QA403: Assistance needed in load configuration

Hi Everyone,

I recently got a QA403 and I would like some verification on my thoughts before I order parts for my loads to do power tests. Is there a sweet spot to get the target voltages for the best analyzer performance?

The maximum voltage input of the QA403 is 40V RMS and I intent to test power levels as follows:

  1. 100W to 1200W RMS into 4 ohm. This translates to 20V - 69V RMS. Attenuation is needed in the load via a voltage divider to get the target voltage under 40V RMS.
  2. 800W to 5000W RMS into 1 ohm. This translates to 28V - 70V RMS. Attenuation is needed in the load via a voltage divider to get the target voltage under 40V RMS.

Is there something like too much attenuation? Can I use the same attenuation design in the load for the 70V tests and 20V tests?

My plan was to buy four 4 Ohm 1500W braking resistors, but if I also need to design attenuation, which affects how many resistors are needed for the voltage divider. Instead, I may need eight 2 ohm resistors instead with one 0.1 ohm resistor. Am I thinking correctly?

Edit: added category

Hi @canasound, yes, you are on the right track. Have you seen the post linked below? It has a bit more on some of the tradeoffs? The max input to the QA403 is 40Vrms, but I’d aim for the your max to come in perhaps 4-6 dB below to ensure margin as distortion will begin to rise as you approach the limit of the input range.

Yes, if you attenuate too much then you hurt your SNR. But realistically, you could even attenuate down to 6 or 12 dBV and still be fine. You might notice the 50/60 Hz hum show up the more you attenuate.

The second link has some info on Arcol loads, which are really inexpensive, durable, non-inductive and nice to work with.

Thanks @matt

Post 1of 2

Here is what I came up with. In this table, I listed the expected loads in Ohms and the power that I am expecting for each device. This is translated to the Voltage, then dBV. Based on your post, I am ideally targeting 6 to 12dBV.

Post 2 of 2 @matt

Then based on the target range, I need several separate attenuation configurations as shown here:


Using the highlighted load configs for the specific load tests, I would end up with input dBV of between 4 and 16dBV.

Is my assumption of needing all these different load configurations correct?
Or am I over thinking this and a more general approach needed?

Hi @Canasound, these tables are very helpful. What I would do is target the highest-power case first (your 37 dBV) and knock that down to 12 dBV, and then measure on the 18 dBV full scale input.

And then, use the lower full scale input settings with the same attenuator and verify you have the margin needed. For example, if you have a 25 dB attenuator (needed for the 37 dBV case), then at your 10 dBV case, the input would be -2 dBV, and then measure that on the 0 or 6 dBV input.

That would give you about 120 dB of dynamic range to measure. And probably 99% of amps will fit comfortably inside that range.

Alternately, at your 20 dBV level, you could go straight in at 24 dBV full scale input and would probably still be able to measure the noise floor on most amps.

Are you looking to automate? Or will these be manual meausurements?

Thanks @matt. I understand now.

I would be doing all these manually since these are different DUTs and needs individual setup in most cases. I just wasn’t sure if there was a sweet spot for getting my input voltages at for the best analyzer performance. I will work with attenuating to target inputs in the range of 6dBV to 26dBV.

Hi @Canasound, your question is definitely the right question to ask. Generally, you can group the full scale inputs into two groups: One group has the attenuator “off” (0…18 dBV full scale input) while the other (24…42 dBV full scale input) has the attenuator on. The self-distortion will roughly be the same as long as you are the same level below your full scale input. That is, when in 0 dBV full scale input mode, then a -5 dBV input would exhibit similar self-distortion as a +1 dBV input in +6 dbV full scale input (assuming you had perfect source with artificially introduced distortion).

The big difference comes in noise floor. When the attenuator is on, the noise floor is higher. So, if you have a super-high-end modern Class D amp you’d probably want to pick an atten that would let you measure with atten off (0…18 dBV full scale input).

Thanks again @matt

For anyone else who are following, this is my decision on the load configurations:
I would be able to test at 4 ohm up to 1200W in two channels, 2 ohm up to 3200W into two channels and 1 ohm up to around 6000W with the following load hardware:

4 x 1500 W 0.9 Ohm
2 x 200 W 0.2 Ohm
2 x 200W 0.4 Ohm
4 x 500W 1.8 Ohm