Revolutions Per Minute

The Giants of Tape, Part Seven: The Telefunken M15A, Part One

Issue 140

To follow a few installments dedicated to Ampex (Issues 136 and 137) and MCI (Issues 138 and 139), it is time to return once again to the Old World, and discover Europe’s best-kept audio secret!

While Studer (and their Revox subsidiary) was the most well-known and internationally successful manufacturer of professional tape machines in Europe, and their A80 (covered in Issue 134) had one of the best tape transports ever made, they were definitely not the only manufacturer worth considering on the continent. There were a few, but today we are going to talk about the one that started it all, having presented their first prototype in 1934, and the first commercial model, the AEG K1, in 1935. The K2 came in 1936 and in 1941, the major breakthrough of AC bias was introduced. By 1944, the K7 was available as a stereophonic recorder.

As you may have already guessed by the dates, the AEG Magnetophon served as an important tool of the Nazi propaganda machine during World War II and the company, with its various activities, had a dark history of supporting the Nazi party, using forced labor during the war, and so on. Their early tape machines are rather crude for my taste, but AEG kept at it for several decades.
The American tape recording industry effectively started when Jack Mullin obtained two early AEG tape machines in 1945, following the invasion of Germany by the Allied forces, and brought them back to the United States, where they were further developed and modified. The result was the early Ampex series of tape machines.

Meanwhile in Europe, the development of the AEG Magnetophon continued after the War had ended. Several models were introduced, but I will skip straight to 1954 and the AEG M5. Originally a monophonic machine with a rather rudimentary tape transport, it remained in production up until the late 1970s, as the M5-B, M5-C and M5-M. The M5-C was also available in stereo. It was built as if it was meant to keep on recording while the building it was in was being bombed, but in terms of sound quality, it was not yet very exciting.

The Telefunken brand name had begun to be used on the M5, with Telefunken originally starting as a joint venture of Siemens & Halske and AEG. In 1941, AEG acquired sole ownership of Telefunken, but it was not until 1967 that the two companies would merge as AEG-Telefunken.

The M10 was introduced in 1960, with a much-refined transport, improved vacuum tube electronics (although the later models came with early solid-state electronics) and a total weight of 180 lbs., continuing the tradition of designing blast-proof machines. However, I am not a big fan of vintage German vacuum tube designs – too overengineered and complicated for my taste. The tape transport was not yet something to go out of your way for, but it was evident that AEG was making huge leaps in this respect.

In 1971, the AEG-Telefunken M15 was introduced, with what I consider to be the best tape transport the company was to design, and one of the finest tape transports ever made, if not the best.

 

Telefunken Magnetophon M15A.

Telefunken Magnetophon M15A.

 

It is very minimalistic, having nothing more than the bare minimum of parts, yet packing all the features it needs to guide the tape confidently, with wow and flutter remaining inaudible even to the most discerning listeners, who claim to always be able to hear it on tape.

Several years ago, a classically-trained musician asked me to demonstrate the audibility of wow and flutter on one of my Telefunken machines, by recording and playing back a 1 kHz tone from my bench oscillator. He said that he could hear it every time on all the tape machines he had ever used.

The idea was to compare the sound of the oscillator directly played by the speakers, to the oscillator recorded to tape and subsequently reproduced on the same set of speakers, switching between one and the other. He claimed that the speed instability of tape transports was always audible to his ears using this test. So we tried it, using an unlabeled switch. After a few flicks of the switch, he looked at me confused and asked, “is this the tape or the oscillator?” I thought it was the tape, but I was not entirely sure. I certainly couldn’t tell by the sound, and neither could he. He then asked, “are you sure one of the two is really the tape?” and I said yes.

So, a couple of flicks later, he just suddenly pressed the Stop button on the tape machine, and the tone stopped. It was the tape, and neither of us could tell. I then explained to him that I didn’t think the test was sensitive enough, and that it would be best to use a 3,150 Hz tone instead, or even 10 kHz, which we did, but we were still both unable to tell which one was which by the sound alone, and we also could not hear any wow and flutter with music.

 

The M15A tape head.

The M15A tape head.

 

All mechanical transport systems have speed errors to some extent. The way these present themselves on the M15 renders them largely inaudible, however, which is what ought to be the aim in the design of a professional tape machine, turntable, or anything else for that matter. The errors will always be there; we just need to place them below the threshold of audibility, which is a complicated non-linear function, largely influenced by psychoacoustics. Scrape flutter is also elegantly dealt with.

The overall construction of the M15 remained faithful to its roots in the M5 and M10, with a military-grade cast aluminum chassis, very large motors, and a belt-driven capstan (with a dedicated motor of course, servo-controlled as was the standard engineering practice in the 1970s). The capstan was massive, with a huge balanced flywheel below the chassis to assist the servo control system with some good old pure mechanical brute force, perhaps with a hint of that medieval Gothic warrior instinct shining through (“I understand that the guided missile system is our strategic advantage, but I still prefer to carry that tried and tested skull-crusher, just in case…”).

The electronics were solid-state, and they were good, but not yet outstanding. The M15 was available in many different versions, covering all possible tape widths, speeds and track configurations. The only thing better than an M15 would be the M15 transport with better electronics.

This came in 1977, as the Telefunken M15A. It had the already-excellent transport of the M15, but with improved electronics. The M15A had by far the best audio electronics of any tape machine of the solid-state era, ever. It was sonically transparent and had tons of headroom.

 

M15A product brochure.

M15A product brochure.

 

The M15A had a rather minimalistic approach to everything, in stark contrast to almost anything else designed in Germany. (With the possible exception of the East German, two-stroke-engined, varnished-cardboard-bodied Trabant, an underpowered lawnmower which they successfully marketed as a car, since that was your only choice if you lived in the GDR, and there was a waiting time of around 10 years from the moment you ordered one to actually taking delivery. The West German M15A was a rather more elegant take on minimalism and at the time it was sold, you didn’t have to wait 10 years for one! Although I must admit that the lines of the Trabant look quite cute, at least compared to a Lada 2103 of a similar vintage and pedigree, but don’t try to fit into one. A single M15A reel motor would have offered a significant performance boost for the Trabant, if used as its engine in place of that two-stroke motor, but it would have to be smuggled in from the West, which was risky business. If you find a Trabant today, which will probably cost less than a Telefunken splicing block, you could try using both M15A reel motors to power it, one on each driven wheel, modifying the motor control boards to provide torque vectoring, which would certainly give Elon Musk heartburn…but not because it would pose a threat to Tesla’s market share, especially if you are to also retain the 10-year delivery date policy!)

 

Worth the wait? A Trabant getting ready to roll.

 

There were no separate input boards and recording circuit boards, as there were also no separate repro and output boards. The repro boards also served as the output boards and the input boards also functioned as the rec boards.

There was one rec and one repro board per channel to keep things simple and easy to keep running in the field. The standard version of the M15A did not come with level-adjusting knobs or any form of metering. The input and output levels were calibrated by means of trim pots, adjustable with a screwdriver, in direct relation to tape levels. There were no in-between stages. Metering was deemed an unnecessary expansion of the signal path, since you were meant to meter on the mixing console, or with a dedicated peak program meter designed for broadcasting use.

The audio electronics were designed to handle signal levels up to  +24 dBm, which was the proper engineering practice for all professional audio equipment, yet not always seen on equipment marketed as such.

 

M15A, tape transport detail.

M15A, tape transport detail.

 

Tape levels could be pushed to insane heights, capable of complete magnetic saturation of even the hottest tape formulations ever made, to this day. The M15A was simply the exact opposite of planned obsolescence; it seemed like the engineers designing it set as their design aim that no matter what was invented over the next century, the M15A had to be capable of going a few steps further. It was to never be outdone.

You want 500 nWb/m tapes? The M15A will do 5000 nWb/m. There were discussions during the late 1960s and throughout the 1970s where audio engineers were complaining about tape machines that were electronics-limited in terms of how hard they could drive the tape, rather than being medium-limited (tape levels should ideally be limited by tape saturation instead of the electronics of the tape machine clipping the signal prior to the saturation limit of the tape). All the tape machines featured in this series are medium-limited, but the M15A went a bit wild on this.

In the next episode, we shall discuss the features and configurations of the M15A.

Header image: from M15A product brochure.

4 comments on “The Giants of Tape, Part Seven: The Telefunken M15A, Part One”

  1. I worked at BBC Radio in London from 1982-1986 maintaining all professional audio equipment. The M15A and Studer A80 1/4in 1/2 track machines were everywhere! There was a mechanical workshop where heads were lapped, motors rebuilt, bearings replaced etc. The M15A was a nice machine.

  2. How do the butterfly playback head and the NAB head compare ? Any advantage in using the butterfly head ?
    The M15A is rather large, but the M20/21 seems more manageable in a domestic setting. How does the M20 compare in terms of the quality of the tape path and the electronics ?

  3. As mentioned, the theoretical (and practical) advantage is the improved S/N ratio, but the better-quality butterfly heads also tend to exhibit a much better LF performance with less pronounced head bumps. In cases of playing back tapes that have been recorded elsewhere, it is important to know that the track width of the reproducing machine must match that of the recording machine, otherwise the LF will either be too weak or overly exaggerated, due to the contour effect. NAB heads permitted a timecode signal to be recorded between the two audio tracks, which was useful in broadcasting, but for the utmost in sound quality, I would prefer butterfly heads. NAB heads do offer a slight advantage in reducing crosstalk, especially on tapes recorded at hot levels.
    For your purposes, Adrian, I would recommend the M15A over the M20 or M21.

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