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In Reply to: RE: Playing with NCore NC400 Class D posted by mitch2 on October 14, 2014 at 21:25:25
boast specs that are way over-optimistic for sensitivity and impedance curve. A speaker advertised as "87 dB and 4 Ohms" more often than not actually measures more like 84 dB dipping into 2 Ohms easily in large portions of the frequency response. In these cases, more power will make a difference.
I did try the NC400 monoblocks on my custom speakers which truly measure at 95 dB sensitivity at mostly 7 Ohms, and they sounded extremely similar to my tricked-out 2A3 SET monoblocks which make 3 watts/channel. NC400 sounded very pure, natural, without overtly spotlit detailing, which means those expecting some kind of fireworks or strong impressions will likely find them too neutral. Of note, these class D amps seem especially sensitive to power cords and power conditioning, so people's experiences will inevitably vary..
Follow Ups:
"A speaker advertised as "87 dB and 4 Ohms" more often than not actually measures more like 84 dB dipping into 2 Ohms easily in large portions of the frequency response."
This is another reason the Ncore's ultra low output impedance is helpful, being 1 milliohm at 16kHz and below. Note it is reportedly not affected by frequency throughout most of the audio band, raising to only 2.5 milliohm at 20kHz.
The first time I had the Atsah's here, I hooked them up with good quality Furutech cords with Oyaide connectors, placed them on amp stands and used large brass pucks on top to help control resonance. I am sure the milled aluminum enclosures also help control resonances, and may be one reason some report those larger amps to sound better than the 200 wpc NC400's.
> This is another reason the Ncore's ultra low output impedance is helpful, being 1 milliohm at 16kHz and below.
That seems awfully low to me. It equates to a damping factor of 8000. I've never heard of anything like that. Have you actually measured it?
That seems awfully low to me. It equates to a damping factor of 8000.
Given the design, I'm not surprised. As Nelson Pass once observed:
"Class D amplifiers are switches with tons of feedback."
"Tons" of feedback buys you very low source impedance.
Oops...a small error.
The DIY module NC400 has an output impedance of 1mohm, but
the larger OEM module NC1200 has an output impedance of 3mohm at frequencies less than 20K Hz, and 2mohm at less than 1K Hz.
Yes, the damping factor is high and a couple of reviewers have reported that, on some speakers, this can result in the perception of less bass.
With my Aerial 9s, the bass was outstanding with a nice balance of sounding full, deep and tight, and excellent drive.
Specs for the NC1200 are covered on the linked datasheet.
Three milliohms gives you a DF=2666 with a nominal 8 ohm load. Anything over about 50 isn't really needed and that data supports the notion that it has significant amounts of NFB.
I'll pass.
Well Bruno Putzey is on record as saying that if you use negative feedback you should use LOTS and LOTS of it. So I wouldn't be surprised to find a boat load in an amp he designed.
Not sure I agree with him but I found the difference between DF200 and 400 subtly audible.
As it happens in every publication I've read which said that a DF above 50 is meaningless they assumed speaker cable with a resistance a couple of orders of magnitude higher than my cheap 11AWG cable.
Not surprising that it does not make much difference in those circumstances or when using passive speakers (that wire in the almost inevitable series inductor on the woofer screws it up).
My experience with class d amps ( tripath, ice, Pascal), is that they are all VERY sensitive to power cords, conditioning and vibration control. If you don't address these, you won't hear it at its best.
Jack
I agree 100% with you on the importance of setting up class D amps for best sound. I bought a pair of D-Sonic m2-600 mono-blocks about a year and a half ago to replace a pair of tube amps that were in my system for 20 years and could not be repaired for less than double the price of the new amps and found that they were tantalizingly close in sound, but seriously flawed in several areas. I almost returned them during their 30 day grace period, but made a major effort to fit them into my system and it paid off. In fact, I spent nearly 6 months on integrating them and applied much of what I learned to setting up all my other components. I can make the following observations about this experience:
1. They are very susceptible to EMI/RF - giving and getting - from elsewhere in their environment. Physical placement in 3-dimensional space in relation to other components, cables, and power supplies. They can't be within 3 feet of my SDS unit or the separate power supply feeding a Thor phono preamp in any direction. Failure to follow this rule (in my system) can result in oscillating whistles, steady hum or buzzing, muddy high frequency play and possibly a White Sox game during baseball season.
2. Isolation platforms under these amps reward one with a more noticeable improvement in clean sound than when used under any other component save my turntable. The success I had when experimenting with some leftover brass cones, isoblocks, and a pair of oak shelves borrowed from my bedside tables was so stunning that I went on to treat every component and power supply with its own maple block platform. The amps are currently on Triceratops brass footers set on 2" X 17" round platforms placed on isoblocks on spiked stands behind the speakers. They seem to sound a bit nicer with a couple of ponds of brass discs on top.
3. They really have a long, long break-in period. I stopped clocking improvements at the 1200 hour mark with mine.
4. My amps are very revealing of other problems in the system, and can easily be blamed for those issues. Dissatisfaction with their low bass and upper midrange during complex passages was blamed on the amps for months until I performed a major cleaning and retuning of of my TT. It turns out that these amps were only revealing to me - for the first time in my audiophile life - that VTA and VTF adjustments are indeed critical and variable from record to record. I had to throw out everything I thought I knew about cartridge setup after realizing that the amps are capable of shining a spotlight on the VTA sweet spot when adjusting it on-the fly.
5. Class D amps really like clean power, but not all power cords are created equal. Mine sound best with an old pair of Top Gun HCFI monsters plugged directly into the wall sockets (Maestro duplex with matching covers) and bypassing the PPP power regenerator.
My whole system really came into its own after I stopped blaming the amps for everything I thought was wrong.
Everything is going to the dogs
FWIW, I'm currently using the D-sonic M3-1500 monoblocks in my main system. They are sitting on a symposium platform that is sitting on a signalguard platform (has visco-elastic fluid). A *wonderful* combination. They also have Herbie's Supersonic Stabilizers on top, which made a huge difference. Mine are plugged into my PPP via Audio Art cables.
This set up has also worked well for other class D amps.
Jack
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