Yulong A28 black fully balanced class A headphone amplifier 

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The circuit, construction and components are fine tuned and selected over years of design and listen experience, aimed for best sound quality within a certain cost, and not just use hifi grade parts in blind. The amplification is OPAMP driven complementary transistor buffer (also known as “Diamond Buffer”). After trying dozens of OPAMPS – from pristine NE5534 to the latest offers from major manufacturers, OPA 2604 was chosen because we feel it sounds most balanced in this particular design. The output is MJE243/253 middle powered transistor.
 
Other details include: Plitron power transformer from Canada, Neutrik XLR connectors, and Alpha 4-gang potentiometer made in Tai Wan. Unbalanced inputs are converted to balanced signal by an OPA1632, and the balanced/unbalanced input selection is done by a group of relays to optimize signal path.
 
The precisely machined aluminum mini case is same foot print as Saber D series: 250*200*60mm. So it is easy to part with those products such as a DA8 to make a complete set. Please note that there are cooling vents at the side of the machine, and please keep free air flow near those vents.
 
Use note: use balance input can get better performance ,  Input signals from the rear panel switch selection. Two headphone jacks of the front panel is double jack, can connect double 3 pin balance headphones, also can connect 6.35 headphones. 4 pin balanced output can only be connected 4 pin headphones, not through the connector to connect unbalanced headphones, otherwise it will damage it
 
Please be noted, if you use a same source to feed A28, do not connect the RCA and XLR interconnects altogether. This will cause common ground interference and degrades the performance of the RCA input signals. (This applies not only to A28 but other similar products in our tests).

Techncial Specifications:

SNR:>120dB
THD+N: 0.0005%
Crosstalk >100dB
Frequency response:20-20KHz-0.3dB
Balance output power :600ohm:400mW 300 ohm:700mW 150 ohm:1.3W  32 ohm:2.5W
unbalance output power: 600ohm:120mW 300 ohm:200mW 150 ohm:350mW 32 ohm:800mW
Power Consumption: <30W.

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