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Old 16th September 2011, 05:02 AM   #401
agdr is offline agdr  United States
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Interesting to note though that if someone stuffed in +/-5V or +/-6V regulators to produce roughly 4.5V or 5.5V rails who would do something like that then with an 8R speaker it looks like you could get to the point of clipping the output NJM4556s without frying them.

With +/-5v regs, the sim output clips at around 0.95Vpeak (0.67Vrms) for a 2.5x gain and an input of around 4.2V peak (0.3V rms). At that point the 8R speaker power is around 56mW, still insignificant but nearly 2x from the 12V version, and the chip power is (4.5V)(84mA) - 56mW = 322mW = a happy NJM4556 chip!

This doesn't have much applicability to 8 ohm speakers since 56mW is still to low to be useful, but it does to low(er) impedance headphones.
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File Type: png O2 into 8R Vin 0.3Vrms gain 2.5x Vout 0.67Vrms plot.png (26.5 KB, 384 views)

Last edited by agdr; 16th September 2011 at 05:05 AM.
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Old 16th September 2011, 08:39 AM   #402
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Quote:
The DC power consumption is about 22 mA idle and typically under 80 mA playing music even at high levels into low impedance loads. The AC power consumption is typically under 200 mA but full power worst case sine wave testing with both channels driven it's 480 mA (including battery charging current).
Much appreciated. I can roll with this. agdr, your sim values are helpful also. Any modding I do is going to target the power supply, just for sake of personal convenience.
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Old 16th September 2011, 12:12 PM   #403
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Originally Posted by RocketScientist View Post
@dewasiuk, the O2 PCB can't accommodate a 1/4" headphone jack directly but you can use a bigger enclosure if you want and panel mount the jack. See Enclosure Options. If you mainly want a desktop amp, I'm working on a desktop-only PCB.
rocketscientist, are you going to modify the desktop version to allow higher voltage swing? it can be very useful for high impedance headphone

from the reviews roaming around in hf, it seems like o2 still doing 'ok' in driving hard to drive headphones, not yet to be called 'great'
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Old 16th September 2011, 12:19 PM   #404
timpert is offline timpert  Netherlands
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Would it be an idea to switch the power supply voltage along with the gain? If you use high gain, that will presumably be because your headphone needs the voltage. In that case, it will be user more friendly to switch the supply voltage along with it, rather than having a different knob for it.
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Old 16th September 2011, 12:36 PM   #405
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Would it be an idea to switch the power supply voltage along with the gain? If you use high gain, that will presumably be because your headphone needs the voltage. In that case, it will be user more friendly to switch the supply voltage along with it, rather than having a different knob for it.
you do realize changing the gain means you're changing the output voltage, right? i think it's easier to switch gain rather than power supply

just implement high voltage supply, then set the low gain to really low level, and high gain to maximum allowed level. then you're good to go for both IEM and hard to drive headphone
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Old 16th September 2011, 01:11 PM   #406
timpert is offline timpert  Netherlands
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Quote:
you do realize changing the gain means you're changing the output voltage, right?
Yes, I do, and that's precisely why I'm proposing it. If you switch to a higher gain in order to achieve a higher output voltage as needed for your headphones, the power supply voltage increases with it, in order to accommodate the larger output swing. You only have the gain switch on the front panel and no other button, because the related function of increasing the supply voltage occurs under the hood without the user noticing.

The reason to use low voltage with low impedance phones (and thus low gain) is dissipation. When the power opamps are used with high supply voltage and low impedance loads, they may become too hot. This has been discussed already in the thread and on RS's blog. Also, when set to low gain, it will be harder to blow up an expensive IEM when the output voltage is naturally limited by clipping to a low supply voltage.

Last edited by timpert; 16th September 2011 at 01:36 PM.
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Old 16th September 2011, 01:16 PM   #407
MrSlim is offline MrSlim  Canada
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RS has already discussed his reasons for keeping the supply voltage at 12V for the desktop version in this thread..
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Old 16th September 2011, 04:10 PM   #408
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hello rocketscientist, I am a hobbyist from HongKong. A group of HK engineers pointed out that U2A and U2B is a voltage comparator and its purpose is merely to shut down the circuit when battery is running low. It is nothing to help with any headphone protection. Do you have any idea?
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Old 16th September 2011, 05:24 PM   #409
MrSlim is offline MrSlim  Canada
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Originally Posted by stockpickerhk View Post
hello rocketscientist, I am a hobbyist from HongKong. A group of HK engineers pointed out that U2A and U2B is a voltage comparator and its purpose is merely to shut down the circuit when battery is running low. It is nothing to help with any headphone protection. Do you have any idea?
Since the comparators are used to prevent the amp from doing bad things when the battery voltages get low (or unbalanced), wouldn't you consider that to be headphone protection?
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Old 16th September 2011, 06:05 PM   #410
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@stockpickerhk, please read the Circuit Description section. The circuit does protect the headphones, and it also greatly reduces the turn on transient every time you turn the amp on. Because the O2 uses a true bipolar power supply, if the positive and negative rails are not perfectly symmetrical on power up and power down, you get a transient at the output. The power circuit reduces that to safe levels.

IN SUMMARY: DO NOT REMOVE THE POWER MANAGEMENT CIRCUIT. It causes no harm and serves an important purpose even without any batteries.

For those of you who want only a desktop amp (no batteries) you might want to wait for the desktop version of the board. It will have a more conventional power supply.

@agdr you did your sims with sine waves I think? If so, those don't reflect dissipation with music so your numbers are significantly off. But it's really pointless. A sports car can't tow a 30 foot boat either. The O2 will drive 16 ohms playing real music to clipping all day long if you don't jack up the supply rails beyond 12 volts.

@idjoel2000 I'm considering the higher voltage option for the desktop only amp. I need to do some testing before I'll know for sure and that might be a while as I don't even have a prototype PCB yet. Most of the headphones people seem to be worried about needing more than 7 Vrms are long since discontinued. So it's a bit hard to justify too much effort to support such headphones. The DT880-600 still seems like the best example of a "high voltage" reasonably popular headphone currently in production. And the O2 can handle it unless someone's really into hearing damage. If I'm missing another current headphone, please let me know?
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