I recently finished another round of modifications to my Musical Fidelity Tri-Vista 21 Class A Tube DAC. In this post, I go into details about the changes and the MIL-spec vintage parts I’ve used!
Actually, I will do some more power supply modifications in 2018, more on that when it happens. This is part three, in an ongoing process to develop and evaluate the best performance mods for the wonderful Musical Fidelity Tri-Vista 21 DAC.
My Musical Fidelity Tri-Vista 21 is serial-numbered in the low 1400’s, of 1500 built. This time I am focussing on signal path integrity, the last round of mods I made were more about local power supplies and decoupling.
New old stock (NOS) green ERO / Roederstein MKT 1822 cap, next to the removed and very cheap yellow film cap Musical Fidelity originally used.
I am especially keen to replace the 0.47uF capacitors in the signal path to the 5703 tube output stage. This will make a real difference, any capacitor in the signal path influences the sound in some way. My video, outlining the first stage of improvements I made to the MF DAC contains more details.
Op Amps
I am also replacing the 6 x LM4562 op-amps I installed in my part 1 changes, with good old NE5532 op-amps in the I/V conversion stages and ridiculously expensive LT1028 op-amps in the balanced to unbalanced conversion stage.
I am implementing this as per the datasheet for the TI PCM1792 DAC chip. I figure that the engineers who’ve specified this arrangement have done so for very good reasons. MF could never have included $20 op-amps in the Tri-Vista 21 for production cost reasons, but we can certainly do it now.
UPDATE: The LT1028 opamp does not work in this application, despite being the preferred solution by TI for the PCM1792. I should have realised it wouldn’t work.
MF has designed the summing circuit suits the 5532, with a unity gain bandwidth of 10MHz. The LT1028, with a bandwidth of 75MHz oscillates and draws too much current. They got so hot after a few seconds that I had to shut her down. I will stick with a pair of the outstanding LM4562 opamps in the balanced to unbalanced summing role.
To summarise, the NE5532 actually features superbly low current noise, much lower than many new, state-of-the-art op-amps. This is why TI recommend it as the I to V converter after most of the DAC chips, even now. However the LM4562 features superbly low voltage noise and lower distortion, far lower than the NE5532, so it is in fact perfect here in its role as a summer.
Coupling Capacitors
I am also trying a third output coupling capacitor arrangement. I have removed the large Obbligato 10uF paper-in-oil caps I previously installed in part 2 of my mods.
I’ve decided to use some very cool ERO MKC film caps that I pulled from some medical gear a while ago. These are meant to sound amazing, perhaps even better than the ERO MKT 1822 caps Ive used elsewhere in this unit. I’ve bypassed them with the same Siemens MKT film caps I’ve used throughout the DAC.
Finally, I’m removing the factory flying leads that connect the output jacks and installing aviation-grade Belden Teflon-sheathed silver-coated copper wire. This is running straight into a pair of gorgeous CMC silver RCA jacks, silver-soldered, of course!
Local Power Supply Decoupling
I have replaced the four filter caps located near the opamps. These provide a local charge reservoir and provide the lowest impedance +/- 15 V rails for the op amps. I had originally replaced the factory Jamicons with 100uF Panasonic FM – superb caps. This time I went with larger 220uF Panasonic FC low-impedance caps. This was a last-minute thing and I did it because I have all these premium parts in stock just sitting there, begging to be used!
The end result is that the Musical Fidelity Tri-Vista 21 sounds sublime, better than she ever has. This results really validates all the modifications I have made to this great DAC. This is now easily the most analogue and relaxed sounding digital system I have heard in my home and one of the best I’ve heard anywhere.
MF Tri-Vista main board after part 2 of the modifications. Note the massive Obbligato oil filled capacitors used as output couplers. These were really too big for this purpose and I think it was a silly move to use them here. You can see the yellow Chinese film caps on the board, around the 5703 tubes and behind them, nearer the back panel.Close-up view of the ridiculous Obbligato paper-in-oil output-coupling capacitors I installed in part 2 of the modifications. They were nice, but too unwieldy in the MF chassis. I also don’t like long flying-leads in an RF-rich environment like a DAC.Mil-spec green ERO MKT capacitors, after installation on the main board of the Tri-Vista 21. The yellow parts are the cheap Chinese caps I pulled out. They are much better than electrolytic capacitors, no dispute, but they are at the bottom of the list of MKT types you would want in or near the signal path.Close-up of the cheapo MKT caps I pulled out. Especially important are the 0.47 uF caps in the signal path from the converter to the tubes.Again, ERO vs Chinese caps. Always go with European NOS vintage mil-spec quality, always.Sexy aren’t they? I am so glad I have large stock of these ERO MKT 1822 caps, they really are amazing capacitors.Gratuitous ERO capacitor shot, for those so inclined… This raises another issue – the twisted pair, non impedance matched digital input and output wiring. I must replace this with MIL-spec mini-coax in my next (2018) round of mods.The new ERO signal path couplers, sitting in between the last pair of op amps and the matched set of 5703 triodes I installed before.The new ERO capacitors sit very well on the MF board, with some slight dressing of the leads to match the hole-spacing on this board.‘Styroseals’ are awesome and I was going to use them to bypass the output coupling caps, but went for the same mil-spec Siemens caps I’ve used throughout the DAC for this role in the end. Next time I will instal these polystyrene capacitors, for sure.This was inspired by a really long read of the PCM1792 datasheet. TI recommended these after 2 x NE5532 opamps per channel.. You don’t take engineers’ recommendations lightly, especially not from those who designed the DAC chip a product uses. The problem is that the MF circuit is not designed to work with these and they are just too fast and have too much gain at high frequencies to be stable here. Oh well, perhaps I will add some ‘puff’ sized ceramics to take the edge of for the next round of mods.Close, but no cigar… YET! The $20 a piece LT1028 instrumentation opamps are awesome, but have too much HF gain to be stable here.Preparing some lengths of Belden silver-coated copper wire, teflon-coated, used for wiring aircraft instrumentation. Hell yeah!Now if we are talking coupling capacitors, ERO are a great choice. Before they made MKT capacitors, they made MKC capacitors. Many feel that these sound better. Some say that these sound the best of any types. I pulled these from some 1970’s uber-expensive medical equipment. I can tell you that the finished DAC sounds amazing.Fitted to the bottom of the board, with the EROs soldered to the flying leads of the 0.1 uF Siemens caps I bypassed them with.Sexy, again..EROs are hot-glued to the board, on the back side. There is no room for them on the top and I always try to use component leads to minimise the number of soldered joints in my modifications.New CMC silver RCA jacks in place. Note the factory output leads are still attached, for measurement purposes.I then replaced the standard leads with twisted pairs of Belden aviation-spec silver-plated copper lead-out wires…They run to these gorgeous CMC silver-plated RCA connectors. These are just so much nicer in every way than the Neutrik types that most people use that you should just always use them. I certainly always use these now, but they are more expensive.The knurled retainer is so nice to use and the quality of construction of these CMC jacks is second-to-noneDetail shot of my twisted-pair Belden lead-out wires.Part 2 of my mods saw me install 6 x LM4562 super-high-spec opamps. These were and are probably the best way to go, but just for something different, I wanted to try a different arrangement.Final opamp iteration – 5532, 5532, as per TI datasheet and MF original design, followed by 4562. The 5532 and 55534 are much maligned but these are still near state-of-the-art for low noise and very low distortion.Note also the new 220uF local filter capacitors I added for the +/- 15 V rails that supply the opamps right there. Best place for them, originals were 100uF Jamicons.New, shorter Belden aviation-spec instrument wire lead-outs. Silver solder used here.The last view of the insides on my Musical Fidelity Tri-Vista 21 Class A Tube DAC. I probably won’t open her up again for quite some time. (Yes I will!)
3 thoughts on “Musical Fidelity Tri-Vista 21 Tube DAC Modifications, Part 3”
Chong K M
It is interesting to note that there is someone is capable of doing this fantastic job and, as well, having the energy and the stock which otherwise would not be available in the market. K M Chong
It is interesting to note that there is someone is capable of doing this fantastic job and, as well, having the energy and the stock which otherwise would not be available in the market. K M Chong
Have you ever modified a Nuvista product from Musical Fidelity such as a CD player?
Hi Richard, I work on quite a bit of MF stuff – why do you ask? Mike