QA401 amp load question

Relative newbie here, please bear with me. I’m trying to understand the implications of putting the sense resistor at the bottom of the load like this:


Is it safe to use such load with QA401 with the internal attenuator off provided that voltage across R2 is connected to +/- differential inputs and doesn’t exceed 6dBV?

Hi gvl

Is your amp single ended or is it a bridged output?
If it is single ended, there is no problem to connect the QA40x in single ended mode across R2 (assumption is, that R3 is replaced by a wire going to the amps minus output).
Please note that with the given resistor values, the load your amp is seeing is only 1.1 Ohm - too low for most amps.
If the amp has a bridged output stage, I would stick with the 3 resistors. The resistor values still give a quite low 2.1 Ohm load.
Good luck


Just trying to build a universal load I can use in a variety of applications, so it’s both single ended and bridged, and class D. I should have been more specific, and R1 will be 4 or 8 ohms, R3 will become a wire, and the value of R2 is TBD.

Is a load that contains 3 resistors preferred for bridged amps because amp output terminals may have a voltage relative to ground that can damage the analyzer? Does the 6dBV limit apply when measured across differential inputs or does it also apply to each differential input relative to ground?

At the end of the month I am putting out a video on my Youtube channel where I show the 4/8ohm load that I built for the QA40x which is not that expensive to build and has taps for measuring higher power levels so the QA40x won’t be damaged.

Hi gvl
The QA40x audio analyzers have a floating power supply for the audio section without galvanic connection to the USB signals and hence you computer. This is important to avoid ground loops, noise and allows for e.g. have the analyzer ground connected to an output of a bridged amplifier.

In case of a bridged amplifier and just R1 and R2 as the load, you can connect the + input of the QA401 to R2 and the - input to the other side of R2 (= amplifier - ouput).
Alternatively you can connect the + input of the QA401 to R2 and the GND of the QA401 to the amplifier - output. In this case use a 50/75 Ohm termination on the QA401 - input.
In any case, do not connect the QA401 GND to the chassis or a ground connection of an amplifier with bridged output.

I recommend using the 3 resistor version for bridged output amps and the 2 resistor version for single ended amplifier outputs. Without the attenuator, the QA401 max. input is just 6dBV (2Vrms). You shoul first measure with the attenuator activated to be sure your signal is not exceeding the 6dBV.

Concerning class D amps you should consider a high order low pass filter (fc around 70kHz…100kHz) to remove switching noise before entering the QA401.

I did not measure high power amps with my QA402 yet so a 50 watts resistor did the job. Nevertheless I plan a higher power amp later in 2024 so I’m looking forward on your approach.

Either I or someone else will post a link to the video once it “airs”…

Is this to avoid ground loops or to prevent damage to the QA401? I thought QA401 GND is connected to amp ground on input.

Is there a technical reason for this?

Hello gvl,
You are correct, the connection of the analyzer output to the amp input ties grounds together (unless you are using a transformer).
Let’s assume you are mesuring a 200W into 8 Ohm bridged output amplifier (an amplifier like the Samson Servo 200). This needs 40Vrms between the amplifier output terminals. Each terminal has 20Vrms with inverse phase on the - ouput. The 20Vrms are referenced to GND.
The 8 Ohm load could consist of something around R1 = 7.6 Ohm and R2 = 0.4 Ohm. If you connect your QA401 across R2, the input voltages relative to GND on the + and - inputs are 18Vrms and 20Vrms (2Vrms differential). For the - input this is about the limit of the QA401.

I don’t know what the input section of the QA401 looks like. I think the differential voltage is feed to the ADC, so a measurement in the +6dBV range should be possible.

Now let’s assume you use a 3 resistor load with R1 = 3.8 Ohm, R2 = 0.4 Ohm and R3 = 3.8 Ohm. The QA401 inputs are connected across R2. With the same 40Vrms across the load, the input voltage on the + and - inputs is 1Vrms each resp. 2Vrms differential.

With the 2 resistor load you can measure bridged amplifiers up to max. 200W into 8 Ohm. With the 3 resistor load you can go way beyond 200W.

So 20Vrms relative to ground is the max voltage on the +/- inputs even if the differential is less than 2Vrms?

Yes, according to the datasheet of the QA401, the max. input is 26dBV resp. 20Vrms.

It’s not clear to me if the 26dBV limitation only applies across +/- inputs or also in reference to ground. Certainly if one of the inputs is grounded and you’re performing single-ended measurements this also becomes the limit for the ground reference but only because it’s also the voltage across +/- inputs.

From the Users Manual:

The analyzer PC interface will share a ground with the PC. The audio inputs and outputs, however, are isolated from the PC. This isolation is limited to 50V. Do not connect the QA401 to a product that has its ground reference more than +/-50V from the PC ground.

I realize this is about PC ground, but does this also imply that it’s safe to have up to 50V on the +/- inputs relative to analog ground as long as you don’t exceed 26dBV across the +/- with the attenuator engaged or 6dBV without?

Hello , i build me long time ago this little Puppy up with 10 8,2 Ohms 50Watt Resistors on an Descend Heatsink so easely you could measure short Bursts with 1000Watts or long time @ 500Watt with aktive Cooling.
Put 2 Resistors In serie with an Connector in the Middle so you could use 2 in Serie or 2 Parallel with Less Conectors needed.

This works great for me, even you dont need to solder big Wire, 1,5mm² (16Amp) is enough.


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