QA451 Programmable 8/4 Ohm Load
26-Jan-2024: The QA451 is on backorder, with stock arriving late March. We'll open orders a few weeks prior to it being back in stock.
The QA451 and QA451B (collectively referred to as the QA451) are the replacement products for the QA450. The QA451 is designed for high-speed testing of amplifiers. If you are testing at the module level (with modules designed for single supply operation), the QA451 allows the on/off control and current measurement of the supply rail. If you are testing finished amplifiers (eg amplifiers that plug into a wall), then the QA451's programmable loads can enable fast automated testing when combined with the QA40x audio analyzer.
The QA451 also offers a 6th order low-pass filter (Fc=67 kHz) to help with testing modern class D amplifiers. The block diagram of the signal chain, from the balanced load inputs to the outputs is shown below. There are three key features performed by the QA451. First, the balanced signal is attenuated by 12 dB. This shifts a 200W signal (32 dBV = 40Vrms into 8 ohms) down to 20 dBV. Second, the balanced signal is converted to single-ended. And finally, the active 6th order low-pass filter suppresses any class D artifacts. Note, too, that the 4 or 8 ohm loads can be bypassed completely, allowing you to substitute external loads.
The QA451B differs slightly from the QA451 in a single area, and that is the current sense resolution. The QA451 has 5-10 mA resolution on the DC sense, while the QA451B has 10-20 mA resolution. This is due to supply issues with the Allegro parts used on the QA451.
NOTE: The QA451 is not designed for extended operation at high power levels. Exceeding the power handling of the QA451 will result in internal load resistors opening up. These resistors are very difficult to remove without special equipment for uniformly heating the board. The manual goes into great detail on power handling capability of the QA451 and how to compute a thermal budget for factory test flow. Do not purchase the QA451 if are looking for a load that can soak up hundreds of watts while tuning an amp for several minutes. The QA451 isn't designed for that. See the thermal discussion on page 14 of the manual. Alternately, you can use external loads and use the QA451 and a balanced to single-ended converter with a steep low-pass filter (Fc=70 kHz). See the link below about External Loads.