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[Rich Mays]

This was posted in the Neumann forum regarding the M149, and I am wondering if anyone might like to comment:

sorry, but this web rumour of:"tube circuitry is only used in the cardioid pattern" is one of the sillier things you find in our world wide rumour web. Both capsule signals, front & rear, pass the tube, always and in ALL pattern settings.

Marttin Schneider / Neumann

[Klaus Heyne]

Please read my post (page five) at: http://recforums.prosoundweb.com/index.php/m/0/139/64/318/?S Q=b440d158d6254c6ff9efbc20a4421542

[Plush]

Klaus,

Where is the answer?
This issue must be uncovered and discussed.

[Klaus Heyne]

Patience, please.
Just because we are now used to instant infos on almost anything, does not mean that certain sensitive infos like a proprietory schematic are obtained instantly as well.

I will report one way or the other as soon as but not before I had a look at the schematic.

That's a promise.

[Oliver Archut]

There were several posts on different pin boards regarding the nature of the signal processing in the Neumann M149 tube microphone.

Aside several advantages in mechanical layout and circuit board design, the mic's circuit eliminates the approx. 4 to 5 dB higher S/N ratio found in classic multi pattern designs.

After several repairs of my (now sold) M149 I finally concluded that the 6111 tube needs a better replacement, I updated it to a good old AC701 (after that the mic worked without any problem). In the process of this modification I stumbled on the rather strange observation, that the signal of the backside of the capsule is not present on the input side of the tube.

I concluded that the back half of the capsule is not passing the tube, not a big deal for an hybrid design. In talks to a tech that works at the Sennheiser/Neumann, he confirmed my findings.

I mentioned my findings to Klaus and some other techs about this mic's unusual set up; somehow this minute detail grew up to become an internet life of its own:

On the Neumann Pin Board, Mr. Schneider's response to a related question was :
"Sorry, but this web rumor of:"tube circuitry is only used in the cardioid pattern" is one of the sillier things you find in our world wide rumor web. Both capsule signals, front & rear, pass the tube, always and in ALL pattern settings."

To confirm or disprove Neumann's assertion, I took apart another M149 to try to figure out what is going on. My findings:

The front of the capsule is going via a decoupling cap right to the grid (input) of the 6111 tube; both systems are connected in parallel, reducing the tube noise into half.

The back side of the M149 capsule is going via a decoupling resistor into the gate of a surface mount array of transistors; then it is leaving the upper circuit board via a quick connect and passing the tube's printed circuit board, disappearing into the lower circuit board.

The final test was injecting a signal first to the front part of the capsule  (capsule removed with capacitor dummies simulating the capsule) where I detected and traced the signal right to the control grid. Then the same signal was injected to the back part of the capsule, with no signal on the control grid present.

There may be a test/set up error, that might explain the different statement of Mr. Schneider, but also there is no circuit for the M149 available that could explain the missing back signal on the control grid of the tube.

To sum it up, in my findings the back part of the K47 capsule is processed by a transistor buffer and is mixed after the 6111 tube with the front part of the capsule, Mr. Schneider/Neumann insists that the signal of both half is going through the tube, a statement that is not reflected in the tested mic.

There is still the chance that Neumann changed the design, the tested mic was manufactured in 1997.

Best regards,

[Oliver Archut]

The pic shows the M149 inners from the front, on the right hand side the white wire/front, red backplate, white wire in the back for the opposite side of the capsule (the one signal path in question).
Next to the left of the wires are three transistors and one opto-coupler that process the back signal.

[Gustav]

Ah! Thanks Oliver it makes sense now the tube is always in circuit even when in other patterns.  The fet? part is added for more than cardiod and summed and/or inverted later in the circuit path for the other patterns.

 I am guessing It is "marketing speak" to say the tube is always in circuit.  I wonder why the more complex circuit!  The tube M147 part Oliver posted before is a charge amp circuit to control gain.  I think the correct way to name it would be a charge amp anode follower.

 In the Neumann infopool section there is a GREAT PDF "Microphones" there are to circuit fragments in the back of the KM84 and Fet47 that are charge amp designs.  With charge amps the capsule capacitance is part of the circuit and adding the other skin would upset the gain,  So I am guessing it would be easyer to sum the buffered skins later in the signal path.

 I wonder what the M149 would sound like with a Brauner VM1 or KH VM1 type circuit dropped in it?

Gustav Smalley

[Klaus Heyne]

Here is a verbatim copy of a correction I just posted on the Neumann Pin Board:

Quote:

I had written in my forum that the M149 only uses its tube for the front diaphragm's processing (cardioid pattern), whereas the rear capsule side's processing is done entirely in the solid state domain. After months of research here in the U.S., and endless trans-oceanic conversations with colleagues, I have to retract that statement and apologize for having posted it in the first place: The routing of the signal is not as easy and straight forward as I first thought. The signal routing in this very sophisticated design seems to be a lot more complicated: Whereas the front side of the M149's capsule seems to be directly connected with the grid of the tube, the rear side first enters a FET impedance conversion stage, then a solid state processing chip, before it is recycled back to the tube for, what I would call, 'dirtying up' the sound, i.e. to give it some "tubey" character that adds color and distortion which the very clean solid state processing does not provide. What complicates the matter is the fact that even in cardioid some small portion of rear capsule information is mixed in with the front capsule's signal, in order to widen the narrow cardioid the K49 capsule posesses. That means, even in cardioid, two differing types of tube/transistor signal processing are at work at all times. What I should have written is: The M149's electronic processing for the rear capsule side is primarily a solid state affair, with the tube having more or less an 'after the fact' coloring function, rather than the classic impedance converter/grid pick up function traditionally associated with tube processing in microphones.

Kind regards,

[Schallfeldnebel]

But what stays is the more you use from the backmembrane, the more you use a combination of two different impedance converters, and that's maybe not such a problem for the omni pattern, but for the figure of eight, I start to scratch on my head.

Those who own a M149, put it on figure of eight and test the front side, then turn it around, switch you microphone preamp in reverse phase, and test the back of the microphone, and now listen to the differences.

Klaus, maybe you were too early with your statement, but Mr. Schneider from Neumann in his reaction did not tell the whole story either.

Erik Sikkema

[Marik]

Klaus Heyne wrote on Sat, 16 October 2004 11:11

Here is a verbatim copy of a correction I just posted on the Neumann Pin Board: Quote:   ....Whereas the front side of the M149's capsule seems to be directly connected with the grid of the tube, the rear side first enters a FET impedance conversion stage, then a solid state processing chip, before it is recycled back to the tube for, what I would call, 'dirtying up' the sound, i.e. to give it some "tubey" character that adds color and distortion which the very clean solid state processing does not provide... Kind regards,

Klaus,

I got little confused, as from Oliver's message I understood that "back" signal does not come to a grid of the tube, so the tube doesn't process it. According to Oliver, this signal mixes with "front" one (tube part) somewhere after the tube. Could you calrify what do you mean by "recycled back to the tube", or did I miss something?

Regards, Mark Fuksman

[Klaus Heyne]

How I understand it: the rear capsule information, after processing by the FET and chip stages, is routed to the grid of the tube, to be processed on more time, this time by the tube, in the same manner as it's done for the front diaphragm information.

[Marik]

Klaus Heyne wrote on Sun, 17 October 2004 01:05

How I understand it: the rear capsule information, after processing by the FET and chip stages, is routed to the grid of the tube, to be processed on more time, this time by the tube, in the same manner as it's done for the front diaphragm information.

Dear Klaus,

I am confused even more. How is it possible? The FET and chip stages will give a significant gain. For both, omni or fig8 patterns, the front and back signals on the grid of the tube should be the same , and the patterns are selected by mere polarity change of rear signal. Any "in between" patterns would mean only certain back signal attenuation. In any case amplitude of rear signal should not exceed the front one. All these would mean that after FET and chip amplification, the signal should be attenuated back down to unity, but this will result in noise increase. The only sensible solution, as I see it, would be mixing the front and back signals right after the tube, as Oliver has said. Kind of "tube blend"--the feature found in many modern cheap quasy "tube" preamplifiers.

I become really curious as for what is actually going on here and what Neumann people had in mind designing it? Also, do they use both halves of the tube, or only one (or halves paralled)?  

Regards, Mark Fuksman

[David Satz]

Mark/Marik, please forgive me if I am underestimating your knowledge of electronics here, but a stage of amplification can be arranged either as a voltage amplifier or as a current amplifier, or in some cases both.

The first stage to which a capsule's output is connected in a condenser microphone--in older circuits, the only stage--is always (dare I say "always"?) a current amplifier. Its main function is to convert the audio signal to a low impedance; voltage gain at this stage isn't generally necessary. The capsule's output at audio frequencies is at extremely high impedance--so high that many undesirable effects are present. Without this impedance conversion the signal would be vulnerable to losses and interference of all kinds, even in the shortest and most "monstrous" of cables.

After a signal has been converted to low impedance, then if you want to bend it in some way, that becomes a much more manageable proposition. The output of any such processing circuitry, such as an EQ network, would most likely be at a relatively low impedance as well. But in any case that signal can very well be fed into the ultra-high-impedance input of a second current amplification stage, such as the other half (or even the same half) of the tube that's amplifying the current from the front half of the capsule in this case.

--best regards

P.S.:  By coincidence I suppose, the ad being displayed at the top of this page this morning is from a dealer offering the M 149 Tube at "low, low prices" (!) and describing it as a cardioid (!) condenser microphone. Maybe they think that "cardioid" means "superior"--like, "Wow, man, that guitar player last night, he was really cardioid, man."

[volki]

Mark,
according to a drawing of the tube section
http://recforums.prosoundweb.com/index.php/fa/106/3055/?SQ=f aa78e7507c36c7d1aa1bfeeaffa4b3d
made by oliver, both triode systems are used in parallel (obviously for better noise figures).

I really would like to hear a report on performance of the figure 8 pattern, though. If the two halves don't perform equally, this could present a problem especially with M/S recordings.

Regarding the summing of rear and front capsule system again: I wonder how it is done impedance-wise? The front capsule, which is coupled directly to the tube, presents a high capacitive impedance.  The rear capsule, after having been processed by the mentioned FET(?) electronics, supposedly has an ohmic and less high impedance. So both are summed at the tube. Wouldn't this "unequal" summing mean any impedance mismatch issues...?

The whole thing doesn't really seem logical. When using FET electronics for impedance conversion in the first place, why not use them for both front AND rear systems, and then run them both trhough the tube? Since the tube seems to be in there mainly for coloration anyway...? (And a couple more SMD-elements shouldn't have taken up too much space, either.)

[Marik]

David Satz wrote on Sun, 17 October 2004 13:54

Mark/Marik, please forgive me if I am underestimating your knowledge of electronics here, but a stage of amplification can be arranged either as a voltage amplifier or as a current amplifier, or in some cases both. The first stage to which a capsule's output is connected in a condenser microphone--in older circuits, the only stage--is always (dare I say "always"?) a current amplifier. Its main function is to convert the audio signal to a low impedance; voltage gain at this stage isn't generally necessary. The capsule's output at audio frequencies is at extremely high impedance--so high that many undesirable effects are present. Without this impedance conversion the signal would be vulnerable to losses and interference of all kinds, even in the shortest and most "monstrous" of cables.

Dear David,

Yes, of course, we need a low output impedance.
All "older" tube circuits used two configurations (out of three tube connections possible)--anode follower (grounded cathode), and cathode follower (grounded anode). While the cathode follower is a current amplifier, anode follower is a voltage one. Both, in a sense have actually two stages. In anode follower first stage--voltage amplifier, and then transformer--current amplifier--brings down very high output impedance of the tube down to more "usable" range. Along the way, of course, it brings down the signal, as well. Since the relationship between impedance and voltage is a square, the signal drops "slower" than impedance. Since cathode follower does not amplify signal (in fact, its gain is slightly below unity) and is essentially 100% local feedback amp, its output impedance is much lower and we need much lower ratio transformer (usually 1:2 or 1:4, vs. 1:8--1:10 in anode follower) and signal does not drop as much as in anode follower.

David Satz wrote on Sun, 17 October 2004 13:54

After a signal has been converted to low impedance, then if you want to bend it in some way, that becomes a much more manageable proposition. The output of any such processing circuitry, such as an EQ network, would most likely be at a relatively low impedance as well. But in any case that signal can very well be fed into the ultra-high-impedance input of a second current amplification stage, such as the other half (or even the same half) of the tube that's amplifying the current from the front half of the capsule in this case.

That's actually the part where I have most of problems. Current amplifiers for "normal" work need load. Feeding its output back into extremely hiZ input of the tube would mean it works unloaded, which will not work "right". Also, summing rear signal with the front one at this stage would mean some kind of mixing network, which will introduce even more problems.

I think these two signals summing is ought to be after the tube, unless Neumann people found some ingenious and elegant solution.

Best regards, Mark Fuksman