SMSL sApII Pro

This amplifier does an excellent job in powering headphones, and for the price, it could be the best out there for under $75 new. Other then power, the noise is not changed from original output. Hissing is present at some levels. Recommend to people looking for cheap power.

I use the S.M.S.L. sApII Pro to drive 300Ohm Sennheiser HD600s and it performs really well. It delivers plenty of clean, flat power to drive my headphones with authority, just what a headphone amplifier should do in my opinion. There is not much information around but I would say the amp is designed for high impedance headphones.  I do not recommend getting this amplifier to use with low impedance headphones as there is channel imbalance at low volume. I can hear this while listening to IEMs at lowest audible volume.
 
The amp comes with a source switch which allows you to hook up two sources and switch between the RCA and 3.5mm input, nice! Negatives would be that the volume pot makes some noise and that the amp does get warm after a while.
 
In a world of overpriced HiFi gear I believe this is a bargain at around 70 bucks.

 

 

 
It is brilliant, having been based on a OPA2134 as input amp that is wired to a TPA6120A2 as buffer (Current amp).
 
It uses a TDA2030 as a rail splitter. Using a power opamp as a rail splitter has it's advantages, mainly you will notice bass on this amp is much more defined than the original sApII.
It DEFINITELY will improve SNR if you use a better power supply than standard, standard power supply is pretty junky. It runs on 24V so if you want peak headphone power (or lower distortion at lower outputs and also higher voltage has a advantage because the lower frequencies can maintain a constant output regardless of impedance as all real headphones have weird ass impedance graphs) stick to 24V don't be a smart alec.
 
The noise is kept low on the sApII
Now that I have K7XX, AD500X (modded cable + rubber band) and M50X (shure HPAEC840 pleather pads + 3.5 mod) I guess I'll comment more
 
This amp provides PLENTY of power, so much that even with a K7XX which requires a lot of power I'm still barely off like what is the equivalent of 2 from my AK4490 line out DAC.
And when it comes to flatness (which is what an amp should be all about in my opinion) it still remains flat all the way up the power range on a K7XX, I have heard less than satisfying amps at huge price brackets as well so if it does well for me it's all good.
 
But, the only issue is the crap power supply. But that's the only issue. SQ wise it has low distortion and tight response. It's not the tightest granted but unless you want to spend over 1000$ on an amp (which I am thoroughly convinced is all marketing and audiophoolery) you can't go wrong with this tiny tiny 50$ amp