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.
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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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'
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
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.
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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?
@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?
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?
i doubt o2 can drive my vintage wharfedale isodynamic to a decent level, but that's not common case since i'm pretty sure the users are pretty few (i use gainclone LM3886 out of the speaker output to power my wharfie)
for modern headphone, i think the hardest to drive would be HE6? i haven't seen any o2 review using hifiman he6 as the benchmark
implementing higher voltage swing i think would not be a problem since the builders can customize them self what gain do they need. if he's not building it himself than of course he can ask the builder to customize the gain for him
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It is slightly more complicated than that. You would have to increase the gain also. Plus using a chipamp is a much more elegant solution to power speakers than trying to force a ton of opamps to do the job.
You are talking about 16 or more 4556 to make a convincing buffer for a speaker amp. You are likely to run into problems with that.
I agree with you but more voltage isn't the answer. The HE6 also needs lots of current and raising the rails on the O2 with that headphone will push the dissipation to uncomfortable levels. I believe the O2 will play at least reasonably loud with the HE6 with 12 volt rails. For those who want more, or more headroom, I don't think any amp with a DIP8 output stage is going to work any better.
To drive the HE6 to higher SPL you need something like a BUF634, LME49600, or a properly engineered discrete output stage which typically means true Class-A, significant heat sinks, etc. There are already several more expensive amps in those categories. For commercial designs the best I know of are the Violectric amps. The whole idea of the O2 is to be significantly less expensive even if that leaves a tiny minority of users out of the game.
All this speaker stuff really is off topic and kind of nonsense. Trying to modify the O2 to drive speakers is like modifying a Porsche to pull a big travel trailer. It's not up to the task in multiple ways. Let's please stay on topic and keep this about a headphone amp driving realistic headphones.
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When operating at 192khz DACs (excluding NOS DACs which simply measure crap whatever you give them) the DAC operates at reduced oversampling ratios, which reduces the quality of the digital filtering.
Don't take it from me, take it from Benchmark Audio:
"To date, Benchmark has no evidence that 192kHz performs better than 96kHz, but we have a substantial body of evidence that shows that 192kHz has defects that are not present at 96kHz. These issues are also shared openly by one of our competitors: Lavry Engineering. We suspect many other manufacturers are aware of these issues, but choose not to talk about them."
96kHz vs. 192kHz
An article by Lavry Engineering says prettymuch the same thing, but goes into greater technical detail.
Hello RocketScientist!
i'm looking for transformer, but not shure what to choose. Please would you give me a tip?
Here are specs of a number of transformers that i can buy from a local store.
http://home.comet.bg/datasheets/Transformers/6VA.pdf
10x!
Sorry for bad english
i'm looking for transformer, but not shure what to choose. Please would you give me a tip?
Here are specs of a number of transformers that i can buy from a local store.
http://home.comet.bg/datasheets/Transformers/6VA.pdf
10x!
Sorry for bad english
I was very confused on that issue becuase I had discussed this function in our local forum with some other Electronic Engineers. They said it is only a battery under-voltage protection.
I personnally would say o2 has some some kind of "earphone protection circuit". They said I was seriously mis-leading people.
They insisted that Do not confuse it with a normal "earphone protection circuit" but I am not sure what it is actually. Could you figure it out the difference?
btw in our country we are holding a group-buy and up to now there is already over 150 orders. O2 is a hot cake now.
The 15V 400 mA transformer would be perfect. Their part: ANG6VA/15V
@stockpickerhk, I'm glad the O2 is popular in HK
I suspect this might be partly a language translation issue. If one battery dies first, or one battery become disconnected during use, the O2 can output 7+ volts of DC into the headphones. This will almost certainly damage many headphones. This is one of the biggest problems, along with lack of output current, of most dual battery Cmoy-type amplifiers. But even if you use the O2 only on AC power you still need the power circuitry. For example, when you turn on the AC power, the positive and negative regulators do not turn on at the same time (due to differences in the regulator ICs). This means for a short time the O2 tries to run with only one power supply, this causes a large transient into the headphones.
In both cases above the circuit is protecting the headphones. As a side benefit, the circuit also helps protect the batteries from being discharged too low and being damaged (by cell reversal). So it's true the circuit also help protect the batteries. The circuit does both things.