automatic tuned notch filter for THD measurement

[PA0NVD]

There are several treads about AF quality, modulators amps etc. I did make quite a few tube HI-FI amps and had the desire to measure distortion, especially so that I could SEE what the source of the distorsion was. So I made a simple self-tuning notch filter based analyser that can measure from 10% down to 0.01%  The construcion has no PCB, the analyser is made at non etched PCBoard, many photos in the link. The analyser has an output after the notch filter so one can compare the amplified distorsion signal with the original signal at a two channel scope in order to see where the distortion originates. Real fun to play with and (for me) an extremely instructive experience
The analyser needs a clean and reasonable stable AF generator tuned at 700 Hz. You can use the analyser to see how clean the signal is. Small variations in frequency are automatically tuned.
Here is a link to the analyser
http://www.diyaudio.com/forums/equipment-and-tools/298895-simple-automatic-notch-filter.html
It did cost me less than 1 day to make the analyser, so acceptable
attached the first 3 pic of the distortion meter
ATTENTION
There is a small error in the second diagram Notch 2. The "NOTCH" signal enters an opamp which is coupled to a second opamp via 3.3 uF. There should be a resistor of 3k3 in series with this 3,3 uF capacitor. My error in drawing Sorry 
As you will see in the next pics in the following post, the building is very simple. gluing the IC's upside down at a non-etched PCB and gluing islands as connection points.
I glued power rais left and right of the circuits for the + and _ 7 Volts
Just build in the same order as the diagram has been drawn. Super fast and the best ground plane possible. May be you should build a little bit larger, I have a problem that I always tend to build too small...

[PA0NVD]

Here the schematic diagrams
I got the comment that the effort isn't worth it because there are many THD analysers available . That was not the case in Europe and I enjoyed myself very much designing , building and using. After all, it is my hobby...The project did cost me some junk box stuff and approx 10$
There is also a scope pic of the measurement of my Levell generator. at the 0,1% THD range. The total distortion was here 0,03% You can see that a big part from that distortion originates from compression of the neg. sine half
I also measure AM distorsion of a small AM transmitter by rectifying the RF at high level in order to minimize the distortion due to the diode. One could clearly see several sources of distortion, one saturation of the Iron (Heising modulation) and positive peak compression due to low drive etc. Very interesting