HeadRoom Headphone Amplifier & Digital-Analog Converter - Manual (Page 5)

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What is the HeadRoom Crossfeed? Electronics Modules for your Micro Amp
The Micro Module
Desktop Electronics Module Upgrade
The Micro Amp can be upgraded to the Desktop Module. This little green mod-
ule contains all the electronics for a Desktop Headphone Amp. There are two
4-layer circuit boards (one each for the left and right channel) that are con-
nected with a series of header pins used to solder the module to the main cir-
cuit board. This module is designed to sound great but must provide at least
20 hours of play time on four D-cells in the Desktop Portable Amp. It uses Burr-
Brown (Okay, now TI) 2134 op-amps for the internal input buffer, crossfeed,
summer, and power amp voltage gain stage. The output current amplifier is
our version of the highly regarded Diamond Buffer discrete transistor design
by Walt Jung. Resistors are 1% metal thin film, and caps in the signal chain are
polyphenylenesulfide (poly film); these are MUCH higher quality parts than
you’d ever find in a typical consumer electronics product.
Imagine you are listening to a pair of speakers. If you turn off the left speaker,
both ears hear the sound from the right speaker. But because the left ear is
slight farther away than the right ear, it hears the speaker’s sound slightly
after the right ear; about 300 microSeconds. This time difference is called the
“inter-aural time difference” and it is the main thing your brain listens for in
order to tell where to place sound left-to-right.
But in headphones if you turn off the left channel, only the right ear hears the
sound. In headphones, if there is any sound that is only in the left channel, or
only in the right channel, then only that ear hears the sound. This is not natu-
ral, and you brain becomes fatigued trying to figure out where sound is com-
ing from when only one ear is hearing it. This tends to create an audio image
that is a blob on the left, blob on the right and a blob in the middle.
HeadRoom amplifiers cure the problem by allowing you to cross-feed a little
of the left and right channels across to each other through a short time de-
lay using the crossfeed switch. The usefulness of the circuit varies depending
on what type of recording you are listening to; mono and binaural recordings
need no processor at all. Old studio recordings that have instruments panned
hard left or right, benefit greatly from the processor. Live and classical record-
ings miked from a distance benefit somewhat less, and can often be listened
to without the processor quite comfortably.
Plain
Headphones
With
HeadRoom
The crossfeed switch in
HeadRoom amplifiers al-
low you to cross-feed a
little of the left and right
channels across to eacho-
ther through a short time
delay.
Far ear
hears
slight delay.
Near ear
hears sound
first.
30 degrees
off axis
Micro electronics module
Desktop electronics module
This little red module contains all the electronics for a Micro Headphone Amp.
There are two 4-layer circuit boards (one each for the left and right channel)
that are connected with a series of header pins used to solder the module
to the main circuit board. This module is designed to sound great but must
provide at least 20 hours of play time on two 9-Volt batteries in the Micro
Amp. It uses AD822 op-amps for the internal input buffer, crossfeed, sum-
mer, and power amp voltage gain stage. The output current amplifier is our
version of the highly regarded Diamond Buffer discrete transistor design by
Walt Jung. Resistors are 1% metal thin film, and caps in the signal chain are
polyphenylenesulfide (poly film); these are MUCH higher quality parts than
you’d ever find in a typical consumer electronics product. The Micro Amp can
be upgraded to the Desktop Module. The result will be an approximate halving
of play time and moderate improvement in sound quality. If you will be using
this amp largely in a portable mode, we suggest sticking with the stock micro
module as its better sounding by far than any portable source (accept maybe
the Micro DAC) and saving batteries is a good thing.