OK guys, I have been looking into building a headphone amp for a while, and i have decided in the next few weeks to get on and build one based around the portable cMoy amp design or something similar considering i have an RS components next door to me almost. Aswell as the amp, i am going to ingerate an alien DAC or Bantam DAC, depends on what i decide is easier later. Now the DAC got me thinking, the device will have a USB port, which i could charge my amps battery up on. So anyone know of a good circuit i could use. The amp requires twin 9v (PP3) batteries. so its fairly demanding on voltage. If anyone can help could you tell me why the cMoy requires the use of 9v batteries.
That's the nature of op-amps I'm afraid (IIRC, the cmoy has a virtual ground from it's power supply so you can get +9v and -9v from the batteries to use as a vRef right?); an OPA132 needs 5~9v. (see here for more info on op-amps (solder a DIL/DIP socket - you may want to try a few op-amps out!)). As for a charging circuit, something like a MAX761 (more) can do a DC-DC boost up to 150mA (dunno how much your battery will need tbh, 150mA might be enough for a trickle charge?). We need to know what sort of battery it's going to be charging, though (NiMH, LiPo etc.) for a specific charging circuit.
The original cMoy tutorial by the designer is here: http://headwize.com/projects/showfile.php?file=cmoy2_prj.htm It seems the 9V actually gets split into two 4.5V supplies. john
Almost all Op-amps (and all quality audio ones) waste a couple of volts at either end of the supply range, so a 9V battery with a splitter to get a mid-way ground would give a ±4.5V supply but only give about ±2.5V peak output on music - just enough for headphones. Using 2 x 9V is surely a bit OTT for a cmoy? The original is a single 9V cell. No harm in ±9V and the Tangentsoft circuit uses that, but a bit OTT. You could of course use 1.5V AA or AAA cells to make up say 12V total (for a ±6V supply) but in-situe charging from 5V USB is going to be tricky.
Ok i think a single 9v battery is enough, any recomendations on charging that, is it possible to alter a MAX716 circuit to deliver 9v, i imagin there to be a slight ampage increase for some reason.. i think NiMh cells are less hassle compared to li-ion, and there easier for me to get hold of. The idea is that the first one will be bulky and stable, so i understand what is going on, then i can look into making it super small, with a tight layout as possible, and start thinking about EMI sheilding... Eventually i want it to fit with in the profile of a credit card, and to keep the height to a minimum. EDIT I found that ipod batteries are easy to get hold of, and there nice and thin. I guess there li-ion, so maybe some help in developing a charging circuit with protection... Maybe i should focus on lithium ion batteries from the start.
