Behringer inuke NU6000 vs KAM KXD7200 bench tested

NU6000:

The instruction manual for the Behringer actually has rms figures in it as well as their advertised inflated figures. I'm still not sure where they get their multiplier from to convert one to the other but it seems to be x1.36.....

Stereo 8 Ohms per channel: 1500W or 1100Wrms

Stereo 4 Ohms per channel: 3000W or 2200Wrms

KXD7200:

Now if these amps were class-AB/H/G etc. with 50Hz transformer based supplies then measuring power output is normally easy, just use a continuous tone and measure what you get. As we know amps with switching power supplies sometimes restrict the output power under sinusoidal tone conditions as they do not have the thermal capacity or overload capacity to suck it up for a minute or so whilst the engineer measures it. In anticipation of this happening here I choose to make two measurement types. Continuous sinusoidal output for 1 minute (i.e. the traditional way) and output under 33% duty cycle conditions, which is a kind of burst rating but where the burst is repeated so as to deliver 33% maximum output. This is similar to saying that an amplifier must deliver 33% of its power indefinitely so that music signals with a low crest factor can still be reliably produced. 1/8th power is typical used for domestic equipment but 1/3rd seems safer for professional use where those dub bass lines can get quite demanding.

Speaking of bass, we also know that amplifiers cannot always deliver the same power at low frequencies as they do at 1KHz so I also measured both the conditions above at 31Hz and 1KHz. In order for the burst rating not to take a unfair advantage of the energy stored in the capacitors I chose the repetition rates for the burst to be relatively long so that the power output is what the power supply can sustain, not what it can do for 1ms!

So the 33% duty test signal used was as follows:

For the 1KHz measurement the signal is on for 70ms and off for 140ms.

For the 31Hz measurement the signal is on for 450ms and off for 900ms.

Sorry for boring those non-techies of you but there will probably be some debate about what tests are appropriate so I thought I needed to explain where I came from.

So, on to the results, first up, the continuous tone tests.

The Behringer was happy to drive 8 Ohm loads at full power (limiter active) for 1 minute without any change in power output. With both channels driven it produced 1.17kW at 1kHz and 1.08kW at 31Hz

The Kam was more interesting, it was happy for 30 seconds at 1kHz but had reduced output slightly by 1 minute. The reduction seemed to steady out after that. With both channels driven it produced 1.05kW at 1kHz at the 30 second mark and then fell to 950W at 1 minute.

Strangely, at 31Hz, the reduction did not happen, possibly because it started at a lower value in the first place. At 31Hz the output was stable at 980W.

The Behringer was happy to drive 1 channel at 4 Ohms with no reduction in output at 1 minute. Power output was 2.27kW at 1kHz and 2.12kW at 31Hz. This demonstrates that the amp section should drive high power into 4 Ohms without any short-term limitation. However, when driving two channels simultaneously the amp would trip its protection and shut down, needing a power on/off cycle to reset. Since the power amp part was happy to deliver the power then I conclude there is a total power limit on the supply part and rather than engaging the limiters to reduce output the amp switches off. This seems a strange decision on the part of the designer, better the show go on at reduced level if the amp finds itself producing sine-wave like power outputs. It is only fair to remember however that this is a harsh test and the amplifier was pulling 24Arms from the mains at the point it cut-out! (Note: the 12A breaker on the rear did not operate as it has a time delay, this was an internal electronic limitation).

Whether this ever occurs in a music situation is debatable and the burst tests below will demonstrate its actual ability with more realistic signals. That said it does mean it is possible to trip the amp if you abuse it into 4 Ohms, not ideal.

If the output power was kept below 1.7kW per channel then the amp would not trip. To leave a margin I left the amp running at 1.5kW per channel to see if I could get a 1 minute figure. Unfortunately after 15 seconds the circuit breaker opened, not surprising at it was drawing 22A from the mains at the time. So in the end I didn't get a 1 minute figure for 4 Ohms running but I expect it will be around the 1.2kW per channel mark as that produces a current draw of 15A which would eventually trip the breaker.

The Kam takes a different and far more cautious approach. Regardless of whether you use one or both channels the output power into 4 Ohms is limited on a time delay basis. The limitation here does not appear to be the supply but the amp sections themselves.

Power output at 1 minute was restricted to 650W at 1kHz and 540W at 31Hz. The limitation comes in after around 4 seconds:

Again, this may not affect the operation with music, see the results from the burst testing below.

At least it doesn't trip out leaving you with no sound however it does seem a large restriction when it should do 1.8kW!

So now it's onto the more music like signals to hopefully get an idea of what these amps will achieve in practice. First up, the Behringer get the 1kHz test.

Unfortunately I've lost the picture that goes with this measurement but the result was 1.37kW into 8 Ohms with either one or both channels driven. At 4 Ohms the output was 2.05kW both channels driven and 2.45kW one channel driven. For the single channel case the waveform is shown below:

The upper traces are the + and - speaker terminals, measured separately as this was a balanced (bridged) output. The lower trace is the AC mains current draw with a scale of 20A/division. The lack of PFC means the current peaks are already 36A for one channel driven.

Now changing to 31Hz, the NU6000 now manages 1.16kW into 8 Ohms with one or both channels driven. The traces below are for 2 channels driven and the current is now at 50A/division!

At 4 Ohms with 1 channel driven we get 2.05kW:

And at 4 Ohms both channels driven we get 1.80kW:

Peak mains current is now at 80A!

The Kam results are similar but lower. Starting with 1kHz and 8 Ohms it will produce 1.12kW with either 1 or 2 channels driven. Trace shows 2 channels driven:

Notice the mains current is more sinusoidal though not perfectly so (more on that later) and a scale of 20A/division is adequate :)

At 4 Ohms the Kam produces 2.08kW with one channel driven and 1.77kW with both driven. Trace shows both channels driven:

Peak mains current is around 30A.

Changing to 31Hz, at 8 Ohms the KXD7200 produced 1.11kW with one channel driven and 1.02kW with both driven:

At 4 Ohms the output was 1.95kW with one channel and 1.62kW with both channels driven. I appear to have lost the trace for 2 channels so here it is for one channel, 4 Ohms, 31Hz:

Neither of the amplifiers showed any signs of progressively restricting the output during these tests so I'm inclined believe that they should be good to do this indefinitely. As a caveat though I did not test them in this mode for long as my load box is not capable of these power levels for more than a couple of minutes at a time.

To summarise, for music signals, both channels driven, you can expect an output of:

Behringer:

1.37kW into 8 Ohms, 2.05kW into 4 Ohms for full-range or mid-top duty

1.16kW into 8 Ohms, 1.80kW into 4 Ohms for Bass duty

Kam:

In the next instalment we will look at the frequency response......

+1

Since I have quite a bit of data to show I will upload it using several messages over time so please bare with me as I'm not glued to my computer!

So firstly, some pics and a general description.

Behringer inuke NU6000:

I'll be amazed if I find anyone who thinks this is attractive? Maybe they let their new graduate designer sculpt these fine lines!

The case is made from folded aluminium (except the front moulding which is plastic fastened to the punched aluminium inside). Despite it being quite thin it doesn't feel too flimsy and given its very light it should be just fine.... The chunky front panel adds a lot of depth to the front of the amp and it ends up with the knobs 42mm from the rack ears so make sure your flight case has enough space!

The non-Neutrik connector worked OK for me but I wasn't keen on the XLRs which have no locking pins!

There is no ducting for the two 80mm fans on this model. All the power devices are clamped to the base plate, which is acting as the heat sink. The fans just push air through the box to take away the heat build-up of the case and the components. The base plate does get warm when delivering a lot of average power. The fans are speed controlled but even at the lowest setting they are quite noisy, when you work it hard it starts to sound like a hair dryer!

The mains input filtering looks chunky enough and the NTC soft-start resistor is bypassed by the relay after start-up. There is a 12A thermal breaker on the back panel which may be there to protect the IEC inlet and power cable as this thing can drink some juice!

The circuit boards all look nicely made with SMD parts used in preference to through-hole. The amp sections are based around the IRS20957S and there are four channels used in permanent bridge mode to give the two outputs.

The Behringer doesn't have any PFC (Power Factor Correction) and would appear to be a fairly standard forward converter. It uses a fairly substantial ferrite transformer with a resonant switching choke (well glued to the PCB). Power rails measure at +/-84V and there is a reasonable amount of energy storage around which is formed of 4 x 2200uF/200V on the primary side and 4 x 3300uF/100V plus 8 x 1000uF/100V on the secondary side. The effective primary capacitance is therefore 2200uF/400V and secondary side is 10,600uF/100V per rail. Total energy storage is therefore approx. 200J at nominal working voltages.

Amp output inductors used a decent gauge of wire and are securely glued to the PCB. Each channel has its own local reservoir caps but they are common to the same power rails.

KAM KXD7200:

The KAM is 1U and thus it has been a lot harder to fit everything in. There are multiple instances of stacked PCBs which appear to be held in place by a posh type of twist tie! Four variable-speed 40mm fans pull air from front to back over the heat sinks and guts. Casework is steel and feels substantial enough.

The KAM has PFC which uses the modest size toroidal choke on the right hand side.

All controls are recessed (to the rack ears anyway) and it looks a lot nicer than the Behringer!

The KXD7200 is a single-ended output type and thus can be bridged if you really want to get some voltage swing! The IEC inlet is fused with an F15A fuse which means it would probably fail before the 13A fuse in the plug, as it should.

The amplifier sections use two boards stacked on top of the main PCB and the top board has been coated in black paint or similar for reasons unknown. Probably to prevent easy copying I guess?

The output inductors here have a much smaller gauge of wire than the Behringer and, as we'll see later, that leads to a lot of heat build-up in these parts.

The power supply in the KAM seems smaller that the Behringer despite its extra complexity. The Ferrite transformer is smaller at just 5cm x 5cm x 3.5cm. Primary side capacitors are 2 x 330uF 400V and secondary side consists of 10 x 1200uF 160V, so 6000uF per rail. DC rails run at +/-145V and seem very stiff even under heavy loading. They could be regulated or perhaps just benefiting from the stable output of the PFC stage. Assuming 380V for the PFC DC bus output (I didn't check it) then total energy storage would be approx. 173J

The secret amp boards complete with what looks like paint brush hairs :)

PFC inductor and part of the input filter. Driver circuit is again painted over to disguise it's secrets!

Certainly an impressive power to weight ratio if it lives up to it's specs....

So in the next section we'll look at power output.....

Stay tuned as I'm off for my dinner :)