Performance Evaluation

I am not sure if this is the correct Thread to post this question, but I will give it a try.

The manufacturers of Audio (Home Stereo) go to great lengths to supply performance specifications of their equipment. How can a small shop implement these testing procedures into daily practices. My primary work is with Receivers, Integrated Amps, Preamps and Power Amps that were produced in the 1970’s & 80’s. Question is how to duplicate these test, procedure and needed equipment? Here is an example of the test I wish to perform:

Output ?

Power Band Width ?

Damping Factor ?

Distortion ?

Noise level S/N ?

Residual Noise Input Terminal ?

Max allowable input ?

Output terminal ?

Max output ?

Frequency Characteristics ?

Tone Control ?

Filter ?

Peak Meter ?

You should be able to do most of the measurements with your QA40x and a proper load. It is a lot of work to measure most of the items on a receiver or integrated amp. I
Would not measure everything that you have listed there. You may want to download a copy of the old IHF standards for reference. You can see what I measured on a receiver a while back that had separable Pre Out/Main In jacks here:

Absolutely exactly what I have been searching for! Now it would greatly help for a step by step breakdown of the QA settings for each test.

For the QA403

I have used this as an example for what I have been asking for “the step by step procedure”. I know this example is for a calibration procedure, but I believe it describes the format.

Preamps, integrated amps and (power) amps all require a different procedure for the QA40x depending on what you are going to measure. There are several papers and a few videos that can be found on the wiki page:

I have thought about using the TRACTOR to automate the measurements, but there are many things that need to be done first so I am not sure how useful it would be for what I do. THe best way is just to jump into measuring a device and see if the data looks reasonable. The big caveat is when measuring power amps to make sure you do not exceed the input voltage rate of the QA40x. Typically when you are measuring over about 100w/8ohms you will need to make sure the signal going into the QA40x is attenuated and that amount is inputted into the dBV sections input gain as a negative number.

Hi @YamahaCA, to add to what VAR has noted, a good place to start is to pick a single measurement you’d like to make, and ensure it makes sense when looking at just a tone. The post below goes into some detail on measuring a 1 kW amp–noise, gain, THD versus power. It was done on the older software and hardware, but much is still applicable today.

So, as a starting point, let’s try and make a gain measurement on your amp. Probably the amp gain will be under 30 dB of gain, so start with a -30 dBV signal into your amp, have the amp connected to the desired load. The QA40x L+ output goes into your amp. The QA40x L- input goes to the speaker (or load) minus, the QA40x L+ input goes to the speaker plus. Crank the amp volume to max.

Then, on the QA40x software do the following:

  1. File->New Settings puts the SW into a known state
  2. Right Click on the GEN1 button and set the amplitude to -30 dBV
  3. Left click on on the GEN1 button to enable the generator
  4. Left click on the RIGHT button to turn off the right channel display
  5. Click on the measurement tile area to add a measurement, and add the L:Gain dB measurement
  6. Do control+space to run a single cycle.
  7. Do an Edit->Copy Bitmap to Clipboard->Copy image 800 pixels wide. Paste that here.

I did the above on a QA461, which has 20 dB of gain. The resulting plot is shown below:

Please see if you can make this measurement on your amp.

Loop Back Test

Yes, OK. So in loopback the gain is shown to be precisely 0 dB, which is what you’d expect. Note the THD and THD+N are being driven by the noise floor of the analyzer, which with the attenuator on is artificially high.

Can you please try the steps I outlined above on your amp so we can learn the gain of your amp?

I am working on getting the Amp hooked up.

The S/N & THD-N numbers in the loopback appear to be very high (way out of range)

Yes, because the attenuator is active. As you increase your input range, your noise floor increases too. That is because the dynamic range remains constant. So, if the ADC has a dynamic range of 120 dB, when your max input is 42 dBV, then your noise floor is 42-120 = -78 dBV.

And if your max input is 0 dBV, then your noise floor is 0 - 120 = -120 dBV.

OK, great! So, now we know the gain of your amp and it’s 23 dB!

Next, keep the same setup. But let’s test at 5Vrms = 13.97 dBV output. Into 8 ohms that will be 3.125W, and into 4 ohms that will be 6.25W.

Keep the same setup as before. And now do the following:

  1. Right click on dBV button and enter your load impedance.
  2. Since we want 13.97 dBV out and your amp gain is 23 dB, we set Gen1 to -9.03 dBV
  3. Add the measurement tiles L Pk Pwr Watts and R: Pk Pwr Watts
  4. Add the measurement tiles L:THD % and R:THD %
  5. Since we know the output will be 14 dBV, set your full scale input to be 18 dBV

With everything set up, hit the spacebar to start running and then share the screen.

Not sure how to set at other GEN values than preset jumps 1dbv

You can set the Gen to a precise level by right clicking on GEN1 and entering freq and/or level dirctly.

If you have a touch screen, you can hit CTRL+1 and enter amplitude directly there with your fingers on the touch screen.

OK! So your plot looks good. At 6.3W out, you are at 0.09% THD, and it’s dominated by the 3rd harmonic. If you’d like a summary of the harmonics, make sure you are running (space bar) and then go to Visualizers->THD Bargraph Display.

What is the max power of this amp?

Now we can do a sweep. We know the gain is 23 dB and we know the load is 4 ohms. Let’s sweep up to 10W. 10W into 4 ohms = 6.32Vrms = 16 dBV. And we know you have 23 dB of gain. So, that means we want to end at 16-23 = -7 dBV.

And let’s start 30 dB below that, or -37 dBV.

Using the same setup as before, do the following:

  1. With acquisitions stopped, click on Automated Tests->Pwr THD versus Power.
  2. Select Defaults in the upper left of the dialog that opens. That puts us to a common point.
  3. Set your start level to -37 dBV
  4. Set your stop level to -7 dBV
  5. Set load impedance to 4 ohms
  6. Uncheck “Measure THD+N instead of THD”

Leave everything else as defaults. Click OK and share that plot. This will step from -37 to -7 dB, with each step 2 dB hotter than the last. With “Enabled Early Abort” checked, if the power out is greater than 1W and the THD is worse than -20 dB, the testing will be stopped automatically.

Max power is 250w into 4ohm

Those numbers are far from spec. Any ideas of why?

OK! So, using a single tone, we know the distortion was 0.09% at 6.4W, and from the plot you just posted, it shows the same. Note that we were aiming to do a THD test (not THD+N) and your plot shows a THD+N. However, the THD+N is dominated by harmonics in your amp rather than noise, so THD and THD+N are going to be about the same. But for next time, make sure you follow step 6 (“Uncheck…”).

Now, if the amp is good for 250W into 4 ohms, that is 31.6Vrms = 30 dBV. That is on the edge of not needing an attenuator. But let’s run it and see what happens without an external atten.

Press F3 to re-run the last test. This time, put your start level to -40 dBV and your stop level at 6 dBV. The last step will be 6 + 23 = 29 dBV = 28.18 Vrms = 198W (into 4 ohms). Remember to run THD instead of THD+N.

And of course, make sure your loads can take this power, make sure you are ready to press the Cancel button while the test is running if something goes south, etc. The abort should kick in if the amp complains.