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Pros/Cons Of Neurochrome 686 vs Purifi EVAL1 Builds

concorde1

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What are some pros and cons of building a balanced stereo amplifier with Neurochrome Modulus-686 modules vs Purifi EVAL1?

The EVAL1/SMPS build is a lot cheaper. The Modulus-686 is Class AB so presumably less efficient. I believe the parts for Modulus-686 amp use better capacitors than the EVAL1 build - by default, anyway.

Anything else?

I don't know which completed amp would measure better? Which is closer to Benchmark AHB2? (I realise differences will be inaudible.)
 

somebodyelse

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Tom publishes fairly comprehensive measurements (see the 'Performance graphs' tab on the product page) so you can compare with the measurements of the other amps here yourself. You can use the modulus 286 review to see how measurements on each site compare.

You need to add the Guardian-686 to get comparable protection to that built into the Purifi, and the Modulus will need more expensive heatsinking than the Purifi.
 

tomchr

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The Modulus-686 is a balanced design. The Purifi 1ET400A is not. Neither is the Hypex NC500. So if that's an absolute requirement, the choice is easy. :) They all have balanced inputs though. Perhaps that's what you're after.

The Modulus-686 is indeed a Class AB design. Theoretically Class AB should get you up to 75% efficiency (vs 94-95% for the Purifi). This means the Modulus-686 will require a sizeable heat sink, whereas the Purifi can get away with a much smaller heat sink. So if efficiency is your ultimate requirement, the choice is also easy.

The AHB-2 has more in common with the Modulus-686 than with any Class D amp.

The Benchmark AHB-2 is a Class H design. Class H and Class G are fundamentally Class AB output stages but they have some tricks applied to lower the dissipated power. They both use multiple power rails. Class G effectively powers the output stage from the lower voltage rails when low power is needed and from the higher voltage rails when high power is needed. Class H bootstraps the power supply for the output stage such that the supply voltage tracks the output voltage once the output swing exceeds the lower voltage rails. Class AB, Class G, and Class H are all linear designs (i.e. not switching like Class D).

Tom
 

fluid

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I have got both the Eval 1 and a MOD686. The EVAL 1 is still in pieces so I cannot offer any comparative opinion.

What I can say is that the MOD686 is a really great amplifier. There is nothing that I dislike about it and I haven't had an amplifier that I could say that about before, the bass being particularly good. They are very different from each other but the measured performance is quite close.

The MOD686 is a lot more powerful so in any price comparison that has to be considered as well as the use case to determine if the extra power is needed.

The cases are night and day in size. The MOD686 is in a 5U Modushop chassis and for the Eval 1 I have a pretty small Aluminium box much like the Ghent case. The 5U might seem overkill but it still gets pretty warm in use, don't underestimate the need for heatsinking.

I got stung with customs charges (AUS) on the Purifi which did spoil the value in the end. Purifi and Hypex both charge very high shipping so take that into consideration too. Sound Imports EU is much cheaper for shipping the Hypex supplies.

The MOD686 does not have to be built with a super expensive power supply or the other boards Tom makes unless you want to. The performance doesn't really change as the amplifier has very good PSRR. I used some power supplies I bought direct from a Chinese manufacturer. They are very good and very cheap for the quality.

I can't see how anyone could be disappointed with a MOD686 if it is within their budget and ability to make. The power of the Purifi is more limited much like the Benchmark and for some that has been an issue.
 

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concorde1

concorde1

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I think I'm going with the Modulus-686 build. I plan to have my own case fabricated from sheet aluminum I will buy, then get it modified to fit the amp components. It seems like the CAD file supplied on the Neurochrome website is only applicable to the Modushop 4U, which I think is out of my budget considering shipping, so I will have to design my own case more or less from scratch.
 

fluid

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Curious why you have both?
Curiosity to see if Bruno has been able to get rid of the hardness that caused me to dislike the UcD amplifiers in the long term in the hope that it would serve well in an active system.
 

Feyire

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Curiosity to see if Bruno has been able to get rid of the hardness that caused me to dislike the UcD amplifiers in the long term in the hope that it would serve well in an active system.
I'd really like to hear your thoughts on the Purifi EVAL1, once you get it up and running and put in some listening time, especially when sonically compared to the 686. I wouldn't be surprised if the 686 sounded subjectively better.

I'll also chime in and say that I'm happy with my two NC400 mono blocks, but only after some extensive modding. It was the only way I could get them to sound satisfactory to me, as there was always this harshness/graininess in the sound which gets annoying over time and which Bruno himself acknowledged was an audible problem that affected all Ncores due to hysteresis caused by the iron core inductor used in the LC output filter. Without the mods to fix this, I would have sold mine.
 
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concorde1

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I have gone with the Modulus-686 option! :)
I'm buying something at this level of performance because I like the idea of supporting cutting edge, and my speakers are hard to drive so an AV receiver will not do.

If best case hearing threshold is -116 dB SPL, then to my understanding even the AHB2 is not transparent, because THD+Noise comes to -113 dB. It does seem to be the best technology available though. And +/- 3 dB SPL seems to be a barely audible difference in volume from what I've read.

The EVAL1 / Mod-686 presumably have similar measurements, so unless there is a big difference in THD+Noise I don't know why you'd hear a difference between them.

I don't even know if I could hear the difference between a cheap AV receiver and this awesome thing I'm going to make.

I'm not an engineer so I'm sharing my ideas without any conviction.
 

fluid

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I have gone with the Modulus-686 option! :)
Nice, I hope it all works out :)

If best case hearing threshold is -116 dB SPL, then to my understanding even the AHB2 is not transparent, because THD+Noise comes to -113 dB. It does seem to be the best technology available though. And +/- 3 dB SPL seems to be a barely audible difference in volume from what I've read.

The EVAL1 / Mod-686 presumably have similar measurements, so unless there is a big difference in THD+Noise I don't know why you'd hear a difference between them.

The standard set of measurements are very useful in designing and testing amplifiers. They are less useful at telling you whether you will like that amplifier with the speakers that you will use them with.

THD has almost no correlation to the perception of distortion or sound quality assessment. Noise is different as audible white noise at the listening position is quite annoying.

There is a post here which I think is quite well written and makes a sensible argument.
https://www.audiosciencereview.com/...w-amplifiers-to-sound-different.23/post-42125

Most equipment that I have disliked or replaced only showed itself to be that way after an extended period of time listening to all different types and styles of music. Adaption would say that I would just get used to it (and I have experienced that too) but some things I have just found hard to put my finger on exactly what it is I don't like about it, but I know that it does not eventually sound right.
 

fluid

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I think you have got a digit wrong there those are 800VA versions the recommendation is for the 400VA 4225

"The recommended power supply for the Modulus-686 is ±36 V with at least 10 A (RMS) available per channel. Thus, for a traditional unregulated power supply, I recommend a Power-686 with a 2×24 VAC or 2×25 VAC, 350-500 VA power transformer. The Antek AS-4225"

Check the shipping on those transformers as it is very expensive. Toroidy in Poland makes some really excellent transformers and their shipping prices are more reasonable but any transformer that is not local will cost more to ship than to buy the transformer.

This one from RS looks suitable too. I have used a smaller version of this and it is good.
https://au.rs-online.com/web/p/toroidal-transformers/1234031/
 
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concorde1

concorde1

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No, I definitely have it right, it is the 8225. I believe the 4225 is for a mono build using 1x Modulus-686. (This info might not be on the website.) I may be best to just ask Neurochrome. I'm not super comfortable getting a transformer unless it's on the recommended list because I'm not savvy with specifications of electronic components.


EDIT: I will find out if 8425 is suitable.

Assuming Antek 8425, or Hammond 1182U24 are suitable (second one definitely is), what might be some more local (RS Online etc etc) alternatives? <----------- my country. Thanks.
 
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tomchr

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The Antek AN-8425 has four windings of 25 V. The AN-8225 has two windings of 25 V. They're both 800 VA transformers. Either can be used with the Power-686 (and Modulus-686).

If you're using a single Power-686 the AN-8225 would be easier to work with. With the AN-8425 and a single Power-686, you would have to wrestle two wires into each terminal of the Power-686 input. Depending on which wire gauge Antek is using that could require that you solder the wires together to a single wire and then connect this single wire to the Power-686 input. I would think that there's enough room for both wires in one terminal, though.

On the other hand, if you're going for a dual-mono build and use two Power-686 boards, the AN-8425 is actually preferred. You simply connect two of the 25 V windings to one Power-686 and the other two to the second Power-686.

Some may argue that using a single transformer in a dual-mono build is not True Dual-Mono™. There's some validity to that argument, but any difference between a two-transformer dual-mono and a single-transformer, four-winding, dual-mono will be at the dBGF level. That's dB referenced to one imperial gnat fart. :) Four windings on one core will couple more than four windings on two cores, but the two cores still couple though their primaries. The Modulus-686 is not very sensitive to the power supply, so I always encourage folks to build with a single four-winding transformer if one is available. It makes the chassis design and layout much friendlier.

Regardless of which configuration you use, do make sure that you get the phases connected correctly to the Power-686. I write about that in the Power-686 design doc. Also note that Antek tins the wire ends. I recommend that you cut that off and strip the wire so you have un-tinned stranded wires into the terminal blocks. Solder compresses in the terminals and over time the connections will become loose if you use the tinned ends.

Tom
 
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