Next goes the incomparably more expensive amplifier from C.E.C. The HD53R-80 differs from most devices of its class with having two stereo amplifiers that allow to connect up to four headphones simultaneously (thanks to some exclusive system called Load Effect Free) or a couple of high-resistance headphones and speakers. Thus, the amplifier’s left and right outputs, differing with the recommended load impedance, are equipped with two sockets for 3.5 and 6.3mm connectors. I found out that the connection of a second pair of 32Ohm speakers to the right amplifier’s outputs (rated for a load of 16 Ohms and higher) did not affect the sound of the first pair. The left amplifier, although rated for headphones with impedance of over 100 Ohms, copes with the Grado headphones but overloads at high volume.
The C.E.C. amplifier proved to be noisier than the Scythe but I could not normally notice that. This amplifier had a surprisingly low growth of harmonic distortions under load: the difference from the distortions at no load was less than 0.0005%! That’s Load Effect Free indeed. What about the shape of distortions introduced by the amplifier?
That’s just amazing! The left section of the amplifier, optimized for high-impedance headphones, does not introduce harmonics higher than the third one, and even this harmonic is missing in the right section. To be exact, the third harmonic is lost in the noise whose characteristic peak is indicative of a ground loop. The user manual strongly recommends using additional grounding (there is a special screw at the bottom of the case) but I hadn’t looked this piece of advice up before my tests. According to the manual, the grounding will help get rid of noises when you are turning the volume controls.
Practice suggests that the volume controls are the weakest spot of the C.E.C. HD53R-80. When the controls point at 1 o'clock or lower, they bring a considerable difference into the volume of the channels. When set above the middle, they begin to lower the level of low and high frequencies as can be seen in the frequency response diagrams for the right and left amplifiers recorded at different positions of the volume controls. There we can also see the distortion of the frequency response under load, which is due to the nonzero output impedance of both amplifiers. However, these deflections are too small to be taken seriously. The nonzero impedance worsens the electric dumping of the dynamic heads, increasing distortions in low frequencies that spoil the result of the intermodulation distortion test. But it should be noted that the right amplifier only has a high second 60Hz harmonic whereas the left amplifier, which is not actually meant for 32Ohm headphones, lowers the harmonics quickly – you can see them up to the fifth harmonic only. It is all right at the high frequencies with the C.E.C. HD53R-80: the slim column of the second 7kHz harmonic is the only indication that this amplifier is not perfect.
Well, the reason for the very clear sound of the C.E.C. amplifier is obvious enough. I will now check out how the sound cards cope with the headphones. First goes the Creative X-Fi Elite Pro which has only served as a registering instrument until now. The measurements were performed at two positions of the sound card’s main volume control corresponding to the two tests of the KamaBay Amp in signal level.
It is clear from the table that this sound card doesn’t like high volume: the distortions in the headphones are growing up like an avalanche. This might have been expected since the operation amplifier at the sound card’s line output cannot provide an alternating current with an amplitude of more than 1V at a load impedance of 32Ohm.
NJM2114 Load Driving
Even at a volume of 65% the X-Fi Elite Pro sound card is considerably inferior to the inexpensive Scythe KamaBay Amp in every parameter, save for signal-to-noise ratio. The difference in terms of channel crosstalk is especially impressive: the amplifier lost about 20dB when I connected the headphones but the sound card lost about 80dB! This is a very poor result since the crosstalk limits the stereo panorama and clarity of musical instruments.
The impedance of this sound card is much higher than that of the C.E.C. amplifier. The harmonics are even higher than with the Scythe amplifier and their spectrum is not much shorter. This result can only be considered acceptable with one reservation: at everyday use I almost never had to increase the sound volume above 45% which was the default value set by the sound card’s software when I selected the Headphones configuration.