Simple bar graph of 2nd, 3rd, 4th, 5th harmonic dist. profile?


I’m contemplating ordering a QA403. Can the software give me a simple graph of the fundamental, 2nd, 3rd and 4th harmonic distortion levels side by side? So as I experimentally change the operating point of a vacuum tube in real time, I can dial in at what point the 3rd harmonic is lowest and the 2nd harmonic is highest? Tube amplifiers sound best when the 2nd harmonic is dominant and 3rd harmonic is lowest. We’d like to be able to experimentally dial-in the bias point, plate voltage, screen voltage, cathode resistor, that gives the harmonic profile we are after, basically high 2nd harmonic that cascades down to very low odd harmonics. If this $600 device can do that one thing, it would be worth a $10,000 audio analyzer. Knowing THD is of little use for us, we need to see the harmonic distortion profile in real time and as a simple percentages bar graph if possible.



Can be done in real time for left, right and both channels with the THD bargraph visualizer.

Now you can send @matt a check for $10,000 and I’ll get some commission hopefully. :slight_smile:


Great! Your check in the mail! This is exactly what we need. The example you posted is typical for a push/pull Amp which will tend to cancel out the “good” even harmonics leaving the “bad” odd harmonics dominant, so you get that high/low/high/low cascade seen here. This is why push pull Amp sound harsher than single-ended amplifiers.

In a single ended amplifier the ideal profile would be to bias so you achieve a high level of 2nd harmonic sloping off fast the odd harmonics, so a downward slope, not the hills seen here. We are working with single ended vacuum tube amplifiers.

Thanks for the help, this is truly going to fill a big gap.

The image I posted for you was not an audio amplifier, it was a wein-bridge based oscillator I had lying around on my bench.

Glad to help.

For comparison, here is a harmonic plot for an old Radio Craftsmen tube amp. I typically use the dBr mode so there is no numbering of the levels.

Is the presented harmonic plot displayed in real time? If it is displayed continuously, in real time, it can be used for adjustments, if it is about short recordings, it is much more difficult.

Hi. Yes, I confirm that it is displayed in real time.

That’s why I wrote “real time” . :slight_smile:

Obviously, you shorten the FFT size for a faster update.

Thank you for the information, in this case I consider it extremely useful.