I think this data might help some of you DIY'ers on a budget in your system design.
Here is the guts of the DUT:
Based on what I've seen online over the years of this amp being looked at or tested, I think this is at least the 3rd version of the amp. The only major mod that I see that they did is to move the toroidal inductors and power resistors into the line of the fan (which in the stock amp blows back to front).
This was reported to significantly help with fan noise. The only heat sink is the bottom of the chassis for both the PSU and amp output silicon so my advice would be not to put this amp on top of something else that generates heat like an AVR. I also wouldn't recommend reversing fan direction simply because there are important components that generate the most heat in front of the fans and this is the most efficient way to cool them.
PSU rail voltage measured -83.8V/83.8V. Maximum clean input before clipping is 6.8Vpk - 4.72Vrms (unbalanced). Volume controls are detented with 20 clicks. A lot of AVR's sub outputs are rated at 1.2Vrms unbalanced and I tested to see if the amp could be driven to maximum output with this signal and it can with the volumes turned up 15 clicks.
The front signal LED's... from the 1st light to where the 2nd lights up is 12dB. From the 2nd to the 3rd is 5dB and from the 3rd to the clip light is 5dB. The clip lights are only accurate at 7.5Hz and below. Above that frequency, the amp goes into clip without the red light coming on. At 100Hz, the clip light is off by 1.5dB. By the time the clip light comes on the amp is already into about 5-10% THD depending on the frequency. This is with no limiter engaged.
I right away noticed that the noise floor of the amp was horrendous. When zooming in on the waveform, I realized that the amp's output was oscillating.
I thought there might be something wrong with the amp since the last thing before the output of a digital amp is a low pass filter to clean up the switching noise. I feared that if after the filter there was still ~7Vp-p on the output at such a high frequency, it might be a much worse problem up stream from that. The amp never kicked the fans up at idle though and there didn't seem to be any abnormal heat. The oscillation didn't change with different loads or at different volume levels.
I decided to contact Behringer about it. After a couple rounds of back and forth describing the issue to them and showing them scope shots, this was the tech's response:
"This is a class D amplifier, Any Class D amps will most likely have this same readings. No speaker could reproduce this, since it's 20 times higher then the highest frequency a human can hear.
rest assured, there's nothing wrong with it, Class D amps do this."
Well in my experience they don't but hey whatever.
I figured I'd carry on with the testing and if it blew up, just send it back. To it's credit the amp survived the usual trauma that goes with max output sine wave testing and now that it's hooked up to a pair of HT-18's it's doing the job.
Another thing that I noticed in the testing was that the outputs of the amp are out of phase.
I remember reading a while back when I think Brian O. was testing the inuke12k, he mentioned the same thing with the polarity being reversed. He guessed they might have gotten the XLR hot and cold wrong if memory serves. I don't really know if that's the case or if it gets reversed further on down the stream but either way, it's a pretty big oversight on the design of the amp and points to a lack of detailed testing of the product.
*Edit*: -At least one owner of a nu6k non dsp version has reported that his amp was in phase when compared with his nu6kdsp which was out of phase when using the Polarity .wav test
. See his post here
At idle, the amp consumed 147.71W of power. Something that I found surprising was that when the power was off to the unit but it was still plugged in, it consumed 16.17W of power. I thought the power switch broke the main wire coming in but apparently it doesn't. I didn't have the time to track down exactly what was being fed the power but this might have something to do with why some report popping issues with a remote start setup.
I tested 30Hz on a 15A power line and the most that I could get out of the amp was 817.51W(average) – 1627.69W(peak) per channel using 5.2 ohm loads. Any more and the amp would power cycle. It would power cycle using a burst test at 30Hz as well at around 1923Wpk. All of the rest of the tests that I did were performed using a 30A 120V line and I had absolutely no issues with power cycling no matter how hard I pushed so anyone with problems with cycling out there, look at your house power.
I have a lot of interesting data that I'll post here including max power, burst power, limiter settings vs. power output, frequency response graphs and DSP tricks for getting less rolloff or cheating the high pass filters to go lower (all with graphs) so stay tuned. I'll try to get it all out there this week as time permits.
*Edit* Links to all data collected:
Power measurements 5.2ohms both channels driven
Power measurements 10.43ohms 1 channel driven
Power measurement TIH hulk punch scene
DSP limiter measurements
Frequency response measurement
PEQ filters measurements
High pass filter frequency lowering cheat
Low end response extension curve/signal chain rolloff compensation curves
Polarity test .wav file
NU6000 vs. NX6000 Internal pics