C12A Sony C37a Hybrid

[Matt Nolan]

A question I've often wanted to ask is how can you calculate the AC input voltage at the grid of a tube from a given capsule at a given SPL? I cant seem to find any equations.

As a reasonable approximation, you could work back from overall mic sensitivity specs, through the transformer ratio and gain (if any) of the tube stage to get the AC input voltage at the grid.

 

[abbey road d enfer]

Ok so CF  mic (with lower winding ratio transformer) performs better with a solid state mic pre (2k) rather than a tube mic pre (200R). Its more load dependent
But that is given that bias point, B+, I/V relationship is the same yes? C37a is biased a lot colder (3k) than C800G (1k) so comparison is not so straight forward.
These 2 mics provide the perfect comparison because they use the same tube (although different capsule).

Admittedly, there are many factors at play. Tweaking the operating point is just one of the optimization elements.

A question I've often wanted to ask is how can you calculate the AC input voltage at the grid of a tube from a given capsule at a given SPL? I cant seem to find any equations.

Quite complicated... The voltage is directly proportional to the bias voltage, to the diaphragm area and its displacement, inversely proportional to the square of the average distance between diaphragm and backplate. The displacement is also proportional to the sound pressure and inversely proportional to the diaphragm tension BUT the damping (induced by turbulent air movement in the backplate holes) messes with that all (by up to about 20-25 dB!) for the sake of increased linearity.
I don't think anyone has a mathematical formula for evaluating the damping factor.
Just the difference in distance between electrodes can easily induce -50/+100% variation.

Also the harmonic distortion signature will be different between a C37a(more 3rd) and a C800G (more 2nd). Would this explain why some engineers love the C37a for toms and other high SPL sources? 3rd harmonic (in small doses) provides a nice edge for toms and accentuates the impact transient whereas 2nd harmonic seems to have a thickening effect but softens the impact transient.

You can't restrict the differences to just the harmonic content, when there are many other factors at play. The dual diaphragm (C800G) vs. single, the bootstrapped 100Meg (C37A) vs. 500 Meg have probably as much or even more sonic impact.