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[soapfoot]

Would love to hear impressions of the subjective and objective differences between these two models. Schematics of both systems attached for reference.

Here's what we can tell from the documents and from examining examples in the wild:

• While the C12 has a fixed bias supply delivered from the N12K power supply, the SM204/23 has a partly-bypassed cathode-biasing scheme with an additional 1k8 resistor and 20nF capacitor
• The aforementioned 20nF bypass capacitor is considerably smaller than similar arrangements found in the C24 and Ela M251E, both of which use 20µF instead of the 25nF. The 25nF doesn't appear to be a misprint on the schematic, as the symbol for the capacitor is clearly drawn as a non-polarized type
• While the C12 has the typical T14/1 transformer, the SM204 specifies the (Henry Radio? Rohde & Schwarz?) V2545. Examining real-life examples bears this out. Curiously, this is not the same part number as the Henry V2148 often cited as the "early C12" transformer)
• Even without considering the lack of a fixed bias supply, the PSU topologies are different, with the SM204/23 system having a 150B2 regulator tube (and some other differences)

Besides these verifiable distinctions, here are some other things I've seen mentioned--

• As part of a web search, I found comments posted by Tim Campbell stating that all SM204s known to exist appear to be from the very first run of C12 production, and that all seem to have been equipped with an early CK12 variant that sounds a bit different (described subjectively as a bit more midrange-forward). Another conversant in that conversation used the term "shallow dish," but that's not a reference I'd ever heard before
• In that same discussion, TC mentioned that the SM204 has slightly lower output than the AKG-spec C12

Does anyone here have extensive direct experience with these models, and what are your impressions? We have a fairly good idea of what some of the technical distinctions are, but in practice, is an SM204 essentially a "rebadged, early-spec C12," or does it make more sense to consider it a distinct model of mic?

The body tube, head grille, etc. are the same... I've seen two SM204s in person, and I believe both even had the AKG logo, so it'd be easy to just assume it's a "C12."

I'd love to get impressions, in particular about how the fixed bias vs. cathode bias might impact the sound/performance/character of the microphone (I acknowledge it's probably hard to parse that out from potential differences in transformer and capsule spec).

I'd also be interested in thoughts about that small cathode bypass capacitor. Wouldn't this have the net effect of a top-end lift above a certain frequency? Could this have been synergistic with the early CK12 variant, but less-optimal as the capsule evolved?

Would love to hear any thoughts!

[RuudNL]

  the SM204/23 has a partly-bypassed cathode-biasing scheme with an additional 1k8 resistor and 20nF capacitor

I suppose that should be a 20 (25) µF capacitor. (20 nF would make a very 'bright' microphone.)

[soapfoot]

I suppose that should be a 20 (25) µF capacitor. (20 nF would make a very 'bright' microphone.)

I'd have assumed the same.

What's interesting is that the symbol on the schematic is clearly for a non-polarized type, when compared to the symbols for the electrolytics in the power supply.

So do we think it's an error on the schematic--both the symbol and the unit? Does anyone have an SM204 handy they could inspect to verify?

[gtoledo3]

Just generally speaking, I think that one would want to track down schematic for the AKG C1 mic, if it exists anywhere to see which more closely resembles THAT mic. It would possibly shed some more light on how to view the Siemens mic in relation to C12.

There has been a lot of talk of shallow dish/deep dish CK12 on forums over the years, you may want to search about that some more. Most people prefer the deep dish, so C12 “clones” tend to be based on the (arguably) wrong version of the capsule along with other physical differences the cloners induce on their builds.

[gtoledo3]

I didn’t have time to dig through my gear data archive, so I tried to pull up a picture of the AKG C1 from the net. I notice there is a pic posted on groupdiy, and I also note that the mic actually has Siemens written on it as well as the AKG logo. https://gearspace.com/board/high-end/1258097-chandler-tg-mic-24.html

This fact, along with my memory of mic listings in old audio mags, would lead me to be willing to bet someone lunch that the order of mic release would have been C1, Siemens SM, then finally C12. AKG was similar to Neumann, Beyer, Schoeps,  in their relationship with distributors and rebadging, so it is very likely the Siemens version would have been prior to a mic with only the AKG branding. Debatable I’m sure.

I think there was some hypothesis about what transformer appears to be in the C1 but I didn’t keep reading the thread, have to go do some stuff!

[soapfoot]

I didn’t have time to dig through my gear data archive, so I tried to pull up a picture of the AKG C1 from the net. I notice there is a pic posted on groupdiy, and I also note that the mic actually has Siemens written on it as well as the AKG logo. https://gearspace.com/board/high-end/1258097-chandler-tg-mic-24.html

This fact, along with my memory of mic listings in old audio mags, would lead me to be willing to bet someone lunch that the order of mic release would have been C1, Siemens SM, then finally C12. AKG was similar to Neumann, Beyer, Schoeps,  in their relationship with distributors and rebadging, so it is very likely the Siemens version would have been prior to a mic with only the AKG branding. Debatable I’m sure.

I think there was some hypothesis about what transformer appears to be in the C1 but I didn’t keep reading the thread, have to go do some stuff!

If you could find that schematic, I'd appreciate having a look. A web search didn't turn up much (it was hard to even find anything about the C1, which is a model I must admit I'm not familiar with)

[gtoledo3]

Oooh, I didn’t mean to suggest I have a schematic for that one, though I think I may have very very little more info on the mic somewhere. Unsure if it would shed any further light on the history between this Siemens SM series and the C12.

Just thinking about this subject more, you may be interested in also being reminded that the earliest CK12 were not only “shallow dish”, they were also made of styroflex. If I recall correctly, styroflex is a brand name for polypropylene, which in my experience tends to have self rigidity, more like PVC. Some of these early CK12 are also a very little larger, though I’m not sure how these various factors overlapped in all cases. I believe some out there wound up having some mixed sub parts.

[soapfoot]

Styroflex is the trade name for polystyrene, not polypropylene

[gtoledo3]

Ahh, thanks for catching that, and just to clarify…that’s what I *meant*, and my description of the physical character is intentional and descriptive of polystyrene. Too many thoughts going down the funnel at once! I was tangentially thinking of some other products that have switched between these, and some of the physical similarities, and then just typed the wrong word.

The “styroflex” branding is obviously derivative of the underlying polySTYRENE, which I am thinking is a good way to keep this straight.

[gtoledo3]

I think that mic in the groupdiy thread may in fact be an AKG C2, mislabeled.

I believe the C1 was a bottle mic ( http://recordinghacks.com/pdf/akg/AKG_History.pdf ), that used an early CK12 type capsule, or related. Anyway, I was just poking around for schematics (unlikely) and noticed this discrepancy between what is called a C1 in the two different pictures.

Now…I have to add that there is some noticeably incorrect info in this AKG history, so I think certain things should be taken with a grain of salt.

Anyway, not trying to mess up the signal to noise ratio of the thread, Brad. But it would be great if a C1 or C2 schematic or other parts blueprints ever showed up somewhere, from a historical perspective. AKG has always seemed much worse about historical documentation being out there than Neumann, even as far as preserving their own internal knowledge goes.

[gtoledo3]

Brad, I wanted to follow up on a phrase I was using, so you understand why…”siemens sm series”.

Check this out, it’s wild, SM205 : https://www.madooma.com/shop/akg/SIEMENS_AKG_SM205_1_Vintage_Roehren_Mikrofon_269_SM-205_C12_C-12_e.html

I’m not 100% sure what is going on with this one, but noteb the more U47-esque headbasket. What seems very interesting as well is, what the heck is going on with the capsule? It looks way too small in the headbasket, so either it’s smaller or this mic is built on a bigger body tube standard than a C12. Anyway, it would be somewhat fascinating to better understand the evolution of all of these related mic, circuit wise, and as far as construction of sub parts (capsule, transformer, etc) goes.

My memory on it’s fuzzy, but I think I’ve seen a couple other siemens sm series mic models that appeared related to all of these.

I’m not sure if I wrote this above, and am kind of waking up, drinking coffee and checking my emails/the forums….it comes to mind that I think maybe some of Oliver’s C12 related circuits may have experimented with different bias methods. I know him as well as David Bock have told me they feel as though the effect of bias method is pretty huge.

[soapfoot]

Thanks, George.

That SM205 does look interesting, and yes it does appear to have a smaller capsule (the mic itself next to the PSU doesn't look excessively large, assuming the PSU scales the same as typical similar units)

Oliver's "C12 Alternative" schematic shows bias derived from heater supply, like a U47. This is different from either the AKG C12 (dedicated fixed bias supply on its own conductor from PSU) or the Siemens SM204 (cathode bias)

[labinquiry]

Thanks, George.

That SM205 does look interesting, and yes it does appear to have a smaller capsule (the mic itself next to the PSU doesn't look excessively large, assuming the PSU scales the same as typical similar units) coreball

Oliver's "C12 Alternative" schematic shows bias derived from heater supply, like a U47. This is different from either the AKG C12 (dedicated fixed bias supply on its own conductor from PSU) or the Siemens SM204 (cathode bias)

Interesing info. Thank for your sharing.

[soapfoot]

To clarify one thing I’ve come to understand better: the C12 isn’t a “true” fixed bias supply, but rather uses a technique called back-biasing. This is similar to cathode biasing in that a voltage drop is created across a resistor by the current pulled through the tube—only it’s executed on the anode side, and the negative voltage developed across the bias resistor is then injected directly onto the control grid.

[gtoledo3]

I think most C12 “clones” aren’t setup with the negative voltage arrangement.