Using the QA451 with External Loads

Post by Matt

One drawback of the QA450 was the fact that when the loads were disconnected, the output was muted. That changed on the QA451, and the measurement path was decoupled from the load path. This means that on the QA451 you can make measurements of an unloaded amplifier. But it also means you can use an external load in place of the integrated 4 and 8 ohm loads.

The QA451's internal loads are designed for fast burst testing, and with an appropriately sized FFT you can quickly make measurements up to 200W into 8 ohms very quickly (4 ohms will give you roughly twice that, but see the manual for the caveats) . But there are times, especially during development, where it's useful to have much larger resistors sitting in a bucket of water to allow extended testing at very high power levels. With the decoupled inputs of the QA451, this is easy to do. A QA451 user (thanks MP!) shared some external load resistors that I'd not seen before: ARCOL chassis mount. These are thick film, non-inductive resistors with the load element encapsulated inside an aluminum extrusion. Three of the LPR100 series stacked together are about the size of a deck of playing cards. They come in wide range of values, including 4, 6 and 8 ohms. The LPR50 can handle 50 watts, and the LPR100 can handle 100W with a maximum temperature rating of 250C. They can handle 5X the rated load for 5 seconds. And--the best part--a 100W resistor (500W peak) is about $6 in small quantities.

With that, the path to extended testing at 1 kW or even 2 kW is suddenly very clear!

OK, so with these loads, why use the QA451 at all for extended duration testing of Class D? Primarily it's the filter. Take a look below at the hash on the output of a Behringer NX1000. The first plot shows a few full cycles (this is 200W into 4 ohms):

In the plot above, we can see it's roughly 20 volts peak. This is one leg of the output and it's being driven differentially. This is about 40 volts peak across the speaker, which is 28.3 Vrms, which means 200W (the QA401 is reporting 206W). Note the tips of the sine are showing a lot of fuzz--that is the the carrier (aka the switching frequency) of the Class D amp. Let's zoom in and take a closer look at this hash/fuzz. Below you can see a zoom near a zero crossing. What you are seeing here is a zoom of the hash. The class D frequency is about 300 kHz, and the digital hash is about 2.5Vpp (5Vpp differential). That's a lot of hash to ask the audio analyzer to handle by itself.

The QA451's 6th order filter was previously discussed HERE. Given the 300 kHz carrier frequency of the NX1000, we'd expect to see roughly 60 dB of attenuation of the switching frequency based on the plot at the link above. That would take the 5 Vpp and knock it down to around 10 mVpp. And it also converts the signal to a single ended signal. 

QA451 Max Input Consideration

Outside of the power handling, the QA451 has a maximum input capability that is largely set by the internal +/- 15.5V rails. Remember the QA451 has a 12 dB attenuator on the speaker signal path (the link above has the schematic). And the opamps used are rail to rail input and output. So, we might expect the max swing to about around +/-15.2V or so. With the 12 dB attenuator considered, this means a max input of +/- 60.8V = 43 Vrms = 32.7 dBV, which is 230W into 8 ohms, 462W into 4 ohms, and 925W into 2 ohms.

Let's take a look at the block diagram of the QA451, this time with an external load connected (NOTE: Do NOT engage the internal loads when using an external load! Check the front-panel LEDs of the QA451 to ensure your internal loads are NOT active)

To verify the 15.2V figure we estimated above, let's compare the output of the amplifier with the output of the QA451 at the onset of clipping. In the plot below you can see the amp output (top trace) and the QA451 output (lower trace, with flattened tips). Clearly, you can see that the QA451 has started clipping before the amp has started clipping. The peak to peak is show as 32V = 16Vp, but that's a bit optimistic and the 15.2V or so used above is the more conservative figure. 


 The spectrum for the above appears as follows: This is about 484W.

So, with the QA451 clipping we know that to hit the 1000W watt mark, we're going to need 6 dB of external attenuation in addition to the 12 dB of attenuation supplied by the QA451.

What we ultimately end up with is a configuration that appears below. Note that the QA451 inputs tap off the lower half the resistors, providing 6 dB of attenuation before the signal hits the QA451. Combined with the QA451's front-end, this gives us 18 dB of attenuation total. This will take the QA451's max handling (due to input swing limitations) from 32.7 dBV to 38.7 dBV = 86.1Vrms. Into 4 ohms, that is 1.8 kW. And it aligns nicely with the 2 kW limitation (5 seconds) offered by the 4 LPC100 resistors. 

And a picture of the setup as connected to the Berhinger NX1000 amp:

And with the above, we can run a THD plot on the NX1000 with the following parameters (this is for 64K FFT, and note we've specified the input gain as -18 dB, which is 12 for the QA451 and 6 for the resistor array. We've also specified that the plugin will manage the attenuator, which means the plugin will engage the attenuator as needed to maximize THD or THD+N. The early abort is enabled, which says effectively to 'stop testing if the output power is greater than 10W AND if the THD has degraded beyond -20 dB'). 

The resulting plot is below with the NX1000 at max volume:


And the THD+N plot appears as follows:


The QA451 is well suited for fast testing on the manufacturing line. But there are times in development where you need to soak up lots more power and for extended intervals. For that, you can use your preferred external load and still get the benefits the QA451 offers: a 6th order filter to knock down Class D switching energy and a clean differential to single ended conversion. With a little bit of wiring, you should have a setup that can test 4 ohms to nearly 2 kW. This same recipe can be readily tweaked to allow 4 kW at 2 ohms, or 1 kW at 8 ohms.

PS. This post can be discussed on the forum at the link HERE.

If you liked the post you just read, please consider signing up for our mailing list at the bottom of the page.