Building a 6:1 Mux for 5.1 Testing

When testing a 5.1 amplifier, it's desirable to perform the tests with all channels under load. The front-panel of the QA440 is shown below. It's a product (to be released in January or February 2022) that permits relay switching using standard "snakes" such as the one located HERE

The front-panel of the QA440 is shown below:


The front-panel of the QA440 has two DB25 connectors. The connectors expose 15 group of conductors as shown below:

Relays inside the box allow you to connect Group 1 or Group 2 to Group 3, and Group 4 or Group 5 to Group 6, etc. The UI for the QA440 is exposed as follows:

From the above, it should be clear how you can select the various routings. From the primitives, some very sophisticated routings can be had. For example, you can create 5 2:1 muxes, or 3 2:1 muxes, and a 3:1 mux, or a 4:1 mux and a 3:1 mux. There are lots of possibilities. The full list is shown at the wiki located HERE.

For 5.1 testing, the aim is to provide 6 channels of measurement. 

The 5.1 Test Board

The test board is show below. The size is 610 mm wide and 145 mm tall.

For the 5.1 test board, there were a few goals:

0) Demonstration only (this isn't a product for sale)

1) Power handling to 500W/channel

2) Built-in attenuation

3) Up to 6 channels

4) Direct connection to the QA440

The power handling was achieved using ARCOL loads. These loads are discussed in more detail in the post HERE. In short, they are very low-cost, non-inductive, and very rugged. The loads are mounted in a piece of laser-cut 1/8" aluminum to help with thermal dissipation. 

Attenuation is achieved using a simple resistive divider, and are discussed in more detail HERE.

Up to six channels can be supported. In the picture above, just 5 loads are shown. Above each load is a red + black screw terminal for connecting to the amp. 

The DB25's shown in the upper right of the test board are for connection to the QA440.

The laser cut aluminum piece is shown below. The various cutouts are relief for the various through-hole connectors. Mounting holes for the loads are shown. Those were tapped upon receipt. 

The PCB connection detail for the loads is shown below. To connect, the load leads are trimmed and soldered to a pad on the PCB:

The routing for the PCB was selected to ensure all traces flowed cleanly without any jumpers being needed and keeping most all routing to the top layer:

Other Options

A 5.1 amplifier has 6 independent amplifiers. Above, we used a 6:1 mux to test each amplifier sequentially, while ensuring all amps were under load. The signal paths were balanced, meaning each measurement had a + and - signal. 

If your amp wasn't balanced, you might be able to do a stereo 6:1, though each channel wouldn't be independent.

Another approach would be a 3:1 mux, and measure as stereo pairs (front left and front right, back left and back right, and then measure center channel and sub together). The wiki linked above shows the various configuration options. This configuration would offer independent control of each 3:1 mux (and you'd get a spare 2:1 mux).

A 7.1 amplifier has 8 channels to measure, or 4 stereo pairs. Since an 8:1 mux cannot be realized with the QA440, if the amp is unbalanced you could do 4:1. If the amp is balanced, however, there aren't enough relays to make the measurement. In this case, the operator could be prompted to manually swap some cabling. Or potentially another QA440 could be used. 

Be aware: the connection choices can be bewildering at times. Draw a picture (as shown above) where you clearly identify the pins for each relay and the connection diagram and double and triple check your work. Connector assignments are the most error-prone phase of design work usually. 


For certain measurements, the standard two channel option isn't convenient. But a relay board and some custom hardware can make it easy to automate more difficult configurations. In the example above, a custom board was made to permit a generic 6:1 loaded measurement. 


Feel free to discuss this post HERE.

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