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Behringer inuke NU6000 vs KAM KXD7200 bench tested

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Old Croc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 23 July 2012 at 3:25pm
Originally posted by audiomik audiomik wrote:



Anyway, an interesting set of test results, but would like to see the sinewave output just into clipping and 33% voltage squarewave responses of both Amplifiers to be able to compare their performance with other Amplifier tests.


Keep up the good work
Mik


+1

A clipping waveform is normally where we separate the men from the boys... Wink  Smile


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Post Options Post Options   Thanks (0) Thanks(0)   Quote teslaman Quote  Post ReplyReply Direct Link To This Post Posted: 23 July 2012 at 6:22pm
I will address the various questions posed at some point but first I want to get the remaining results out (which may answer one or two anyway Smile).

The frequency response of the Behringer is load dependant because it takes it's feedback before the output filter. You can see the result here:


The flattest response is for 8 Ohm loads where the response is just 0.27dB up at 20kHz and otherwise flat to below 10Hz, -3dB is at 42kHz. 4 Ohm loads over-damp the output filters causing a loss of 2.2dB at 20kHz, -3dB is now at 22.7kHz. Fortunately there are virtually no speakers out there with a 4 Ohm impedance at 20kHz as the voice coil inductance, even for compression drivers and tweeters, has started to push up the impedance.
For open circuit loads you can see the output filter is very under-damped, with only the Zobel network for damping. Probably no importance in practice however as long as you don't drive it with a 38kHz sine wave with no load Tongue

The Kam amplifier uses a different technology and obviously takes it's feedback from after the filter as evidenced by the well-behaved response:


The response is pretty much load independent with a -3dB point at 39kHz. Since a high-pass filter option was included I measured it also and you can see it produces a second-order roll-off with -3dV at 33Hz, handy to protect speakers if you're not using an LMS.

The response variations can also be illustrated with a square-wave measurement and the Behringer shows the expected results:


The low frequency square wave (110Hz) shows gentle sloping due to the high-pass filter at sub 10Hz. You can see a slight transient at the start because this is for an 8 Ohm load. To see more detail we go to a 5kHz square wave:


So the slight under-damping is evident. Changing to a 4 Ohm load:


The over-damping is also apparent as well as the bandwidth limitation giving a nice slope to the edges. With an open circuit:


The excessive ringing can be seen, this is due to the energy content of the square wave exciting the 38kHz resonance. In practice, despite its appearance, this is not really an issue as there is not likely to be any frequency content in the signal to set off this resonance. Remember also that this is with no load......

The Kam, as the frequency response tells us, shows a much less variable output. Firstly the 110Hz square wave:


This does not have the slight overshoot at the start because of the controlled damping but otherwise is the same as the Behringer. The 5kHz response at 8 Ohms:


and 4 Ohms:


and open circuit:


Spot the difference Smile OK, so there isn't any change..

So to summarise, the Behringer has a somewhat load dependant response at the highest frequencies which may mean a little tweaking of the EQ is required for different speaker systems, no big deal. The Kam however has an outstanding performance being essentially tolerant to any load condition with little effect.

Of course there is a secondary result of these two approaches, the output impedance of the amplifier itself (Damping factor). The Kam, with its feedback after the filter, should have a very low output impedance across the band but especially at low frequencies (where open-loop gain is at its highest). The Behringer cannot correct for the impedance of it's output filter so will have a higher output impedance across the board but will get especially bad at the highest frequencies, hence the variability with load. The real significance of this is probably minor for most users but it's useful to know. Incidentally, the damping factor specified by the manufacturers is 120 for the Behringer and 800 for the Kam which ties into the results here.
If I find time i'll measure and plot the output impedance against frequency for interest.

Next up will be THD....
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 23 July 2012 at 6:43pm
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Nice. Thanks for taking the trouble to do all this. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote teslaman Quote  Post ReplyReply Direct Link To This Post Posted: 23 July 2012 at 7:59pm
THD

In the absence of an AES17 filter option I used an external 30kHz second-order low-pass filter to reduce the switching frequency content and allow the AP to measure the distortion. Analyser bandwidth was left at 80kHz (In hindsight the 30kHz setting would have been more appropriate but hey-ho, it doesn't make a big difference).

I did some scope traces of the residual so the nature of the distortion could be viewed at specific conditions. Then two measurements were undertaken, THD+N (Total Harmonic Distortion + Noise) versus frequency at 1/3rd power and THD+N vs output power for 1kHz and 31Hz.

First up, Behringer:

At low powers the Behringer is actually pretty good. At 50W, 8 Ohms, 1kHz the reading was 0.009% Smile


(Note: For those who don't know, the top trace is the amplifier output signal, auto-ranged by the AP. The bottom trace is the distortion residual, i.e. all the parts of the signal that shouldn't be there. They are not to the same scale and the actual scale on the screen is irrelevant due to the auto-ranging of both traces)

The distortion is mainly 3rd harmonic although some noise and higher harmonics are present. This is reassuring as I'd been using the amp in my workshop to provide background music for a couple of days and it was actually fairly sweet sounding through hi-fi speakers Shocked.

At 50W into 4 Ohms the THD+N was 0.017%:


You can start to see the higher harmonics building but 3rd is still dominant.
Pushing up the power soon starts to bring out some more nasties though and at 1/3rd power into 8 Ohms (380W), still 1kHz, the measurement is now 0.0217%:


Plenty of higher harmonics there now so I'd expect the sound to harden up a bit when the amp gets pushed but the total THD level is still reasonable.
At 4 Ohms, 760W, 1kHz the THD+N is 0.08%:


3rd harmonic still a big contributor but plenty of other stuff there still.

Moving to 31Hz, 8 Ohms, 380W, the figure is 0.0616%:


The distortion residual is smoother and so harmonics will be mainly low order, looks like 3rd and 5th mainly. Should make for a reasonable sounding bass amp!

At 4 Ohms, 31Hz, 760W the figure was very similar at 0.0618%:


The residual also looks similar so I have a hunch that this distortion might be caused by something other than the power amp section itself. It kind of reminds me of the distortion you get when using SMD ceramic capacitors as DC blockers for audio signals! I wonder if they have skimped on caps with the design and used SMD for signal coupling Confused This would also explain the fact that the THD is rising as the frequency goes down since the AC component of signal across the caps increases with decreasing frequency.

So, at 1/3rd power the THD+N versus frequency looks like this:


Nothing special about this, in fact it's not great considering a good class-AB amp will do 0.005% at 1kHz and maybe 0.02% at 20kHz. The increase in distortion at low frequencies is interesting and unusual, it would normally plateau out at the noise floor of the measurement but there is clearly another mechanism at work in this design, maybe those ceramic coupling caps at work??.

Finally for the Behringer is the plot of THD+N versus output power:


Because the limiter comes in as clipping starts the THD doesn't actually rise too much, we see the level just stops increasing and the curve fluctuates around a constant power point. The limiter is not defeat-able so I couldn't generate the classic types of curves you normally gets from this measurement with a sharp increase in THD above clipping.

Onto the Kam:

THD was consistent with the Kam pretty much regardless of level, so there was no sweet spot to be obtained at low power outputs.
At 1kHz, 8 ohms, 340W, the THD+N was 0.11%:


Quite a bit of harmonic content and fuzz here so not as impressive as the Behringer.

At 1kHz, 4 Ohms, 640W the figure was 0.126%:


Similar to the above and perhaps looks indicative of some induced distortion from power supply tracks since its asymmetrical and has a large component at the zero cross points.

At 31Hz, 8 Ohms, 340W the THD was 0.115%:


This makes it very clear that the positive half cycle has a much larger interference effect than the negative half despite there similar shapes, re-enforcing the thought of power trace related distortion. Pure speculation based on personal experience but it may be down to circuit layout allowing a node to be disturbed by current in another part of the circuit.

31Hz, 4 Ohms, 640W:


No real change.

Plotting the THD+N vs Frequency we get:


This shows the independence of THD on frequency further suggesting a well defined coupling mechanism of distortion into the signal.

Moving on to the THD+N vs Output power. This caused a headache since, as we have already seen, the amplifier limits power into 4 Ohm loads after 4 seconds. Unfortunately the measurement of THD vs Power output takes longer than that to do so the results are messed up. Still, for the fun of it, here they are:


Both 8 Ohms traces are OK and you can see the power limiting on 1kHz but not on the 31Hz as already noted in the power testing. The 4 Ohm traces get messed up by the power limiting kicking in during the test. At 1kHz it tops out at 650W but it maintains the 31Hz output for long enough to get up to reasonable power but it wasn't stable.
The traces are, in the main, flat again at around 0.13%. It does increase at high frequencies but not until 10kHz so the top end is not bad at least.

You can draw your own conclusions but I think the Behringer has the better chance of sounding nice.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote snafu Quote  Post ReplyReply Direct Link To This Post Posted: 23 July 2012 at 8:46pm
so it doe's roughly what it says on the tin ?.
As expected.
Good on behringer, keep up the good work.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote U.Viktor Quote  Post ReplyReply Direct Link To This Post Posted: 23 July 2012 at 10:38pm
Anything bellow 1% THD is OK.
THD is not the most important thing how well a professional amp would perform.
However the lack of real output feedback in the iNUKE is a serious problem and renders this amp USELESS anywhere above driving sub sections. The undamped/uncontolled output LC filter causes unpredictable signal level variations with different speaker loads probably creates huge spikes in the mids/high freq. range.
I have seen/heard first series PKN amps ~10years before with similar problems so Behringer should have learnt something :-)
That KAM has at least two blatant design mistakes 1. see those electrolyte caps in the HOT airflow(!) 2. see the "PFC inductor" has made from single layer insulated magnet wire (light yellowish). Also the winding technology of that inductor deserves a possible failure due insulation weakness+bad construction.

Those design mistakes will be paid by poor guys without understanding of electronics and believe that a sub ~200EUR amp could be reliably good for their applications. Of course NOT.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote teslaman Quote  Post ReplyReply Direct Link To This Post Posted: 23 July 2012 at 11:04pm
To finish up for now there's a few other bits and bobs to mention.

AC Mains draw

As already noted the Kam has PFC which really helps keep the AC input current under control. However, it's not a perfect looking PFC performance so I recorded a couple of shots of AC voltage and current with a 1.05kW output power (single channel).

So, the Kam with output of 1kHz, 8 Ohms, 1 channel driven. The Ac voltage and current waveforms are shown:


The current waveform is hardly a perfect sinusoid but at least it's drawing current for virtually all the cycle. The scale is 5A/division so roughly 10A peak and the rms current draw was 5.85A.

The Behringer uses a simple rectifier/capacitor arrangement and hence draws short bursts of high current. Under the same conditions as above:


Apologies the traces are aligned differently but you get the idea. Peak current for the same power output as the Kam was about 18A (10A per division for this reading) and the rms current was 7.69A. It should be noted that this does not mean the Behringer was using more power than the Kam, in fact both amps were drawing about the same (I forgot to note the reading but it was around 1.2KW), it's just the power factor of the Kam is better due to the PFC.
It does mean that the AC supply cables, connectors etc. get a harder run with the behringer as it is the rms current that causes the heating (power loss, I2R loss) and its also more likely the behringer will trip a circuit breaker or blow a fuse.

People often say how come they can get away with a 10A IEC inlet then? Well thats because the safety regulations require an audio amp to be tested at 1/8th its rated power as this is generally agreed to be representative of normal music played at just clipping levels. Thats why the behringer says "620W" on the back of it: 2 channels of 2.3kW = 4.6kW, divide 8 is 575W and hence with some losses it will draw 620W from the mains.  In theory this should be the "worse case" but as I've already said, used as a bass amplifier and driven into limiting the average power output will be much higher than 575W! It would be interesting to see what power it can be made to pull when used with a system and driven hard, I suspect it will be lower than you think!

EMC

Now I don't have any fancy EMC test equipment but I can connect a scope to the speaker outputs (with no input signal) and see what's leaking out! We expect to see some carrier signal remaining after the output filter but it should close to sinusoidal as any high frequencies are likely to cause common-mode or differential-mode radiation from the speaker leads.

So the behringer was scoped up (one channel on + and one on - speaker terminals due to bridged output arrangement):


The waveforms look nicely smooth and show a 348kHz carrier is used for the power amp stages (at least at idle). Since there are no strong high-frequency elements present I would surmise the Behringer is not going to create much interference, a good result!

Now the Kam:


Hmmmm, not so good. There's a good breakthrough of switching noise and ringing from the output stage suggesting a less than ideal layout and probably a lot of contamination of the internal ground reference. It's possible the Kam could cause issues with some equipment and in fact I did experience this when I did listening tests using this along with a Void QX5 amplifier. The Void amplifier was emitting a quiet but audible tone through the speakers when the Kam was running and this tone varied as the unit warmed up. The effect was just like an intermodulation tone or beat frequency as it started high, reduced right down to DC and then started climbing again, classic symptoms...  The noise ceased if I disconnected the Kam from the LMS and so just a ground connection into the system was enough to cause an issue (the amps were not fed from the same LMS output).

DC Offset

The DC offsets were noted:

Behringer Ch1: +11mV, Ch2: +56mV
Kam Ch1: 6.5mV, Ch2: 2.2mV

Nothing too nasty there then.

Limiter response speed

The Kam limiter was speedy, cutting down a 200Hz signal which was 6dB too hot in two half cycles (~5ms):


Apologies that it was 200Hz, it was supposed to be 1kHz but I messed up!

I then tested the Behringer at the originally planned 1kHz (D'oh!) and it was also fast, taking 2 half cycles to drop the gain:


This speedy response, without overshooting on gain reduction, should help keep signals clean when overdriven.

Thermal issues

I took some thermal images of both amps whilst running a fairly high output level for a minute or so to see if anything was getting worryingly hot!

The Kam had the hottest component, the output inductors were wound with quite thin wire and when driving 4 Ohm loads the wire was very hot and the core not far behind:


118 degrees for the wire itself and the core was getting up to 100 degrees as well. A short time later I stop the test as I was getting worried:


138 degrees!

The power supply heatsink also warmed up nicely but the transformer itself was also hotting up:


Transformer:

Overall view from top:

Otherwise it was OK but those output inductors certainly do get warm! Cramming so much into 1U certainly doesn't help get the heat out but under music conditions I assume it works OK otherwise we would have heard about it wouldn't we?

The Behringer fairs better, no doubt due to the colossal airflow that takes place when you pump the signal up a bit, those fans sure are noisy!!

Transformer:

Bridge rectifier:

Fan control circuit with dropper resistors:

Power amp section top view:

Output inductors:

Close up of FET driver chip:

Nothing too alarming here at least Smile

Well thats about it for what I've done already. I'll re-read the other posts and see if theres any test I can do to answer question. I did do some listening tests but since these things are subjective I haven't decided whether to write about them or not. Better you make your own minds up with these things Big smile



Edited by teslaman - 24 July 2012 at 10:00pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote spongebob Quote  Post ReplyReply Direct Link To This Post Posted: 23 July 2012 at 11:32pm
Teslaman - I can't confess to understanding all of the technical details that you have provided in these posts, but I would like to thank you for such an in-depth contribution to this forum. Do you think you will be reviewing other amplifiers in a manner similar to the above?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote infrasound Quote  Post ReplyReply Direct Link To This Post Posted: 23 July 2012 at 11:53pm
Thanks as well.

Great writeup, but the current comparison in the PFC section isn't so clear (although I can tell, doesn't make clear which is Berry, and which is Kam)

Feel free to give your opinion on sound quality (perhaps compared to the QX?), complete with disclaimer Smile
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Muckerbarnes1 Quote  Post ReplyReply Direct Link To This Post Posted: 24 July 2012 at 9:02am
Good work David.
 
Totally agree with the overating IEC connectors and heat on the reservoir caps. Not to mention the temperatures on poor quality inductors.
 
I agree with U Viktor. A crash is inevitable.
 
With such high current variation I would not like such a device in any rack with any brand name. Also miffed at the linearity of the amps. Very poor.
 
The Behringer's storage is somwhat less at operating voltage. 10.6F x 84V = 890.3 Coulombs x 2 (rails) = 1780.8 Coulombs = 74.8 Joules. This is 17.9 calories... They are deffo LOW CALORIE  LOL


Edited by Muckerbarnes1 - 24 July 2012 at 9:03am
Billy Dawg.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote nuclearbass Quote  Post ReplyReply Direct Link To This Post Posted: 24 July 2012 at 10:33am
Originally posted by spongebob spongebob wrote:

Teslaman - I can't confess to understanding all of the technical details that you have provided in these posts, but I would like to thank you for such an in-depth contribution to this forum. Do you think you will be reviewing other amplifiers in a manner similar to the above?

totally agree with this, not seen a post as usfull and as filled with actual facts for a long time on here.... well done Clap
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 24 July 2012 at 5:51pm
im confused about the storage figures,surely ripple is a better measure than capacitance?
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