QA450 Programmable 8/4 Ohm Load

We're putting the finishing touches on a new product, the QA450. This product is conceptually pretty simple: It's a programmable load that will provide 4 or 8 ohms, 100% resistive, to an amplifier.  

There are a few twists that make the product more interesting, and we'll go through those below. Right now, we'd like like to find another dozen beta testers that are focused on fast, automated testing of amplifiers that could benefit from a programmable load. If you or someone you know are facing this challenge and they are already a QA401 user, please send them to our support email alias.

QA450 Front Panel

The front panel of the QA450 is shown below.

Starting from the left, we have the following:

Status LEDs: The LEDs indicate the connection to the desktop app, and whether the QA450 is providing a 4 or 8 ohm load to the DUT. If the 4 and 8 ohm load LEDs are flashing, then it signifies a thermal fault has occurred. We will discuss thermals below in more detail. 

Current Sense: The QA450 has the ability to sense DC current flowing from a DC power supply to an amplifier (see the larger system diagram below). This is achieved with an ACS711 Hall Sensor (+/-15A full scale). The noise from this reading is about +/- 30 mA. Currently, we're not calibrating linearity, but that will change in the future. Additionally, we can sample up to 200 Ksps or so, and thus there's improvements that can be made to the noise too. But in the first release, treat this as a DC current meter with perhaps +/- 50 mA of accuracy. It's probably not enough accuracy for measuring small amplifiers, but for large amps drawing north of 10A, the resolution should be sufficient for making reasonable efficiency measurements. The current sense section is galvanically isolated from the USB ground, and has a sense resistor that is about 600 micro-ohms. 

DUT Power: This LED indicates if the power to the amp is enabled or not. If you use the current sensing feature on the QA450, then you also have the ability to turn on and off the supply to the amp. This switch is part of the current sense path.There's another feature here--soft-start--that we cover below in more detail. 

Load Inputs: This is where the loads are connected to the QA450. The connector used here and for current sense is a Molex Eurostyle.These are the type of connector where you insert the wire, and then screw down a terminal. The connector can be removed, making it easy to leave an amp wired for testing. The current rating on these connectors is 15A/300V. We de-rate both figures considerably. 

Output:The output of the QA450 is an attenuated and filtered balanced replica of the input. The attenuation is 6 dB. The filter is a first-order LPF with a corner at 33 KHz (100 ohm series, and 0.047 uF NP0 C0G). This is the filter TI recommends in their app note for measuring filterless class D. This filter has about 0.4 dB of rolloff at 10K, and 1.2 dB of rolloff at 20K. If you want a ruler flat response and have no interest in measuring filterless Class D, it's easy to bypass the filter with a soldering iron: just remove the shunt C (easy to do as they are large 1206 packages). Alternately, we might ship with the C's removed. It's still TBD. 

Other Considerations


From a thermal loading perspective, the box can handle extremely higher power levels for a short period of time. For example, on the front panel it's shown that the box can withstand 300W/channel for 300 mS. The load resistors used are "pulse withstanding" types, which means they are happy taking many times their steady state rating. Specifically, we use Bourns PWR163 series. Each of these is rated for 25W when the case is at 25C. 25W is impossibly optimistic as a steady state rating. In practice, the box can handle an average power of a few watts over long periods of time. But momentarily, the resistors can handle enormous loads. Pulse power ratings on these resistors is about 10J for 100 mS, or 100W for 100 mS is 10 joules. We use 16 resistors for 4 ohms, which means the resistors are happy seeing as much as 16*100 = 1.6 KW for 100 mS (800 W for 100 mS in 8 ohm mode). The limit on the QA450 proto units is 300W for 300 mS (90J across 8 devices, or 11.3J per device), and we've delivered thousands of cycles to the loads as part of endurance testing. We may de-rate this further, it's still TBD.  

The QA450 has very accurate digital temp sensors buried in the resistor load array. This allows us to ensure the power resistors are never above 60'C or so--a fraction of their 155C rating. If the temp exceeds the 60C threshold, the mechanical relays are opened until the resistors have cooled. 

Average Thermal

When building your testing profiles, you should expect your average testing power to be just a few watts over longer periods, otherwise the thermal protection will shut down the QA450. Remember, the QA401 tests with shorts bursts of audio, and the length of the burst is related to the selected sample rate and FFT size. So, at 48 Ksps and 8K FFT you get a 200 mS burst of audio. Things scale linearly from there. But if you are making a measurement at 300W with a 200 mS burst of audio, and the rest of the measurements are at 10W or so, then you are easily below the average thermal the box can readily support. Remember, average includes the time to load/unload the board you are testing from the test jig, too. 

Current Sense and Soft Start

A challenge with big amps is finding an economical power supply for testing. A nice lab supply that can output 700W or so, with adjustable current limiting and a 50V output will cost around $1000. The lab supply lets you set the voltage of operation, measure the supply current, provides over current protection and soft start (via current limiting)--all features needed for amp testing.

The soft-start (current limiting) feature is needed because the amp capacitance is usually massive. For example, TI's TPA3255EVM has 10 mF of rail capacitance. If you just connect that to 50V supply, the output caps of the supply will dump straight through your wiring into the amp, and you'll see 600A amps flow for hundreds of microseconds and big sparks. The correct way to power up these amps is to disable the supply output, connect the amps, and then enable the amp. This will give a controlled, current-limited startup. 

On the QA450, this is achieved via the on/off switch that is part of the current sense path. This switch is solid state (high-side PMOS) and the gate drive is isolated from the host PC ground. Additionally, there's a second switch with a series resistor (about 10 ohms) that is energized prior to the main switch closing. This allows the amp (DUT) to be gently brought up to operating voltage and limits the stresses put on the DUT. While the soft-start operating is in progress, you'll see the front-panel DUT POWER LED flash quickly. After 1 second, the LED stays on continuously indicating the soft-start has completed. 

The QA450 also provides over current protection. If current exceeds +/-15A for more than 2 mS, then the power switch is shut off. This protects against DUTs that have come off of the assembly line with unintended shorts. The algorithm has the ability to respond much more quickly with some tuning.  The MOS used as the power switch is rated for routine operation at 200A if the pulse is limited to 100uS, and 80 to 100A if the pulse is limited 1 mS. We'd like to get to the point in software where we can survive a direct short with lots of source capacitance present. We're not there yet. But we see a path. 

Attacking Supply Cost

With the above features in place--the ability to measure current, the ability to soft-start and the ability to protect against over current due to faults on the DUT--we have addressed many of the issues that require a lab supply. The ultimate goal here is to permit testing from low-cost 12, 24, 36, and 48V DCDC power supplies that are available everywhere. You can readily find a 1000W 48V supply for under $100. True, this doesn't give you the ability to test at several different voltages. But increasingly, amps are always running at a single DC voltage as part of a larger system installation. 

QA450 Software

The software for the QA450 is shown below:

The software allows you manually turn the DUT on and off, select the desired load (4 ohms, 8 ohms, or open), and monitor the temp of the left and right loads and see the measured current.

Behind the scenes, there are APIs present to allow to you query and control each of the features. 

The Big Picture

Below is a block diagram showing how the QA401 and QA450 work together to test an amplifier:


If you are designing and shipping high-powered amps and looking for an economical testing solution, understand we are putting the pieces in place to deliver what you need. Combined with our QA401 audio analyzer, the QA401 and QA450 working together will facilitate inexpensive, rapid testing of a range of amp power levels and topologies. 

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