Baxandall Tone Stack

[jonrhee]

Could someone explain the function of IC2A, C9 and R16 in the Baxandall section of the Glitterati? The parts in question are missing from the Quasar and i'm curious what the difference is.

The part in question is circled in the schematic i've attached.

[jonrhee]

My best guess is that it's boosting the signal before the eq section? Would that be correct?

[MullisMan]

R16 and R15 set the gain for this half of the OpAmp, it's set at 4.7 (Rf/Ri in an inverting configuration, 47K/10K) The capacitor is there to control high frequencies. This stage is boosting the signal a little before the active tone stack section.

Jeremy

[jonrhee]

That makes sense, is that the same thing that is happening on the Kingslayer?

And if so is the gain set at approximately 8 or 392k/47k?

[stecykmi]

pretty much, but the gain is more difficult to analyze because of all the components also connected to those nodes. it would be safe to say that op amp has a gain of x, and x _has_ to be a negative number, because it is inverting configuration. because it's an AC signal, a negative gain basically means the output signal is 180 deg out of phase from the input, but scaled to (absolute) value of x.

Additionally, because capacitors are involved, the gain is now also _very_ frequency dependent, meaning gain for low freq signals will not be the same as high freq signals.

[jonrhee]

Gotcha, the components involved meaning the C12, C13, R16, R17 and R18?

Are those components an EQ section themself?

[stecykmi]

Gotcha, the components involved meaning the C12, C13, R16, R17 and R18?

Are those components an EQ section themself?

they seem to eq the clean/clipped mix, although they are not related to the tone control itself.

just as an aside, the arrangement of C14 is classic in opamp circuits, nearly every opamp-based design uses them, even low gain circuits. you typically see them in the 100's of pF range and they are used to cut high end and prevent possible oscillation.