Switch on Marshall MG-30DFX power amp section

[Agung Kurniawan]

hi, I just a little bit corious on that switch. did anyone know what does that switch do?

[Phoenix]

hi, I just a little bit corious on that switch. did anyone know what does that switch do?

That switch is Marshall's "FDD - Frequency Dependant Damping".
The circuit snippet you show does not include enough of the schematic to get a full understanding of what it does.
Basically it alters the output impedance of the power amplifier IC by changing the resonant filter in its feedback loop. The amp uses some current feedback from the speaker to achieve this. The idea is to make the speaker more interactive with the power amp by changing the damping factor, and this is intended to make the solid state amp sound more like a tube amp.
Here's a link to the full schematic for anyone who wants to study it in more detail: MG-30DFX.

[dschwartz]

I saw the schem, and correct me i I'm wrong, it already has mixed mode feedback. And the switch activates a bridged T feedback filter that boost a specific frequency..100hz perhaps? For a tighter punchier sound?

[Phoenix]

I saw the schem, and correct me i I'm wrong, it already has mixed mode feedback. And the switch activates a bridged T feedback filter that boost a specific frequency..100hz perhaps? For a tighter punchier sound?

Yes, but since the load forms part of the feedback loop, it's not quite that straightforward. But you are correct in that it always has some mixed-mode feedback, regardless of the switch setting. The switch just alters which frequencies are more or less effected.

[Agung Kurniawan]

And the switch activates a bridged T feedback filter that boost a specific frequency..100hz perhaps? For a tighter punchier sound?

I get it, its a bit look like your equalite

is that any way to lower the freq boost, maybe down at 80Hz and (about) 750-1KHz for crunchier sound?

[antonis]

I think you're a lazy guy, Angung.. 

Calculate C41, R41 & R42 for bridged T, C40, R39, R38, R12 & R40 for LP and R37 & C39 for HP..

(keep resistor values the same and mess up with caps..)

[Agung Kurniawan]

I think you're a lazy guy, Angung.. 

Calculate C41, R41 & R42 for bridged T, C40, R39, R38, R12 & R40 for LP and R37 & C39 for HP..

(keep resistor values the same and mess up with caps..)

ahahah, sorry Im not into study lot of electronics formula on school. I do my best with my experience.
by the way, thanks. that helping