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miniDSP SHD Power & Hypex NC400 Class D Amps


Langston

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7 minutes ago, richieb said:


- and as I recall the nCore nc400’s were a very close second? 

The Benchmark wasn't out then, nor was there any sort of Data on the NC-400 about 6 years ago when I took a chance and bought mine. Never regretted it.

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16 minutes ago, ClaudeJ1 said:

The Benchmark wasn't out then, nor was there any sort of Data on the NC-400 about 6 years ago when I took a chance and bought mine. Never regretted it.

I bought the NC1200 based on your recommendation. I love it and have no plans to upgrade or try anything else.  Thanks for the tip. 

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54 minutes ago, ClaudeJ1 said:

The Benchmark wasn't out then, nor was there any sort of Data on the NC-400 about 6 years ago when I took a chance and bought mine. Never regretted it.


I have the nc400 monos and a pair of FW F3’s used with passive Jubes and/or LS1’s. I can easily live with either 

and do rotate them when the mood strikes. Currently nCores with LS + sub. While not a Jube performance I really like 

this combination. Overkill, yes but provides the LS with added oomph on the bottom. 
Running the F3 as monoblocks brings their 15 watts to a whole ‘nuther level of bass response, pronounced midrange and 

much more authority a soon the entire band. F3’s are had to come by - grab one if you can. A tremendous sounding SS amp that so closely mimics single ended tubes. I read where Pass said this remains one of his favorite FW designs and at the time one the finest sounding SS amps in existence. 

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7 hours ago, richieb said:


I have the nc400 monos and a pair of FW F3’s used with passive Jubes and/or LS1’s. I can easily live with either 

and do rotate them when the mood strikes. Currently nCores with LS + sub. While not a Jube performance I really like 

this combination. Overkill, yes but provides the LS with added oomph on the bottom. 
Running the F3 as monoblocks brings their 15 watts to a whole ‘nuther level of bass response, pronounced midrange and 

much more authority a soon the entire band. F3’s are had to come by - grab one if you can. A tremendous sounding SS amp that so closely mimics single ended tubes. I read where Pass said this remains one of his favorite FW designs and at the time one the finest sounding SS amps in existence. 

The F3 is one of the best sounding SS amps I've ever owned. But in my quest for efficiency (and not a Space Heater I can use to cook my Eggs on) I had to cut it loose. That is my ONLY complaint about Class A. Obviously others don't mind that, which is OK by me if it works in their system as they like.

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5 hours ago, ClaudeJ1 said:

The F3 is one of the best sounding SS amps I've ever owned. But in my quest for efficiency (and not a Space Heater I can use to cook my Eggs on) I had to cut it loose. That is my ONLY complaint about Class A. Obviously others don't mind that, which is OK by me if it works in their system as they like.


No doubt they do run “warm”. But hey, it’s Sunday morning and bacon and eggs seems appropriate. Bacon on the right channel, eggs on the left. And Miles Davis providing the “heat”. It’s very Zen like — 

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I've finished the measurements unless (we) think of something else. This is the most interesting amp measurement session I've ever had, and I've measured dozens of these things over the last few decades, though mostly professional stuff.

 

I haven't listened to the NC400 yet and to tell you the truth, I'm not looking forward to it. Why? Because it measures, overall, about the same or better (!) than the AHB2. In my experience, these are the two best measuring audio amplifiers in the world. I'm just worried the NC400 isn't gonna sound good, as 100% of my experience to date with Class D amps has been. If it doesn't sound good it's going to show me that I'm no closer to correlating amp measurement to the listening experience than I was 30 years ago!

 

Let's start with the easy stuff.

 

The Bench

475259618_AmpsonBench.thumb.jpg.a493d5cdbf5cc22729b882c35e2f514b.jpg

 

The Power Dedicated 120/240/30A. Nope - I haven't messed up YET. The NC400 negotiates world voltages automatically, the AHB2 has to be ordered to spec. Mine are 120V.

208258718_AmpPower.thumb.jpg.3ea3508a2459a349f331f9bd4ee19f38.jpg

 

Single frequency power into a resistor at 120V and 240V. 20Hz and 50Hz were at 3% THD, 1kHz and 5kHz were at 1% THD. The AHB2 performs to spec, thus isn't included.

 

NC400 120V 4Ω 20Hz

2142703739_NC400120V4Ohms20Hz.png.cef339c64b8125d1893b76f2dd255dac.png

 

NC400 240V 4Ω 20Hz

938801903_NC400240V4Ohms20Hz.png.509cd43cbff14fc0dd888aaf549c22cf.png

 

NC400 120V 4Ω 50Hz

1273098612_NC400120V4Ohms50Hz.png.337291a51c563ea15813f1bd2bece3c7.png

 

NC400 240V 4Ω 50Hz

1112574212_NC400240V4Ohms50Hz.png.fdc3099cb6b26587e0d05fdf07e38dfc.png

 

NC400 120V 4Ω 1kHz

1501707438_NC400120V4Ohms1kHz.png.3a5c2f2e6c002476986cf6692a358cf4.png

 

NC400 240V 4Ω 1kHz

648803241_NC400240V4Ohms1kHz.png.a48cad9c8373585f4c116c621eca6438.png

 

NC400 120V 4Ω 5kHz

2097580034_NC400120V4Ohms5kHz.png.c1ad934bfeba8d0e97c3aa922edb1f43.png

 

NC400 240V 4Ω 5kHz

907134726_NC400240V4Ohms5kHz.png.d2293332aeca057f27ba92b9b9f93c63.png

 

Finally, let's channel a little Nelson Pass and look at the 1kHz 1% THD sine with residual amplified 10x for both amplifiers. And then let's look at the same at 1dB lower than these clipping levels.

 

AHB2 1% THD 1kHz

1059615489_AHB21THD1kHzScope.png.2832e2dc7d6343169941ee573fd6f8b7.png

 

AHB2 1dB under clipping

1787132576_AHB21THD-1dB1kHzScope.png.5837d20bf8914d2e733ee4b62308bae9.png

 

NC400 1% THD 1kHz

180902831_NC4001THD1kHzScope.png.0e3249ac6da2253bfb128d5bb1e20359.png

 

NC400 1dB under clipping

1471460685_NC4001THD-1dB1kHzScope.png.d83f292056d60e299bfe6c5382e09ea1.png

 

Much more interesting stuff to come..

 

God bless you and your precious family - Langston

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5 minutes ago, ClaudeJ1 said:

And so the mystery shall remain and be resolved by the Ears some other day!

 

I love surprises, but I'm getting antsy too, maybe tonight, but first more useless trivia! : )

 

Another easy one - gain comparison with the AHB2 switched to low-gain:

 

1480433351_GainCompare.thumb.png.69f70a8f8f4e750ec3668f2e9536e72c.png


Now for something interesting, an output magnitude comparison using the 4Ω resistor and the 4Ω nominal moving coil loudspeaker load simulator. The resistor is of course a flat line at 4Ω, but the simulator has the following impedance:

 

1855324991_LoudspeakerLoadSimulator.thumb.png.7dbe48b8363660b530d89773f653e45b.png

 

The AHB2 has a very low output impedance which minimizes changes in the magnitude response of the loudspeakers it drives down to inaudible levels. The NC400 takes this much further and behaves almost like a short circuit. This is achieved by its ground referenced (non-bridging) design, simple circuit path (more on this in a bit) and lots of feedback.

 

234052124_OutputZCompare.thumb.png.dce19cbc26435f814c49bd500eda0e7a.png

 

Something else the NC400 stands alone with in the realm of Class D amplifiers is an extremely low and well behaved noise floor with great suppression of the SMPS power line fundamental and harmonics. The AHB2 completely suppresses SMPS power line noise, which I have yet to see elsewhere.

 

2095403176_FFTNoiseSpectrumCompare.thumb.png.3c09111853d021e45db76dcd4a142cec.png

 

Now we're getting close to an the most surprising thing I learned about the NC400, and a hint is provided in the following phase vs. group delay plot. The blue traces of the NC400 show evidence of the benign 65kHz noise peak seen above, but notice the absolutely flat LF group delay. That's impossible UNLESS..

 

469126772_PhaseGDCompare.thumb.png.aca0e422dbbc2321fd177e50172fb9ce.png

 

..UNLESS this thing is DC coupled from input to output - which it is! This amp just became my first choice for powering low frequencies. The following shows DC amplification at precisely the same gain as AC.

 

1545686611_NC400DCAmplification.thumb.png.6e4c028b2020466c85606f2408aabb36.png

 

You are going to have to be careful not to feed this thing DC from a pre-amp, etc., but it would probably have to be broken to do that. Folks with loudspeakers without passive crossovers are going to have to take the most care because there's no capacitor to interrupt DC going to their delicate HF driver(s). I restored an old Crown DC300 to use on my bench to test woofers for rub & buzz and offset distortion experiments. One of the hassles with rub & buzz testing is the noise generated by the portion of the test that takes the driver near Xmax. What you can do with a DC amplifier is offset the driver near Xmax and then drive it with a much smaller signal AC test stimulus, then repeat using the opposite offset.

 

Which is what's gonna happen to your drivers without capacitors in series with them if you feed the NC400 with a mix of AC (music) and DC from a downstream component. Best case will be distortion, worst case.. Well you know.

 

A DC amplifier like this is simpler to design, requires fewer components (such as capacitors), and has the best LF transfer function in town. Both my NC400 amps are completely DC-free on their outputs when feeding them AC, so there's some fancy servo (feedback) going on I'm sure.

 

The NC400 runs stone cold into low loads at high levels. It's quite the reverse of Class A. Fascinating amp. Wonder what it sounds like? : )

 

One more set of measurements to come.

 

God bless you and your precious family - Langston

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5 minutes ago, Langston said:

The NC400 runs stone cold into low loads at high levels. It's quite the reverse of Class A. Fascinating amp. Wonder what it sounds like? : )

You're killing me man! Can't wait for you to hear it. Mine have never been turned off but once when I moved, over 6 years running, since I use a battery backup for brownouts. LOL.

 

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Last set of measurements..

 

I don't like the shape and pattern of the THD I'm seeing here from the NC400 at approx. 1 watt, but it's so far under the probable threshold of audibility that I'm stretching things to complain.

 

345711724_1WattTHDCompare.thumb.png.d7c3695ea8f8702bbca3d06e1592463d.png

 

Classic 1kHz S/N at full throttle (1% THD) for both amps into 4Ω.

 

AHB2

1404352969_AHB24OhmsMaxSN.PNG.dc74386ec836effa56da92e4ae4af86a.PNG


NC400

1193847300_NC4004OhmsMaxSN.PNG.5f89864fd73ef70abc619cda29c66df6.PNG


I selected two swept twin-tone IMD measurements for the amps and the NC400 is neck and neck with the amazing AHB2, both at 1 watt output.

 

First up is DFD, or difference frequency distortion measurement that begins at mean (average) frequency of 20kHz with twin-tones 40Hz below and above that mean. Thus measurement #1 is actually equal levels (1 watt) of 19,960Hz and 20,040Hz. Distortion components to the 5th harmonic are recorded and plotted. This is repeated for 31 logarithmic steps down to a mean frequency of 250Hz.

 

1214393745_IMDDFDCompare.thumb.png.05b85b653b755f579958a7c4ed356175.png

 

The second IMD measurement is classic SMPTE, but with the addition of a swept modulation frequency. Normally twin-tone SMPTE distortion measurements use 40Hz at a 4x (12dB) higher level than the 7kHz frequency of interest and a single value is calculated. In the following the same method applies, except that the offending (modulation) frequency begins at 1kHz and logarithmically steps down 31 times to 40Hz. Again at 1 watt. Much more revealing, just as the swept DFD measurement is much more revealing that the typical 19kHz/20kHz twin-tone.

 

1275170243_IMDSMPTECompare.thumb.png.c80fc191b20050cd8ca8118128313b3c.png


Finally, the multitone FFT plot has the NC400 as the winner. Fascinating amp!

 

2071762744_MultitoneCompare.thumb.png.f685c91438e58890460c72abf75ac214.png

 

God bless you and your precious family - Langston

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12 hours ago, Langston said:

 

You are going to have to be careful not to feed this thing DC from a pre-amp, etc., but it would probably have to be broken to do that. Folks with loudspeakers without passive crossovers are going to have to take the most care because there's no capacitor to interrupt DC going to their delicate HF driver(s). I restored an old Crown DC300 to use on my bench to test woofers for rub & buzz and offset distortion experiments. One of the hassles with rub & buzz testing is the noise generated by the portion of the test that takes the driver near Xmax. What you can do with a DC amplifier is offset the driver near Xmax and then drive it with a much smaller signal AC test stimulus, then repeat using the opposite offset.

 

Which is what's gonna happen to your drivers without capacitors in series with them if you feed the NC400 with a mix of AC (music) and DC from a downstream component. Best case will be distortion, worst case.. Well you know.

 

Please explain how not to feed NC400 with DC from a pre-amp.

What to watch for, where and how to find parameters to avoid the worse?

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7 hours ago, Langston said:

First up is DFD, or difference frequency distortion measurement that begins at mean (average) frequency of 20kHz with twin-tones 40Hz below and above that mean.

Paul Klipsch always maintained that THD was more benign than Intermodulation Distortion, which is likely about 1,000 Times higher in Loudspeakers (yes even horns) than in Modern Amplifiers! He claimed it was more audible and at a certain level, Irritating! So all the levels shown are so LOW that, claims of audibility would be questionable.

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3 hours ago, parlophone1 said:

What to watch for, where and how to find parameters to avoid the worse?

 

Good question - I measure everything before using it and rarely see DC offsets on the output of line-level audio components exceeding about 3mV. The rule of thumb is that anything under 50mV is acceptable, under 20mV is good and under 10mV is great - but based on my experience I'd fix it if it were above 5mV. Just my opinion - I'm no expert.

 

You'd make the DC measurement between center and ring on an RCA and between pins 2 and 3 on an XLR output. Give the component 5 minutes to warm up, disconnect anything from its inputs and if it has a gain knob mfg's say to leave it at minimum, but I sweep it through its range. When you sweep the gain you need to let it stay put for a few seconds to let the circuit settle.

 

The NC400 compares the DC on its inputs to its outputs. If it sees DC on the output when it's not being fed to the input, it'll fault and go into protect mode. If it sees DC on its input, it'll obediently amplify it along with AC, if any.

 

One story from my concert production days - I had a friend who ran another company that was using the same EAW line array at the time that I was. EAW released updated firmware and presets for the UX8800 processor (included FIR processing for the mids and highs), and I of course measured the thing to study the update differences prior to feeding my amp racks with it. My friend just did the upgrade and took out (48) high frequency drivers at the beginning of a show. EAW replaced them all for free, but man what a hassle.

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Warning! Bogus subjectivity ahead! : )

 

I finally gave the amps a listen and I don't hate Class D anymore. I ran through my usual test songs and the first thing that hit me was that I could hear everything. "Wow, that's good, I didn't think Class D could do that" was my first response.

 

My Klipschorns are sounding very good with the Crites cast frame woofers and the Dave Harris Tractrix horn and B&C DE750 in a two-way system. I'm not finished tinkering, but these things are so good that they've allowed me to do other things for a bit, such as mess around with amps. The loudspeakers are sealed against their corners and the ceilings are 10'. This isn't the end-game room for them, but it's the most convenient for experimenting.

 

Klipschorns

Klipschorns.thumb.jpg.83eb8bdb5a91aaf6b72aab4ac58ad9ce.jpg

 

NC400 Setup

556905317_AHB2NC400Rack.thumb.jpg.b56aafceadde9f895a37a1b92c545fe3.jpg

 

AHB2 Setup

1619376951_AHB2Rack.thumb.jpg.be45a02da048c729287bdeaa3181ef11.jpg

 

After a while of listening, "dryness" became my primary impression, everything was there but it wasn't pulling me into the music like usual. Maybe I'm just tired or something I thought, so it was time to switch the AHB2 back to the horns. The moment I started playing music again it was over. The AHB2 brought back the liquid, not more detail, but the smooth spread of the recording environment was filling the space between the loudspeakers again with increased depth and pulling me into the music. The difference was pretty significant, though I could easily see myself enjoying the NC400's if I hadn't heard the AHB2. The NC400 reminds me in a way of my MC275 tube amps, just the opposite end of the spectrum. The MC275 is too liquid and too soft, the detail is there but too laid back for my taste. Whatever that means. : )

 

I have a huge, long-term, personal bias against Class D amps and this is a big change for me toward respecting the technology. For now I'm going to use them for LF.

 

Then I had an idea, why not arrange a way for you guys to hear the difference for yourselves? In my next post you'll get access to (3) music files:

 

1. The original 96k/24bit test track.
2. The same recorded from the output of the AHB2.
3. The same recorded from the output of the NC400.

 

God bless you and your precious family - Langston

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Here's what I did:

 

1. I chose a single, well-recorded 96k/24bit track freely available from HDTracks in their 2020 Sampler for the test.

 

2. I played it back with no EQ, etc., from a PC using Roon playback software through a DAC3 (USB connection) and into the amps while they were connected to a pair of unmodified Heresy II's. I matched the output of each amp within 0.01dB using a -1dBFS swept sine at a voltage equivalent to 1 watt into 4Ω. This resulted in the test song RMS being closer to 0.1 watt during playback, which sounded about right for a critical listening volume.

 

3. I simultaneously recorded the output of each amp loaded by the Heresy's with the Audio Precision analyzer set for 96k/24bit at a fixed input gain. Given the effort they made to perfect the audio interface, it makes a phenomenal recording platform.

 

4. Finally I recorded the original test song at the same level, but directly out of the DAC3 to allow you to hear it without the amplifiers in the circuit.

 

Recording Setup Block Diagram

934459268_AmpTestBlockDiagram.thumb.jpg.c463b3b60bb8a3324771999030a10c35.jpg

 

Recording Setup Front

1129772119_RecordFront.thumb.jpg.92f014ec6c7420eb3bf4955fa46a3ef2.jpg

 

Recording Setup Rear

1993108961_RecordRear.thumb.jpg.57211ea28055aac1f67a1db2de9c0b72.jpg

 

Music File Downloads

 

1. Original Song (DAC3 Output without Amp)
2. AHB2 Song Output
3. NC400 Song Output

 

God bless you and your precious family - Langston

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On 8/9/2021 at 9:05 PM, Langston said:

The difference was pretty significant, though I could easily see myself enjoying the NC400's if I hadn't heard the AHB2. The NC400 reminds me in a way of my MC275 tube amps, just the opposite end of the spectrum. The MC275 is too liquid and too soft, the detail is there but too laid back for my taste. Whatever that means. : )

 

I have a huge, long-term, personal bias against Class D amps and this is a big change for me toward respecting the technology. For now I'm going to use them for LF.

Then I had an idea, why not arrange a way for you guys to hear the difference for yourselves? In my next post you'll get access to (3) music files:

Well then, mission accomplished by you with positive conclusions.

 

As long as one is willing to spend 8X the cost per Watt of the NC-400 Mono Blocks, you can have a Wet Sound vs. a Dry One.

 

Since they achieve their respective signal paths in 2 Radically different Transistor Designs,  measure so close to each other, the difference is in the listening, the final arbiter of all things Audio.

Bruno Putzey's "Neither Dirt nor Fairy Dust" is an accurate assessment of his creation for marketing purposes.

 

In line with that philosophical conclusion, one could look at it going the other way for NC-400 owners. You already have the best VALUE in amplifiers, geting 6 dbWatts more Output, then stick to what you have.

 

The only other SS Amplifier in the World,  with 6 dbWatts Less output (who cares about watts if you own Khorns or Equivalent anyhow) that sounds better, then upgrade to the Benchmark if you can afford it.

 

For those like ChrisA, who are lucky enough to buy a limited production First Watt F3 for $3,000, the cost of more power (if needed) is the same with the Benchmark with more power than the F3, with only the Ears as the final judge of "better or worse than what I already have."

 

Your objective and subjective exercise here supports my original point about ChrisA's High Resolution speaker setup and the 4 ears he had available to assess his own comparison.

 

What you and they heard was obviously not more distortion, but a less EUPHORIC experience that can only be determined by the 3 Dimensional ear/brain mechanism vs. 2 dimensional measuring equipment.

 

This ultimate arbiter of sound allowed y'all to hear that "something was missing" compared was already lived with over a longer period of time and hearing acclamation.

 

Same conclusion coming from different places and people.

 

Thanks for the thorough evaluation performed here. Good work!

 

I'm also glad you left your Bias behind, since, according to your earlier post, I was the catalyst that prompted this whole exercise to begin with. I wasn't wrong after all.

 

However in the interest of "the latest and greatest" Bruno Putzey's managed to improve his design with his new company Purifi! So I wonder if that particular class D offering (fully assembled like the Benchmark AHB2) and likely more expensive, just might beat it. Who knows?

 

Any ABX boxes for rent?

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I looked at the Purifi specs and noticed Bruno eliminated the instrumentation amp with it's 4.7x gain (as found in the NC400) in front of the differential amp's 4.5x gain.

 

460638836_InstrumentationAmp.jpg.71bcb70ab341647e2585599278ee5824.jpg

 

Thus the Purifi amps just have the differential amp at 4.4x or 12.8dB gain, which is a helpful move toward further noise reduction (gain optimization). You won't need to pad the input of these new amps, but you will need a preamp that'll output 9.6V (19.6dBV) cleanly to get full power out of the amp.

 

The advantage of the instrumentation amp frontend is that you get transformer-like common mode rejection due to the MegΩ range input impedance. The loss of this feature is no big deal for home audio IMO. The new 4.4kΩ input impedance will push the current capability limits of some preamps, but it's likely a finished product will add a buffer between the I/O connectors and amp module.

 

Back to the issue of padding the inputs of amps that have too much gain - the $37 passive attenuator that Claude provided a link to really is amazing for the money. I got one just to measure it and it's pretty good. You can't get matching impedances between pins 2-3 with these things, so CMRR is compromised, but again I don't see the harm in that with the short cable runs in the home.

 

If you want to go there, switched (matched) resistor passive attenuators can compete or better high-end active preamps in my experience. Not cheap. I had Shallco build me one in the 80's for around $500 as I remember. They don't make old school attenuators anymore, but there are plenty of others, both kits and prefab.

 

If you just need a set-it-and-leave-it pad for an amp, I'd use a matched 4 resistor double L-Pad inside the male XLR connector feeding the amp.

 

$37 Little Bear passive attenuator (balanced I/O) gain reduction vs. magnitude

1814187336_LittleBearAttenuation.thumb.png.398984b439bad5d88c922c7b1337d966.png

 

Little Bear gain reduction vs. crosstalk

873949681_LittleBearCrosstalk.thumb.png.89bb18c1f2f9129c3f7a70c200dcd827.png

 

Switched, matched resistor gain reduction vs. magnitude to half volume

1969583550_ShallcoAttenuation.thumb.png.f985a96e99c77e25b5cfc8eea3feacc4.png

 

Switched, matched resistor gain reduction vs. crosstalk to half volume

1418688445_ShallcoCrosstalk.thumb.png.c21fef404203787728c497872ebc3cac.png

 

God bless you and your precious family - Langston

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