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

Musical Fidelity Trivista 21 DAC re-engineering pt 6 (final) by Ed Kwok

In this final part of the series I will talk about the (in)famous TriVista tube buffer and why you should bypass it.

What! I hear you say. Bypass the tube buffer? But the tubes are the reason for Trivista 21’s existence, right? I thought so too. Let me take you through the effort and expenditure I took to reach that conclusion. Let’s talk circuits. In the DAC section above we already discovered the output from the third op amp in the filter circuit is ready for output to the preamp. But instead of being made available at the back panel this signal goes through a unity gain buffer that contains the 5703 triodes. The 5703 triodes are not wired as traditional (and good sounding) cathode followers. Instead each channel has a pair of 5703 which form a long tailed pair in a discrete op amp configuration that also includes 7 bipolar transistors. Below are the improvements I made:

a) Matching the input valves: Performance of any long tailed pair hinges on perfect matching of the input devices so I purchased twenty JAN NOS 5703 and hand matched 2 pairs using an AVO valve tester. I did find the original valves were not matched.

b) Matching the NPN and PNP transistors in the second stage and output stage: originals were not matched at all.

Despite all this effort at a formal A/B test with and without the valve buffer all the Society members preferred the bypassed output with its much better transparency, communication of musical performance and far greater width and depth. I would like to try the tubes in a better circuit configuration in future and for now it was decided to permanently bypass the tube stage and disconnect the HT transformer to further reduce noise.

Figure 7: Trivista 21 (re-engineered)

Conclusion
After formal A/B comparison in a reference system we have determined that the re-engineered Trivista 21 is fully competitive with current DACs from Chord, Naim, Merging, Lumin. The secret is to correct the mistakes and use the correct (not just expensive) components. One last problem: now that the tube stage is disconnected do we still call it a Trivista?

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),

Musical Fidelity Trivista 21 DAC re-engineering pt 5 by Ed Kwok

Now we get to to the heart of the project – the DAC section.

MF made some mistakes in the analog power supply and the op amp filters.  Below I will show you how to correct these mistakes and unlock tremendous improvements. Trivista uses a Burr Brown DSD1792A 24/192 DAC in PCM mode. This chip was state of the art in 2003 and boasts 127dB dynamic range and 0.0004% (-108dB) THD+N, which is comfortably beyond the performance required for CD audio. DAC chips are all quite cheap (tens of USD) on their own but they need good supporting circuits to make them sound good. TI published a reference circuit in the 1792 data sheet and I will refer to parts of it as we proceed.

+5V DAC chip analog power supply. DAC chips need separate clean +5V for the analog power pins. So why did MF supply the DAC chip’s analog power pins VCC2L and VCC2R from the dirty digital 5V power rail? After analysis confirmed by listening tests I concluded this was indeed a mistake probably made when they designed the PCB. There actually is a clean +5V rail supplied by a 7805 regulator with choke filtering that takes power from the already clean +15V analog rail but it is only used for the DAC chip’s VCC1. Get out your soldering iron and connect the DAC chip’s VCC2L and VCC2R pins to the clean analog +5V rail. You have just doubled your Trivista 21’s sound quality. For further improvement replace the 7805 regulator with something better. In this critical application the difference between a fairly good LM340A and a state of the art (in 2017) LT3045 is clearly audible in terms of transparency, resolution and spaciousness. If you want perfection you can use 3 LT3045 to power VCC1, VCC2L and VCC2R but you would need to make a piggy back board. Finish off with a sprinkling of OSCON on the DAC chip power rails. Don’t use OSCON for the internal bias decoupling caps because they have high leakage current. You have now doubled the sound quality again.

+15 and -15V op amp power supply. Each rail is provided by one 78/79 regulator taking power from the +-24 volt rails. Each regulator supplies both left and right channels. Putting in separate regulators for the left and right op amps would have cost USD5 more in components so this kind of cost cutting by MF is surprising. Unfortunately, there’s not much that can be done without seriously cutting up the PCB so I have left as is and merely upgraded the regulators to LM340. At least there is room to put in decent electrolytics.

Op amp integrator and filter. MF uses the reference circuit and I agree 100%. Some hobbyists claim they can come up with a better circuit, if they are so good they can go and design the DAC chip as well. However here is where MF made another mistake on the PCB (but not on the circuit). For some reason MF uses half of a 5532 dual op amp in all six positions instead of the recommended single 5534. That in itself is not a problem and maybe they had a lot of 5532 in stock. But they failed to disable the unused half of the 5532’s in the correct way. They wired both inverting and non-inverting inputs to ground, which ensures the output is driven at open loop gain and will peg itself at the + or the – power rail at random. Thus the non-working half of each 5532 compromises the working half. Correction requires track cutting to reconnect the inverting input to the output to regain stability. While you are working on the op amps why not use something better? The data sheet specifies the 5534 loved by big Japanese electronics companies who are the main customers. But the same circuit appears in OPA1611/12 data sheet so TI engineers know better. I am using LME49710 and LME49720 but if you use AD797 I would love to hear from you. While you are in there replace those cheap resistors and capacitors in the filters with your favorite precision types. I went with 0.1% Dale metal films with 5 and 15 ppm/C and hand matched 1% Phillips NOS polystyrene. There’s not much space to put in anything bigger/better.

Output buffer added. The output of the third op amp is ready to feed into your preamp. In the Trivista 21 this signal is connected to the input of the tube stage and is not available at the back panel. I added an LME49720 unity gain buffer stage so I could bring this clean signal to some extra phono sockets on the back panel to facilitate A/B comparison with/without the tube stage.

Figure 5: DAC  original                 Figure 6: re-engineered

Musical Fidelity Trivista 21 DAC re-engineering pt4 by Ed Kwok

Now let’s turn our attention to the main PCB.
Figure 3: Main PCB (original)

Decoupling capacitors – stick with ceramics but use low-loss non-microphonic C0G types
If you are reading this you are no stranger to capacitors. I will leave you to decide whether you are going to increase capacitance or stick with original values. I would like to offer the following suggestions.
a. High frequency decoupling: There are about 50 low quality 0.1uF disc ceramic capacitors providing power supply decoupling onto the ground plane. Replace these with 0.1uF ceramic C0G. Remember this is a digital board with a 50MHz clock so you need caps that are good to at least 1GHz. Save your plastic films for analog stuff.
b. Low frequency decoupling: There are about 25 Jamicon brand electrolytics ranging from 10uF to 1000uF. I would go for 105C rating since you don’t want to do this again in the next 10 years. I stayed with Nichicon UKA because some 3 terminal regulators are said to be unstable with OSCON on the output. If you use OSCON, please let me know how it went. Some electrolytics provide critical voltage reference to the DAC chip and I will talk about these in the appropriate section.

Figure 4: Removing the cheap original capacitors

Musical Fidelity Trivista 21 DAC re-engineering pt3 by Ed Kwok

1) Main power supply – quality choke filtered PSU, leave as is
Figure 2: PSU PCB (Original)

Power supply fanatics will be delighted to find large chokes on both the low voltage and high voltage analog power supplies. These chokes are bigger even than the power transformers. There are 2 main reservoir capacitors for each rail and they are wired in C-L-C arrangement with the chokes for maximum rejection of ripple, hum and other noise. The 24V rails will feed the 15V regulators for the op amps and the 90V rails will go through additional filtering before reaching the tube section. A 10V rail feeds the 5V regulator for the digital section and a 15V rail feeds the 12V regulator for the tube heaters; these rails do not have choke filters and probably do not need it. Best not to mess around with this superb design and just replace the old Jamicon electrolytics with new 105C rated components. I used Nichicon LGU at 50% bigger capacity and you should use your own favorite.

Musical Fidelity Trivista 21 DAC re-engineering pt2 by Ed Kwok

Trivista 21 circuit has several mistakes!!
I could not find any circuit diagrams on the internet, so I had to draw them myself by studying the circuit board. In the process I found several mistakes and shortcuts that absolutely prevent the Trivista reaching anywhere near its full potential. I was initially baffled and tried hard to convince myself that they were intentional. But after exhaustive engineering analysis backed by listening tests I had to conclude that they were indeed mistakes. I will highlight these in the appropriate section and show how they can be corrected.
Let’s get started with something all of us can see and that is the build quality. Trivista 21 build quality is excellent. Look at those beautiful double sided fibre glass PCBs with wide tracks and extensive ground planes. The wiring is terminated on the PCBs via proper termination pins and all soldered wiring is stress relieved with heat shrink. The last time I saw something this nice may have been during my days in military electronics.

What follows is quite a long article as I take you through each section and explain what I discovered.

Musical Fidelity Trivista 21 DAC re-engineering pt1 by Ed Kwok

Can the 2003 Trivista 21 be modernised to compete with today’s (2017) reference DACs? Enthusiasts have tried putting in expensive capacitors and op amps and some even offer their services for a fee. I applaud their efforts and generosity in sharing their work on the internet. But I wanted to go much further. When you peer inside the Trivista 21 you realise it is an extremely serious piece of equipment that demands an equally serious engineering effort to modernize it. This is not going to be a case of just swapping capacitors – this is going to be a total re-engineering. We will have to reverse-engineer the circuit, identify and correct all the weaknesses and deploy our components budget where it matters. With some work and USD 500 of components the Trivista 21 can emphatically withstand A/B blind testing against the very best DACs available today (2017) at any price. Provided you also bypass the tube section entirely!
To be continued.

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.

Playback Blog moderated by Ir. Ed Kwok, C.Eng, MHKIE

The Playback Blog is for the technically minded audiophile who is prepared to go beyond commercially accepted standards in search of engineering perfection. Moderated by Ed Kwok, a UK qualified Chartered Engineer and patent holder with a military electronics background. If playback to you means spending hundreds of thousands on a system and having it set up by the dealer then stop reading now. However if you like meticulously engineered equipment based on proper application of scientific knowledge and you are happiest when you have a data sheet in one hand and a soldering iron in the other then read on!

Recording Blog moderated by Glenn Fok

The Recording Blog focuses on technique and equipment to make great live recordings of classical music performances. It is moderated by Glenn Fok. Glenn has an encyclopedic knowledge of concert hall sound gained from a lifetime of attending concerts all over the world. He has personally recorded many live orchestra and chamber performances. Glenn serves as a Member of the Board Of Governors of the Hong Kong Philharmonic Orchestra.