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miniDSP SHD Power & Hypex NC400 Class D Amps


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Following Claude's strong suggestion for a volume control preceding an N-Core amp I ordered a pair of "FV3 SMD Dact 21 Stepped attenuator 50K Passive Pre-amplifier preamp HIFI audio" to connect between the outputs of the miniDSPs and inputs of the Hypex Ncore NC122MP amps in my tri-amp setup. I chose these attenuators on the theory that being stepped they would be reset-able (by counting the steps from off to the desired volume) with more precision than a standard potentiometer.


Using REW I measured one K-Horn, removed the L-Pads from the mid and hi DCX464 in the Top Hat, cabled the volume control in between the miniDSP and the N-Core amps, set the stepped vol to maximum and measured again. Clicked the vol down one, measure, repeat until the SPL of the mid and hi were exactly the same as with the L-Pad. I expected the SPL to be 1/2 a db or so out and require some dsp gain adjustment but SPL was spot on.

I swapped in the second volume control, set it to the same attenuation as the first and ran another REW sweep. Exactly the same SPL in the mid and hi, so I installed the stepped vol control in the second channel.


I had used L-Pads with the DCX464s to lower their sensitivity and therefore lower the miniDSP noise.


Following on from the opinions on N-Core Hypex amps, the next day I re-arranged the tri amping to use my NAD C725BEE as the mid amp and the N-Core amps for Bass and Hi. After getting the gains adjusted the music sounds a bit warmer, not as dry and sterile as it did with the N-Core amps for Mid range. Time will tell.

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  • 2 weeks later...

Below are new measurements of my modified NC400 as discussed here.


I originally purchased the AHB2 for loudspeaker measurement use due to its amazing specs, sufficient power, convenient size and weight. It didn't have enough power for the loudspeakers I was using at the time, but the Klipschorn changed that. The modified NC400 is better than the AHB2 in every way that matters for measurement use, so it gets that duty now and my Klipschorns get the AHB2.


The better specs of the NC400 include more power, equal (and in some cases lower) noise, very similar CMRR and (in some cases lower) distortion, better transfer function with DC coupling, lighter weight, lower output impedance and lower cost.


I added a high-current version of the NL4 connector on the back because they are superior and will directly interface with my cabling. BTW, I noticed that the screws on the OEM binding posts that clamp the 12AWG cable from the amp module needed tightening even though I torqued them as much as I dared during the build. Not an ideal design. If you built one of these amps I'd suggest you check on those screws.


120V was used for all measurements.

Plenty of room for the NL4.



Also plenty of room for additional wires on the amp module screw terminals.



Gain and phase response for each measurement. The near zero output impedance results in overlapping gain traces regardless of load impedance.



This little amp is a beast. It performs best on the most difficult loads. The following IMD distortion plot is a little busy, but the take-away is that the 1W distortion decreases at higher load impedances as expected, but at 100W and max output (the level required for 1% THD from 10Hz-20kHz) decreases with lower load impedances. This makes me suspect that the input stages are approaching clipping with the higher drive voltages allowed by the higher load impedances.



1W, 100W and max output at 2Ω. (-40dB = 1% THD)



1W, 100W and max output at 4Ω. (-40dB = 1% THD)



1W, 100W and max output at 8Ω. (-40dB = 1% THD)



An amazing amp. : )


God bless you and your precious family - Langston

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In other news..


I learned something about resistors recently that overstated all my amplifier distortion measurements prior to the modified NC400 post above. I'm seeing a reduction in IMD and THD between about 3dB and 10dB using a new set of well-designed power resistors. The good thing is that my past distortion measurements are, of course, comparable with each other.


We all know that resistors increase in value with heating, though some are designed with reduced thermal drift, the better/cheaper way to deal with this is to oversize the resistors and/or provide thermal transfer (cooling). Thermal drift tends to affect all frequencies equally in the audio spectrum and it's gradual and predictable.


I accidentally discovered something entirely different recently while trying to buy 200W power resistors from Parts Express to "upgrade" the 100W versions I previously bought. For some reason the 200W version showed much higher IMD and THD readings than the 100W version even at very low levels - less than 1V. I thought something was wrong with the amp at first, but found it was entirely due to the resistor and it's nonlinear distortion derived from something called the Voltage Coefficient of Resistance (VCR). Even the 100W power resistors exhibit significant VCR distortion, but I didn't know it until I spent real money on resistors that act like resistors. VCR apparently is present to a minor degree in most resistors, but particularly in the cheap crap that is so hard to avoid these days. VCR distortion is independent of temperature, varies with frequency and can occur in the millivolt range. This isn't something simple like parasitic inductance or capacitance. It generates IMD and THD and probably other bad actors. I thought you could relax when it came to resistors, but you cannot.


I sent the 200W resistors back to Parts Express and got some good ones, ARCOL NHS300 4Ω 1%, through Mouser per the recommendation of the ASR guy (Amir). These things are linear to 2,500V. I should have asked John Siau at Benchmark what he used because that's apparently how the Amir figured it out. I also designed a jig for (4) of these resistors that allows me to switch between 2/4/8Ω without removing cables. Very convenient. : )


God bless you and your precious family - Langston


Edit: here's a link to the 1966 paper that Amir referenced. The authors think measurement of the 3rd harmonic distortion component can reveal many of the evils of passive components (L,C,R), though they spend most of the time discussing resistors. I'm always late to the game. : )


Edit 2: here's a link to Bruce Hofer's take on passive component distortion. His math supports the 1966 paper above in that the 3rd harmonic is the tell-tale. So does my experience at this point - keep an eye on the 3rd harmonic and keep the resistors cool and all will be well.

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