Power Amps : Updated for QA403?

This article >> https://quantasylum.com/blogs/news/basics-power-amps covers a lot of exactly what I am trying to do with the QA403/451. However, most of the screen shots and menus have changed considerably from the original 403 when this was put together.

I’m having some difficulty getting the levels calibrated to the point where they make sense. For one example, the 451 states an attenuation of -12dB through the load box. Ok, fine. So I put that into the DB offset and the results are not correct (verified quickly by an oscilloscope reading as compared to the RMS volts indicated, and yes corrected for the peak to rms difference :slight_smile: If I put in an offset of -18db, it’s closer. Still playing with those numbers, but what’s the correct way to actually calculate the proper offset? I’m using the load box at 8 ohms at the moment.

Etc … the real question was, if the original author had any time or intention to re-create this excellent tutorial for the newer software. Hmm, maybe I should find a 401 with the original software lol.

Anyway, thanks for looking :slight_smile: Barry

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I put together a video that may help a bit with the QA403 basic operation, but I don’t have the QA451 so that is not covered. IF you go to Youtube search for “Using the QuantAsylum QA40x Audio Analyzer Pt. 1” and you should find it- hopefully it will help…

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Hi @BBTV, yes, this should be replicated for the QA402/QA403. Another user recently asked for more info on grounding strategies two that should be a part of this article.

For the QA403 and QA451, you would do the following:

  1. Right click on the DBV and enter -12 as the external input gain


Now, if you are in single-ended loopback (no QA451) with a 0 dBV signal, you’ll see:

There are a few things to note here. First, our generator is at 0 dB, but we’re showing 12 dB of amplitude. This is because we’ve told the analyzer that something external to the analyzer has -12 dB of gain. And so, the 12 dB measured reflects that.

Now, take the L+ output of the analyzer and route it to the L+ input on the QA451, and take the BNC shell ground of the L+ output and route it to the L- input on the QA451.

In the QA451 app, make sure both 8 ohm and 4 ohm loads are off.

In this configuration, the QA403 is driving the QA451. Since the 4 and 8 ohm loads are off, the QA451 is presenting a ~4K ohm load to the QA403, which is no problem.

Now, run another acquisition and you’ll see the setup shows 0 dBV out, we’ve specified an external gain of -12 dB, and we measure -0.3 dBV.

We could further tweak the gain to get a true zero. For example, if I specify the input gain is -12.3 dB, I’ll get a reading of 0 dBV input. Your unit will vary a bit so tweak as you see fit.

With the setup above, I can now connect the QA403 output to an amp (remember to set level to a very small number for starting out: -30 dBV or so). And then connect the amp output to the QA451, select a load in the Q451 app (4 or 8 ohms) and run a test. Remember to set the load impedance you have chosen in the DBV context menu (right click on DBV). This will then let you read power in watts if the add the Watts measurement.


Hopefully the above answers your question on how to calculate the proper offset.

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Scott, thank you, that’s a helpful video.

Matt, thanks again for the QA453 :slight_smile: And your excellent support. I will probably follow up with more questions. One at the moment; I am using 75 ohm terminators instead of the suggested 50 ohm. I will make up some 50’s and test this myself, but would this difference cause any of what I’m seeing as db offset above the nominal -12dB of the 451?