Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on September 14, 2025, 05:05:39 pm

Title: What Is this circuit ???
Post by: kagliostro on September 14, 2025, 05:05:39 pm

I've find the attached schematic on the web

There were no explanation and I'not able to understand what exactly It Is

The upper section looks like a Preamp with lo impedance output

But about the lower section I'm not able to understand what It do, which Is the pourpose

Any explanation or idea about ?

Thanks

Franco

Title: Re: What Is this circuit ???
Post by: SEL49 on September 14, 2025, 06:02:41 pm

Looks like a preamp for a phono magnetic cartridge (pickup). It has switch selectable equalization.

Title: Re: What Is this circuit ???
Post by: HotBluePlates on September 14, 2025, 07:24:29 pm

RIAA Equalization Curve (https://en.wikipedia.org/wiki/RIAA_equalization)

Notice when Recording that Bass is cut (to keep the needle from jumping out of the groove), and Treble is emphasized.

On Playback, Bass is boosted to restore "natural Lows" and Treble is cut (reduces Hiss while also restoring "natural Highs").

There were several different curves used by different record-manufacturers, or standards-setting groups.


A Phono ("PH") signal was also weaker than other sources, and needed extra amplification.  The output of the PH-channel's EQ section feeds the rest of the preamp when the Selector is in the "PH" position.

Title: Re: What Is this circuit ???
Post by: kagliostro on September 15, 2025, 12:58:25 am

Many thanks Friends, now I understand

Thanks you very much

Franco