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8. Features Explained
8.1 Extreme Power Headphone Outputs
During the development of the ADI-2 Pro an extensive research on today's headphone amp tech-
nology as well as headphones has been carried out. Many (many!) headphones later a maximum
output level of +22 dBu (10 Volt) was set as development goal, as it will drive even insensitive
headphones sufficiently, while a maximum output current of around 320 mA per channel will result
in lots of power for lower impedance phones (2.1 Watts @ 32 Ohm).
Limiting the current makes a lot of sense. It is needed to control the internal power supply, to not
fully overdrive (and destroy) weaker headphones, and prevents malfunction at short-circuit state.
The Extreme Power output stage acts like a small power amplifier, so it got a similar feature set:
a relay that mutes and interrupts the connection to the phones, a DC sensing circuit to prevent
DC at the output (DC destroys your precious phones already when the rated watts are not even
closely reached!), and an over-current protection circuit that will notice when a short-circuit causes
too high current, preventing the output stage to get destroyed. In light of the malicious treatment
of this output stage during development it needs to be mentioned that it can not be destroyed by
a short-circuit at the output nor by overheating. Still some extra safety won't hurt, and the over-
current protection circuit got in.
A goal during development was to build a headphone amp that not only reaches very low THD
un-loaded (the typical way of measuring it), but very low THD values with a 32 or 16 ohm real-
world load. This was achieved in the new Extreme Power headphone driver output stage. It uses
6-fold spread power technology, improved thermal conductivity and a special super-low distortion
driver design. The result is THD below -110 dB at 32 Ohm load even near full output level (clip-
ping), the same SNR as the DAC provides (122 dBA), an output impedance of only 0.1 Ohms,
totally stable operation, and a frequency response from 0 Hz up to 80 kHz, with just 0.5 dB de-
crease at the top end. The result: No audible hum, noise or distortion, fully transparent and crystal
clear sound at any volume setting, for any personal taste in any application.
And there is more. The headphone sockets of the ADI-2/4 Pro SE have sensor contacts. The unit
always knows when a headphone jack is inserted or removed. The DSP uses this information for
several superior, partly never-seen before features. For example when inserting the headphone
jack into Ph 3/4, the ADI-2/4 Pro SE activates the mute relay after half a second, then the DSP
ramps up the volume slowly from lower level to the last used state. Comfortable? Luxurious? Yes,
but the main reason for it was to give the user a chance to react. Extreme Power headphone
outputs set to full output level, music already playing at full level, inserting the phones, and the
moment the relay switches on the doctor is called, diagnosing sudden deafness - this should and
can not happen with the ADI-2/4 Pro SE. When the volume is ramped up one has the time to
either quickly set the phones off, unplug the phones again, or to grab the Volume knob to quickly
turn it down.
To guarantee that the Volume knob will be set to control the correct outputs in that moment, the
DSP also sets the Volume knob automatically to the output where phones had been plugged in.
And even returns the setting when the phones are unplugged again.
This is just an example of how intelligent and elaborate the control logic of the ADI-2/4 Pro SE
has been implemented. There are lots of such functions and features that might even stay un-
noticed, making the unit behave fail-proof as well as easy to operate.
But aren't up to +21.5dBu, or Hi-Power as it is called in the menu, much too loud for modern
phones? That depends. There are still phones that need higher levels. Music can be low in volume
but consume a lot of power, especially with lots of sub-bass. And lots of headroom is always nice
to have. Typically with Hi-Power off, which equals +7 dBu maximum output level, modern music
and modern headphones, Hi-Power is mostly not needed. But you will notice that even with Hi-
Power active, which requires to use a volume setting 15 dB lower as usual, the sound stays the
same, and there is no audible noise or hum at the phones output (provided the source is clean,
of course). So even at a Volume setting of -40 dB the ADI-2/4 Pro SE delivers perfect sound
quality, being a no-brainer in daily use as how to set it.
User's Guide ADI-2/4 Pro SE - v 1.0
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