T15 build notes and minor errata

[Philo]

This was my third SCA preamp, but first T15. Basically everything went smoothly, but I caught a few minor mistakes in the instructions worth noting.

Step 24, page 13:
Reference to U4 and U5 should be to U5 and U6.

Options, last paragraph on page 16:
Reference to U5 should be to U4.

The biggest problem I had was thinking I had lost R28 and looking all over the workspace for it. Of course the fact that R28 is optional and not included is explained fully in the instructions, but perhaps you could add a note in the reference column in the bill of material indicating that the part is optional and not included.

Something like: "R28 [Optional--not included in kit]"

Anyway, it's working fine!