M49C circuit details / questions

[Migs 31]

Hey Migs, thank you for your reply. Yes, I also thought about omitting C4 and R3 - just as all the other measurement input related parts - to preserve maximum gain and bass, but for a first shot I wanted to keep the option to maybe be able to level match my pair a little better with C4.

By the way, did you stick with two 10nF caps C2/C3 (=5nF in total) regardless of the missing R3 output link?

Yes,
I kept 10nF - only C3: connecting it from Backplate/R2 nod, then to ground.
So literally making C2 & R3 disapear.

Check this post (#12 ) in the thread, what Oliver A. did. Except that his R-C numbers DO NOT match the original Neumann Schem. Re-read it, if not done already:

https://groupdiy.com/threads/m49c-circuit-details-questions.80237/#post-1108723

M

 

[Roman Beilharz]

Yes,
I kept 10nF - only C3: connecting it from Backplate/R2 nod, then to ground.
So literally making C2 & R3 disapear.

Check this post (#12 ) in the thread, what Oliver A. did. Except that his R-C numbers DO NOT match the original Neumann Schem. Re-read it, if not done already:

https://groupdiy.com/threads/m49c-circuit-details-questions.80237/#post-1108723

M

Thank you, yes, I had read this posting and I know about Oliver's choices, but then again, I am using a Phaedrus AC701, which at least from the specs seems to be way closer to the original tube than any of the popular substitute tubes.

Btw. on the topic of the self-oscillation/"motorboating" issue of the Phaedrus Audio AC701, it's developers describe the R3 feedback mechanism like this:

"The Phaedrus Audio AC701 has rather better LF characteristics than many Telefunken AC701s (due to the absence of initial-velocity current which is inevitable in a tube). This extra gain at LF is enough that when the phase-shift due to the LPF R3/C3 reaches 45 degrees, instability results. The cure (RB: of some M49c variants creating self-oscillation with the Phaedrus AC701) is to remove R3 (or, at least, un-hook it). The existence of the instability indicates that the LF response of the microphone is already excellent with the Phaedrus Audio AC701 on its own and this palliative positive-feedback is unnecessary to prop-up LF response with the new tube fitted. In fact, measurements here in the workshop have shown the positive feedback causes the dip in the frequency response prior to the resonance frequency and the removal of R3 guarantees a maximally flat response (see below).



It's worth pointing out that other M49s have not demonstrated this problem. Given that the positive feedback is taken off the junction of C5 and the output-transformer, it is quite possible that the age/version/ variation of the output-transformer has a significant bearing on whether or not the Phaedrus Audio AC701 provokes instability when substituting an original Telefunken tube since the voltage at this point demonstrates very significant phase-shift at LF."