Phono 3 Trimmer

[Richard]

Hi craigtone and welcome.

The test/adjustment set up is,

Sig gen > inverse RIAA filter > Phono3 > scope loaded with 50K (to mimic pre-amp)

Set the scope input to display 1 or 2V p-p sine wave at 1KHz. Increase the sig gen level to display it on the screen. Switch to SW and check/adjust the trimmer cap for the best corner appearance at 1KHz, and/or try 5KHz if you wish, but it is the high harmonics which are making up the SW corner so 1KHz should be fine. It is very obvious and you will see it over/under shoot as you turn the trimmer.

Given a -40dB inverse RIAA filter such as the Hagerman; the sig gen puts out a flat signal approx 1V > the inverse RIAA filters it to the shape of a cartridge output and attenuates it -40dB to mimic a cart > Phono3 amplifies it +45dB and filters it back to a flat signal > the scope displays what should then be a flat signal again at approx 1V. That way Phono3 sees approx MM cart voltage on its input and puts out a level well within its 5V capability.