■ FIGURE 7. Close-up of
the audio amplifier section
of the populated PCB.
aluminum boxes, and used
the 8. 6 x 6. 3 x 2 inch
HM905-ND with aluminum
front and rear panels. For
the front panel (see Figure
11), I splurged and designed
a custom blue, white, and
red one, using the software
from Front Panel Express
com). For the rear, I used
the panel that shipped with
the Hamond box and ironed
on lettering from a laser
printout on Staples Photo
Paper. Clean the panel with
acetone before you iron on
the lettering. Soak the paper
in lukewarm water and peel it from the panel to expose
the black lettering. Use your fingertips (not nails) to rub
any paper remaining on the panel. Dry the panel and
finish with Krylon satin clear coat.
■ FIGURE 8. ExpressPCB layout showing ground plane
configuration and thermal vias for the TPA6120A2 (U5).
Subjectively, the amplifier performs as well as my
expensive commercial audio gear. It’s obviously much
quieter than an LM386-based headphone amplifier that
I built for another project. Music from my tube-type
preamp, as well as guitar tones, are clean and crisp.
A proper comparison of this amplifier with other
amplifiers involves objective measurements of factors such
as THD+N. If you don’t happen to have a room full of
audio test equipment, I suggest you use an inexpensive
PC-based audio spectrum analyzer, such as TrueRTA. A
low-resolution, fully-operational version of TrueRTA is
available for free download from www.TrueAudio.com.
Figure 12 shows the noise level of the amplifier from
0– 50 kHz. As you can see from the figure, the noise level
is constant across the measurement range at about - 93 dB.
As described in the TrueRTA documentation, the noise
floor of your PC sound card limits the minimum noise that
can be measured. Because of differences in sound cards
and the impedance mismatch between the sound card
input and amplifier output, I consider comparative results
more meaningful than absolute measurement values.
TrueRTA showed the noise floor for my 386-based amplifier
was about 20 dB greater than for the precision amplifier.
Figure 13 shows the frequency response of the
amplifier, again using TrueRTA. With the low-pass filter
■ FIGURE 9. Amplifier with standoffs and toroidal
transformer, ready for mounting.
■ FIGURE 10. Wiring of step attenuator and
instrument input jack.
June 2008 53