Original AKG C12a mod

[Andrewfrommontreal]

Hi all,

I have here an original AKG C12a microphone. Klaus Heyne has confirmed that the capsule itself is from a C414EB though.

I have also read that he suggests that taking out the cathode follower - which the audio first meets anlong its path - will produce a louder output and improve the dynamics of the microphone.

My question is…

Has anyone done this to a vintage C12a?

If so, was it complicated?

Might you have schematics to show what was done?

Thank you in advance,

Andrew

 

[Khron]

... But why?

Unless you rewire the nuvistor to act as a gain stage (AND replace the transformer with a higher-ratio one), i don't think there's much other circuitry you can shoehorn into that tight casing, really.

Why not sell this and buy something else that might suit your... desires (i didn't wanna say "needs") better?

 

[Barry Hufker]

I am also against modding the C12A. I think they are fine as is... but please do what makes you happy.

 

[Andrewfrommontreal]

... But why?

Unless you rewire the nuvistor to act as a gain stage (AND replace the transformer with a higher-ratio one), i don't think there's much other circuitry you can shoehorn into that tight casing, really.

Why not sell this and buy something else that might suit your... desires (i didn't wanna say "needs") better?

I was under the impression that by removing the cathode follower, which surprisingly is the first thing the audio hits along its path, one gets more gain and thus less noise. Is this not the case?

 

[rock soderstrom]

I was under the impression that by removing the cathode follower that surprisingly is the first thing the audio hits along its path, one gets more gain and thus less noise. Is this not the case?

This is not so surprising, as the differences between a cathode follower and plate out circuit are not that great in this application.

Both act as impedance converters in conjunction with the OPT.

A headamp with a lot of gain and a high step down transformer result in a quite similar output level and driver capability as a no gain CF with a low step down transformer.

 

[Andrewfrommontreal]

This is not so surprising, as the differences between a cathode follower and plate out circuit are not that great in this application.

Both act as impedance converters in conjunction with the OPT.

A headamp with a lot of gain and a high step down transformer result in a quite similar output level and driver capability as a no gain CF with a low step down transformer.

And what of the sonic qualities described by Klaus? He speaks of the mic’s original configuration as sluggish.

 

[Barry Hufker]

I get what he means by "sluggish," but in my experience, all mics have their uses. It's not the first thing I would reach for to do some things. For instance, I might not use a stereo pair of them to record a choir (and that's just my opinion), but I used a stereo pair to record a brass quartet and they sounded marvelous. Just get to know and appreciate its qualities, and employ it where it makes things shine.

 

[kingkorg]

And what of the sonic qualities described by Klaus? He speaks of the mic’s original configuration as sluggish.

Be wary of these claims, people have businesses to run, reputations to build/keep...

There are some definite and very practical diferences between circuit topologies, they vary in performance in certain occasions, in others they perform exactly the same. Attribute "sluggish" and similar IMHO can't be part of any serious discussion. Either use familiar terms so people understand what you are talking about or bugger off. Excuse my French. I hope you get what i mean.

Try staying away from audiofoolery, you'll lose time, energy, money instead making music with a marvelous mic you already have. "I thought my mic sounded awesome until someone told me it doesn't" is a common syndrome.

If you have too much time on your hands, build couple of circuits where topology is the only difference, feed audio through them and see how they vary in performance when it comes to THD, frequency response, how they respond to different preamp impedances/topologies, phase... You will learn much more than spending time reading ridiculous, and misleading info by "gurus" invested in the business.

"I have also read that he suggests that taking out the cathode follower - which the audio first meets anlong its path - will produce a louder output and improve the dynamics of the microphone."

Output will depend on transformer used, and the output level has nothing to do with quality of a mic, in certain cases it might drive some preamps differently. Dynamics will actually be reduced, cathode follower has larger dynamic range than plate follower.

 

[rock soderstrom]

And what of the sonic qualities described by Klaus? He speaks of the mic’s original configuration as sluggish.

I was talking more about technical things. Different topologies can sound different, whereby sound perceptions are rather subjective and difficult to describe verbally.

 

[rock soderstrom]

Has anyone done this to a vintage C12a?

If so, was it complicated?

Might you have schematics to show what was done?

It's not complicated, you choose the circuit you want and build it.

As described above, apart from the body, parts of the PSU and the capsule, everything else will be needed new for a different topology.

I would rather build a new microphone and leave the C12a as it is.

 

[Barry Hufker]

@Andrewfrommontreal: I get it - it “has a sound…” but all mics have “a sound.” If you think you can use it, keep it. If not, sell it and buy something else you want. Even if you modify it you may not like how it sounds and then what?

 

[Andrewfrommontreal]

Thank you for everyone’s input. Considering the many other lovely mics I have, the C12a shall remain as is. Not to mention, I have a lovely C414EB with the same capsule and it too sounds great though different.

I should mention that I do have two 1968 U87s which Klaus had modded and without question they were both greatly improved due to the work he did.

 

[micolas]

I should mention that I do have two 1968 U87s which Klaus had modded and without question they were both greatly improved due to the work he did.

Hi Andrewfrommontreal!
I don't want to go off topic, but can you tell us what changes Klaus made to your 1968 U87 microphones?
Someone mentioned changing the jFET with BF256,rebiasing and some capacitors.Have you noticed any changes to the capsule or the position of the capsule in the head basket?

 

[JeffKall]

I would rather build a new microphone and leave the C12a as it is.

Amen !!!

I see little value (pun intended) in “modding” a classic vintage microphone …

While I absolutely value Klaus for his knowledge and experience … it doesn’t seem worth gouging a working classic to “update” its performance …

All the originals have different character that can create a gambit of opinion as to their sound and application … but unless they are in need of service from tired or failed components … I would just build the contemporary version with the topology you seek … and mod your clone until you get the sound you want … and let the classic be …

I built two of the Matador/Chunger C12 clones and absolutely love them … but I would happily have taken two originals had I the means and the luck to acquire them …

Just my humble opinion …

Best Regards

 

[Gus]

I worked on a number of U87s from the 70s(this was about 20 years ago)
they had 4.7k to 11k source resistors The one I like the best had the 4.7K source resistor 3819 type JFETs.
FWIW if you can find the Neumann paper with the title "Microphones" the 84, 87 circuit fragment is shown with a 3.9k source resistor
I bought 1000 3819s (years ago when you could buy 1000 for $.06 because of the IDSS Vgsoff specs) and only found 4 that biased up with a 3.9k source resistor. The IDSS was about 2.5mA at 9VDC drain to source.

I have not seen inside a KH modded u87 I would guess the source resistor is in the 4.7k to 6.8k range and maybe a tant or 2 was changed to a film with a different source bypass cap..

 

[Andrewfrommontreal]

Hi Andrewfrommontreal!
I don't want to go off topic, but can you tell us what changes Klaus made to your 1968 U87 microphones?
Someone mentioned changing the jFET with BF256,rebiasing and some capacitors.Have you noticed any changes to the capsule or the position of the capsule in the head basket?

From what I understand, he:

- removed RF filtering that he felt was unnecessary and tarnished the quality of the signal;

- removed low-pass filtering that was once useful for the German radio (or something like that);

- changed the high-pass filter that is now activated with an internal switch - the actual switch on the side of the body is used to go between two different cut-off frequencies;

- got rid of the attenuator because, according to him, it was useless seeing as it came after the point that limited the mic’s headroom… meaning if the signal was going to distort, it would already have with or without the attenuator being engaged;

- converted that switch into one that is used to go from cardioid-only (using only one side of the capsule) to its usual configuration - the cardioid-only mode produces a signal that is 6dBs stronger;

- changed components… where there was once the battery holder, there are now large (what I imagine are) capacitors, as well as using an output transformer from Oliver Archut;

- of course he also cleaned the diaphragms.

Honestly, the modifications turned what was a lovely mic into a very impressive mic. The low end is more open. The transients feel less held back. But it retains that lovely U87 low-mid magic. One could say it moved the mic towards the U67 zone.

 

[Recording Engineer]

He definitely didn’t replace a damaged diaphragm. That is not something Klaus believes in. He’d recommend a replacement.

 

[kingkorg]

- removed low-pass filtering that was once useful for the German radio (or something like that);

These were his typical u87 mods, where he basically removed de-emphasis. In the analog age extra high end was appreciated and not associated with harshness. This led Røde and many chinese manufacturers to manufacture k67 based mics with reduced of even removed de-emphasis. But somehow that became the "China" sound, or harshness... If it's Klaus' or C800 it's extra detail.

It certainly didn't bring it towards u67, as u67 has even more LF and HF roll-off. He brought it more towards C800-g territory.

 

[paulcheeba]

He's a good guy. He's generous with his time.

 

[Andrewfrommontreal]

He definitely didn’t replace a damaged diaphragm. That is not something Klaus believes in. He’d recommend a replacement.

It's true. Upon looking at documentation, this was not done. I shall correct in my list.