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Originally Posted by FallenAngel /img/forum/go_quote.gif
That's a quite different design.
The M3 uses a pair of MOSFETs to create a push-pull output stage, that HA171 uses a monolithic buffer. It's also a LOT easier/cheaper to use a 2-channel design with a passive ground (ground signal goes to PSU). The M3 uses an actively driven ground channel.
You are correct in that there are two camps - 3-channel vs 2-channel. My only argument against the 2-channel design is that the sound REALLY depends on the PSU quality while a 3-channel does not depend on it as much.
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Well, if I may peel open the LME49600 buffer, it does have a push-pull stage at the output IIRC. If we compare them in a block diagram sorta way, the two will have a fair bit of similarity. (well, most amps on the market are like that anyways...
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Since the transformer, power supply circuits, regulator circuits and the whole works are in our control. We basically implemented a very low noise split rail power that works very well with our DACs and headphone amps. The difference between DAC's power and headphone amp's power is the size of the regulator's transistor. DA-152/HA-171 use higher powered TO-220 Toshiba transistor while the DACs use a medium power transistor that I can't recall the model number at the moment. As a personal preference, I prefer to use a big burly power supply when I have the room that is available on most desktop amps.
I personally have nothing against virtual ground, but I feel it is better suited to portable application when it is not feasible to build a power supply with a ground that can sink and source like ones on desktop units.
Some may say that virtual ground will almost always guarantee near-perfect split of the rail voltage, yes that is true. However, what is the point of absolute perfect split when a manually adjusted rail comes within 0.1V difference? The difference in the rail does not produce any output bias on a properly functioning amp, and does not really do much other than having one side of the output transistor/mos to dissipate more heat/power due to the voltage drop. But that point is also irrelevant if the rail difference is very small and the two side of the output device is in the same package (as is the case with LME49600).
As for M3's power, I'd say it IS really dependent on the power, I've actually built two M3s myself. One planned with moderate parts cost and high input capacitor value (I think in the order or 20000uF, but that was a while back) and powered by a traditional transformer power supply. I initially plan to use a run of the mill switching power, but the noise and interference generated by the power just wreck the sound of the unit, and introducing a LOT of noises into the system (Yes, I did try all kind of filtering techniques, none came even close to acceptable). Being unemployed at the time, I need to sell some toys for cash, so I basically drop the switching power and use a traditional transformer power supply.
The second one was built with higher priced parts and lower input cap size (BC 118 AHT, total 6600uF I think), this time I paid about 10 times the price for a supposed high quality switching power (well, I got a job when I was building the second one). Lo and behold, it was one of the cleanest sounding headphone amp/M3 that I've heard up til then.
How does those two M3 compare to HA-171??? Well, I have NO IDEA
Since both units are long sold to my friends before HA-171 was ever materialized.
I also have built two Mini3s, with identical parts except the op-amps... Interestingly I find the lower power/lower performance National Semi op-amp version to sound better than the supposingly high performance AD op-amp version... Anyone felt the same about the Mini3?