The Objective2 (O2) Headphone Amp DIY Project

[MrSlim]

The 8.4V NiMH are the ones RS has included in the BOM so you should probably stick to them.

See his posting #10 on this thread re his comment on LiPo batteries. LiPo can't be charged the same way as NiMH, so they can''t be used in the O2. They require a specific charging circuit. See http://www.rchelisite.com/lipo_batte...fety_guide.php

[RocketScientist]

The iPower Li-Poly battery is interesting, but MrSlim is correct, you need a special charging circuit. I even question if the $35 iPower charger listed for that battery is correct as it's using Delta-V and that's not how you're supposed to charge Li-Poly. Even if the charger tries to figure out if it's charging a Ni-Mh or Li-Poly battery I'm not sure how it can do that reliably enough under all conditions including a defective battery.

They also claim it's 8.4 volts nominal but Li-Poly chemistry is around 3.7 volts per cell yielding either 7.4 or 11.1 volts. There's also no mention of a protection circuit in the battery. Overall, I would say those batteries are a significant fire hazard when used in devices without over current protection and with that charger.

[timpert]

8.4 Volts is the typical maximum voltage of the battery during charging of a 2-cell Li-ion battery. Usually, Li-ion batteries are charged with a constant current until 4.2 Volts per cell is reached. It is then held at that voltage, after which the charging current naturally drops. If the current drops below a preset value (somewhere in the order of C/20), the charge process is terminated.

Li-ion batteries have a relatively larger voltage variation during charge and discharge than, say, an NiMH battery. The 3,7 Volts rating is a representative value somewhere around 50% state of charge, at end of charge, the voltage may be as low as 2.8 V per cell.

EDIT: Do not use any Li-ion type cells (Lithium polymer cells are just a flavor of them) in the O2. They WILL be overcharged and they WILL be damaged and and become dangerous. The O2 has no provision for the proper charging method, so stick with NiMH batteries.

[musical]

Thanks guys for the answer. After seeing all the spectacular explosions and fire in youtube, I'll keep away from the Lithium polymer cells.

[goldkenn]

I have a JVC HA-FXT90LTD, it is a very special earphone which may change your mind about high sensitivity earphone, since although it is 107dB and 12 ohms, but it do need huge power to control it, I tried a DIY TPA6120, Mini3 with 2x gain, E11 with low gain, and SR17, nothing satisfy, the JVC just like a current hunger loudspeakers, you don't really need high watts, but big current with excellent damping will be needed, could O2 drive it good?

[ianrt]

Let's see.

107dB/mW needs 2mW to give 110dB.
2mw into 12 ohms requires 155mV at 13mA.
Clearly, at unity gain O2 is still 13 times too high (assuming 2V in) so you will be well below the optimum volume setting (into the noise region?).
In fact, 1.3V will give the earphone maximum power (150mW) which equates to 129dB - hearing damage territory! Be very careful.

Ian

Originally Posted by goldkenn

I have a JVC HA-FXT90LTD, it is a very special earphone which may change your mind about high sensitivity earphone, since although it is 107dB and 12 ohms, but it do need huge power to control it, I tried a DIY TPA6120, Mini3 with 2x gain, E11 with low gain, and SR17, nothing satisfy, the JVC just like a current hunger loudspeakers, you don't really need high watts, but big current with excellent damping will be needed, could O2 drive it good?

[musical]

The Maha Imedion NiMH 8.4V 250 mAh is a low discharge battery. Could someboady confirm that it is okay before I buy some?

[idjoel2000]

Originally Posted by musical

The Maha Imedion NiMH 8.4V 250 mAh is a low discharge battery. Could someboady confirm that it is okay before I buy some?

it's an NiMH battery, compatible with o2. what other confirmation do you need?

[MrSlim]

I asked RS about this a while back and he said there wasn't any problem with using them, but there might not be any real advantage either, unless you intend to have your O2 on the shelf, unused for a long time, and what fun is there in that..

[agdr]

Originally Posted by musical

The 9V NiMH battery is usually either 8.4V or 9.6V. Which is more suitable for O2?

I'm using two Maha 9.6V NiMh in mine and changing R1 and R2 to 150R:

MAHA / POWEREX 9.6V 230mAh Rechargeable NiMH Battery

but that is an unapproved off-label O2 mod.

I didn't see the use in paying the extra $2 for the low discharge 9.6V maha cells since I don't plan on having it sit, charged, on the shelf for months on end. EDIT: just saw MrSlim's post - we were composing at the same time. So there you have it from two folks.