preamp stage circuit analysis

[joesatch]

what is going on here after the 500k volume pot aside from the mute to ground switch?

[Dave]

A little bit of treble bleed and then nothing.


Dave

[pdf64]

Frequencies below about 9Hz get a 4dB boost.

[shooter]

below about 9Hz get a 4dB boost.

must be a subliminal messaging app

[Dave]

Maybe I'm looking at it with my morning brain. I realize now that 47nF bleeds about everything, not just treble, but bow does it boost?


Dave

[pdf64]

It’s a shelf filter, a frequency dependent potential divider.
Content much below 9Hz doesn’t get divided, whereas above 9Hz it gets reduced, 390/(220+390) = 0.64 = -3.9dB.

[sluckey]

Frequencies below about 9Hz get a 4dB boost.

Maybe a good addition to a weak tremolo circuit.   

[PRR]

Frequencies below about 9Hz get a 4dB boost.

How? 4dB re: what?? The signal never gets out at all.

[pdf64]

Frequencies below about 9Hz get a 4dB boost.

How? 4dB re: what?? The signal never gets out at all.

But by the same token, the signal never gets in either.
And the valve has no grid leak path shown, so the stage couldn’t operate.

We do our best with the info snippets that slip past the gatekeeper of the opening post 

Which in this case mentions a mute switch. So I assumed that the output was taken from the contacts of the only switch shown 

The 4dB boost was with respect to the level of audio signal.

[joesatch]

well that was the last stage and output of the clean channel .sounds pretty fantastic. i'm incorporating this 10ohm isolated ground on everything going forward

[shooter]

10ohm isolated ground

most everything I worked on where digital and analog lived on the same board/environment used a ground lift/isolation for the Digital side, but i'm confused how you got here from the shelf filter?

[joesatch]

if you look close i have a floating chassis inside the chassis. plastic standoffs. no part of the preamp circuit is touching the outer chassis. silent as a mouse. power section (rectifier, PS) is grounded to the outer chassis via Hum-loop block. All preamp grounds measure 10 ohms resistance to outer chassis

[shooter]

oh I found the resistor ok, have much experience with the benefits, just trying to figure out how the snippet from the pre lead to the 10 ohm R.  That's the kind of "thought process" I have, and there are few people that i've met that use that process