@ethanolson, you're welcome. While it's certainly true that some amps have significantly more distortion at very low levels, often due to crossover distortion, the O2 is not one of those amps.
The rise in the O2 THD+N plots is almost entirely noise (the "+N" means "plus noise") not distortion. If you plot only pure distortion it's relatively flat with respect to output until you start to hit the analyzer's noise floor at the low end and until you get close to clipping at the high end. Using a distortion magnifier ahead of the analyzer, it's possible to demonstrate even at very low levels, the op amps in the O2 are not contributing meaningful distortion.
So that just leaves noise and that's entirely related to the sensitivity of the headphones and where the audible threshold is. If the O2 is completely silent even at full volume with the worst-case most sensitive IEMs in an extremely silent room with nothing playing, one can make a case it's plenty quiet for any headphone at any listening level.
All that's a long way of saying I don't think there's any meaningful difference in the O2's performance from a few millivolts up to 4+ volts RMS.
I think the differences between the Sony V6 and HD650 headphones in Benchmark's measurements can be explained by their different impedance, driver mass, inductance, and back EMF. Together those things can easily create very different damping requirements.
@thirdman, Epiphany in the UK or JDS for a completed board in the USA. See O2 Resources
The rise in the O2 THD+N plots is almost entirely noise (the "+N" means "plus noise") not distortion. If you plot only pure distortion it's relatively flat with respect to output until you start to hit the analyzer's noise floor at the low end and until you get close to clipping at the high end. Using a distortion magnifier ahead of the analyzer, it's possible to demonstrate even at very low levels, the op amps in the O2 are not contributing meaningful distortion.
So that just leaves noise and that's entirely related to the sensitivity of the headphones and where the audible threshold is. If the O2 is completely silent even at full volume with the worst-case most sensitive IEMs in an extremely silent room with nothing playing, one can make a case it's plenty quiet for any headphone at any listening level.
All that's a long way of saying I don't think there's any meaningful difference in the O2's performance from a few millivolts up to 4+ volts RMS.
I think the differences between the Sony V6 and HD650 headphones in Benchmark's measurements can be explained by their different impedance, driver mass, inductance, and back EMF. Together those things can easily create very different damping requirements.
@thirdman, Epiphany in the UK or JDS for a completed board in the USA. See O2 Resources
@ethanolson, I plan to run some comparison tests (having a Benchmark DAC1 with the HPA2 headphone amp, an O2, and the HD650) of the exact measurement John ran at Benchmark. I suspect there won't be any significant difference between the 0.5 ohm output impedance of the O2 and the 0.05 ohm resistance of the Benchmark. I believe both are sufficiently low to provide plenty of damping.
The only way to lower the O2 output impedance further would be to use just a single section of each op amp (remove one of the 1 ohm resistors for each channel and jumper the other resistor with a piece of wire). That will drop the output impedance down close to 0.05 ohms. It will also cut the maximum output current in half but that won't matter for a lot of headphones including the HD650. But I don't think it will change the sound quality.
The only way to lower the O2 output impedance further would be to use just a single section of each op amp (remove one of the 1 ohm resistors for each channel and jumper the other resistor with a piece of wire). That will drop the output impedance down close to 0.05 ohms. It will also cut the maximum output current in half but that won't matter for a lot of headphones including the HD650. But I don't think it will change the sound quality.
Pimping the O2
How about these as volumeknobs?
tenayo_wdk_300
tenayo_wdk_600
tenayo_wdk_200
Not shure if the diameter is right and if it uses a screw to fix it in place. But cheaper than metal and very fancy.
How about these as volumeknobs?
tenayo_wdk_300
tenayo_wdk_600
tenayo_wdk_200
Not shure if the diameter is right and if it uses a screw to fix it in place. But cheaper than metal and very fancy.
Thats counter-intuitive to me, I would think that parallel output buffers would have lower output impedance than a single (thats what I get for being a tube guy.) Can you explain this one ?
This is one of the best qualities of this amp on paper for headphones and why this tube guy is messing with opamps
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@regal, I can see how it might be confusing if you don't know about the output resistors. The 1 ohm series output resistors in the O2 form the bulk of the output impedance (with the resistors effectively in parallel the output impedance is 0.5 ohms plus the amp's output impedance of about 0.05 ohms for a total of 0.55 ohms).
As ethanolson pointed out, if you lower the resistors, or get rid of them completely, you have a problem with different DC offsets between the amps. I talk about it in the O2 Circuit Description. If the DC offsets differ between the two halves of the DIP8 package by even a few millivolts, that creates substantial DC current between the two op amps. Right now a 2 mV difference will generate 1 mA of constant DC. With no resistors it would be more like 10+ mA of constant DC which exceeds the entire idle (quiescent) current of both op amps combined.
The current is flowing between the two op amps, not into the load. From the headphone's perspective nothing has changed. But the two op amps will, in effect, both be constantly fighting each other trying to maintain their specific DC offset.
That DC current, if high enough, sufficiently shifts the DC operating point of both op amps and increases distortion. It also increases the idle current and hence decreases battery life.
Long story short, the resistors are responsible for 90+% of the output impedance. And when you parallel two op amps, the resistors are required. If you want to get rid of the resistors to lower the output impedance you can only use a single op amp.
As ethanolson pointed out, if you lower the resistors, or get rid of them completely, you have a problem with different DC offsets between the amps. I talk about it in the O2 Circuit Description. If the DC offsets differ between the two halves of the DIP8 package by even a few millivolts, that creates substantial DC current between the two op amps. Right now a 2 mV difference will generate 1 mA of constant DC. With no resistors it would be more like 10+ mA of constant DC which exceeds the entire idle (quiescent) current of both op amps combined.
The current is flowing between the two op amps, not into the load. From the headphone's perspective nothing has changed. But the two op amps will, in effect, both be constantly fighting each other trying to maintain their specific DC offset.
That DC current, if high enough, sufficiently shifts the DC operating point of both op amps and increases distortion. It also increases the idle current and hence decreases battery life.
Long story short, the resistors are responsible for 90+% of the output impedance. And when you parallel two op amps, the resistors are required. If you want to get rid of the resistors to lower the output impedance you can only use a single op amp.
Front faceplate : drilling holes
Guys and gals,
I've been advised that using a step drill bit for drilling holes is the worst option for a clean hole. Has anyone actually done this with their o2 BOM-enclosure faceplate? How is it looking? Clean, needs a filing, debur (whatever that means, I've no idea)? Close up pics gratefully recieved (remember to use the 'flower' symbol on your camera for close focussing).
Just trying to sound out where I need to go to get the job done cost effectively but also neat enough. I don't much want to purchase several hole punches: the total comes to much more than a single step bit. But then again my drill doesn't have speed ratings: it depends only on the pressure on the trigger as to how fast it goes, so I don't have fine control when it comes to the drilling speed. I could be looking at a mess.
Guys and gals,
I've been advised that using a step drill bit for drilling holes is the worst option for a clean hole. Has anyone actually done this with their o2 BOM-enclosure faceplate? How is it looking? Clean, needs a filing, debur (whatever that means, I've no idea)? Close up pics gratefully recieved (remember to use the 'flower' symbol on your camera for close focussing).
Just trying to sound out where I need to go to get the job done cost effectively but also neat enough. I don't much want to purchase several hole punches: the total comes to much more than a single step bit. But then again my drill doesn't have speed ratings: it depends only on the pressure on the trigger as to how fast it goes, so I don't have fine control when it comes to the drilling speed. I could be looking at a mess.
I briefly experimented in metalwork before settling with my plastic front panel. Anything you drill will need deburring, which is the process of removing the excess metal sticking out from the hole.
I was also using a hand drill, which is hugely suboptimal (should really use a drill press) and found it a nightmare. The drill slipped constantly, even when drilling through wood on top, the holes were incredibly messy and only a few of the drill bits I was using could actually penetrate the metal (Annoyingly, the bits designed for drilling metal that I had were completely worn out)
I don't think what you are doing is likely to work out well, but then again I'm pretty terrible at this sort of thing - you might have better luck.
I was also using a hand drill, which is hugely suboptimal (should really use a drill press) and found it a nightmare. The drill slipped constantly, even when drilling through wood on top, the holes were incredibly messy and only a few of the drill bits I was using could actually penetrate the metal (Annoyingly, the bits designed for drilling metal that I had were completely worn out)
I don't think what you are doing is likely to work out well, but then again I'm pretty terrible at this sort of thing - you might have better luck.
+1 for Willakan's comment about self-drilled panels not turning out well. Unless you have experience doing similar metal work and access to a drill press, etc., it's far more difficult than some might imagine to get a good result.
It's also dangerous if you don't have the means to properly secure the small front panel while drilling. Most try to use their fingers and drill bits have a way of "grabbing" in metal at the end of the hole. The drill usually wins over fingers and, in a split second, the panel becomes a spinning "blade".
Another advantage of the ODA is suppliers like Flynhawaiian, JDS, Jtktam, etc. will hopefully be selling versions already optimized for desktop use so there should be much less need for custom panels.
It's also dangerous if you don't have the means to properly secure the small front panel while drilling. Most try to use their fingers and drill bits have a way of "grabbing" in metal at the end of the hole. The drill usually wins over fingers and, in a split second, the panel becomes a spinning "blade".
Another advantage of the ODA is suppliers like Flynhawaiian, JDS, Jtktam, etc. will hopefully be selling versions already optimized for desktop use so there should be much less need for custom panels.
@qusp, good point. I'm not against custom panels and some of the pictures I've seen of custom enclosure designs have been great. I'm just suggesting using Front Panel Express or one of the laser cut plastic vendors to make custom panels will likely yield better results then a hand drill. And for those not able to buy a ready made panel because they want a desktop O2, the ODA should solve that problem.
