HA75-DAC HEADPHONE AMP
Hafler
9
THe VoX ConTRoL
This three-position VOX or loudness control switch serves two functions.
The first is as a traditional loudness control, the second to enhance bass.
Back in the 1970's, most integrated receivers were outfitted with a
loudness switch that would enhance bass at low volumes to make up
for loudspeaker inefficiency and the human ear’s greater sensitivity to
mid-range frequencies. This countered the Fletcher-Munson curves
that demonstrated how the human hearing changed based on sound
pressure levels. We felt that when listening to music at low levels, say
while reading a book, enhancing the low frequencies would enhance
the listening experience. To set the HA75-DAC’s VOX switch to normal
loudness, set the switch in the down position.
The other issue is one that has come about in recent times - headphones
with exaggerated bass that are now all too common. When comparing
headphones, the difference can be astonishing. Some have extended
high frequencies, others so much bass they can cause your head to spin.
Another problem is listening to music that may have been recorded in the
1960's or 70's that may lack bass.
Set the VOX switch in the up position to increase the bass and simulate
the effect of these over-exaggerated headphones or offset bass-lacking
mixes. This also allows the recording engineer to simulate listening to
various headphones to get a sense of how the program material will
translate.
For the purist, simply set VOX control to the middle position (bypass) to
hear the natural mix without enhancement.
VOX Switch
A - Bass enhancer
Ø - Bypass / Off
B - Loudness
AbouT THe Tube
Most of the voltage amplification inside the HA75-DAC is done with a
12AX7 dual triode tube operating with a B+ of about 140 Volts. This tube
may be unplugged and replaced with any other brand of 12AX7 if the
user has some other preference. It is however important to note that the
HA75-DAC is biased for the 12AX7 and use with a 12AU7 or 12AT7 will
not sound good at all.
Note: High voltages are present inside the HA75-DAC.
For safety we
suggest that the tube only be changed by a qualified technician.
