Total Harmonic Distortion—Rough Measurement for Better Cartridge Alignment and Sound

Reading another thread about alignment methods, I remembered a discussion on another audio forum where I had been using a test record and the Vonexgo SPAN, free VST plug-in. I noticed that a 1K tone showed a second harmonic at 2K etc on up the frequency scale. I assumed it was harmonic, but had no idea. Well, I discovered that it was harmonic distortion AND it could be used to make a rough calculation of THD (Total Harmonic Distortion). I decided to use this as a tool when doing cartridge alignment, and it has proven useful.

If you don’t know about this and would like to try it, you need an audio program that uses VST plug-ins (there are many free ones), the free Vonexgo SPAN frequency analysis plug-in, and a test record with a 1K tone. I have a few, and do note some variance depending on where the track is located on the record. The following data is captured from the Hi-Fi news test record (level -14db). Note this is not a calibrated measurement, but it is useful in comparing alignment methods if you accept that the data is relative.

Here's the 1K tone, and the harmonic. I've expanded the window and used the crosshairs to get as accurate number as possible.



Now take the difference between the peaks. In this case, -42.8 db, and plug that into this handy calculator.

http://www.sengpielaudio.com/calculator-thd.htm

The results:



Not too bad. It is commonly considered that 5-10% is audible, and my ears confirm this. I've used this method to catch a bad alignment before. Anyway, it is useful to me, and so it may be to others. Please do post your own findings and/or add info that has helped you get better cartridge alignment. Cheers.

 

Total harmonic distortion isn't just the 2nd harmonic - you probably have higher THD.

Not sure I really care much for THD as an indicator of much. It's the nature of the harmonic distortion that matters (even/odd/etc). Cue all the folks who will tell you how low THD in early SS amplifiers didn't necessarily translate to better fidelity...

 

I agree, pretty much. Just looking at some ways to fine tune a cartridge. It is easy to chart a long series of harmonics, so methods of calculating would be of use.

 

This isn't a new notion. Sound Technology promoted it in the 1970s or early 1980s and it was generally found to be not terribly effective. Of course ST was trying to sell its expensive test boxes.

 

Right. Didn’t intend to imply it was a new notion. As far as effective, it is just a marker. For example, if my overhang was off I would be showing much more distortion, maybe upwards of 6 to 10%. I read all sorts of posts about alignment, and this is a super easy way to check. It also takes a total of arm/table/phono pre so you can check your source. No expensive test box required.

BY ALL MEANS, DO NOT DO THIS IF YOU DO NOT WANT TO.

 

Yes I think it´s good using measurements when they are applicable. To have a number one can rely on is satifying.

 

Simple tools. While I was testing I reworked my grounding to minimize hum. I can’t tell absolutely walking from speaker to speaker and listening close. But, I can see in real-time as I move cables and ground the tone arm or the shielding, to see what the effect is on each channel. As I stated in the original post, it is a relative measurement, variable to each system, but it can show you if you are making an improvement. I can see the whole frequency spectrum of system noise down around -90 db, including a 60 Hz hum that sits at around -87 db. Being able to see this displayed in real-time is helpful to me. No cost. No agenda.

 

I'm not saying this should not be done in a defined manner with appropriate equipment, I'm just saying a distortion box may/probably is not the thing to use.

Also, I think PC based test using commodity sound cards is one thing that should be regarded with skepticism until and unless tested against proven calibrated lab gear, and probably even then. I agree it can be used for relative comparisons but it is not designed for the same characteristics as the correct equipment, whether a standalone physical box or a dedicated card.

 

Good to emphasize. I tried to do that in the original post. I’m using a USB audio interface, and have been able to calibrate it using FFT/loopback, so I have seen how it looks—flat enough, but the noise that it is introducing is part of the tally as well.