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 | | | | Jun30Written by:Bill Park
Saturday, June 30, 2001 6:00 PM  In the Field
So we have had the C-28s at the Benedum Center for a couple of weeks. The Pittsburgh Opera is presenting Puccini's "Turandot". Through the good graces of house sound engineer Chris Evans we have used the C-28 in the orchestra pit. We started out putting one on the violins. We could have moved the microphone all over the orchestra, but the characteristics seemed rather obvious. Thus we decided to live with the C-28 in one spot in order to gather a better feel based upon a longer-term observation. Our reference microphone for comparison is the AKG 460, and we have nine of them scattered about the orchestra. Where we have replaced a 460 with the C-28 the results have been very interesting. We carefully matched the levels. There are no effects in play, and we are using a Midas XL-4 console.
What I expected: I thought that the C-28 might be a tad noisier, with a little less low end. Since these microphones are an evolutionary progression from the C-28 through the 480, I anticipated that the earlier iterations would not stand up to our modern expectations.
What we found: The C-28 is just as quiet as the 460, with a very similar over-all sound and apparent frequency response. But there is a slight roundness of tone… nothing hugely obvious, just a subtle but definite difference that both Chris and I found appealing. They are also very hot, almost 10 dB hotter than the 460s and 480s that we are using.
In the Studio
Moving on to a test that would be more typical studio usage, I used the C-28 to record acoustic guitar and vocal. To make the test more interesting I also set up an AKG 451 with a CK-1 capsule, a Neumann KM-84i, and a new Octava 012 with a cardioid capsule. I added the other microphones in order to achieve some sort of balance and to allow myself a reference. I'm well aware how easy it is to fool one's ears without a reference from which to compare.
I used pink noise to match the gain structure through my Tascam TM-D4000 console, with no effects, equalization, or compression. I fed this through an RME 9652 to SAWStudio, and recorded each microphone to it's own track. I set the microphones about two and a half feet from my chair, with about two and a half inches between each microphone. This is not a typical placement for recording acoustic guitar, but I wanted to push the microphones back far enough that the distance from the guitar body would mitigate any side-to-side differences between the placements of the four microphones in order to even out the tonal character that reached each microphone diaphragm.
I played some delicate finger-style with lots of harmonics, some hard strumming, and I sang a few tunes of various types. I used two different guitars with different sounds. I then punched through the various takes, listening to the tracks individually and in combination.
So???
The results were interesting. First lets talk about the C-28. It was very warm and deep. It contained enough top to satisfy, and much more low end than I expected. It was also richer than I would have expected from a small diaphragm condenser. But it is a tube microphone, after all. The interesting part was that on playback, it almost sounded to me as if I was sitting beside me, singing and playing. But I'm not as schizoid as you think we are…. Very natural.
The AKG 451/CK-1 combination had a very similar sound to the C-28, without the warmth and roundness. It is easy to see how this became an industry standard. Mine are some of the earliest, with serial numbers in the 800s and early 1000s. They stopped making the 451 quite a few years ago, replacing it with the 460 during the 1980s, and the 480 about ten years later. I'll admit to being a fan of the AKG sound. I have much experience in using 451s and 460s both live and in recording. Great sound, and exactly what I expected to hear. This was my reference microphone.
Now the dark horse of the group… the Oktava 012. This is a surprisingly good sounding microphone, given its extremely affordable price. It stood up well to the competition in this unofficial shootout. It has a nice, even tone and a very useable, pleasant sound. But I'd have to admit that it was a little bland in comparison to the other microphones. But it was in no way shabby.
The Neumann KM-84i is another vintage microphone that has been out of production for some time. It should not be confused with the newer, less expensive KM-184. This microphone really has a wonderful sound. This microphone was directly adjacent to the C-28, so one would expect it to have received about the same sonic content. But the output was far different. It is not as warm or rich as the C-28 or the 451, but it has a top end sparkle that just blew me away. When I punched up the C-28 together with the KM-84i, I was floored. I have found a new micing combination for acoustic guitar.
Conclusion
The AKG C-28 is a very nice sounding microphone. It was wonderful on the orchestra. It's innate warmth and roundness were a relief from the usual thinness that generally accompanies distance micing. I liked it on acoustic guitar, and it even worked well on vocals.
I've spent quite a lot of time on this project. It was nowhere near as easy as it would have been to have just gone to the local audio shop, plopped down a couple of grand, and walked out with the current media favorite. But I have had a lot of fun, not to mention some small degree of frustration, in this experience. I have ended up with a pair of somewhat rare and unique microphones, with a great history and a great sound. I learned a lot.
Would I recommend a restoration project like this to anyone else? Only if they have a lot of patience. I had the advantage of a couple of resources to which others might not have access. But if you have the time and the patience and you want to end up with a piece of gear that is a little out of the norm, go for it. I can see that these microphones will get a lot of use in my studio, and that alone is what makes the effort worthwhile.
Footnote
I found myself unhappy with the physical aspect of dealing with tube microphone power supplies. Anyone who has more than one or two will know what I am talking about. It was particularly a pain when the mics were in the orchestra pit at the Benedum. On one hand, you don't want the power supply to run all night. On the other hand, you've got a stage full of people waiting for you to stop pissing around so that they can go home. Besides shutting down the entire audio, communications, and video system, now you have to crawl around the pit turning off microphone power supplies to try to preserve tube life. But if you have seen the price of the vintage vacuum tubes that go into these microphones, you know that you want to shut the power down when it is not needed.
So I came up with an idea. Why not build a 48 volt detection circuit into the power supplies? When the supply detects the presence of phantom power, it turns on the power to the microphone. Turn the phantom power off, and the mic power supply detects the change and turns power off to the microphone, protecting the tube life. Since all pro consoles have phantom power, it then becomes possible to turn the tube microphones on and off remotely. I approached Mike Mazur about the practicality of doing this. He thought about it for a while and came up with a simple addition to the power supply design that would do just what I asked for. I'm pretty happy that this turned out to be easy to design and practical to implement. I expect to see this modification turning up all over the place. If you have a couple of tube microphones, check with the guy who keeps your gear running. Undoubtedly he can install the same modification to your microphones. Save tube life, save money.
Acknowledgements:
Thanks are due to many people for helping to make this article possible. Mike Mazur and Tracy and Nadine Korby of Korby Audio offered much help, suggestions, and support in addition to building the custom power supplies and restoring the microphones to factory spec. Australian Glenn Knight provided the original microphones. Karl Peschel of AKG in Austria provided schematics, catalogs, and additional documentation from the period, and Australian Greg Simmons managed to also dig up several documents from the 1960s for me. Alison from the English company Gear On Line found the extension tubes (albeit, incorrect ones) and the W17 windscreens, as well as some other pieces that I wanted. Chris Evans, House Sound Engineer of the Benedum Center for the Performing Arts in Pittsburgh Pennsylvania was unstinting in his willingness to experiment, and offered us every courtesy and accommodation. The help of all of these individuals was necessary to the success of my endeavor to buy and restore these microphones, and the subsequent article that evolved from that experience.
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