Hi,
Going with a big driver is generally a bad idea. You need to look at a few parameters. The BL factor which is the force in Newtons per Amp of drive current is very important. Large drivers will generally have a high BL which is a good thing. However, that typically comes at the cost of large moving mass. Since F=MA, the gain due to high BL could be out weighed by the large moving mass. A tiny driver with a lower moving mass might just work out better than a large driver. You really have to run the numbers to determine what works best. Be sure to consider the driver power limits. Using a big driver might also make the physical head much larger along with all of the coupling components (e.g. torque tube, coupling wires and/or cones) . That could once again add too much moving mass. You really need to keep the moving mass in the head as low as you can (e.g. <= 1g total). Also you need to be aware of the mechanical stiffness of your coupling from the driver to the stylus holder. larger generally means less stiff or again more massive and that results in secondary resonances that creep into the upper range of the head between 5-15Khz. You will find its hard to eliminate them and the result is that the high frequency response will suffer on the other side of these resonances. As far as the driver resonance is concerned, you don't really care too much about this this other than it is set by the total moving mass of the driver and the suspension stiffness. Once the driver is coupled to your system, the main resonance will be the the combination of the driver moving mass and stiffness and all of your added external components. It may go up or do down depending on the parameters. Generally, you want the final built main system resonance to be in the 1-2Khz range. How you get there matters. If you start out with a high moving mass, then you will have to add an very high stiffness to raise the resonance to that range. If your mass is low, you don't need much stiffness to get a decent main resonance point. If you set the system resonance too low, then it will take large amounts of power to accelerate the head on the other side of the main resonance. At low frequencies below the main resonance the sensitivity of the head is controlled by its stiffness. On the other side of the resonance it is mass controlled. For example at DC, if you put an amp through the driver voice coil, you will generate a force equal to the BL product of the driver. The driver will deflect based on the stiffness of the suspension. There is no acceleration present. Once the head is moving, the peak acceleration due to it's back an forth motion results a peak force equal to the moving mass time this acceleration. At low frequencies, the component is low. On the other side of the main resonance, it dominates.
Don't get too worried about the RIAA curve. You want a head that is easy to drive and has good high frequency response with as few nasty secondary resonances as you can get. You will generally not apply the textbook RIAA curve to any open loop moving coil head. Instead you will map an EQ to give you a composite calibration that takes the head response and the RIAA into account. Spinnertown has a great post on mapping the head.
This is the executive summary of head design 101. There are lots of interdependent tradeoffs to be made. You can just build and wing it and hope you get a good result. But if you have the chops to do a deep analysis of the physics, you will save a lot of lost time and money. Hope this helps you out.
Mark
Re: Stereo Cutting Head - based on Grooveguys design
Welcome to the world of lathe cutting acemc. I can certainly send through some STL file for the 3D printed parts, you may need to modify them to fit your lathe once you've built this but it should be enough to get you going. I agree with Markrob's suggestion of going smaller rather than bigger with your drivers. You'll see on this thread that I made a head using a much larger set of drivers. While these can work with some modification (see farmers plough thread) My experience is that out of the box, the smaller drivers get you much closer to where you need to be. That isn't to say that the small exciters are the best thing for the job, there may well be better options out there and I know that Flo recommends a good quality tweeter, but these are what I have used and I think the results are good. One thing I absolutely recommend is to apply damping to these drivers as I've described in this thread, or do your own experiments. The diaphragm of these drivers is quite soft so resonance and distortion can be really hard to manage. I've found that the damping method I used eliminates a huge amount of resonance. If you do build one of these heads please share the results, always good to hear about other peoples experiences.