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|24th April 2014, 12:20 AM||#1|
O2 headamp PM latch PC board to stop O2 low battery buzzing
Here is a small PC board that replaces the U2 comparator chip on NwAvGuy / RocketScientist's O2 headphone amplifier. Just unplug the U2 chip and plug in this board. It uses the same pluggable headers in the O2 booster board. The board solves the problem where the O2 will oscillate (make a buzzing sound in the headphones) when the batteries get low and the O2's mosfets try to shut off.
The board implements one of the O2 mods I published a year or two ago here that latches the O2's power managment circuit "off" once the batteries get low and the O2's PM circuit cuts off the mosfets. Once the latch happens, turning the O2's power switch off for 10 seconds or so and then turning it back on resets the latch. But, of course, if the batteries haven't been recharged it will just latch again.
The O2's problem here is that the batteries rise up in voltage slightly when their load is cut off (when the O2's mosfets turn off when the O2's batteries are low) which is common behavior for batteries. NwAvGuy / RocketScientist tried to compensate for this by including some hysteresis in the O2 circuit.
But unfortunately some batteries rise up in voltage when cutoff more than the amount of his hystersis circuit. This seems to happen more often when the batteries get old. That in turn causes the O2's power management circuit to turn the mosfets back on, which causes the battery voltage to quickly drop again, which causes the PM circuit to cut the batteries off again. The cycle then repeats at roughly a 60 Hz rate, hence the buzzing that is heard in the headphones.
This board just adds a circuit that latches the mosfets permanently off the first time they turn off, at least until the O2's power switch is turned on and off again. The board plugs in for the O2's U2, replacing it (the board contains the same chip, a LM2903, just in a smaller package) and requires one wire to be run from the board to the O2 PC board.
I've just sent the board out for fabrication and will post the Gerber files when it gets back and if it tests OK, so anyone can fabricate boards. Remember that you can hit the "+" zoom on the PDFs below to zoom then up to readable size.
This is the same optional circuit I had added to V2.0 of the O2 booster board, but then realized that I screwed up after it came back from fab, since the circuit has to wire in before the O2's mosfets. The booster board wires in after. I considered just making the O2 booster board longer, to cover the O2's U2 chip, and add this circuit back to the board. But one of the mosfets that NwAvGuy / RocketScientist has specified is too tall and would hit the board. I could have specified a shorter mosfet to replace it, but I think a lot of folks would rather have something plug and play rather than having to mess around with replacing an O2 mosfet. Then there is the issue that some folks have installed RCA jacks or a power jack in the back of their O2 that may stick out enough around U2 to prevent using this board, so best have it separate.
If anyone is interested in spare boards when they get back, assuming it tests out OK, please post in my vendor thread rather than here, or PM me. The vendor thread is at:
Parallel NJM4556AL two stage amp
Looks like the at-cost fabrication price is around $2 USD since the board is tiny and just two layer, plus $1.50 for the connection pins. Interesting that 30 of them were priced exactly the same as 20 of them at Seeed Studio.
Last edited by agdr; 24th April 2014 at 12:41 AM.
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