Benchmark DAC1 now available with USB - Page 88

This sounds like a good setup. You also have the option of utilizing the DAC1's volume control and decreasing digital attenuation, but I doubt that will improve SNR much, if at all. I'd say you're good as you are...

Thanks,
Elias

The XLR outputs on the DAC1 USB are not designed to drive headphones. The output impedance is too high (anything over 0.5 ohms is too high for driving headphones). The HPA2 headphone amp has <0.01 ohm output impedance. This is one reason it achieves ultra-low-distortion headphone amplification.

The 4562's are great line amps for the line outputs (XLR and RCA), but they are not meant to be headphone amplifiers. They will do better then the 5532's at that job, but still not ideal. The HPA2 headphone output driver is not a 5532...it is a BUF634, which is extremely well suited for driving the awkward load that headphones present.

There's a lot of discussion about balanced headphones earlier in this thread, but I'll give a brief description of the problems with balanced headphones...

ALL headphones have non-linear mechanical impedances (that is, the mass and shape of a speaker will resonate more at certain frequencies and much less at other frequencies). This means the physical build of the headphones (as well as other physical impedances, like your head and ears!) will try to override the electrical system (amplifier and speaker coil).

To create low-distortion headphone response, one must consider 'damping factor'. A high damping factor will control the response of the speaker, thus preventing the physical impedances from dictating frequency response. Damping factor is the ratio of speaker (load) impedance to amplifier (source) impedance. In other words, the best damping factor will result from a low source impedance. Again, the source impedance from the HPA2 is less then 0.01 ohms...as low as gets!!

Balanced headphone amps will double the source impedance of an unbalanced headphone amp. No matter how low the impedance of a balanced headphone amp, it could be half that much if it was unbalanced. This is one reason balanced headphone amps are not a good idea. (It should also be noted that the balanced output of the DAC1 / USB / PRE is 60 ohms or greater, depending on the attenuator settings).

Not only will the source impedance double with balanced headphone amplifiers, but the total distortion and noise of the amplifier will double as well!! Every output device (opamp, transistor, tube) creates some distortion and some noise. If there are two opamps or transistors or tubes driving each headphone speaker, twice as much distortion and noise will be added!!

The result of balanced headphones is less damping factor, more distortion, and more noise. Also, balanced headphones configurations offer no real benefits, to boot.

Feel free to use the XLR outputs of the DAC1 / USB / PRE for balanced headphone outputs (as mentioned above, the DAC1 USB and DAC1 PRE will do better then the DAC1 at this task, because of the 4562's). It won't damage anything to operate in this configuration. But, for the reasons above, I don't recommend it.

Thanks,
Elias

XLR-to-RCA will be equivalent to RCA-to-RCA. Just remember to make sure the cable has pin-3 floating!

Thanks,
Elias

L-e, Gordon, et. al.,

With the AD1896 (the ASRC used in the UltraLock clocking system of Benchmark converters), the jitter of the incoming signal will not affect the conversion, it will merely (and inconsequentially) affect the amount of data in the input buffer between samples (as l-e correctly stated).

The speed of the Fin/Fout ratio updater of the ASRC will dictate the amount of jitter seen by the converter. The AD1896 has a fast start-up (higher jitter), but settles very quickly (~500ms) to a slower mode (low jitter). In the slower mode, the jitter is attenuated VERY effectively. Specifically, it begins rolling off at 1 Hz, and is lower then -100 dB at 1kHz. (All of this can be seen in the performance charts of the AD1896 ASRC: http://www.analog.com/UploadedFiles/...ets/AD1896.pdf)

The bottom line is that the effects of jitter is a constant, known quantity that is completely independent of the amount of incoming jitter. In other words, the incoming clock could have 0.05 UI of 15 kHz jitter or 12 UI of broadband jitter, and the amount of distortion will not change AT ALL!!! Pretty amazing... This is an important performance characteristic because it eliminates the differences between cable quality, cable lengths, environmental EMI, AC-induced signal noise, transmission methods (optical vs. electrical vs. USB), etc, etc, etc...

It should also be noted that the amount of jitter from the local on-board clock also affects the amount of jitter-induced distortion during conversion. This is why intelligent clock circuitry and board-layout is extremely important for low-distortion performance. All-in-all, for all levels of incoming jitter, the amplitude of jitter-induced artifacts of the DAC1 is ~-133 dB...well below the threshold of hearing. (btw, this performance spec has been verified by many reviewers...see John Atkinson's tests in Stereophile, for example).

L-e, the RAM in the AD1896 is 512 words per channel.

Thanks,
Elias