Questions about QA451

I bought a QA451b at the same time I got a QA403 and I haven’t used it quite yet. I’m hoping that you all might be able to answer some questions for me.

I plan on only using external loads when using it, but having it in the line between the amplifier and the QA403, will it add to the distortion or noise?

It provides 12 db of attenuation, making it easier on the QA403. The example used on the product page is that it shifts a 200w (32 dBV, 40 vrms @ 8 ohm) signal down to 20 dBV, so the QA403 basically sees 10 vrms or 12.5 watts. Much further away from the limits of the QA403.

So is my math correct in this? Let’s say I want to measure an amp at 800 watts into 8 ohms. My external load can take it. 800w is 80 vrms into 8 ohms, or 38 dBV. So attenuating by 12 db takes us to 26 dBV or just shy of 20 vrms (50 watts) which is well within the limits of the QA403. With the QA451 I should be able to measure well over 1000 watts into 8 ohms. Is this correct, or am I missing something here? I’m wanting to measure some high power amplifiers and was looking into buying a bunch of smaller value resistors and tapping off of them, but with the QA451 it looks like it isn’t necessary.


Hi @saabracer23, from the product page there is this picture:


The 12 dB attenuator is formed two 3 resistors, R126, R33 and R130.

The resistors are 0603, and the 3 in series gives 4.34K. We know P = I^2 * R, and so:

Max I in 1.58K => 0.1W = I^2 * 1.58K, or I = sqrt(0.1/1.58K) = 7.95mA

Max I in 1.18k => 0.1W = I^2 * 1.18K, or I = sqrt(0.1/1.18K) = 9.2mA

We’ll take the lower limit of 7.95mA.

We know V = IR, and 7.95mA * 4.34K gives V=34.5V.

So, if the voltage exceeds 34.5Vrms, we know 34.5Vrms / 4.34K = 7.95mA will flow, and 7.95mA flowing in 1.58k is 100 mW.

In other words, if you are using an external load, you’d want to limit the voltage across the inputs to 34.5Vrms for indefinite operation. Note that the attenuator resistors aren’t part of the thermal protection. So, you need to ensure your input is below 34.5Vrms.

That 34.5Vrms will be knocked down to 8.66Vrms by the input attenuator, and that sine will have tips hitting +/- 12.2V. The opamp is a OPA1656, which has a 2.25V input rail limitation, suggesting the rails need to be better than 14.5V. On the QA451, they are about 15.5V. So, you shouldn’t be seeing a rise in distortion at 34.5Vrms input.

I appreciate the response Matt. I picked up the Q451 thinking that the built in attenuator would help with higher powered amps, but it seems the QA403 can handle more Vrms than the QA451. I figured the 12db attenuator was there to help the QA403 see lower voltages, so I guess I don’t see the benefit of the 12 db attentuator.

Not saying I’m going to, but if one were to increase the size of those three resistors to like 1/2, 1, or 2 watt resistors, would that increase the wattage the QA451 could see? Kinda bummed, thought the QA451 was going to help me with the high power amps. Guess I’ll have to figure something else out.


Hi @saabracer23, the QA451 is really designed for fast automated testing of an amp, and if you don’t need the higher order filter and/or the DC current measurement, it’s hard to beat just running scope probes from the QA403 to a beefy load (with or without an attenuator) and doing whatever attenuation you need there.

Why don’t you make some measurements of your using external loads and just the QA403? And if that suffices for your needs (eg you don’t need the QA451), then contact us as Sales and Jan can work something out!

Hi Matt, I appreciate that, I intend on keeping it regardless, I have a lot to learn and I’m
Sure it will be useful to me in the end, especially when measuring class D. When measuring amplifiers the chirp method of the QA403 works well enough, so as far as I know I don’t need continuous output from the amplifier. I guess I’m just going to have to bite the bullet and buy many smaller 100w resistors and tap off of them to get attenuation, but at the moment that is confusing to me.

Just out of curiosity, it I were to replace the 0603s that make up the 12 db attenuator with larger more powerful resistors would that allow the QA451 to see higher voltages safely?


Hi @saabracer23, yes, that would definitely help a bit. But remember you have the supply rail limitations inside the QA451. So, just handling more power still means you are clipping if you push much more.

What you could is double the two input resistors (R126 and R130). Replace the 1.58K with 4.2K or similar (keep the 1.18K the same). That will give you more attenuation which is what you need. And the higher R means the power dissipation in those two resistors is lessened.

Swap the R’s and then sweep the QA451 without any load active and make sure the response is flat and you can read the attenuation at that point. I’d guess that will bring the total attenuation up to 20 dB or so.

And then check THD and THDN.

And it’s very easy to go back if you don’t like it.