Interview with Andreas Grosser


Andreas, how did you get into the world of microphones – technically and commercially?


In 1980 I began working for Hausmann Elektronik Berlin, we were selling mixing consoles, outboard gear from Klark Technik and others  and I was working in the service department to handle the repair of units. A few months into that, a few customers were also bringing in certain microphones which I had not seen before. So i asked friends at Neumann Berlin if these were older Neumann products, which they were not. That triggered my curiosity and so i became aware of east german manufacturer Gefell and their shared history with Neumann.


You have to understand, a few years before, about 1974 or 75 someone handed me my first ever microphone  to  repair – while I found it interesting to study the technology behind this and actually repair the mic, it took another 5 years or so for me to get back to microphones. In between I studied, i was an active musician and did a few other things, but I still had that wonderful experience in the back of my mind: my first technical encounter with condenser microphones. Back then i hadn‘t fully grasped the beauty of them. Do you know what I mean? Like you would enjoy playing a piano piece for the very first time and later you couldn’t say why you never played it again after that.


Once I had begun working in West Berlin, word of mouth led to more and more people entering my office with these exotic microphones. It made me want to fully understand every detail of microphone technology. That was in the early 1980s. In 1984 I started my own company.


Do you see microphones more from a technician‘s point of view or with the eyes/ears of a musician?


Hm, a long time ago I did write and record music, this is also a reason why I care for microphones. It is a natural thing for me to discuss with musicians anything soundwise – for instance, I have my favourite set of headphones which I prefer because of their musicality… in recent years I have not had enough time for the music side in me, but that’s life. These days I am mainly in my workshop and enjoy repairing mics. So quite a few times per year i hand out microphones to musician friends with critical ears to confirm my own observations.


I also used to do recordings with Klaus Schulze. In my adolescent years i had piano teachers for many years, so there is a good musical background for what i am doing today. Whenever i have time, i am going to concerts, visit recording studios where my microphones are used and so on.


I used to enjoy being in the studio and compare microphones until five in the morning… A real discovery for me was, that i realized it is really much better to repeat the same test one or a few days later again. It is in an effort to minimize the psychoaccoustic effect, you check again with different (day)light in the room , probably no one or other people around you, a different situation. The conclusions have more weight this way.


Take your good friend Don for example, who is also customer of me. He tested the AG 47 with different tubes. At first he liked the FET version best. Then after a while he prefered the glass tube version. At one point he found the original VF14 to be the best, followed by a conclusion that all of those give great results. So he thought he was done and erased all audio test files from  the forums, but some people requested to put them back online. Finally he called me last week to say, that after listening to the same files again 6 months later, he now likes the FET version best. Interesting. Why? I guess the real difference comes from testing those mics and actually USE them and live with them in the studio for a longer period of time time. There is a psychological thing going on under time constraints, other people around you talking which affects your own opinion… ultimately you need to spend time with those microphones and get to know them well.


Are you a specialized in a certain brand, like Neumann or Gefell or AKG?


Hm, back then i was starting out with Neumann and Gefell microphones. Over the years all sorts of different microphones were coming in. So while I enjoy Neumann and Gefell microphones, i feel as familiar with any other mic manufacturer, like  Schoeps or AKG.


What i find remarkable is the highly professional built quality of Neumann and Gefell. Others like for example AKG were not on the same level. Schoeps is also very professionally manufactured but the aforementioned seem to be a little bit better- all the mechanical parts, the way their mics can be dismantled for service is just done a tad better. You can tell that those factories made a lot of microphones and know a lot about mechanics, a lot about isolation… simply the best.


To be more specific, what are typical problems that you have to deal with regarding vintage microphones like U47, M49, U67, U87 from Neumann,  AKG C12 (AKG), and CMV 563 or UM57 from Gefell?


The most common U47 problems are noise, crackling , microphonic tube, distortion. These problems are usually related to faulty contacts. Most  people think immediately the tube is broken, while the problem is rather the dark female socket for the tube and/or uncleaned pins. Another common problem is the tub . A lot of tubes are noisy but I have my special method of revitalizing the VF14, which means I am able to bring about 75-80% of VF14 back to life. So they should work fine for another 5-10 years. So there is hope if you have the infamous “crackling noise”. On the other hand, if the tube is microphonic or distortion kicks in much too early, there is not much that can be done.


What about tube substitutions for the VF14 valve?


The key to that question is: you have to understand the working principle of tubes. A tube with low noise, low microphonic values can give great results with the right amplification, once you find the right bias, the correct „Arbeitspunkt“ (operating point). So once you understand under which condition K2 distortion kicks in,  and at which point K3 distortion, you get an idea of the overall level amplification profile of that tube. Under these circumstances i could make any good tube work, for example EF 86. Once it is tweaked to the right level and the right bias point for distortion, the behaviour of one good tube should not be so different from another good tube under the same condition.


Take guitar amplifiers for example, guitarists usually swap tubes and think „it is a telefunken and  that is why the tube sounds better than another valve brand.“ But in reality each specimen behaves more or less different, some tubes of the same brand have distortion kicking in 2db earlier, others some 2db later.  So while the the tube has the same name its individual behaviour is different. Which means there is no point to prefer a certain brand like Telefunken, Mullard and so on just for its name. In reality regardless of the brand all tubes behave slightly different and need to be adjusted. What i also find sometimes is that some tubes are outside of the working range, so they simply can not be adjusted.  But in general you can use any GOOD tube.


I did a lot of tests, in which i also used small glass tubes and under normal conditions you would not hear any huge difference.


So while it is correct to say that tubes produce a characteristic sound, it is not correct to say that for an specific brand of tube will produce a specific sound that would not be achievable with other types of tubes.


So you are saying if is the BIAS and other parameters are adjusted properly, one can use any tube as substitute?


Yes, as long as it is a good tube.


Let’s be more specific regarding the U47, how did you go about making a good substitute for the original tube?


I was not merely interested in replicating the VF14.  Of course it helps if you know how to manufacture a 100% identical copy. That’s a good start, but it is as important to be aware of the problems the original VF14 had and how to avoid those problems.


Not every old microphone is in best condition, obviously. There are problems with noise,  with hum, the tube being too sensitive for distortion kicking in, the amount of K3 (third harmonic) distortion. You should know what sounds right and where the problems are coming from. And since you are building something from new parts and you are able to resolve those problems – eliminate hum, reduce K3 distortion – if I am able to do it technically,  i HAVE to do it, for myself.  So I was not interested to get a picture perfect copy including the mistakes.  I wanted an improved copy – better than the original.


By the way, the VF 14 is far from being a perfect tube. Why ? The idea behind it was to produce a tube which would work with just 2 supply lines plus shielding. The EF 12 for example is more comfortable but needs a separate supply line for the heater. You have to understand under which conditions microphones had been developed in the years before: at the time of WW2 there was a need for a simple mic for reporters, which would work with a single voltage for the anode and heater. Back then the military used a „battery suitcase“ which supplied 100 V and with that the mic was working fine. After the war most of the electrical infrastructure had been destroyed in Germany and so totally new and incompatible electrical networks were build from the ground in the british, french, russian and american sector. All of a sudden you had to deal with 100 V DC, 110 V DC, 110 V AC and 220 V AC.


For that reason it became necessary to develop a mic which required only 100 V and still could be used anywhere. That was only possible with the VF14.


Lets talk about the M49.


A common problem of the M49 is usually the acryl plate inside, often covered with lots of dust, thus being noisy. So the plate has to be cleaned professionally, the insulation of the plate being recreated, which also occurs with U67 mics by the way. Other than that M49 problems are only broken valves or capsules. Another source of problems for many old microphones are the transformers. Transformers are hard to be repaired – they need to be rewound and often the  color lacquer on the cable has stiffened and dried out over the years. So you can‘t touch it without breaking the copper wire inside.


Like the U47, many different versions of the M49 came into existence, so often people think their M49 does not sound as it should. While in fact later versions were built with filters inside in contrast to earlier models.


Sometimes I am asked to rework a microphone to the latest version of that model, the final revision. Over 20 -25 years  of production the manufacturer has made lots of changes – as with the U47 it is hard to compare the first version to the final version of U47. They changed  a lot of parts inside, not only the transformer and this is why the last version has not such a big low end.


How about the U67 and U87?


U67 has not that many issues, sometimes there are problems with the switching point, so good insulation is required. Some of them become noisy over time. People sometimes dismantle the head and touch the capsule. So besides a dirty capsule that needs to be cleaned, there are not too many problems.


87 ..there are so many  different versions. I have seen around 21 different versions.


Early versions had the same switching unit as the U67 so it is the problem with insulation i mentioned earlier. The later yellow version of the switching unit  is easily  to break and the final (grey) switching unit is ok. There are usually not so many problems with the U87 ..if you dont touch it, it should be fine. Problems arise if a customer tries to repair a capsule on his own. Older capsules are mounted on 4 pins, it is a very sensitive capsule in regards to dust, so the problem appears sooner. The AI capsule is the same as in U67 and that version is more reliable. Sometimes if a capsule is broken, the level is going up and down and sometimes distorted… but as i said usually there not that many problems…


What about AKG C12 and Elam E251?


The biggest problem is the capsule holder – it is a slightly unusual construction highly sensitive in regards to pressure and it is easily destroyed. Also the capsule is often a big problem…


What do you find interesting about repairing microphones these days?


Let me put it this way: I am very happy with clients sending in their microphone with the expectation to get it it back in fully working condition, especially if i can achieve that while making the microphone less noisy. Then there are other clients who love to say „german vintage equipement is  the best in the world“, and yet ask me at the same time:“ but you must change all tubes, you must change all transformers, all other parts“, so in other words they want me to build a different amplifier. Which makes no sense, when those people are after the german vintage sound. I am sorry, but i won‘t  change that sound for no reason! It is something else of course, if a customer tells me to match two microphones, let’s say two U87 which sound different from each other. In that case, i might find that some capacitators have different values and i will offer to match those microphones. Once it is clear what a customer wants, i can do it. But I will not alter the classic sound without an indicated need. This might be different to other technicians, some out of Germany, who are changing the sound of a mic with the attitude they know better what is good for the client than the client himself.


I don´t know what is  best for YOU and YOUR needs. I can give you ideas, and inform you what the manufacturer had in mind when they designed the microphone, and of course I can tell you what I would personally like, but the decision is ultimately your decision.