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September 27, 2022, 03:46:44 AM

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Vox Pathfinder 10 as a modding platform

Started by dimkasta, May 09, 2021, 05:52:05 PM

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dimkasta

Slow day at work today, so I had some time to experiment.

There are two interesting parts in the first gain stage.

The first is the capacitor C5 and how it interacts with the gain pot. At first glance it looks like a coupling cap. But it's in the feedback loop, and interacts with the pot. As the gain increases, the upper part of the pot divider decreases, and the cut off frequency of the high pass filter is increased (less bass passes the filter). But because the filter is in the negative feedback loop, this means that low frequencies are increased at the output. This works exactly like a resonance control. But the problem is that bass increases with gain. Which is quite counter intuitive, especially at higher gains, unless you like fuzzy distortion. Removing C5 removes that weird interaction and high gain sounds tighter.

The second area is C4, R4, R5 and the switch. The switch acts exactly like turning up a presence control at high gain. R4 sets a minimum resistance so that things do not go too far. Ideally, we would replace R5 with a 100K or 50K pot wired as a variable resistor, and then remove this pole of the switch so that we have full control both at high and low gain. This would give us a very versatile presence-like control.

Puguglybonehead

Great thread! Thank you for sharing your progress. I hadn't realized that the Pathfinder 10 was such a simple circuit and  all thru-hole to boot. Wishing I'd kept mine now. Eliminating that ridiculous 'Boost' section is a great idea. I never found it to be realistically useable. I'd thought about re-homing mine in a new cabinet with a decent speaker (8"or 10") but never got around to it.

At 18 watts you've got the perfect amp for small club gigs. Currently, that's mostly what there is around here. (besides some occasional busking) Hipsters in our neighborhood want wallpaper music to hang out to so little amps like this are really useful. I hope you'll have some sound samples for us in the near future.

dimkasta

To be fair, the 18W are peak to peak. The RMS rating should be closer to 10W, and is probably further limited by the small transformer, the small power bank and the distance/inductance from the power bank to the chip itself.

TBH, by just removing the leds and adding an external 12" cab, you already have a decent amp  if you have a good set of pedals to complement and keep your expectations realistic.

For gigs I would probably consider it only if you are very budget constrained, and then I would mic it to avoid overheating the chip.

Unfortunately I cannot provide samples since my p10 is now just an empty chassis for experimenting with other circuits.