#12 [Repair] DIY Pass F5 audio amplifier repair and distortion measurement

I repair a F5 audio amplifier that a friend built with me many years ago. It turns out to have a faulty design which I partially remedy. I then use an external Creative EMU 0404 USB sound card to measure the distortion and frequency response of the amplifier. I use the Room Eq Wizzard software to make these measurements and confirm the amplifier is "in spec" while also noting some other "issues" for future work.

The analyser software I used https://www.roomeqwizard.com/

The DIYAudioStore links for everything I used (no commission, they're just a great outfit to support)
The F5 boards (and guides) https://diyaudiostore.com/collections/power-amplifier/products/f-5
The amplifier case https://diyaudiostore.com/collections/frontpage/products/deluxe-ultimate-amplifier-chassis?variant=39313326211145
The matched JFETs (you need an NP quad) https://diyaudiostore.com/collections/frontpage/products/matched-jfets
The power supply https://diyaudiostore.com/collections/frontpage/products/universal-psu
The soft-start and speaker protect boards https://diyaudiostore.com/collections/printed-circuit-boards/products/soft-start-speaker-turn-on-delay-and-dc-protector-boards

Mr Pass' F5 Manual https://www.firstwatt.com/pdf/prod_f5_man.pdf

And as an added bonus, his board to add(!) second harmonic distortion (!!) https://www.firstwatt.com/pdf/art_h2_v1.pdf

0:00 Intro
1:27 Teardown
5:31 Soft-start testing
8:03 The dropper capacitor
9:57 Simulations
13:49 Testing with the HP35665A
16:50 The EMU0404
17:53 Setup of REW
20:44 1kHz distortion test
22:42 Sweep response
24:57 Outlook/Conclusions