@Spacehead the O2 measurements speak for themselves. While I agree the PC board layout is cramped and less than optimal, I worked really hard to make it still perform well.
The desktop version will have a better layout and I expect it to measure even better in at least a few areas. But both amps are well beyond the point of diminishing returns in terms of performance. So, short of more the more optimal gain structure and perhaps a few other minor things, it's not likely the desktop amp will sound better than the O2. But it will be a more optimal design and should provide some additional "piece of mind" for audiophiles who want more than just great measurements.
The desktop version will have a better layout and I expect it to measure even better in at least a few areas. But both amps are well beyond the point of diminishing returns in terms of performance. So, short of more the more optimal gain structure and perhaps a few other minor things, it's not likely the desktop amp will sound better than the O2. But it will be a more optimal design and should provide some additional "piece of mind" for audiophiles who want more than just great measurements.
Yeah, I look forward to listening to it. I have only Superlux HD668B but they have been praised everywhere. It is very good to have all connectors and switches on the PC board, I guess you will try to achieve that also in desktop version. I think you need to change the load sharing resistors in desktop version, or how much the power dissipation would be at max level , with 15V rails?
@Spacehead, despite concerns raised about obsolete 600 ohm K240 headphones, I still believe 12 volt rails, and 7+ Vrms of output, is all 99% of us need. So I'm not planning on 15 volt default rails for the desktop amp. The load sharing resistors can safely stay at 1 ohm for a 0.5 ohm output impedance. I've run an O2 on AC power, pushed to clipping into a worst case 15 ohms with typically heavily compressed pop music, for hours and the 4556's were still within their max temp limits.
If someone into serious hearing damage, or with really unusual headphones, really needs more than 7 Vrms they can either modify the amp at their own risk, or simply use a different amp.
If someone into serious hearing damage, or with really unusual headphones, really needs more than 7 Vrms they can either modify the amp at their own risk, or simply use a different amp.
@Spacehead, I assume that's addressed to me? I think it sounds great! To pick one word, the O2 is "effortless". With any headphones I own it doesn't come close to any of its limits, is dead silent, and delivers the best sound I've ever heard from any of those headphones. That said, my $1600 Benchmark DAC1 Pre sounds equally as good. As the O2 reviews by others have confirmed, the amp just disappears and lets you enjoy the music as the recording engineers intended. That's more than can be said for LOTS of popular amps that let their presence be known in one or more ways.
Yeah, total transparency. Then some audible differences can be found and heard from different audio sources and of course changing headphones. Have you thought about designing a DAC ?
I have this amplifier here that is built on protoboard (don't laugh) and with NJM2068 buffered with BUF634 on each channel, and virtual ground (don't laugh) buffered with NJM4556 (parallel, 0.47 ohm load sharing resistor). It is quite effortless too and silent. Usually new amplifiers sound better , because the subjective experience changes.
@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.
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). There's more in the Circuit Description section.
Last edited:
I was thinking, and reasoned that the amplifier can drive small 8 ohm speakers, but for how long, I am curious. Many high end DIY amplifier designers advertise that their amplifier can drive speakers too.
@Spacehead, claims aside, it's best to use a tractor for plowing fields and keep the Porsche on the roads. I can't recommend the O2 for driving speakers. There are plenty of chip amps, class-D modules, etc. for that. The O2 will only produce about 1.2 volts RMS into 8 ohms and get rather hot in the process.
Any headphone amp that can do a decent job driving speakers is designed to the wrong criteria. Everything is a compromise.
Any headphone amp that can do a decent job driving speakers is designed to the wrong criteria. Everything is a compromise.
If user plugs speakers, will the amplifier survive? I have read that op amps can work at quite wide temperature range. There is no thermal protection in NJM4556 I think.
For example in AMB M3 the discrete Class A Mosfet output stage can deliver the current for 8 ohm loads. But that amp is flawed anyways.
It might survive for a little while, but I suspect the 4556s will fry pretty quickly. RS has designed the O2 to handle down to 16 Ohm headphones, halving it again to 8 will probably be ill advised..
I really don't get the point.. use the amp for the use it is designed, or get another amp..
There are enough amp modules of various sorts around that you could build something decent for only slightly more, and do a much better job for speakers..
I really don't get the point.. use the amp for the use it is designed, or get another amp..
There are enough amp modules of various sorts around that you could build something decent for only slightly more, and do a much better job for speakers..
Never being one to shirk from abusing the O2 in sims, it looks like the NJM4556 chips would fry at right around 0.21Vrms input (0.3V peak) at x2.5 gain with 8R speaker loads, if I've done my math right. 
That results in about 0.7 peak or 0.49Vrms out (61mA rms), which would only be 30mW into the speaker. Over on the NJM4556 side of things though that wold be (12v)(61mA) - 30mW = 700mW = max chip dissipation.
The NJM4556's don't actually start clipping in the sim until about 1.3Vpeak output (0.92Vrms), but by then smoke is probably issuing out of the 4556 chips.
That results in about 0.7 peak or 0.49Vrms out (61mA rms), which would only be 30mW into the speaker. Over on the NJM4556 side of things though that wold be (12v)(61mA) - 30mW = 700mW = max chip dissipation.
The NJM4556's don't actually start clipping in the sim until about 1.3Vpeak output (0.92Vrms), but by then smoke is probably issuing out of the 4556 chips.
Attachments
Last edited: