Hypex NC400 - DIY Mods

[Feyire]

I have recently spent a lot of time researching various possible NC400 DIY modifications, for the purpose of improving the listening experience with my amplifiers, but also having some classic DIY fun.

Warnings before we continue:

• Attempt these mods at your own risk.
• These mods will almost certainly void any warranty.

For the amplifier module, based on what others have successfully implemented and documented in the past, I have noted the following modifications:

- LC output filter

1. Replacing the iron core litz wire output inductor (0.01mH)
The goal here is to remove the ferromagnetic inductor, and replace it with a copper air core conductor, therefore eliminating the hysteresis problem. @Bruno Putzeys himself was quoted in an article back in late 2019 saying that an investigation he made into complaints about a "sporadic granularity" of the sound from the ncores, was due to the ferromagnetic material of the output coil.

I believe that something similar was already attempted in the past by @Apollon Audio, but with the inductor in the NC1200 module. That unfortunately did not end in a successful result. There are however others, who have successfully made this modification on the NC400 and Purifi modules, with very positively reported results.


2. Replacing the 3 parallel output capacitors (0.68uF each)
The capacitor brand used seems to vary, with earlier produced modules using what I believe is the WIMA MKS4 (large and red in color), while the later produced modules use the Vishay MKT470 (smaller and black in color). Both are metallized polyester film capacitors.

There are numerous goals with this modification:

1. Replace the polyester capacitors with equivalent polypropylene ones (which have a lower dielectric constant than polyester)

2. Check the leads of the polypropylene capacitors, and if they contain ferromagnetic leads (e.g. steel), replace them with copper ones. Just like with the inductor, we want to remove ferromagnetic material from the output path.

Note that on Douglas Self's book on "Audio Power Amplifier Design", he mentions that magnetic distortion "...arises when a signal at amplifier output level is passed through a ferromagnetic conductor. Ferromagnetic materials have a non-linear relationship between the current passing through them and the magnetic flux it creates, and this induces voltages that add distortion to the signal." This is another reason why we want to avoid ferromagnetic material in the output stage or anywhere after it (e.g. in binding posts).

3. Find and mark the outer foil of the capacitors, so that when they are soldered back into the module, their outer foil is facing towards lowest potential/ground, in order to minimize noise. Additional information on a capacitors outer foil can be found here.


- Removal/tweaking of SMD resistor R141 (Sort of optional - depends on your overall system configuration)
The goal of removing/tweaking R141 is to be able to adjust the gain of the amplifier module. Typically, you'll want to remove/tweak R141 in order to lower the gain. Full removal of R141 results in a 10dB reduction. This has a number of benefits:

• Higher feedback
• Higher signal-to-noise ratio

So given the list of mods I specified above, I personally was able to successfully implement the capacitor mod so far, with a very positive result on both of my Hypex NC400 DIY kits. I'd describe the perceived listening difference as a more pure and smoother sound with enhanced clarity. I find it most noticeable when I focus on listening to the vocals. My experience in this regard is very similar to others who implemented this mod and reported their experiences.

In the near future, I also plan on attempting the inductor mod and the gain resistor mod. I plan to report back to this thread with my experiences of both.

I'd appreciate it if anyone else on these forums, who has modified or attempted to modify their NC400 modules, share their experiences and thoughts.

Thanks everyone




WARNING: This thread contains information about practices or circumstances that may lead to personal injury or death, property damage,
or economic loss. If you are not experienced with electrical safety practices, please do not attempt to modify your equipment.

 

[dc655321]

Have you measured what effect your mods have versus the stock board? Minus the obvious gain changes...

 

[Zoomer]

That would be quite the thing if you improved measured performance of the NC400!
Modding fun is one thing, claiming audible improvements resulting from said fun is quite another matter though.

 

[TimoJ]

- LC output filter
There are however others, who have successfully made this modification on the NC400 and Purifi modules, with very positively reported results.

But wasn't Purifi modules already designed to fix this issue even when using ferromagnetic inductors?

 

[Feyire]

Have you measured what effect your mods have versus the stock board? Minus the obvious gain changes...

I don't have any before and after measurements, simply because I don't have any analyser/measurement equipment. When I did the capacitor mod, I was relying on the positive feedback of others who have implemented it - basically a leap of faith. If there is anyone in the Netherlands who has a high precision analyser, and is curious and willing to come by and measure the effect of the mods, I'd be happy to invite them over for a chat and measurement session.

That would be quite the thing if you improved measured performance of the NC400!
Modding fun is one thing, claiming audible improvements resulting from said fun is quite another matter though.

I believe Bruno stated a long time ago on another forum that removing R141 would increase the feedback loop and therefore also increase the measured performance. Whether that performance improvement is significant, I don't know, perhaps not, but I'd consider any improvement most welcome. My positive subjective experience of the mod is backed up by others who did the same. If you asked me if I'd want to go back to the un-modded sound of the module? I'd give that a definitive no thanks.

But wasn't Purifi modules already designed to fix this issue even when using ferromagnetic inductors?

This is a good question. As far I as can tell, it still looks like a similar type of iron core litz wire inductor is used in the Purifi 1ET400A module - so no? Maybe this issue will be addressed in the more powerful Purifi module that is coming soon. Perhaps @Bruno Putzeys could shed some light on this.

Below on the left is the NC400 inductor and on the right is the Purifi 1ET400A one (The stickers seem to suggest something has changed, but physically they look very similar):

 

[boXem]

 

[Feyire]

For those interested, an excerpt from the NC400 schematic showing the LC filter circuit with the appropriate inductor and capacitors highlighted:

 

[Feyire]

@boXem | audio, would you care to elaborate on your thoughts? Since I get the impression you don't approve of what is being said/discussed.

Perhaps you can address the following questions:

1. Do you disagree with Bruno's research and conclusion in regards to the output coil and the issue of hysteresis?
2. Do you disagree with the statement that capacitors with lower dialectic loss are better and that having their outer foil orientated toward lower potential/ground reduces noise?
3. Do you disagree with Douglas Self's research that ferromagnetic material in the amplifier output stage introduces distortions to the output signal and therefore should be avoided?
4. Do you disagree that lowering the gain of the amplifier results in increased feedback and improved signal-to-noise ratio (both of these things being beneficial)?

 

[Dave Zan]

- Removal/tweaking of SMD resistor R141 (Sort of optional - depends on your overall system

It's fun and educational to modify equipment.
But there's a fine line between educational experiments and self deluded audiophile "tweaks".
In answer to your question 4 above, increased feedback can destroy the stability of the amp.
If you have to ask questions about this then you probably don't have the skills to fiddle with it yet.
Gain is almost free, the problem is stability, if there was easy distortion reduction from increased feedback then don't you think Bruno would have already done so?
The NCore amps have a particularly finely tuned feedback system where the feedback is already close to the limit.
Do you have a Spice simulation of the entire amp?
Without this, and some excellent instrumentation, your odds to improve on Bruno's work are pretty slim.
The odds you will provoke instability and destroy the amp, however are not slim.
If you are lucky then there will only be transient instability as the amp clips or on difficult speakers.
Then you can say the modifications help "reveal" the different sonic characteristics of the speakers!*

If you do understand conditional (Nyquist) stability and 3rd order (and above) feedback loops then have at it.
I would love to see your results, multipole feedback is a bit of a hobby of mine.

Best wishes
David

* Richard Lee did proper double blind tests on speakers and amps back in the 1990 as a professional in the British speaker industry.
He says this really happens!

 

[Feyire]

In answer to your question 4 above, increased feedback can destroy the stability of the amp.
If you have to ask questions about this then you probably don't have the skills to fiddle with it yet.
Gain is almost free, the problem is stability, if there was easy distortion reduction from increased feedback then don't you think Bruno would have already done so?

Bruno himself has stated in the past on other forums that the performance of the module is slightly improved after removing R141, as the amount of feedback is increased and that this did result in a measured improvement. (I can't find this quote right now, but when I do I'll add a reference here, or @Bruno Putzeys himself can comment to verify).

However, consider that for the average consumer, the additional gain is far more important than some small measured improvement. Therefore, if someone's system doesn't require this high level of gain, why not give the DIY enthusiasts/tweakers the ability remove the resistor and enjoy the increased signal-to-noise ratio and better performance? That's exactly why I'm going to do this mod.

Please also note that Hypex themselves have an application note on the topic of voltage gain in their modules, and how you can tweak it available here. Do you think Hypex would provide public information about this type of modification if it caused any stability issues in the module? Even if it did, you'd at least expect a warning about it in the document. From what I understand, there is no major concern here.

 

[boXem]

@boXem | audio, would you care to elaborate on your thoughts? Since I get the impression you don't approve of what is being said/discussed.

Perhaps you can address the following questions:

1. Do you disagree with Bruno's research and conclusion in regards to the output coil and the issue of hysteresis?
2. Do you disagree with the statement that capacitors with lower dialectic loss are better and that having their outer foil orientated toward lower potential/ground reduces noise?
3. Do you disagree with Douglas Self's research that ferromagnetic material in the amplifier output stage introduces distortions to the output signal and therefore should be avoided?
4. Do you disagree that lowering the gain of the amplifier results in increased feedback and improved signal-to-noise ratio (both of these things being beneficial)?

In general I consider that pretending to dramatically improve the work from one of the best audio EE is utterly presumptuous.

1. I am not sure you understood the conclusions from Mr. Putzeys work on hysteresis distortion
2. If you could point to any serious research paper on the subject, I would be glad to read it
3. I didn't know that Mr. Self was both a class D amplification and EMC specialist. Could you point to any of his work on the subject?
4. I don't know the particular architecture of the NC400 DIY. I know the one from NC400 OEM, NC500, NC1200, NCxxxMP. In each case the gain is shared between the input stage, where removing one resistor set it to unity gain without side effect, and the amplifier itself where changing the gain is both more complicated and quite dangerous for stability reasons. So in this case your mod may improve the SNR of the amplifier. If it does more it's frankly dangerous.

 

[Feyire]

1. I am not sure you understood the conclusions from Mr. Putzeys work on hysteresis distortion

Then please correct me as appropriate, what have I misunderstood?

I'll start with these two statements from the article:

• "Sporadic reports of a sort of “granularity” in the sound reproduction"
• "Putzeys finally found a culprit: hysteresis in the ferromagnetic material of the amplifier output filter coil."

So the inductor is the main source of a known audible flaw - specifically because of its iron core. Please let that statement sink in real deep for a minute, because it is literally an admission from the designer himself that the NCORE modules with iron core inductors have a known audible flaw - yep, that definitely hurts.

So given that, let's move on and see what we can do about it. Consider two logical ways to address this:

1. Remove the source the problem (i.e. prevent it from ever occurring in the first place)

There are a few benefits that I see in taking this approach. Firstly, pure copper inductors inherently don't have hysteresis issues (because the material is not ferromagnetic), and secondly, air core inductors also don't suffer from core losses like iron core inductors do. This solution also allows NCORE owners such as myself to address this problem, without having to go and purchase the new Purifi modules or other amplifier solutions. As a bonus, we get to have some DIY fun.

2. Apply measures to partially or fully mitigate the problem coming from the source (i.e. a partial or full cure)

This is the approach that Bruno suggests in the article. In other words, rather than removing the source of the problem, he has instead decided to make a new design which increases the loop gain/feedback in order to try and partially or even fully mitigate the problem and then release this solution as part of a new amplifier module.

However, please note in the article that his solution is not perfect: "...with nonlinearities in the audio band strongly suppressed at the expense of amplifying errors far outside the audio band". Ok, sounds good, but the garbage (i.e. non-linearities) is now moved from one spectrum of the signal to another. Yes, we can rejoice that they are no longer in the audio-band, but personally, I'd rather have that garbage not exist anywhere in the spectrum in the first place, wouldn't you agree?

So coming back to your question, are you suggesting that I don't understand the conclusions from Bruno, simply because I think that removing the source of the problem logically makes more sense than trying to work around it? It is of course great that his design has increased the feedback which also resulted in proven improved measurements, but why not go for both? (i.e. remove the iron core inductor AND enjoy the benefits the improved feedback design). That would logically make more sense to me, but I'd imagine this would probably come down to an issue of cost and availability, as copper is quite simply very expensive and air core inductors require more physical space on a PCB and also emit more EMI. Perhaps in the future Bruno will let us know why he specifically took the approach he did.

2. If you could point to any serious research paper on the subject, I would be glad to read it

I assume you are looking for peer-reviewed articles? I don't have any to refer to at this moment, but when I do, I'll add it here. If you like, you can take a look at this article for a brief overview. A very quick practical example of the noise reduction from correctly orientating the outer foil is demonstrated here.

3. I didn't know that Mr. Self was both a class D amplification and EMC specialist. Could you point to any of his work on the subject?

Certainly, I suggest you take a look at his book "Audio Power Amplifier Design - 6th Edition". He briefly discusses magnetic distortions in chapter 5 and then covers it in more depth in chapter 11.

4. I don't know the particular architecture of the NC400 DIY. I know the one from NC400 OEM, NC500, NC1200, NCxxxMP. In each case the gain is shared between the input stage, where removing one resistor set it to unity gain without side effect, and the amplifier itself where changing the gain is both more complicated and quite dangerous for stability reasons. So in this case your mod may improve the SNR of the amplifier. If it does more it's frankly dangerous.

Removing the resistor increases the feedback (so going by Bruno's conclusion, this mod will already help to mitigate the hysteresis issue) and improves the SNR. I'm not aware of any other side effect, other than the loss in gain of course.

Feel free to take a look at the application note from Hypex (which I link to in my previous post), where they document how you can adjust the gain for the NCORE modules. In my opinion, they wouldn't make this information publicly available if it was inherently dangerous to do so.

 

[Dave Zan]

Please also note that Hypex themselves have an application note on the topic...and how you can tweak it available here....

Ok, now there's a reference to read and it is a little different from what I understood from your first post, thanks for the clarification.
My comments referred to modifications to the power amp section because that is what I understood you planned
But R141 is in the input buffer rather than the power amp section.
So removal of the resistor will only affect the input buffer and should be safe - as you pointed out, Hypex would hardly have the note otherwise.
The corollary is that any improvement is likely to be small because only the input buffer is involved.
1. Any improvement in distortion in the whole unit (power amp + buffer) is likely to be very small because the input buffer distortion is already very low.
And you have moved the required gain from the input buffer of the power amp to the output of the pre-amp, you will need to be careful that this doesn't worsen the pre-amp performance more than any benefit from the improved power amp input buffer performance.
2. The lower gain may allow potential improvement in S/N from better gain structure if you have a pre-amp that can put out sufficient volts and is extremely quiet.
If so then yes, it's a possible improvement, it would be in line with the current trend to "hotter" pre-amps and lower sensitivity power amps.
I assume the trend is mainly for noise performance - I have very efficient (sensitive) horn loaded speakers and plan to do this in my own system for just this reason.
So I understand the attraction of the modification but unless you have very sensitive speakers or some special use case then I doubt it will be audible.
Similarly I suspect that the "audible" improvement you say you already hear is placebo.

There is a certain satisfaction to performance improvements even if they are essentially theoretical.
( I trim the aerodynamics of my race bike by fractions of a percent just to feel happy that it's as perfect as possible, so I understand!)
But don't expect to post claims that these improvements are audible without a backlash.
I say "backlash" because most of the posters in this forum are tired of unsubstantiated claims - so the response will be to undermine your own credibility, even for other issues where you may have valid points.
If you do have properly conducted tests (unbiased, with decent statistics) then post that by all means.
People here love ♥ real data.

Best wishes
David

 

[boXem]

Then please correct me as appropriate, what have I misunderstood?

I'll start with these two statements from the article:

• "Sporadic reports of a sort of “granularity” in the sound reproduction"
• "Putzeys finally found a culprit: hysteresis in the ferromagnetic material of the amplifier output filter coil."

So the inductor is the main source of a known audible flaw - specifically because of its iron core. Please let that statement sink in real deep for a minute, because it is literally an admission from the designer himself that the NCORE modules with iron core inductors have a known audible flaw - yep, that definitely hurts.

Wrong. Amplifiers with output filter outside of loop are susceptible to have an audible flaw. This is mitigated by UcD, Ncore, Eigentakt and others.

So given that, let's move on and see what we can do about it. Consider two logical ways to address this:

1. Remove the source the problem (i.e. prevent it from ever occurring in the first place)

There are a few benefits that I see in taking this approach. Firstly, pure copper inductors inherently don't have hysteresis issues (because the material is not ferromagnetic), and secondly, air core inductors also don't suffer from core losses like iron core inductors do. This solution also allows NCORE owners such as myself to address this problem, without having to go and purchase the new Purifi modules or other amplifier solutions. As a bonus, we get to have some DIY fun.

If it was that simple, why didn't Mr. Putzeys use air core inductors since UcD instead of spending 25 years of his life refining the concept of self ocsillating amplifiers with filter inside the feedback loop?

2. Apply measures to partially or fully mitigate the problem coming from the source (i.e. a partial or full cure)

This is the approach that Bruno suggests in the article. In other words, rather than removing the source of the problem, he has instead decided to make a new design which increases the loop gain/feedback in order to try and partially or even fully mitigate the problem and then release this solution as part of a new amplifier module.

It is not a new design. It is the same design supported by a full mathematical model allowing a much more extreme setup than Ncore that still has a hole in it's mathematical model.

However, please note in the article that his solution is not perfect: "...with nonlinearities in the audio band strongly suppressed at the expense of amplifying errors far outside the audio band". Ok, sounds good, but the garbage (i.e. non-linearities) is now moved from one spectrum of the signal to another. Yes, we can rejoice that they are no longer in the audio-band, but personally, I'd rather have that garbage not exist anywhere in the spectrum in the first place, wouldn't you agree?

This is the concept of noise shaping. You change almost nothing to the amount of crap outside the audio band by replacing the inductors. But you add a lot of other crap. See next.

So coming back to your question, are you suggesting that I don't understand the conclusions from Bruno, simply because I think that removing the source of the problem logically makes more sense than trying to work around it? It is of course great that his design has increased the feedback which also resulted in proven improved measurements, but why not go for both? (i.e. remove the iron core inductor AND enjoy the benefits the improved feedback design). That would logically make more sense to me, but I'd imagine this would probably come down to an issue of cost and availability, as copper is quite simply very expensive and air core inductors require more physical space on a PCB and also emit more EMI. Perhaps in the future Bruno will let us know why he specifically took the approach he did.

You got the whole point of using iron core inductors. The output filter is not here for audio, it's here for EMC. By wanting to solve a solved problem, you create a huge one.

I assume you are looking for peer-reviewed articles? I don't have any to refer to at this moment, but when I do, I'll add it here. If you like, you can take a look at this article for a brief overview. A very quick practical example of the noise reduction from correctly orientating the outer foil is demonstrated here.

So you hope to counterbalance the EMC mess you created with the air core inductors by properly orienting the capacitors? Interesting.

Certainly, I suggest you take a look at his book "Audio Power Amplifier Design - 6th Edition". He briefly discusses magnetic distortions in chapter 5 and then covers it in more depth in chapter 11.

Is there a chapter about Class D and EMC?

Removing the resistor increases the feedback (so going by Bruno's conclusion, this mod will already help to mitigate the hysteresis issue) and improves the SNR. I'm not aware of any other side effect, other than the loss in gain of course.

Feel free to take a look at the application note from Hypex (which I link to in my previous post), where they document how you can adjust the gain for the NCORE modules. In my opinion, they wouldn't make this information publicly available if it was inherently dangerous to do so.

I already told you. @Dave Zan just told you. It's not the loop gain that is modified, it's the input buffer gain. Again, if it was that easy, why didn't BP do it 25 years ago?
Since you found this folder, you can spend time reading the white papers about UcD and Ncore. Then spend time trying to understand the math behind these amplifiers. You can also try to get an UcD amplifier running in LTspice (I did it, it's feasible).
There is a presentation about all the class D challenges too.
It is very instructive. And brings a lot of humility.

 

[Feyire]

Ok, now there's a reference to read and it is a little different from what I understood from your first post, thanks for the clarification.

My comments referred to modifications to the power amp section because that is what I understood you planned
But R141 is in the input buffer rather than the power amp section.

My apologies. I agree that I was unclear in that regard, since I just generically referred to the "amplifier module" as a whole, rather than a specific section. I appreciate you taking the time to read the Hypex note and therefore clear up the misunderstanding.

So removal of the resistor will only affect the input buffer and should be safe - as you pointed out, Hypex would hardly have the note otherwise.

The corollary is that any improvement is likely to be small because only the input buffer is involved.
1. Any improvement in distortion in the whole unit (power amp + buffer) is likely to be very small because the input buffer distortion is already very low..

I also concur. The performance improvement will be small, but an improvement nonetheless. I'll psychologically feel better knowing I squeezed out just a little bit more performance than other NC400 owners without the mod .

And you have moved the required gain from the input buffer of the power amp to the output of the pre-amp, you will need to be careful that this doesn't worsen the pre-amp performance more than any benefit from the improved power amp input buffer performance.

2. The lower gain may allow potential improvement in S/N from better gain structure if you have a pre-amp that can put out sufficient volts and is extremely quiet.

If so then yes, it's a possible improvement, it would be in line with the current trend to "hotter" pre-amps and lower sensitivity power amps.

I agree with your points. In the case of my own system, I don't have a separate dedicated pre-amp. I'm going straight from the DAC (which has a volume control built into the chip) to the amplifier. My DAC, the Okto DAC8 Stereo has a 131dB (A-weighted) dynamic range, so well suited for this scenario.

I also agree with your warning in regards to someone who wants to use a dedicated pre-amp with this mod. I have read feedback from others who did the mod, state that it wasn't a success for them, in terms of the sound with the pre-amp in the chain being audibly worse when compared to being directly from DAC to amplifier. As you allude to, a likely reason for this issue could be a weak pre-amp with poor noise performance.

I assume the trend is mainly for noise performance - I have very efficient (sensitive) horn loaded speakers and plan to do this in my own system for just this reason.

Do you also have the NC400s? If so, and if you end up trying this mod, then I look forward to you sharing your experience here.

So I understand the attraction of the modification but unless you have very sensitive speakers or some special use case then I doubt it will be audible.

Similarly I suspect that the "audible" improvement you say you already hear is placebo. But don't expect to post claims that these improvements are audible without a backlash.

I say "backlash" because most of the posters in this forum are tired of unsubstantiated claims - so the response will be to undermine your own credibility, even for other issues where you may have valid points.

People here love ♥ real data.

No worries - I understand what ASR is about. All my purchases so far, apart from my speakers, have been due to ASR reviews, so yes, I certainly appreciate the objective data as well, but the subjective experience also has its place.

As I stated previously, even though I don't have any precision analyser equipment to provide measured proof of any improvement, if someone is interested enough and willing to come by with their analyser and measure my amplifiers with the mods, then I'd be more than happy to accommodate this.

 

[Dave Zan]

....Do you also have the NC400s?...

No, when I said "I plan to do this" I referred to the trend towards "hotter" pre amps and lower sensitivity amps.
I have my own circuit worked out for the amplifiers - a hybrid with switch modulated power rails for efficiency but an ultra low distortion class A/B+
output.
I have simulated it with the most accurate models that I know and it looks excellent - efficient and ultra low noise.
Theoretically it should surpass the PuRiFi for SINAD at most power levels.
Whether I can achieve this in practice remains to be seen!
I still haven't decided the exact sensitivity.
Your Okto DAC8 stereo is what level output?
The standard 4 V RMS won't fully drive the NC400 with R141 removed I think.

As I stated previously, even though I don't have any precision analyser equipment to provide measured proof of any improvement...

I think the reaction you received is not so much about the lack of precision measurements.
It was the claim of audible improvements.
You don't need an analyser for this, just an ABX test box and repeatable, statistically valid comparisons.
Modify one amp module, do level matched comparisons with the other.
If you can repeatably pick the difference then you can impress everyone with your improvements.

Best wishes
David

 

[Feyire]

I have my own circuit worked out for the amplifiers - a hybrid with switch modulated power rails for efficiency but an ultra low distortion class A/B+ output.

I have simulated it with the most accurate models that I know and it looks excellent - efficient and ultra low noise.
Theoretically it should surpass the PuRiFi for SINAD at most power levels.

I certainly hope your circuit reaches the performance levels you are anticipating, and given the claims, I'm sure people will want to see it measured at ASR.

Your Okto DAC8 stereo is what level output?
The standard 4 V RMS won't fully drive the NC400 with R141 removed I think.

Okto's standard is 4.2V - Amir's sample also measured 4.2V at max output, but strangely enough Okto's website says 4.1V is the standard. Anyway, mine should be 4.2V.

In my current setup, my DAC is set to between -20dB and -40dB at my typical listening levels, depending on source material of course.

So referring back to Hypex's notes on voltage gain, gain is 4.17*(1+2*Rf/Rg). Rg=1.2k, Rf=2.2k. Rg is R141 and it's the one furthest to the left of the input power connector. The max gain reduction you can get is 13.4dB, which is achieved when you remove R141 completely.

So if I completely remove R141, which is the plan, I can bring the output level of my DAC closer to its optimal performance envelope (i.e. closer to 0dB), while still giving me some headroom to play with. If you look above, I have at least 20dB headroom right now, which I don't really need.

Highlighted in red below is where R141 is located:

If you can repeatably pick the difference then you can impress everyone with your improvements.

Perhaps one day, a curious stock owner of Hypex's NC400 DIY kit will come visit, and I'll gladly audibly compare it with them.

 

[Feyire]

In the near future, I also plan on attempting the inductor mod and the gain resistor mod. I plan to report back to this thread with my experiences of both.

I'm happy to report that I have finished all my mod work. Here's an in-progress picture after the original inductor was removed:

You can also see the stacked and modded WIMA capacitors in red. The output cabling (solid core) is also new and directly soldered to the board, instead of being screwed down on the inside of the original brass output bridge.

Here's a shot of the underside. The 4 point output bridge was removed on the top side, so I now use the top side to create a vertical bridge, while the bottom side has a horizontal bridge (thus connecting all 4 output points again):

Since I'm sure some of you will be quick to point out this note from the Hypex NC400 sheet:

It is very important to use twisted pairs for the output cabling, at the very least anywhere near the PSU cabling. The distortion of the NC400 is so low that it is easily exceeded by magnetic coupling between the supply wiring and the audio wiring.

Note the way I have orientated the output cables, they are no longer in close proximity to the PSU cabling, and and as such don't need to be twisted together anymore.

With the new air core inductor installed (yeah, that's right, it's absolutely massive and no, I haven't experienced any EMI issues):

R141 (top left) removed to lower the gain, which allows me to bring my DAC closer to optimal performance (i.e. 0dB output):

So you are probably wondering, was all the effort worth it and does it now actually sound better? Absolutely. I'm finally happy with the sound I'm hearing - The true NCORE potential revealed

 

[Matias]

If you were using digital volume before heavily, like -30 dB, then surely removing the resistor and lowering the buffer gain will bring big improvements: you would be using less digital attenuation and reducing noise floor a lot. Makes a lot of difference in my experience. But then again a simple in line attenuator or full analog preamp would do the same.

If you were not using digital volume then forget all this.

 

[Feyire]

If you were using digital volume before heavily, like -30 dB, then surely removing the resistor and lowering the buffer gain will bring big improvements: you would be using less digital attenuation and reducing noise floor a lot. Makes a lot of difference in my experience.

This was indeed case for me, as I was using the digital volume control in my DAC and typical listening levels meant attenuating between -20dB to -40dB.

But then again a simple in line attenuator or full analog preamp would do the same.

Perhaps, but that is unnecessary in my case, as I already have a DAC with an exceptionally low noise floor. You'd need an equally reference level pre-amp to match the objective performance and I'd rather spend my money elsewhere (such as for mods! ).

While on the topic of mods and for those interested, another quick and easy one is to upgrade any fuses. In the case of the NC400 DIY kit, there is 2 fuses, 1 in the IEC connector and 1 in the power supply. Note that in my case, I replaced the stock IEC connector and the new one does not have a fuse anymore.

In the image below, I replaced the stock ceramic fuse (8A slow blow) in the SMPS600N400 power supplies with an equivalent one from HiFi-Tuning (in my case the Supreme 3 Silver Fuse). Note that this fuse is directional, meaning that the arrow on the body of the fuse needs to point in the direction of conventional current flow as shown below. If you place it the other way around, it just won't sound right (strange eh?).

Safety note: Replacing a fuse can be dangerous if you don't know what you are doing. Always ensure that you disconnect all power from your amplifier (i.e. Don't just use the off switch if there is one), but completely unplug all power cables and let the module rest/cool off if it had been switched on previously, before attempting to do anything. As previously stated for the other mods, attempt at your own risk and be aware that your warranty may be void.