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The Objective2 (O2) Headphone Amp DIY Project
The Objective2 (O2) Headphone Amp DIY Project
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Old 3rd November 2017, 05:49 PM   #5021
Mooly is offline Mooly  United Kingdom
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The Objective2 (O2) Headphone Amp DIY Project
The O2 charging circuit is a current limited constant voltage design of the most basic kind. It is only suitable for Ni Cad and Nickel Metal Hydride types, not Lithium Ion. It has around 11.2 volts available open circuit. The higher the cell voltage reaches, the lower the current becomes... it is a tapered charge.

Lithium Ion cells must be charged with a circuit that cuts off the current when full charge is reached. You also can not trickle charge them... it destroys them in very quickly.
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Old 5th November 2017, 04:19 PM   #5022
randomlf is offline randomlf
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But without charging (taking the batteries off for charging) how long run time from using the 18650's? Or does it need some regulator much current?

Last edited by randomlf; 5th November 2017 at 04:33 PM.
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Old 5th November 2017, 04:31 PM   #5023
marioneta is offline marioneta
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Default remote power switch

hi everyone,

i would like to mount the switch remotely but i cant figure out which pins go where.

would this switch do the trick?

what do i have to connect where?

Thanks.
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Old 5th November 2017, 05:35 PM   #5024
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by randomlf View Post
But without charging (taking the batteries off for charging) how long run time from using the 18650's? Or does it need some regulator much current?
I looked up the batteries and they are rated at 3000mAh. So if you measure the current draw per rail and divide that into 3000 you get you run time in hours.

I would guess for the 3 opamps and a comparator you are probably looking at 150 to 200 hours depending how loud you play it.
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Old 5th November 2017, 05:41 PM   #5025
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by marioneta View Post
hi everyone,

i would like to mount the switch remotely but i cant figure out which pins go where.

would this switch do the trick?

what do i have to connect where?

Thanks.
It seems OK although the description is poor. It needs to be a double pole double throw type (6 pins in total).

Wiring it is easy.

The middle pin on one side of the switch goes to the power supply (junction D1 and D2). The top pin on the same side goes to pin 8 of the comparator (look at the circuit).

The other middle pin goes to D5 and D6. The top pin of that side goes to pin 4 of the comparator.
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Old 10th November 2017, 03:28 PM   #5026
Cal42 is offline Cal42  Sweden
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Hello, been lurking a long time here but finally time to post.

I tried OPA1662's as output chips on my O2, after reading about it here: A version of an O2 Desktop Amp (ODA)

However, my DC offset rose to 53.5 and 55 mV. With stock JRC4556 I have only 3.5 mV offset and with LME49720, 1.5 mV both channels.

Now OPA1662 have a lot higher offset and bias currents than 4556, but I'm still surprised. Can anyone confirm this is enough for the big difference or is something else going on?
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Old 10th November 2017, 04:50 PM   #5027
Mooly is offline Mooly  United Kingdom
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It sounds high but not beyond the bounds of possibility given the specs of the device. Worst case of 1200nA across the 40.2k would give 48mv and on top of that there is a possible imbalance in input offset currents to make things worse.

Try shorting R12 (or R13) and see what the offset falls down to.
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Old 13th November 2017, 04:11 PM   #5028
Cal42 is offline Cal42  Sweden
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Quote:
Originally Posted by Mooly View Post
It sounds high but not beyond the bounds of possibility given the specs of the device. Worst case of 1200nA across the 40.2k would give 48mv and on top of that there is a possible imbalance in input offset currents to make things worse.

Try shorting R12 (or R13) and see what the offset falls down to.
Hi Mooly. I shorted R12 and DC offset is down to -0.5 mV. Other channel stayed at 53.5 mV.
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Old 13th November 2017, 06:53 PM   #5029
Mooly is offline Mooly  United Kingdom
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Interesting. So it looks like this is going to be normal behaviour and purely the result of the relatively high input bias currents for these devices causing this offset. The good news is that although it sounds high, in reality little current will flow into a typical headphone DC resistance. You can measure how much for your particular headphone by measuring the volt drop across one of the 1 ohm resistors of the appropriate channel and calculating using ohms law. The voltage will be very tiny though and perhaps not resolvable on a typical DVM.

If the phones have say 20 ohm DC resistance then something around 2.5ma would flow.

No easy fix I'm afraid although it could be made 'better' by lowering impedances of the surrounding circuitry.
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Old 13th November 2017, 06:58 PM   #5030
Mooly is offline Mooly  United Kingdom
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Although I say there isn't an easy fix... well the 'obvious' one would be to add a similar value resistor to R12 and R13 into the feedback path to equalise the error and make both inputs see the same offset voltage. You would have to do it for all four opamps. The downside of this is that the opamp node becomes sensitive to stray noise pickup, and also the step response (squarewave testing) of the circuit could be altered although probably not in any audible way.
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