The Objective2 (O2) Headphone Amp DIY Project

[thirdman]

Anyone still building these complete? If not maybe someone bought all the parts and has shelved the project? I really want one of these.

Thanks
Shane

[qusp]

@ ethanolson: you could get some acoustically transparent custom in ear attenuators/plugs from Westone and then crank it =)

[RocketScientist]

@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

[ethanolson]

So how would one go about bringing the O2's output impedance to <0.05 Ohm and still address the offsets in the opamps being quelled by the output resisters? I assume whatever the solution is it probably goes against the design philosophy of performance for low dollars.

[RocketScientist]

@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.

[ethanolson]

That's an easier solution than expected. I look forward to the test data.

[jtktam]

Originally Posted by thirdman

Anyone still building these complete? If not maybe someone bought all the parts and has shelved the project? I really want one of these.

Thanks
Shane

did you check the swap meet area... i am offering kits and pcb if you need them

-joe

[Jelle Schrijver]

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.

[regal]

Originally Posted by RocketScientist

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.

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 ?

Originally Posted by RocketScientist

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.

This is one of the best qualities of this amp on paper for headphones and why this tube guy is messing with opamps

[RocketScientist]

@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.