Travels of a Vintage Microphone Junkie – Part 2

PLEASE NOTE: This article has been archived. It first appeared on ProRec.com in March 2001, contributed by then Senior Editor Bill Park. We will not be making any updates to the article. Please visit the home page for our latest content. Thank you!

Last month I detailed the fun that I had in acquiring a pair of vintage AKG C-28 microphones. This month I have done a lot more research into this model, replaced one of the capsules, corrected some errors in some of my earlier information, and spent a lot of time bending Tracy Korby’s ear about my new acquisitions. I had hoped to have the mics and power supplies in hand by now, but unfortunately they are not ready yet. Still, I have discovered a lot about these microphones. Let me share some of what I have learned with you.

AKG: A Little History

The two men who started AKG got together in 1945 with the plan to make theatrical loudspeakers, projectors, exposure meters, and other products for movie theaters. Post-war Europe was rebuilding its destroyed theater industry, providing a fertile market for the right company. Ernst Pless and Dr. Rudolph Goerike made several profitable products between 1945 and 1947, at which point they founded AKG, which is the acronym for “Acoustic and Cinematography Equipment”.

AKG made a handful of microphones in its early years that were unique and popular. In 1953 AKG introduced the C12, which became an international best seller and was a turning point for the young company. This microphone was particularly popular with the engineers at the BBC, which led to a long-standing arrangement between AKG and the BBC. The company reputation was set and its financial foundation was solid. And its direction was clear…. The future for AKG was in acoustics.

In the ensuing years AKG has produced some of the most popular studio quality microphones in the world, and has been a leader in headphone, phonograph pickup, and other audio industry technologies. This is a far cry from the original company product line, which included car horns, door intercoms, telephone handsets, and anything else that they could sell. I’m sure that many of you small studio owners can relate to that concept…. I know that I certainly do.

Customer Support? And How!

AKG Service Manager Karl Peschel was fast to respond to my initial queries, but I had no idea how really helpful Karl was going to be for this article. No, he wasn’t able to find a pair of VR-30 extension tubes or W17 ‘poppingscreens’ for me. However, since my last installment he did send pages and pages of information about the C28, and photocopies of documents from as early as the 1950s describing not only the microphone, but also some of the developments that led to the microphone’s creation. He sent detailed exploded-view drawings, flow diagrams, vintage dated schematics of each revision of the microphone, original company promo brochures, and a copy of a paper by Konrad Kimla of AKG that was printed in the Official Journal of the British Sound Recording Association in August of 1960, “New Microphone Developments”.

Among the treasures Karl sent was an owner’s manual and tweaking tips for the power supply. He also sent along a pile of information in German, which I have only partially translated. I can get through some of the German, and I have a friend who’s field is technical German. If there is any information worth adding to the discussion in the German language papers, I will post it in the next installment.

A New Design

Many of you know about the C12 but fewer know the history that surrounds this classic piece, and the Siemens and Telefunken-branded versions of the same microphone. I’m not going to go into much of that here, except to say that a large part of the design of this microphone was an attempt on the part of AKG to respond to the requirements of the Austrian national radio network. It had requested a studio quality microphone designed to be “as slim as a pen.” Head of the original AKG C12 design team Konrad Wolf remembers, ” Nobody was able to do that at the time. But in any case we came up with the slender, cylindrical shape of the C12.” The C12 broke design ground for its size and shape while maintaining top-flight performance.

This shape and the general design characteristics of the C12 are carried over to the C28, on a smaller scale. The original C28 was introduced in late 1955 or 1956. The condenser transducer used in the CK28 capsule came from an even earlier AKG design completed in 1951. The C28 is part of a series of microphones that were designed as broadcast products, specifically for their slim attractive appearance. An AKG goal was to maintain the studio quality of the product within the size constraints required. This from a Service Memo from AKG:

“The reason to develop a C28 simply was the ambition of the development engineers to make a small sized condenser microphone of studio quality, with linear frequency response and changeable capsules. As in these days condenser microphones were all rather big and had large diaphragm capsules, their use was limited to certain recording areas…..”

Although these microphones seem to be huge by our standards for small diaphragm condensers, when they were brought to market they were considered to be so small as to be invisible on camera. I guess when compared to a Neumann U-47 or RCA 44, these babies are tiny. Perhaps on a 1959 8-inch black and white television screen with rabbit-ear reception, they ARE invisible.

The C-28 Series

This series included the C26A omni, the C28A cardioid, the C29A short extension tube microphone, and the C30A long extension tube microphone. This is a very modular system. The C29 and C30 can use either the CK26 omni or CK28 cardioid capsules. The C26 and C28 can be converted to a C29 or C30 by adding either the VR29 or VR30 extension tubes. To convert a C28 to a C30, remove the windscreen from the C28 body, remove the CK28 capsule, screw the VR30 in place of the windscreen, and screw the CK28 capsule to the end of the VR30. The VR30 extension tube is three foot ten inches long and very thin, and provides a subtle way of sneaking a microphone into a film or video shot. The VR29 extension tube is one foot one inch long and also very thin, and made a great way to convert the C28 to a desktop microphone for television usage. There is an optional W17 ‘poppingscreen’ that can be put over the capsule. The AKG catalog of the time makes specific reference to ‘windscreens’ and ‘poppingscreens’, in case you wonder about my use of the term.

This C30 version is the vocal microphone shown in the Beatles rooftop sessions and on the “Hey Jude” films, and on other Beatles films of the time. Adding the W17 ‘popping screen’ completes the transition visually, but the photos that I have found show the microphone without the W17 as often as with it. Some of the Beatles photos and videos show the mic capsule with a round bit of foam taped over it.

The C26 and C28 are identical in appearance, the size of which I mentioned in the last installment. In order to tell them apart at a glance, the windscreen on the C26 is black. The owner of a number of C26 and C28 microphones could swap capsules between amplifier bodies, and still know which was which –if- he remembered to change the windscreens, too.

The original C28 microphone windscreen is threaded and easily removable. With such small capsules housed in such a significantly larger body, it occurred to me that it might be desirable to remove this windscreen on occasion. I discussed this with Tracy Korby, who says that he uses the C28 with or without the windscreen, depending upon what he is recording. AKG advertising literature of the time claims that the windscreens do not affect the acoustics of the C28, but Tracy says that the windscreens change the characteristics of the standing wave, and divide it. He also points out that the windscreen will change the pressure of the wave that hits the capsule. I’ve found a difference in using or not using the windscreens with other microphones, so it is not hard to believe that it will make an audible difference on these microphones, too. It also occurs to me that perhaps we were not the first people to think of this, and maybe that is why there was a different capsule in one of my C28s…. just maybe AKG realized that people were using the microphone with the windscreen off, and decided to put some protection over the capsule itself. Perhaps the second unlabeled capsule was simply a CK28 in a different housing. I have found no available information to prove or disprove this idea.

Another useful piece of information from the AKG literature involves the length of the connecting cable between the microphone and the power supply. Changing the length of this cable also requires an adjustment of the power supply to maintain the correct voltage to the microphone. Tracy and Mike already knew this, of course, but I didn’t. Let’s face it; there just isn’t much opportunity for me to swap out power supply cables between my various microphones, since they all seem to have different connectors. I’ve always used them with whatever cables came with them, and I just never thought about it before. You might want to consider this if you plan to change the length of the cables or replace the cables on any vintage microphones that you may own that require separate power supplies. Check the manual or ask a service tech familiar with your microphone to see if a voltage tweak is needed.

What’s the Difference?

There were various improvements and modifications made to the series over it’s history. The significant changes were marked with sub-letters in the microphone designation. A 1958 schematic of the C28A shows a handful of electronic parts hung off of a General Electric 6072 tube. Those who follow such things will know that the GE 6072 is the same dual triode used from 1960 onward in the large diaphragm C-12s, and the Siemens and Telefunken branded versions of the C12.

The C28B has some small circuit changes and a few more parts than the C28A, replacing the bulkier 6072 with a 7586 Nuvistor, which is a much smaller steel-incased tube. The C28C, which is the model that I have, also uses the Nuvistor and has a slightly smaller parts count and somewhat different mechanics. Earlier I had been told that the 28 and 28B had tubes, but from the schematics, this is incorrect, although the Nuvistor is really a tube, just not in the classic packaging.

The C28 with the CK28 capsule has a frequency response of 30 to 30,000 cycles. Each microphone produced was sold with an individually plotted frequency response chart. Unfortunately, mine did not survive the ensuing years, so I do not have the original plots for my C28s. But the general frequency response characteristics of the microphone from a company-provided plot show about a 4 dB rise from 30 to about 150 cycles, flat from 150 to approximately 3,000 cycles, and about a 3 dB hump up to 5,000 cycles, then a gentle drop-off from 6,000 cycles ending 8 dB down by 20,000 cycles. These specs aren’t world shattering, but were impressive for a 1951 design and are not bad even by today’s standards. The plot is almost identical to that of a modern Neumann KM-184.

Due to the comparative rarity of this microphone, I have been unable to come up with the usual band of adherents to one model version or another. The same holds true for the C12. No one has ever mentioned to me that a C12 made before 1960 sounds any different than a C12 made during or after 1960, yet in 1960 AKG changed the tube used in the microphone. The C28 series moved from the 6072 tubes to the 7586. Parts were added, some values were changed, and parts were removed. One would think that there would be a difference in sound, however small.

There is no doubt about the additional cool factor of the added VR30 extension tube and W17 poppingscreen, though. Electronically, the VR30 appears to be a horrible idea due to the very high impedance of the capsule. But there seems to be enough shielding in the VR30 to make the idea workable. Still, Tracy shakes his head. I don’t care, I WANT ONE! Two would be better!

What You Got for Your Money

The C28 was sold with an N-12 power supply, MK-28 cable, a 5-B17B stand attachment and 8-S26B plug connector, a V28A amplifier, an A28A adapter, a CK28 capsule and GK28A light colored windscreen, an NF28 cable with 6-S-28, a power cable, and the KS28 ‘elegant wood box’. Typical of the boxes supplied by AKG at the time, this is a gray cloth-covered wooden box with a red velvet interior. The standard C28 weighs about a half pound, but the shipping weight of the mic, power supply, and cables is a whopping 29 pounds. Besides the extension tubes and windscreens mentioned above, other optional accessories included the stand adapters, floor and table stands, and two different matching transformers, the U 3027 and U 3053.

Gone and Mostly Forgotten

AKG stopped producing the C28 by 1968. The very popular CMS system, which replaced it, was introduced the following year. The CMS system includes the 451 body, which uses an FET solid state preamp instead of the tube preamp of the earlier AKG condensers. The 451 body is significantly smaller than the C28 body, getting closer to the customer requirement of wanting a microphone “as slim as a pen”. The 451 uses phantom power instead of a separate power supply and is connected directly to a console by (what is now the) standard XLR microphone cable, at least on the US model. It uses capsules of the same size and threading as the earlier C26/28/29/30 system. This enables the CMS system to use the same capsules and many of the same accessories as the C28. This also means that the C28 can use any of the capsules designed for the CMS series. Backward compatibility in 1969? Cool.

The 451 became the world standard by which all other small diaphragm condensers were measured. This microphone fit the perfect break point between performance, reliability, flexibility, and price. The optional field replaceable capsules, extension tubes, windscreens, pads, and right-angle adapters assured maximum return on investment. This trend has continued with the AKG microphones that successively replaced the 451 and each other: the 460, and the newer 480. These microphones have lower noise, extended low end, and smoother frequency response than the original 451, and have quite a number of available capsules, including short and long shotguns and most popular patterns.

In this evolution the C28 has been lost in the shuffle. I found a handful of C28 devotees, but this classic piece has not managed to garner the same mystique as it’s larger brothers, the C12 and C24, or even the desirability of the early 1970s C451 with original CK1 capsules. How has such a beauty been overlooked for so long?

I hope to have more coherent answers with our next installment, when I get the new power supplies from Mike Masur, and get my refurbished pair of C28s back from Tracy Korby and we have a chance to put these vintage microphones to the test. See ya then, and don’t take any wooden 414s.

Let It Snow

Well, it’s like Christmas here at Welcome Home Studios. We’re at the last days of March, and it is snowing. I just got back from Tracy Korby’s, where I picked up the now fully restored AKG C-28 microphones with their brand new Korby power supplies. He also had finished approving the pair of Neumann KM-84i and the KM-83 that I bought from db Engineering in New England. UPS delivered the PI Engineering X-Key Pro programmable keypad that I bought for running SAWStudio. In fact, this feels better than Christmas!

Since Our Last Visit

I have received quite a few emails from all over the world about this project. Apparently, many of you have enjoyed my trip. A couple of you also pointed dealers in my direction. As a result, I have been able to pick up one each of the long and short extension tubes and another pair of Neumann KM-84i microphones. I also picked up an AKG CK-5 capsule, a vocal mic capsule head, which fits on the C-28 and the AKG 451s that I own, and a pair of W17 wind screens for the C-28. This is another story of long distance commerce. All of these parts came to me from a vendor in England.

Fortunately, this dealer accepts Mastercard, and the transaction could not have been simpler.

C-28 microphone shown with optional extension tubes

But guess what? The extension tubes don’t fit the C-28! As I said before, buying vintage gear long-distance can be a bumpy ride. I’m trying hard to find out what these extension tubes were originally meant for. They are too large for the AKG C-28, 451, 460, or 480 but they are the right length and over-all appearance. They are also very obviously old, but have been well cared for. These tubes have no identifying markings that I can find. I have seen extension tubes for the Neumann KM series microphones, but they were only about six or seven inches long. Other than that, the C-28 and the 451 are the only microphones that I have seen with such extension tubes. Very strange.

The vendor has offered to take them back, as they were advertised as being for the C-28 and obviously are not. But before I pack them back up and ship them back to England I want to know what they are for. Who knows, maybe another American customer has the right microphones for these tubes, and I can pass them on to someone who will use them. I am still looking for a pair of CK-26 omni capsules and original stand clips for the C-28 microphones, as well as a pair of extension tubes for the C-28.

Return to Oz

Tracy and I talked about the restored C-28s for a while, and I gathered his impressions. This is a guy who works with the world’s most popular and valuable microphones on a daily basis, and he was quite complimentary without being overly excited about the sound of the restored C-28s. I guess that when you are rebuilding the best and designing and building your own, it takes a lot to impress.

Earlier I had mentioned that Karl Peschel of AKG had provided several schematics reflecting changes made in the C-28 design over its lifetime. Tracy identified my microphones as being from the August 1964 design change, as reflected in the schematic of that date provided by Karl.

Before Tracy actually did the restoration work, he had suggested that I remove the windscreen when I use them. Since the microphones have been restored he re-stated that suggestion, and emphasized it. The photos that I have of the C-28 in use all have the extension tubes installed. When the extension tubes are in place the factory windscreen does not fit, and a different windscreen, the W17, or a simple piece of foam wrapped around the capsule is what is pictured. I figure that since Tracy knows what he is talking about, and I have visual evidence that the C-28 was used as he has suggested, I’ll follow the program. All of the testing that I do on these pieces will be sans the large windscreen that is a part of the C-28 as-shipped factory configuration. I have a pair of W17 windscreens, but I don’t think that I will use them during the tests. They are not part of the original microphone, are difficult to come by, and are not likely to be a part of any C28 that you might find for sale.

Another point: Many of the C-28s out there seem to have CK-1 capsules in them. I don’t know why this has happened. I mentioned earlier that my C-28s came with one each, a CK-28 on one and a CK-1 on the other. I was able to get a second CK-28 capsule for my pair. If you have a C-28 with a CK-1 capsule, or any of the other capsules that fit the C-28, your results may vary from my own.

C28 with CK-28 capsules

Okay, how do the C-28s sound? Well, I don’t exactly know yet. I have to burn in the new tubes in the microphones, and I want the new power supplies to burn in, too. I’m going to keep them running over the weekend, and on Monday I’ll start dragging them around town. Through the course of this piece you will discover the results as I do.

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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