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Old 15th May 2016
  #3087
Lives for gear
 

I’ve checked and like I thought Dennis owned a GZ67fet. He had no (hands-on) experience with the GZ67tube. Neither do I BTW.

Also, he had two M269 clones; no Neumanns FWIW. One was a –c version, made by Dany Bouchard. This one had the correct circuit of the original (which is not as obvious as it may seem). I followed the development of it closely. The other was an RMS. I don’t know how close to the original the circuit in that one was/is. But no doubt it emulated the better known –c version as well, in any case regarding frequency curve.

Somewhere earlier in this thread Dennis explains that the M269 was the broadcast version of the U67 and that it had a somewhat different circuit for that reason. The first bit is fully true, the second bit isn’t the whole truth and/or it needs some elaborating. Since Dennis can’t do that anymore, I will give it a try.

German broadcast had its own system and standard and the tube mics they used typically had the AC701k tube in them. To apply to this standard, Neumann had to make a U67 version with an AC701k tube in it, which they did. This was the M269. Please note there’s no –c suffix there yet. Both the U67 and the M269 had a fixed bias circuit and they sounded very much alike. (!) Experts like Klaus Heyne have confirmed this. The AC701k tube has a hair more clarity and a little less character than the EF86, but this difference is very subtle, especially if the EF86 is a Telefunken.
The M269 soon became the M269b; still fixed bias. Then Neumann changed all their mics with the AC701k (including the M49, KM54 etc.) to self bias. They all got a –c suffix. There’s a noticeable difference in tone between the fixed and self bias (-c) versions, the latter being lighter in the low mids and airier on top. IMO this is the reason, or in any case the main reason, that the M269c sounds airier and lighter than the U67.
Do keep in mind that even the M269c has a de-emphasis c ircuit plus negative feedback and a tertiary winding on the transformer. The original K67 capsule is very bright (pre-emphasized) and really needs de-emphasizing to sound balanced. This was omitted in many Chinese mics that have a K67 style capsule, hence their extreme brightness. Harshness in those has another reason (too) BTW.
The negative feedback is a separate matter. Dave Thomas recently explained that this helps to match the EF86 tube to the relatively low ratio transformer. Although this comes in handy, IMO this wasn’t the real reason (after all, they could have made a different tranny). The whole negative feedback scheme including the tertiary winding was put there with a plan in mind. It effectively kills sibilance and other transient related artifacts that condenser mics can be prone to. Neumann tried to take engineers’ complaints about the U47 into account and this was one of them. Another was the noise floor (pre-de-emphasis took care of that one, not unlike Dolby) and there was built in pop filtering and proximity effect reduction.
However, he most important part of the intricate U67/M269 circuit IMO is the active and dynamic transient control as described above. This sets it apart from all other condenser microphones. The brighter and louder the transients you fire at it, the harder it fights back. This is the reason why some experience it as a dark microphone (especially the U67 and fixed bias M269). The frequency plots in the catalogue don’t show this at all, though. The subtle hills are a little above sea level rather than below it, if that makes sense. Yet another example why frequency plots can be deceiving. But be ready to get hit by some if you say this out loud.
This getting rid of transient overload artifacts so early on is also the reason why the U67/M269 takes a treble EQ (air band) lift like no other condenser mic. And no, filtering (after the mic) is not always better than amplifying. Dennis felt the same way about this BTW.

OK, now let’s look at the 3U Audio GZ67 (tube version). The name could suggest it has something to do with a U67 (or an M269 for that matter). But if you read the above, as well as the GZ specs, you’ll understand that this cannot be the case. Just like it’s not the case with the Peluso 67 and a host of other mics that have 67 in the name. Granted, most of those do have a K67 style capsule. But the majority of LDC mics do anyway. Next to none of them have the special negative feedback circuitry, though. And many don’t even have a proper de-emphasis circuit. Some do however. And others tame the capsule itself so it becomes less overly bright.
This is an interesting matter, particularly if we look at Guoshengs mics. We know he’s an experienced capsule maker who can tune or voice them. The GZ67(tube), apart from having no negative feedback, likely has no de-emphasis either. We know this because he offered a member to replace the K67 capsule for a K47. If the circuit had any de-emphasis at all, the K47 would become too dark. So he tames the K67 capsule is my conclusion. It doesn’t have to be flat and it isn’t. But the extreme lift is taken out. This leaves an airy microphone, just as described.

Another guy who does it like this is Dirk Brauner. He always uses a tamed version of the K67 capsule. He combines this with a relatively simple, flat circuit. All with high quality parts and clean sounding transformers. And like the GZ67, the Valvet actually uses a Lundahl tranny.
So although the GZ67 is not like a U67 at all IMO, on paper it reminds me very much of Brauner mics. Less expensive, though.
To be fair to Dirk, the metalwork of the Brauner bodies is really something special and nothing like anything I’ve seen coming from China (although I’m sure the Chinese could do it if they wanted).
Also, the noise floor is incredibly low for tube microphones.
Finally, Brauners are good investments that hold their value pretty well. This may become the case with 3U Audio tube mics too, but it’s too soon to say that.