Using the QA451 with External Loads

Post by Matt

This post is under construction. Please check back.

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. 

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 very quickly (8 ohms, roughly twice that into 4 ohms, 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, this is easy to do on the QA451. 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.

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? 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 is around 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 modulating frequency or the carrier aka the switching frequency of the Class D amp.

Now, you can see in the plot above there's some high frequency hash on the waveform--especially the tips. Let's zoom in and take a closer look. Below you can see a zoom near a zero crossing. What you are seeing here is a zoom of the has. 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 6th order filter was previously discussed HERE. Given the 300 kHz carrier frequency of the NX100, 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

The QA451 has a maximum input 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 attenuator considered, this means an input of +/- 60.8V = 43 Vrms which is 230W into 8 ohms, 462W into 4 ohms, and 925W into 2 ohms.

Let's take a look again at the block diagram of the QA451 with the external load shown:

We can sanity check by comparing 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. Clearly, you can see that the QA451 has started clipping. The peak to peak is show as 32V, 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, to hit the 1000W watt mark, we're going to need 6 dB of external attenuation. 

The new resistors have been ordered. Stay tuned. To be continued...

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

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