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Adding current feedback to a Marshall MOSFET

Started by FleshOnGear, April 06, 2023, 12:03:05 PM

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FleshOnGear

So, I went ahead and did the mod. It worked pretty well, and there aren't any stability issues. Thanks for letting me bounce these ideas off of you folks.

phatt

Great to hear it worked out well, thanks for the feedback as it will give others clues as to how to do such things.
Phil.

FleshOnGear

I guess I never really detailed what exactly I was planning to do. Here's the schematic:

On the pcb, the negative side of the speaker wires and external speaker jack go through the headphone Jack, then there's one trace from the headphone jack that connects to the ground fill of the pcb. I cut the one trace and attached a 0.20 ohm 10W resistor from the sleeve tab of the headphone jack to ground. Originally planned on using a 0.22, but mouser was out of them.

From the connection of the 0.20 resistor and the headphone jack, I attached a 100uF cap, ran in series with a 220 ohm resistor, and wired that to the junction of R6 and R8. I changed R6 to 1k ohms. The 220 ohm resistor was originally a 330, but I replaced it with the lower value after a play test.

I will say the difference was not dramatic. The lows are definitely deeper, the highs were a bit more open. In addition to the power amp mod, I changed a couple values in the preamp to boost the treble and enhance the presence of the amp. Now it sounds quite to my liking.