Introducing the QA451

This week we begin shipping the QA451. The QA451 replaces the QA450 programmable load. When combined with the QA401, the QA450 and QA451 products make it very easy to quickly test amplifiers at both 4 and 8 ohm loads, and make a range of measurements quickly, including:

  • THD(N) versus Output Power
  • Efficiency versus Output Power
  • Amp Output Impedance versus Frequency

The QA451 offers improved performance over the QA450 in the following areas:

  • Integrated Switch. The DC switch on the front panel makes it easy to turn your DUT on and off remotely. A pre-charge function is included that will allow the gentle charging of rail capacitors up to 10mF at 50V. The QA451 extends the low-voltage operation of this switch, ensuring that amps running from any supply between 0 and 50V can be handled.
  • Output Filter. The QA450 had a single-pole low-pass filter with corner around 33 kHz to support "filterless" class D amplifiers. In the QA451, this single pole filter was replaced with a 6th order filter, deliver substantially more attenuation at amplifier switching frequencies and substantially less attenuation in-band at 20 kHz. The corner is around 70 kHz, permitting measurements of 20 kHz 2H and 3H harmonics.
  • Current Sense Resolution. The QA450 had about  +/-15 mA of current sense resolution. This worked fine on big 300Wx2 amps, but for 10+ watt amplifiers, the resolution wasn't sufficient to make accurate efficiency measurements. On the QA451, additional resolution and accuracy (about +/- 2mA) has been provided that should give good confidence on lower-power Class D amps.
  • Single-ended Outputs. The QA451 outputs are single-ended, allow the QA451 to be used on low-cost soundcard setups that generally only have a single-ended input. The QA450 outputs were differential and relied on the analyzer to provide the CMRR performance that is so important when measuring Class D

QA451 Front Panel

The front panel of the QA451 is shown below. It's slightly different than the QA450 in the DC connector (two pin instead of 3), and in the outputs (single ended versus differential).

The DC switch provides a means for turning the DC supply to your DUT amplifier on and off. The amplifier inputs are the connector in the center of the panel. The two BNC connectors on the right side are the outputs. 

The block diagram of a single channel is shown below:

The amp is connected directly to the channel input on the left. There are dual 8 ohm loads that can be switched in. Switching in just one gives an 8 ohm load, while switching in both gives a 4 ohm load. Because the 4 ohm load uses twice the number of load resistors, the power handling at 4 ohms is nearly twice than of the 8 ohms limit. The QA450 manual has a good treatment of the max power handling capability of the loads (and the loads are the same between the QA450 and QA451).

On the right side, you see the 6th order low pass filter. The schematic for that section is shown below:

On the left side, you can see the connections from the inputs. Unlike the QA450, when the loads are disconnected, you can still measure the incoming signal. On the QA450, if the loads were disconnected, then it wasn't possible to observe the incoming signal. 

R31, R37 and R40 form a 12 dB attenuator, and working in conjunction with C23 you get your first pole. This passive low-pass is useful here to ensure any Class D slew rates are limited so that the subsequent active stages can correctly track and cancel them. On the QA450, this was achieved using the first-order low-pass filter recommended by TI in their app note located here

C27 and C28 are DC blocking caps. These have a voltage rating of 35V which ensure that Class D amps powered from 50V supplies can be handled, as their outputs nominally idle at 25V.

U14.1 and the surrounding circuitry comprise a differential to single ended two-pole low pass filter. U14 is a common second-order Sallen-Key low pass filter, and a final pole exists on the output, working in conjunction with a 50 ohm output resistor. 

Combined, the above give the following SPICE response:

The lower 3 dB point is around 2-3 Hz, and the upper 3 dB point is around 65 kHz. Measured at 200 kHz, with 5Vrms into the QA451, the output is 21 mVrms which is 48 dB of attenuation overall (including 12 dB attenuator)--this agrees well with the SPICE simulation. Class D parts like TI's TPA3255 will have a carrier frequency from just over 400 kHz to just over 600 kHz. At 500 kHz, SPICE puts the attenuation at 94 dB (including the 12 dB attenuator).

In most measurements, you won't see much difference between the QA450 and QA451, in spite of the QA451 having significantly more rejection up around 200 to 500 kHz. This is because most Class D amps are pretty well behaved. But the silicon vendors continue to push on reducing output filter cost and that means they will continue to find ways to get creative with where they dump unwanted switching energy. 

If you are interested in more detail behind the types of filters used for measuring class D amplifiers, I strongly recommend Bob Cordell's book "Designing Audio Power Amplifiers (Second Edition)" for a detailed and extended treatment on the subject. Make sure you get the second edition, as the topic of Class D amps has been greatly expanded over the first edition.

System Block Diagram

The system block diagram between the QA450 and QA451 is unchanged. Below you can see the diagram for the QA401 and QA450. When using the QA451, you won't need the ground wire on the DC switch connector, and the outputs on the QA451 need a single BNC per channel instead of two. These both make for slightly simpler cabling for test installations.

Why a QA451?

The QA451 isn't the tool for tweaking your amp while pumping out 200W steady state for minutes on end--it cannot handle those power levels for more than a few hundred milliseconds. Instead, it's designed for fast, burst testing of amplifiers, and making plots like the ones below. These plots can be made in just a few minutes. 

This first plot is a look at amp output with a 4 and 8 ohm load attached. As expected, there's quite a difference at the high end as Class D amp frequency response is very sensitive to load. This is pretty typical for Class D.

In the plot below you can see the THD of the same amp driving into a 4 and then an 8 ohm load. 

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

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