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."

 

[Murdock]

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).


View attachment 139607
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."

I’ve never tried any Phaedrus product but I find their marketing pretty dubious… let alone their prices.
Member Ruud found out about the instability of their tube with the original circuit.
Also to say that their tube is so excellent that “this palliative positive-feedback is unnecessary to prop-up LF response” is just plain wrong. The goal of this feedback circuit was to reduce low frequency response, not to prop-up. It sais it right there in the original manual of the M49.

Here is an excellent post from Abbey. There also is a good explanation in this thread of how this high pass filter works in the M49.
https://groupdiy.com/threads/feedback-in-mic-buffers.67325/post-854889

 

[rmburrow]

Three or four

Murdock, thank you so much for sharing; your post finds me just in time as I finished building a M49c clone without the 5M/R3 feedback/low cut, but still using a 3-12pF trim cap for C4.

As I am building a matched pair, I was assuming, I could simply level match both mics using C4. Now looking at the schematics again, I realize C4 is part of the feedback loop and now I am wondering, whether I should rather abandon C4 completely?!? A side effect would be I could get rid of the specific trim cap, which is a little too high, so that I was about to dremel away a couple of mm of the headbaskets's interior rim to make this bugger fit into the body...

Any recommendations, anyone?

View attachment 139587

Three or four turns of hookup wire twisted together can give 4 pF.

 

[Roman Beilharz]

I’ve never tried any Phaedrus product but I find their marketing pretty dubious… let alone their prices.
Member Ruud found out about the instability of their tube with the original circuit.
Also to say that their tube is so excellent that “this palliative positive-feedback is unnecessary to prop-up LF response” is just plain wrong. The goal of this feedback circuit was to reduce low frequency response, not to prop-up. It sais it right there in the original manual of the M49.

Here is an excellent post from Abbey. There also is a good explanation in this thread of how this high pass filter works in the M49.
https://groupdiy.com/threads/feedback-in-mic-buffers.67325/post-854889

I was wondering about the "prop up" as well. Sure, the LC implies a dip and some overshoot, but the main goal was to suppress sub bass noise, of course.

I'll check out Abbey's thread, thank you!

Well and Phaedrus' marketing... I don't know. From what I gathered it's obviously a complex task to build and QC such high quality tube substitute solid state circuits. The lack of competition speaks for itself: If it was easy to come up with a product of similar quality for a fraction of the cost, it would be available.

The most important thing for me is that the Phaedrus AC701 seems to be a decent low-noise, high quality, long-life replacement option. As long as timbre, harmonics and dynamics end up sounding great, it's been worth the investment regardless. We will see...