QA402 Feature Request

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I’ve been using the QA402 for some time now and am happy with it BUT!!!

It would be really, really nice if the displays read out distortion as a percentage and voltage as actual volts in addition to (or in place of if desired) dBV, which honestly is rather useless to me. Audio Precision analyzers do this and of course other applications do also. I’d like to use this here at Amplifier Technologies, but converting units is time consuming and a deal killer.

Apologies if this feature request has already been mentioned - if so, I’ll shut up and wait for these. :smiley:

22

You can display rms voltage and THD as a % in the measurement window:

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I am not a fan of dBV but have a spreadsheet formula) that I can easily plug a value into and get watts into 8ohms (or whatever).

23

Thanks - its obvious now, but it would be more intuitive to be able to right-click on the THD measurement button on the left panel to get this option.

Another thing which would be nice is if the program remembered its last-used settings when re-launched (rather than having to manually load them each time the program is launched), and especially if the automated tests graphs remembered their X and Y ranges or at least that these could be saved for quick retrieval.

24

So, you can save your settings and then re-load them (FILE>Save Settings), but it does not remember your X & Y ranges- that would be nice. A trick that I was told is is to draw a rectangle over your area of interest, say a Frequency Response plot, and it will zoom in. But if you then adjust the X or Y axis it will zoom back out. Also, if you hold the ALT key down while changing the up or down Y axis key you can get 1dB increments.

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Thanks, that’s helpful. They need a comprehensive manual so we don’t have to discover this stuff on the street. :thinking:

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I agree. I am retired and have a lot of time and asked a lot of questions and Matt has been pretty good about getting back with me and so you are benefitting from that :slightly_smiling_face: For me, a hobbyist, the QA402 has been some of the best spent $'s ever! Hopefully as time permits, Matt will make some changes to Axis resetting and keep the limits in the Save settings. At some point a Youtube video on its use would be great- a future project for me, maybe…though there are some videos on some applications:

27

The alt and ctrl keys work on almost all of the up/down toggles to change range, so that’s good. Still there are weirdnesses such as having to go to “dBV” in the axis settings to change the speaker impedance for wattage measurements. That’s not necessarily intuitive.

I took the 402 out into the lab and its readings matched up well with our Audio Precision APX analyzer, so that’s assuring.

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That is good to know that the readings matched the way more expensive AP! I am curious why you have two analyzers, if you are able to share that. If you have never seen this review of the QA401 you may find it interesting:

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We are an audio power amp / HT processor manufacturer so we have probably half a dozen APs in the lab and in the production area.

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Pretty cool! I am guessing you need more test capability and wanted to try out something that was more cost effective (assuming it has the performance you need) than the AP. I certainly understand that! I am an RF/Microwave Hardware and test engineer (retired for now but that looks like it will be changing) and RF Test equipment is pretty expensive, by and large. Sometimes I was able to find something from a small outfit that had the performance I needed at a WAY lower cost.

31

The idea was to be able to have a portable measurement rig to be able to take out to the production line to test a suspect unit instead of lugging a 100+ pound amplifier to the lab. The QA402 appears to be adequate for this task with the exception that most of our amplifiers exceed 200 watts output (40V) so couldn’t be full-power tested. This isn’t a deal killer but it is an unfortunate limitation.

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Actually, regarding the 40V limitation, do you know if this is for each side of the differential input or differentially across the +/-? If each +/- side side could tolerate 40V, would that mean that a fully balanced signal of 80V could be tolerated?

33

I have not tested anything over about 60w with the QA402. The top of the QA402 pretty well describes the voltage limits. I built my own 400w, 8ohm loads which have a 0.1ohm sense resistor in the middle which you monitor across , connecting to the QA402 when measuring High Power amps. You can offset the measurement for this. Here is the app note for it:

I have not tried it yet, but have a 200w/ch amp that crying out to me to test it soon. I assume the QA451 programmable load, which can do 200w for 220ms, would not work for your application…

34

Hi @Rammis, just to capture things here and build on @VARs comments:

  1. the @VAR noted there’s peak voltage, but there isn’t currently an RMS voltage. But you are right, the RMS voltage display is needed. it will be added to the next release

  2. agree, the THD and THDN buttons should let you specify the defaults you prefer and persist those.

  3. There are some UI changes coming for cursors and markers, and until the UI gets settled for the 1.0 release, the manual is a second priority. But please feel free to ask questions here as they help everyone. Or, send a note to support

  4. I hear you both on axis settings persisting on through re-start!

  5. The DBV button is the one Y reading that is absolute, which is the rational for some stuff fitting in there. But agree, there are a few corner cases that aren’t intuitive.

  6. There is a mode on the QA402 that you can specify from the command line with the flag -nolimits.

With that mode active, you’ll see a startup message and then you’ll see the “CAUTION!” message on the display (picture below). In this mode, the attenuator will always be on and the lower input ranges are are disabled. But, you can use this mode to hit higher levels. We have verified up to 125Vrms, but it’s not something verified on each unit. Remember the analyzer is a CAT I device (the weakest safety rating). the input caps are bipolar and rated to 50V. But short periods at higher AC voltages they don’t mind (DC voltages much beyond 50V does cause them stress, however).

The voltage limit of 32 dBV takes into account that you might select the 0 dBV input range with with 32 dBV applied. Beyond 32 dBV with the attenuator off, you begin to overstress the input protection. So, software artificially limits the input signal to just a bit beyond 32 dBV.

Let me know if you’d like to understand this mode a bit better. In short, in this test mode, you can go to much higher levels but you have to make sure the attenuator is on (LED active on front panel).

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35

Matt,

Thanks for the reply. Yes, it would be very helpful to be able to measure beyond the 40V limit since many of our amplifiers go up to and beyond 1000 Watts; if we could measure 500W, that would be nice. So if you could go into the setup for this a bit more, that would be appreciated. For our case, a menu item to turn the attenuators ON all the time would be great, but I totally understand the safety/certification restraints would probably prohibit this.

Since we’re on the topic of wish list features, I’d like to see the main analyzer display and control panel be separate windows, so those of us using dual displays could drag the controls to a secondary screen, thus allowing the analyzer display to be maximized on the main display.

Thanks again!

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That is a good idea, but I hope the FR markers come first. :slightly_smiling_face:

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Dear Matt, That is great news, however I still do not see, wether the 32dB (42dB) input limit is for one single ended input or between the two inputs?
So with the 32dB Limit being 40Vrms is it then 80Vrms if I measure a fullbridge amp connected to - and + symmetrically? And is it then 250Vrms with the attenuator always on?
Thanks!

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FOR THOSE JOINING LATE, THE DISCUSSION BELOW IS FOR A SPECIAL TEST MODE ONLY THAT WILL REMOVE ADC LIMITS. IT SHOULD ONLY BE USED BY ENGINEERS AND TECHNICIANS THAT UNDERSTAND THE SAFETY IMPLICATIONS AND SHOULD ONLY BE USED ON LOW-ENERGY CIRCUITS. THE NORMAL MAXIMUM INPUT TO THE QA402 IS +32 DBV AS WRITTEN ON THE TOP CASE AND FRONT PANEL.

Hi @ttako,

Let’s start with an example: When you apply 1V rms (no attenuator) to L+, that is converted by a fully differential amplifier (FDA) to two 0.5Vrms signals balanced that go into the ADC.

Similarly, if you apply two 0.5Vrms differential signals (no attenuator) to L+ and L-, that will be mostly untouched by the FDA and will gain result into two 0.5Vrms signals into the ADC.

So, the ADC doesn’t know if you are inputting an unbalanced (single-ended) or balanced (differential)

The internal rails on the QA402 are about 13.5V, and the front-end passive attenuator is 24 dB. So if you input a single-ended 24 dBV signal with the attenuator active, it will be reduced to 0 dBV by the attenuator, and the FDA will convert it to two 0.5Vrms signals (balanced) just as above.

In the test mode, if you input a 42 dBV signal into the L+ alone (with L- grounded) with the attenuator active, the attenuator will knock that down to 42-24 = 18 dBV = 8Vrms. The tips of this signal will be around 11.2Vp, and this is within the common mode range of the opamps with the +/-13V rails. That 8Vrms signal will be attenuated by an active attenuator to 0 dBV for the ADC.

So, the short answer to your question is: In the special test mode, you can apply up to +42 dBV to either the L+ input (with L- shorted) OR the L- input (with L+ shorted) OR a fully differential signal of 36 dBV = 63Vrms to L+ and L-. All of these will be reported as a +42 dBV signal.

If you are not in the special test mode (and thus +32 dBV max), then the maximum input to L+ is +32 dBV (L- shorted) OR the L- input (with L+ shorted) OR a fully differential signal of +26 dBV = 20Vrms to L+ and L-. All of these will be reported as a +32 dBV signal.

Note that the QA402 overload characteristics (in terms of signal quality) will be similar to applying 0 dBV to the QA402 with the 0 dBV full scale mode selected. In other words, when you are at the limit, clipping may be happening.

Finally, remember to limit the time at the higher AC voltages. The longer you subject to the QA402 to the special test mode voltages, you increase the risk of the front-end aluminum caps degrading. This will manifest as the low-end response of the QA402 increasing.

If you need to make repeated measurements at very high voltages an external attenuator is your best choice.

Let me know if you have more questions!

1 Like
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Dear Matt,

Crystal clear! Thanks for the detailed answer! I will go with an external Optional Attenuator! Similarly I also plan to use a low noise amplifier (10 to 20 dB max gain) for higher precision Noise and THD measurements. One more question tough about the inputs:
Its written, that they are isolated. What exactly does this mean?
Are they isolated from each other as well as form the Ground or just from the ground?? So Can I measure fullbridge amps with a single ended input? I know, I could test it as soon as my QA402 arrives, but that takes still time, as I ordered it just today :slight_smile:

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Hi @ttako,

The “isolation” means that if you take DVM and measure from BNC shell to USB shell, you should see an open. More specifically, if you connect ground to USB shell, and apply 1 KV to BNC shell, zero current should flow indicating >10 GOHM. This test is usually done with something called a “high voltage insulation tester” or “megger”. In any case, I don’t recommend doing the insulation test yourself. But you can use a DVM at anytime to verify there is no connection between USB shell and BNC shell. This helps to avoid ground loops with the PC.

All the BNC shells are connected together.

The link below discusses considerations for amp measurements and how to make the measurements. How many watts is your amp? When you say “full bridge” do you mean the + and - of the amp are equal and opposite?