Question on QA400

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I went through a few threads on this here but I still have questions. I just started looking into method to test THD, my experience in the pass was using spectrum and network analyzer at work. Obviously those are a little too expensive to buy for home use. So I am absolutely a newbie in all these sound card, and low budget equipments.

I read complains that the QA400 has only single ended input. For power amp, why is it important to have differential input? Signal is large. AND even if the ground noise show up, it's going to be a peak at 60, 120Hz. People should know those are ground issue. It should not affect reading at 1KHz and up.

I don't want to build a differential probe with opamp if I can help it, also I try not to spend $79 to buy the QA190. Attached is my interface circuit I think will work.

The top one is from the output of the power amp to input of the QA400. I use a 4ohm 100W load, then I use lower resistance value to do the dividing. I put the 470ohm in series to current limit and use the zener to limit to 4.7V or so. I have a rotater switch to select divided by 2, divider by 10 and 100. Don't read too much in the resistor values, it's approx.

For input to the amp, I take the frequency output from the QA400, just drive through a volume pot and drive the amp.

I do this is all passive so I don't add any distortion. I don't isolate ground. My laptop is powered by two prong plug, there is no ground. I don't need to worry about ground loop. worst come to worst, I unplug the laptop and run on battery!!

What do you guys think?

Thanks
 

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I also have a THD power attenuator actually designed for guitar amps to attenuate the power driving to the speaker. It is basically a power volume pot between the amp and the speaker. It has compensation inside to make the load reactive like a speaker. It even has line out. I have to look deeper into whether this can drive the QA400 directly.

This is for guitar amp, I don't know how well it will work. Also sadly, it's a 8ohm load, not 4ohm that I want.

Any guitar player have experience with this?

Mind you, this is for guitar amp with speaker that has NO CROSSOVER, usually the speakers are 12" with no mid and tweeter. The impedance might be a whole lot different from a hifi speaker. It's just so nice to all the load resistor, switches all packaged up in a box.
 

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I know this thread is long, but towards the beginning, there is voltage divider information and a schematic by 1Audio. Good reading, but a bit long.

I read it and I downloaded the schematic. That's the reason I posted this. I don't think I need all that as described in detail in the OP.

On top, the schematic shows it drive through a 1K resistor. The AC input impedance is 10K, that itself creates error.
 
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Your attenuator will work fine. I would skip the divide by 2 step. There is still potential to damage the QA400 (2VP max really) and its noise and dynamic range is good enough to work with a divide by 10, and then the display is scaled correctly if you follow the instructions for setting it.

Add a 100 Ohm resistor in series with the ground so a balanced amp or other mishap won't fry anything.
 
Your attenuator will work fine. I would skip the divide by 2 step. There is still potential to damage the QA400 (2VP max really) and its noise and dynamic range is good enough to work with a divide by 10, and then the display is scaled correctly if you follow the instructions for setting it.

Add a 100 Ohm resistor in series with the ground so a balanced amp or other mishap won't fry anything.

Thanks for your answer, you are the one that really tested the QA400. I am waiting for answer from QuantAsylum about the 10K input impedance. In my circuit, I actually has 470ohm to try to protect the input, I hope it's a typo and I can raise to 1K like yours or even higher. They are just very slow in reply from reading the other posts.

I am still reading your long thread a little at a time.
 
For what it's worth...it took me a while to get to where I am at with device testing. Many members have helped and I thank them all the time.

Here is one of my amps after a repair. I wanted to see when I would reach 1% distortion; this was around 250 Watts. Top image is ARTA scaled to match my HP3850a spectrum analyzer (bottom).



 
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