Objective 2 Headphone Amplifier

3 reasons... expectancy bias (looks and what has been written about it), wide bandwidth and above all low output resistance.

The design is not much different from C'Moy designs with a few execptions.

The volpot for instance is behind the voltage gain stage.
Advantage is less noise when the volpot is turned down.
Disadvantage is (depending on the power supply voltage) the input stage can easily clip when the gain is set in the high position. Setting it to low should cure the problem.

Technically lower gain should have lower noise and a wider bandwidth but the noise floor is already below audible limits (with T50RP) and bandwidth is limited by the used opamps and is well over 100kHz, so doesn't have me worried and well over what any analog source (recording) can contain.
When calculated it should be capable of reaching 1.5 MHz (-3dB), can't find any references of people having measured the limits.
It could make the amp susceptible to common mode garbage ( diyaudioheaven.wordpress.com/tutorials/power-supplies/ ) when connected to mains.
You can make the gain every value you want if you change a resistor value.
The clipping behaviour 'trick' NwAvGuy used is a good idea for very high sensitivity IEM's but a real pain for the users that have different sources (portable DAP and using it with a decent DAC the next day).
I would never use such a design topology myself for this reason, nor is it the reason it performs as it does.
It does have a lower noise floor when the volume is turned down which is the only advantage.

These are only technical things though (NwAvGuy style) and not subjectively found differences in which the miles may vary....

Opamp rolling (not needed acc. to the designer):

In the voltage gain stage you can pop in almost any dual 8-pin opamps and they will work.
Even TL072 could be popped in although this will increase THD considerably.
Something like the NJM4558 is not recommended due to the low speed it has.

Some of the super-wideband (high slewrate) opamps may not be suited and could oscillate (which may or may not become audible) if the proper decoupling measures (so called compound capacitors) aren't applied in the proper position.

In the voltage gain stage I recommend to look for at least the following properties:
at least +/-12V (or 24V)
Unity gain stable or at least stable at gains < 2x
input topology is not of that big an importance for operation (value of R14/R20 is 'low') but those opamps with relatively high input offset voltage spread MIGHT cause the volume pot to become slightly 'scratchy' when moving it without any music on it (due to DC on it)

For the output stages the choice is more limited as it will greatly affect performance when paired with LOW impedance headphones, as the NJM4556 has higher output current capabilities than most other opamps (at least 2x higher than most other opamps).
These opamps must be unity gain stable and be able to provide current (at least +/- 40mA)

When opamps are used to deliver current (not needed for the voltage gain stage) things change for these opamps.
cable capacitances above say .. 100pF CAN cause instabilities in combination with HIGH impedance headphones (not with low impedance ones, these will have other problems). Experiments done myself indicate some amplifiers don't even care driving 10nF loads (1000x as much !)
It is recommended to increase the output R's value to counteract this effect (in the spec sheets they call these isolation resistors and can be used to prevent oscillations) BUT this will also increase the output R which DOES have a profound influence on the sound.
In spirit of NwAvGuy's 'legacy' these resistors, however, should not be increased in value which is supported by the 'damping factor' or 'grip on drivers' fans.
Personally those playing with K701's and some other HP's might be better off changing R14/R20 to 220 Ohm ! (see: diyaudioheaven.wordpress.com/tutorials/ resistance impedance and power articles)
This will decrease maximum output levels though and a lot of headphones will HATE this change (voltage division artifacts) but for some HP's it is a vast improvement.
I am not as scared of increased output R as most people (and NwAvGuy is) and believe optimal sonic enjoyment with some headphones can be found there (technically easily proven as well).

I say, just experiment with some opamps but watch the minimal voltage range and unity gain stable remarks in the spec sheets.
I recommend not to use the highest slewrate/super wide bandwidth opamps out there as well (>20V/us) unless some measures are taken.

Indeed some opamp's sonic changes will border on the realm of placebo while others can have quite measurable effects within the audible band.
We won't know until someone measures relevant specs and listens (preferably NOT knowing what is popped in, ask someone else to pop X or Y in)

the 'expectancy bias' thing is another subject. Consider NwAvGuy to be in the opposite camp of the subjectivists camp. Everything that has been 'invented' in the opposite camp is about to be trashed by the opposite camp in all cases. Note that this is true for both sides of the 'camps'.

Ultimately the only real test is to listen to it and decide for yourself it is your cup of tea.
Some LOVE it , some are DISGUSTED by it, the hype mainly or the designer himself that has disappeared into thin air after too much critics sabred him down.

Well, I've spoken to someone that has one and he says that it is actually very good, but he does find it 'thin' sounding.

Anyway, one is coming this way for me to hear perhaps in two weeks or so after I return from work.

I'm very interested in how well this drives a difficult load, since it opens the possibility of being able to drive decent headphones mobile. (Away from home - not on the street!!)

Hi Ian, who built this one?

I think Frans mentioned it delivers twice the output of a 'normal' portable due to the configuration of the opamps.

I'd be interested to know how long it lasts - perhaps higher capacity batteries (5-600mAh)

I never tried any of the Bravo/Indeed amps but I know what you mean. My B-1 portable was exactly the same until I put a log pot in.

If it all pans-out with this will you buy from JDS Labs? It could get pricy with shipping.

I remember the opamp swap in the Neco! They were OPA111BM

I really don't need another portable amp!! But I wouldn't be averse to building one if it was significantly cheaper and outperforms my Neco V2. I'm thinking of reverting my DT1350 filter/amp back to just a filter (and using it with a stand-alone amp) as even at maximum gain and using NJM opamps it really doesn't seem to get a grip of the Beyers.

I assume you're using it with your T40 filter?

Initially I found the T50s a bit odd - but I quickly tuned into them; so I can understand why you don't 'need' the filter. And, as you say, they improve with volume.

However, with certain music I just found that I had to reach for my 990s - not so much now the T50s are modded, of course.

I've never liked bright sound and I guess I'm a bit of a bass-head but I think the T50s are educating me.