!