Another C-37/CF Idea

I have a pair of Dale's C37 capsules and I was just going to do the usual Royer circuit with them, but I wanted to try something different, so I played around with some numbers and came up with this:



It seems to make sense (the PSU is regulated so I can use it for either one or two mics at a time). I'm fairly confidant in all my numbers, but I'm left with two questions -

1) According to my math, I'll end up with about 80V on the capsule. I know Dale mentioned that he runs these capsules at 70V, but I'm worried 80V is too high. Any opinions on this. Is my math right?

2) Also according to my math, the Zout of the circuit is around 400Ω, so I set the transformer to step-down 2:1 so I'd have a nice 200Ω output impedance. The trouble is that I've heard that Dale's capsules are already low output, and I don't want to lose more level unless I have to. Is 400Ω too high an output impedance? What about 800Ω (xfrmr wired 1:2)?

Lastly, anyone see any screw up? My tube-circuit designing is a little rusty.

The original C37 has 120V bias. Not sure how much Dale deviated from the original design, but Sony will have way too much noise and low output with 80V.

As for the CF transformer ratio look here:

https://www.tubecad.com/2005/June/blog0049.htm

Also, from memory, Radiotron 4th Edition insists the CF output transformer should have a "usual" high ratio.

Best, M

Thanks for the response Marik!

The transformer impedance is definitely something to think about, and maybe I'm misinterpreting what that article says, but it seems to be focused on power amplifiers driving speaker loads (i.e. 32Ω and less). If I use the formula he mentions:

Ipk = B+/(rp + Rload)

I get roughly 25mA of current into a 150Ω load. That makes 3.75V P-P, which is more than I would expect on the output of a microphone. Maybe I'm missing something?

As for the capsule bias, I know Dale mentioned 70V polarizing voltage on his capsules in another thread, but I don't think he's on this board anymore to comment.

Bump.

Any other insights?