I have modified a pair of NC400 amps with a stacked pair of polypropylene .33mf =.66mf
the replace the three polyester .68mf capacitors
My current setup with the NC400 –
The four NC400 are located in a single aluminium container. Each pair of NC400’s is powered by a single SMPS600. This is bi-AMP arrangement to power a single speaker.
The interesting thing is I have been able to compare here the modified AMP with the stock AMP. The modified AMP is significantly better in all respects bass weight is better defined the mid-range and vocal is awesome. And a significant factor is that the highs are wonderfully beautiful this is really opened up the NC400.
The modification is done using Ric Shultz WIMA capacitor.
I found NC400 to be robust to the modification. If you have good skills with a soldering iron you can do this modification. “But at your own risk” you could perhaps pass on your NC400 to someone skilled in this area to do the job.
I’m only advising this here because this community at PS audio forum are a friendly bunch and I care.
For those who have used the MK1 PS audio digital board in the DAC and swapped out to MK11 PS audio digital board. By changing these capacitors and soldering the speaker wires directly into the NC400 board in my opinion has resulted in significantly more improved sound than compared to going from MK 1 to MK11. Both consider the NC400 now amplifying the beautiful sound that the MK11 board is making.
I have never heard my music as I do now. You can see on the link on this forum in the section Ya’ I got a system
http://www.psaudio.com/forum/#/discussion/4109/johno-low-power-music-system
I realise I take a risk by commenting as I have here, and this is a home modification. However I’m 100% certain that this change has resulted in a significant improvement and I would expect existing NC400 owners to hear the similar improvements as I have. Don’t think of it like just changing a cable - this has been a significant change, and musically better far better than the stock board. I can only conclude that the real estate on the existing board did not allow the manufacturer to put in place what would be six capacitors the polypropylene capacitors. The biggest value is only .33 µF the required value is .68 therefore you need to stack the two capacitors. See - read Ric Shultz WIMA capacitor modification where he has replaced the leads with copper leads. I can confirm one of the six capacitors I swapped-out, one only had steel leads. But also was the fact that the capacitors are orientated foil to ground. I have not picked up any audible noise which could be expected to occur with the setup. It is assumed that the power supply noise could be picked up by these capacitors due to increased surface area. . I find no increase audible noise the highs are better defined and way more natural sounding. I will be modifying my remaining 2 AMPs soon as the capacitors arrive in the mail. 
This change poses a very interesting question.
The manufacturer has created a near perfect looking amplifier on the test scope. Though I have modified the existing on-board capacitors I do not know what the amplifier would look like now on the test scope. However to the ear there is no doubt at all and I would be interested to hear comments from the manufacturer in this respect perhaps they could modify an NC400 as such and put this on a test scope, perhaps there will be no measurable change as the values are the same there is just a slightly bigger surface area of these capacitors. However with a copper leads improving the capacitors over steel and putting the foil side to ground, perhaps those benefits out way any increased noise, I can’t say for certain.
Anyhow if this interest you make the change at your own risk. You could destroy the your NC400 and be left with nothing. I would not want that to happen to you so words of caution please take care. have a professional do the job if you decide.
Following you are successful in your modification, I would be confident you will concur with me the benefits after making the change. I would even suggest having it done by a professional and paying them - the cost would be worth it in my opinion.
If you have questions feel free to ask. There was never any intention to disrespect the designer of this awesome amplifier NC400. HYPEX did aim this module at the DIY market, I guess that’s just what’s happened here a small modification has been done I don’t think I would do anything other mod to the board, I am that impressed with the sound.
I do not modify PS Audio gear, I am completely happy with the sound of PS Audio gear,
Research told me the installed capacitors, polyesters are good but better yet were polypropylene, sure there is better capacitors that are reputed to sound better still. But I have found my happy place.
-Components-
PS Audio power plant
PS Audio MK11 DAC with Ethernet Bridge
Software – Player - Jriver - driver bit perfect PS Audio WaveStream out- on windows 8 CPU 3770k and 16 gigs ram CPU shows <1% utilisation streaming 2496 bit album
Amplifier 4 X NC400 bi-wire pair power speaker
Cables - silver
Cables - balanced
Speaker- Theophany Rhapsody
Cheers
John
Yeah, the usual modification path:
o- upgrade capacitors
o- disconnect unused blocks
o- upgrade the power supply (diodes, filters, connectors, shielding)
o- add proper shielding wherever it’s possible
o- add damping material if needed
o- upgrade all connectors
o- upgrade cabling
o- upgrade opamps (if any)
DIY rules 
Nice job, John!
Look familiar? I had a long talk with Ric one evening and he convinced me to try the cap mod. I’ve had these about 6 months… clearly your stones are bigger than mine. :))
So you replaced all of them, not just the last one?
Attached files 
@wglenn
Did the modification of one cap in each NC400 improve the sound significantly for you?
What would you say about the sound compared to the stock unit
Did you also do the speaker post
Cheers
John
I should have been more clear. I haven’t done it yet. ~:> At the time that I talked to Ric no one had replaced all of the caps yet, including himself. Sounds like you went for the whole set. Thanks so much for sharing this information. I’ll order more caps from Ric before I get my desoldering wick out!
Paul, maybe you should look at the caps in your Hypex modules and give 'er a try if the components are similar to the NC400 at the output.
@wglenn
I used an electric pump solder sucker and not the wick, my opinion there is more direct heat control with the solder sucker and less time heat time on the PCB. I wanted to shorten the heat time on the PCB and this cost is a reasonable cost to the project. I purchased the S-995A model
other components are very close by on the PCB beneath I would recommend the use of the Electric Vacuum De-soldering Pump.
See ebay for various models. Even if I used this once it was well worth it
S-993A 110V 90W Electric Vacuum De-soldering Pump Solder Sucker Gun $87.00
S-998P AC220V 100W Electric De-soldering Gun Vacuum Pump Solder Sucker 350℃~450℃ $125
If you remove the brass screw of the speaker post and place the solder iron directly into the hole where the screw came out, the heat of the iron quickly heats the entire post and you are able to wiggle the post out fairly quickly. I then strapped all four holes underneath to form a single path - to enable additional heat dissipation for when you solder or de-solder the speaker wires as required." then the wire popping up through one hole and this allows a good platform to solder your speaker wires too. This could be a significant benefit to the sound
Cheers
John
@alekz
Hmm well yes to what you say
though the stock NC400 on the test scope is perfect, it is interesting such a change has resulted in SQ far beyond what the stock unit was delivering.
The manufacturing process leaves out’ I refer the assembly of the components to the board do not take into account the foil to ground side of the capacitor or particular attention to steel leads on the capacitors - it is truly DIY when the side of the caps have been dremeled and copper leads attached.
http://www.tweakaudio.com/EVS-2/Modded_Wima_Caps.html
this is not a straight capacitor swap. this is a modified capacitor 
The NC1200 are reputed to sound better, I can completely understand that now. They use the same cap. I think the PWD mk11 direct is a beautiful marriage of components and not to forget WaveStream.
I would love to hear the modified NC400 next to the NC1200. My NC400 have taken a significant leap forward in SQ. there is just no comparison to the stock unit I would consider the stock unit to be lifeless in comparison a big thanks to Ric Schultz is in order from me :-bd
Hello,
i wrote to Hypex staff regarding the Ric’s mods and here their answer:
"We do not endorse any modifications to the NC400. They will void warranty and most effects they claim to achieve are bogus. The NC400 was designed as a no-compromise amplifier.
The only things that make sense are balanced input configuration and proper cabling."
I recap the Ric’s mods:
1. Use an audiophile fuse…makes a big difference, much more full, and lower distortion…we sell the Furutech fuses.
2. Bypass the binding posts using my binding post bypass system. Depending on the connector you use can vary from better to way better.
3. Bypass the output connectors on the NC400 board. Makes the amps much more full and palpable. We remove the stock connectors and replace them with OFC 16 gauge wire that you solder the output wire to. Pic below.
4. Change one of the Polyester .68uf output filter caps to 2 modded .33 uf Polyprop caps. Lowers noise way down…Way more transparent. Its as if you scrubbed the noise away and it is sooooooooo much more palpable and real…amazing amount of space/reverb/air now. The Polyprop caps have the stock leads removed and replaced with 6N copper leads and the caps are marked for outside foil polarity. The caps are mounted so the outside foil is oriented to ground. Pic below.
5. Use good IEC and input connectors.
6. Use good wire on the output (soldered to the new bypass connectors on the NC400 and run outside the amp and wrapped around the binding posts).
7. Add an AudioMagic Pulse Gen ZX to the input power (two if dual mono).
8. Exotic feet and damping
9. Balanced configuration. More air, more detail, more dynamic and more vivid
Source: http://www.audiocircle.com/index.php?topic=110782.0
fabien44 said: most effects they claim to achieve are bogus.
This is not the first time I see such a response from a manufacturer :D Basically, they contradict themselves. If design "A" is perfect and can't be improved, why do they have design "B", which is much more expensive and ...hmmm... more perfect? =))
Or another example (taken from SlimDevices forum):
1- changing parts does not make any sonic effect, DIYer claims are bogus
2- if you modify our units, they will sound worse
1 and 2 contradict each other.
@Alekz
If the mod of changing caps reveals a significant improvment, why Hypex does not include it in the stock unit? The new caps cost few dollars, it does not change anything for their margin, does not it?
fabien44 said: If the mod of changing caps reveals a significant improvment, why Hypex does not include it in the stock unit?
They have much more expensive NC1200 ;)
Also there are so many various capacitors (opamps, resistors, etc.) , designs and manufacturers. You can always find better, more expensive, different sounding, more appealing for various reasons, but not necessarily more practical.
This is why it's called "tweaking" after all. "Tweaking" is very "individual" thing.
yes Alekz, i agree with “different” sounding but not “better” sounding, as you said it is very “individual” (subjective) thing
So i am very suspicious with the first post and the “is significantly better in all respects” affirmation.
I do not question the improvements. I only stated that it’s not practical (from the manufacturer point of view) to use exotic parts. The stock may be limited. Think of Black Gates capacitors as an example. I do have them in my Transporter, they are fantastic, but the production stopped in 2006. Or another example - Jensen paper in oil caps are a bit too big, what is also not practical. And, finally, some caps are extremely expensive. This fact also does not make them practical.
BTW, have a look at this comparison:
http://jimmyauw.com/2009/08/16/extreme-capacitors-battle-1st-session/
http://jimmyauw.com/2009/08/18/extreme-capacitors-battle-2nd-session/
johno said: . . . though the stock NC400 on the test scope is perfect, it is interesting such a change has resulted in SQ far beyond what the stock unit was delivering.
Flat frequency response only tells us about the bandwidth and how well a simple, single tone sweep is reproduced. Music is a tad bit more complicated. :)
Well, I’m going to give it a go. I’ll just order four more caps from Ric so I’ll have a complete set before I bother to tackle it. I have some time in October to play around and I’ll have the parts by then. I’ll let you know what it sounds like.
Has anyone tried this on the PWD
7. Add an AudioMagic Pulse Gen ZX to the input power -
Would it be similar to Noise Harvestors?
@fabien44
I am not surprised to hear these responses from the manufacturer,
a resounding yes these amplifiers measured perfectly on a test Scope. Both reading and research led me to replace these capacitors which I’ve detailed above and the result has been nothing less than positive. If I felt uncertain of this change and it was not positive I would not consider to write what I have.
The real estate available on the PCB of this amplifier does not allow for 6 .33 µF capacitors some ingenuity is required in a way around it is stacking the capacitors. Foil to ground and copper leads some other benefits. These types of things can be quite complicated for a manufacturer to control in production. Simply anyone in the DIY community can perform this and reap the benefits.
I look forward to hearing wglenn results when he gets round to doing similar to what I’ve done. I am keen and eagerly await my other six caps to arrive so I can do my last two amplifiers. If I had any doubts I would not mess with them. For that matter want to place interest here on this forum that may cause others not to be able to enjoy the music. I am certain wglenn will have a positive result and that would make me happy.
If anyone was to contemplate doing this modification I would expect they have the necessary skill or get a professional to make the change. It’s your decision if there is any hesitation or doubt I would not recommend you do the change. I took the risk myself and as I say I am hugely delighted having done so. I have been enjoying getting the sonic improvements through the journey with PS audio. In fact this is what led me to write here on this forum of my small project and the benefits after changing the capacitors and hardwiring the speaker outputs.
It would be very interesting if Hypex put the polyester WIMA capacitor into their NC1200, this was the trigger for me that made sense and there are those additional benefits foil to ground and copper leads. There are plenty of other things one could tweak and adjust. I don’t see myself doing any of the other the highs are extremely good I think this is the biggest improvement from the modification both of mid range and bass are improved as well, I think WaveStream and the MK11 DAC are the icing on the cake here
excuse any typing errors I’m using dragon dictating - some may have guessed by the long windedness of my posts :-))
It is a lot of fun to hear of your good experience and success.
I used Dragon extensively during recovery after I lost the use of my arm and hand in an accident. It is an amazing program once you get it dialed in.