Nagra T-Audio Adventure by Adrian Wu

My first open reel tape recorder was an Otari MX-5050 that a friend gave me in the 1990s. It was a solid machine that could record and play both 2 and 4 track 1/4″ tapes. However, after I started doing live concert recordings, I needed something better since the Otari was not exactly portable and the quality of the recordings made on this machine was not great. The Nagra IV-S had been a standard for location sound recording since the early 1970s until digital came along. These machines were built like tanks and still sound marvelous today. I bought mine from a studio equipment store in the UK. It was an ex-BBC unit that came with the QGB 10.5″ reel adapter. It had served me well for more than a decade, making many recordings of mainly classical concerts. Last year, I learned that the engineer who was in charge of the analogue recorders at Nagra, M. Herbert Bartels, had just retired. I therefore called Nagra (now called Audio Technology Switzerland) to ask if I could send my IV-S in for a major service. It was about time since I had been using it for 12 years without any professional servicing. M. Bartels was very graceful in agreeing to return to work part-time for this. After I sent them the recorder, an idea came to me. The IV-S is a fantastic analogue recorder, but the playback function is a bit of an afterthought given its main function as a location recorder. As the analogue tapes I made were for archiving, with the multitrack digital files being the source for mastering, playback function was not important other than for my own amusement. Nagra of course had their famous studio machine, the T-Audio, since the early 80s for mastering function. This was a marvel of Swiss engineering, and apparently cost £23,000 when it debuted in 1983. For that amount, one could buy a luxury car in those days or even a small flat in London ! This was simply too expensive for most music studios, especially for a two-track machine, but it became the darling of movie studios with their much larger budgets. It also gained traction in the scientific community as a data recorder. So I figured I could ask if they had one available, and if the price was not crazy, I would consider buying one. It turned out that they did have one available, and M. Bartels could work it over and change all the necessary parts including the heads, rollers and belts. The price of 8500 CHF seemed very fair to me, especially since the man who wrote the original service manual would be giving it a once over.

After I received the machines back, both in brand new, original shipping cartons, I immediately made comparisons. While the IV-S playback is competitive with an excellent turntable set-up, the T-Audio is in a different class altogether. The sound is huge, with more weight, more solidity and more power. It is as if the energy of the music has been cranked up by an order of magnitude. However, while it is widely acknowledged in the pro audio community that the T-Audio is one of the best transports available, its playback electronics are not up to the same level of performance. Soon, audiophilia nervosa compelled me to find ways to improve the playback performance. The major criticism I have with the machine is that it sounds a bit electronic, and what is missing is the organic flow of the music and the natural tonality of the instruments. This would not surprise anyone who has looked at the schematic of the repro board. The circuit design is heavily reliant on op amps, mainly LF353N. Whereas one can design pretty respectable audio circuits with modern op amps nowadays, this was not the case in the early 1980s. After doing a bit of research, I found a trove of information on how to wire the repro head of tape machines to outboard electronics. Dr. Bottlehead even has a T-Audio thus modified to work with his tube repro electronics. I therefore called Tim de Paravicini, whom I have known since the 1980s. He was not keen on this idea, since he thought noise could be a problem. He suggested that I send him the repro board and he would work his magic on it. I called Nagra to see if they have spare repro boards, just in case things go wrong. The answer was no. Therefore, if something goes wrong, if the board gets lost in the post, I am toast. The transport simply won’t work without the board, even if I use outboard electronics. Unless I bring the board over to Tim and wait while he modifies it, there will always be a risk. I also don’t want to do irreversible modifications on the machine. Well, I guess I would have to do some experimentation myself.

First, I had to wire the repro head out. The original coax cables from the heads were connected to the vertically placed main circuit board and easily accessible after opening the back cover, with the various daughter boards inserted horizontally from the front. The two cables for the repro head were unsoldered, and I soldered a pair of twisted, teflon-sheathed solid silver wires onto each cable (Fig. 3). I then connected a ground cable to the chassis ground. The signal cables were terminated with Lemo plugs in a single-ended fashion.

I had an Allen Wright RTP-3C phono preamp that I painstakingly built over the course of two years sitting idle on the shelf, as I was using the RTP-3D as my preamp. Built entirely by point-to-point wiring, it was easy to modify. All I had to do was to modify the RIAA section for tape equalization instead. Since the design used passive RIAA, it made life a lot easier. As I only use Nagramaster and CCIR EQ, there was no need for NAB. I determined the input impedance of the stage following the EQ section using a trim pot, and calculated the values of the capacitors and resisters. I used latching relays to switch between the different EQs so that connections are kept as short as possible. As I did not have a test tape for Nagramaster, I recorded the test tones using my IV-S, reasoning that all the Nagramaster tapes I have were recorded using this machine anyway. I then adjusted the resistor values using trim pots until I could get a satisfactory frequency response curve for both EQs. The Nagramaster EQ gave a very extended response, with a slight +2dB bump at around 15 KHz, 0dB at 18KHz and then steeply drops off thereafter. I managed to get the bass response to -3dB at around 30Hz. For CCIR, -3dB was at around 18KHz. The sound of the playback was much improved. The hard edge was gone and the tone of instruments was much more organic.

Good things don’t last forever, and in this case, with the 15-year-old tube preamp having been sitting idle on the shelf for about 8 years, something was bound to go wrong. One of the regulator boards went up in a puff of smoke after about 6 months of use. It was a shunt regulator and ran hot normally, as I liked to keep a fairly high shunt current for better dynamics. One of the resistors has turned into charcoal, damaging the board at the same time. I tried repairing it to no avail. As my dear friend Allen had passed away some years before, I did not know whether I could get a replacement board. Fortunately, Mrs. Wright has continued to run the business and I managed to get another blank circuit board from her. After installing all the components, and setting the correct voltage and shunt current, everything checked out. I had just received the latest installment of the Analogue Production tapes and I was eagerly looking forward to hearing them. I sat down to listen, and halfway through the first tape, the new board went up in a puff of smoke again ! A new board with new components lasting only 15 minutes ? It must be the amplifier circuit that had a fault and somehow shorting out the regulator. The fault was probably intermittent, and therefore everything checked out when I first installed the new regulator. As all the tubes were new (less than 6 months) and soak tested beforehand, it was likely due to some other components and I probably have to rebuild the whole channel.

However much I like my soldering iron, I was getting fed up. Truth be told, I still get pretty nervous when testing live equipment with 400V DC rails after all these years; another good reason to go solid state. In fact, I had been reading a lot of good things about Charles King’s King-Cello tape preamp. This preamp is based on the legendary Cello Audio Suite tape preamp circuit, updated with modern components. It is built to order with a lot of customization possible. I wrote to Charles and told him what I needed. He had quite a few orders to fulfill, as these preamps are hand-made one by one, but he got to mine in November. We decided to have precision pots installed for EQ adjustment, so that I can dial in the precise corner frequencies. The only caveat is that the preamp is single-ended. My whole system is balanced differential, and I was a bit worried about noise with a single-ended connection. Given the arrangement of my set-up, my main preamp is about six feet away from the tape preamp, as I have to accommodate my turntable as well. I generally prefer unshielded cables to reduce capacitance, and the better than 60dB of noise rejection in a balanced connection is highly welcomed in a system as sensitive as mine.

Even before the preamp arrived, I wanted to prepare for the worst. I rummaged through my parts boxes and found a pair of line input transformers salvaged from a Neve mixing desk. These are quite excellent transformers, and since most music recordings during the golden age were mixed using these desks, they can’t be too bad. I therefore hooked them up using a plastic food storage box as chassis to give me a balanced output to drive long cables. When the tape preamp arrived, I quickly connected it up to the system and my worst fear was confirmed. There was a hum. I connected the output through the balancing transformers but the hum remained, so the ground loop was not between the tape preamp and the main preamp. I disconnected the earth at the power plugs of the tape machine and the tape preamp alternately to no avail. I asked Charles for advice and he recommended that I connect the headblock ground directly to the preamp chassis. I located the ground wire coming out of the headblock, followed it to its connection at the chassis ground, lifted this and extended it with an extra length of wire (Fig. 2) to connect to the tape preamp chassis ground. This significantly reduced the noise. I then star grounded the chassis grounds of the tape preamp, the tape machine and the main preamp to a CAD (Computer Audio Design) Ground Control device, and disconnected the earth connection of the tape machine and the tape preamp at the mains plug, so that everything is grounded through the main preamp only. Now, there is total silence even through the single-ended output connection.

The sound of this set up is definitely an improvement over the Nagra’s repro electronics. The scale and dynamics are preserved, but the electronic character is gone. Compared to the RTP-3C, the instruments seem to be more focused and there is more treble energy. The tube preamp did sound more “organic”, for lack of a better word. Listening to the Analogue Productions Power of the Orchestra Ultratape, the dynamics appear to be limitless. The build up of tension during long crescendos can be both exhilarating and foreboding, as one never knows whether the rest of the system could handle it, and the climaxes could make one’s hair stand on end. Aside from the other Ultratape releases, the only recording I have heard that is comparable would be Analogue Production’s own DSD release of the Dorati Firebird. I only hope Chad will make the Mercury Living Presence recordings available on tape format. Listening to my own session masters, my team’s original intention during the making of the recordings came through clearly. The King-Cello preamp adds or subtracts very little from the signal, resulting in a very neutral representation of what is on tape, which is what one wants for mastering purposes. I am therefore not surprised why so many professional mastering engineers are outfitting their decks with Charles’ electronics. With the quality of the sound and the ability to customize, it must be one of the great bargains in audio today.

Fig 1. The Nagra T Audio with the King/Cello tape preamp and the CAD Ground Control device. The four dials allow users to set the LF and HF equalization accurately.


Fig 2. Cable bundle from the headblock, showing the ground cable (red arrow) with the extension cable spliced in.


Fig 3. The red arrows indicate the connection points of the repro head cables to the main circuit board. The cables have been disconnected from the board and a pair of teflon-sheathed silver wires spliced in to connect to the tape preamp inputs.


Fig 4. The two signal cables, the headblock ground cable and the chassis ground cable are brought

Get to know our Vintage Blog moderator, Dr. Adrian Wu

Welcome to the Asia Audio Society, our little virtual community of audio fanatics for sharing ideas and experiences in sound reproduction. I am one of the contributors/moderators of the site and I would like to take this opportunity to introduce myself.

I grew up during the 1960s in Hong Kong, and my first introduction to music came from my piano lessons. It was de rigueur in those days for kids to take up a musical instrument, and my mom probably chose the piano because we happened to have one at home, and the noise it generates is easier to put up with, as compared to say a violin or a trumpet. At the beginning, my enthusiasm was lacklustre to say the least, until I was sent away to an English boarding school and met my second teacher. He was a retired concert pianist who devoted his later years to nurturing the next generation. And for his sins, he ended up with me as his pupil. Nevertheless, his enthusiasm was quite infectious and I soon became fanatical about the instrument and music in general. In those days, having a boom box was a real luxury, but after the Sony Walkman became available, it was a revelation. In sixth form, a few of us in the A-level physics class formed an electronics club, and our physics teacher would teach us all the basic skills in soldering and putting things together. One Sunday afternoon, we went round to his house to help him work on his speakers. It turned out that he had these large, bronze coloured flat panels that looked like space heaters. He took the grill off one of them, put his hand in to disconnect a panel, and suddenly he was thrown back about 5 feet, landing on his backside with sparks flying. He did not discharge the things overnight as instructed, and all of us were duly impressed. From that point on, I lusted after a pair of Quad ESL57. The electronics training was very valuable, as proper technique and a good understanding of fundamentals is essential for our hobby, and so is the appreciation for safety, undischarged electrostatic panels notwithstanding. We learned to build power supplies, radios and even a robot controlled by a Motorola 6800 microprocessor.

I was able to secure summer jobs during my university days in Scotland and finally saved up enough to buy myself a music system. It comprised of a Systemdek II turntable (the “pressure cooker”), with Mission 774 tonearm and Audio Technica AT33 cartridge, which apparently is still available new 35 years later. I had a Mission Cyrus integrated amp and a pair of KEF Coda 3 speakers. All second hand, of course. I spent most of my spare cash buying LPs. I could not afford the flimsy new LPs, especially those miraculous digital recordings, which cost about ₤5 in those days. CDs just started appearing at that time, and they sounded so horrible to me even though they were supposed to have perfect sound, and they cost more than ₤10 each, so I opted to stay with imperfection. Sadly I had to settle for second hand LPs such as the narrow and wide band Deccas, postage stamp EMIs, Columbias SAXs and Lyritas etc. One would pay 50p to ₤1 for these. Fortunately, the Scots were frugal people and they took care of their possessions. LPs from those days still form the bulk of my collection. I used to salivate over my classmate’s Linn/Naim system. I remember him dragging me along to audition the LP12 at a dealership in Edinburgh. The salesman (a kid actually) brought out the turntable, had nowhere to put it and plonked it on top of the cardboard box it came in. The subchassis was bouncing literally sideways, and he made no effort to set it up whatsoever. He played a few tracks for us casually, probably thinking that these poor students were just wasting his time. The turntable didn’t sound right to me, but my friend had already made his decision long before we set foot in the store. He was brainwashed by his hero Ivor to think that even a poorly set up Linn Sondek was better than anything else out there.

After I started working, I was able to save up and buy something better. I sold my system to a friend and bought a Roksan Xerxes turntable with Artemis tonearm and Sumiko cartridge. I had a Musical Fidelity integrated driving a pair of Linn Tukan bookshelf speakers. These proved a considerable upgrade to the sound quality. I then got a job in the US and after moving there, I decided to buy some new amplification. The salesman at the secondhand shop convinced me that a tube preamp with a solid state power amp was the way to go, and I bought a Conrad-Johnson PV10 and an Aragon 2004. It was actually quite a nice combination, and sounded more musical than my previous integrated. I had a very busy job and soon got married, so there was no time to tinker. After 6 years, I returned home. Feeling more settled and with a more stable job, I became interested again in experimentation. It might be my early experience with the Quad ESL, or a romantic attachment to the golden years of high fidelity, I started looking into vintage gear. I studied circuits and learned all about vacuum tubes, transformers etc. I started looking through classified ads and secondhand shops. I bought several classic vintage amps including the Leak TL12.1, the Brook 12A, the Quad II, the Pye PF91, and the Telefunken V69a. I experimented with different passive components and tubes. Some I would restore and then sell, others I have kept. I also bought a pair of ESL57 while visiting my sister in Leicester. I came across an ad in the local paper and asked my sister to take me to see the seller. It turned out to be at a public housing estate and I managed to buy the pair for about 200 pounds. They needed a lot of work, but the exterior was in very good condition, which was exactly how I wanted it. Sending them home took more work, and I also ordered some new panels and EHT units from One Thing Audio. Over Easter holidays, I changed all the panels as well as the EHT unit and brought them up to spec. They sounded gorgeous with certain types of music, horrible with others. I tried driving them with different amplifiers. The Quad II was a bust. They sounded slow and anaemic. The TL12.1 were better; they sounded more transparent and lively, but bass was still lacking. The V69a were better still, giving the speakers more energy and better extension. The best match though was with my friend’s Mark Levinson ML2. With these amps, the speakers were transformed. Suddenly, the bass extension improved by at least one octave and sounded tuneful and solid. These speakers are fully capable of producing fairly deep bass, but most amplifiers cannot cope with the high impedance at these frequencies. It is probably a sacrilege to some people to drive the ESL57 with solid state amplification, but it works. Another idea came from Tim de Paravicini, which is to drive the panels directly with output tubes, which is not a bad idea considering that the output transformers are usually the most expensive components in a tube amp. I never got around to experimenting with this, since the memory of my physics teacher was still haunting me.

After a couple of years in Hong Kong, my turntable gave up the ghost. Probably due to the humidity, the subchassis warped. It was apparently a common problem with the early Roksan turntables. I was becoming intrigued by idler wheel turntables anyway, so I bought a Garrard 301. It had a grease bearing, and came with a slate plinth, an SME 3012 S2 tonearm and a fairly new Clearaudio cartridge. The whole package was ₤1000 from a secondhand shop in London, and as I did not like the cartridge, I sold it on Ebay. Over time, I have changed the main bearing and the tonearm bearing, rewired the tonearm and changed the arm base. The idea was to correct the weaknesses of these vintage components while preserving the characters made them great.

On the electronics front, in my quest to learn about tube circuits, I befriended Allen Wright of Vacuum State Electronics. Allen had his firmly held beliefs on circuit design, based on sound engineering principles and impervious to trends and fashion. I became his “beta tester” for his RTP-3 preamp. During this two year period of experimentation, when I built the preamp by point to point wiring, much was learnt about circuit topology, components, and the relationship between measurements and sound quality. We ended up with a superb sounding preamp that has won much critical acclaim. This was followed by the differential 300B power amps. Having worked with these amps, which use tubes for signal amplification and transistors as regulators and current sinks, one can appreciate that each kind of technology has its place. Each type of device has its advantages, which should be exploited to the full in a circuit. The criticism directed at tubes of being “soft”, “coloured” or warm sounding is not due to the inherent characteristics of these devices, but the way the circuits were designed. With proper implementation, tube amps can sound as dynamic, neutral, extended and speedy as their transistor counterparts.

I moved into my current home 10 years ago. As the flat needed to be completely renovated before I moved in anyway, I asked an acoustic architect friend to design the living room. He was more used to designing concert halls and music studios, but he obliged. The journey was another wonderful learning experience. He did a superb job, and it is an example of how one can marry good acoustics to an aesthetically pleasing living environment. Unfortunately, the living room has outgrown the Quads. The much larger space, coupled with the acoustic treatment, meant that the Quads were not able to produce enough sound pressure. I was sad to see them go, but this also presented me with an opportunity to experiment with horns, something I had always wanted to do. I have had some experience with different horn components, having listened to various vintage speakers, mainly Western Electric, Altec and JBL. After evaluating different drivers and horns, I decided to use the wonderful Electrovoice T350 tweeters, JBL 2450H mid-range compression drivers with 500Hz rectangular wood exponential horns and Altec 515C bass drivers in reflex cabs. Frequency divider duty is relegated to an Accuphase F25 analogue active crossover. A pair of Townshend ribbon supertweeters add some airiness at the top end. The horn drivers, with their 110dB sensitivity, are merciless in revealing any shortcomings and a very useful tool for reviewing components.

Today, I am still experimenting. With our hobby, the possibilities are endless. A high sensitivity horn system is like a microscope, and faults become very obvious. This could be frustrating and yet exhilarating when progress is made. I have experimented with making interconnects and speaker cables, which are finally getting to the point of being acceptable. There is still much to do, and I will share my experience here during this unending quest for perfection.

Analogue: Garrard 301/SME 3012-S2/Ikeda 9TT, Nagra T Audio tape recorder, Nagra IV-S tape recorder.

Digital: Lampizator Level 4 DSD DAC, Microrendu, MacBook Pro with Audirvana Plus v.3, Tascam DA-3000 Stereo Master Recorder.

Preamp: King/Cello tape preamp, Vacuum State Audio RTP-3D.

Crossover: Accuphase F25

Amplifiers: Vacuum State DPA-300B, Mark Levinson ML27.5

Speakers: Altec 515C (below 500Hz), JBL 2450J (500-3500Hz), EV T350 (above 3500Hz),

Vintage Blog moderated by Dr. Adrian Wu

If the mere mention of valves and horns is enough to raise your pulse rate, this section is for you. Hosted by Dr. Adrian Wu, this section offers a modern take on the classic technology. Adrian’s own system is a self-built tri-amped horn system. That’s SIX valve amps! Oh, he also likes DSD.