■ FIGURE 11. Front (left)
and back (right) panel.
front end shown in the
schematic with the 220K ohm
resistor, the frequency response is flat from 0 to about 18
kHz. If your sound sources include signals above 18 kHz
(and you can actually hear the signals), then consider
modifying the input circuit. Removing the 220K resistor
and using the 50K step attenuator results in a virtually
flat frequency response from 0 to about 20 kHz, as
shown in Figure 14. Note the curious frequency response
between 20 kHz and 50 kHz associated with each input
■ FIGURE 12. Amplifier noise level, 0– 50 kHz.
■ FIGURE 13. Frequency response with front
end, as per schematic.
The amplifier described here is meant to be modified
to suit your needs. For example, I built two amplifiers,
one configured as in the schematic, and mounted it
permanently in my tube-type preamplifier.
I built a second amplifier with 50K stepped attenuator
front end and mounted it in a Hamond case to use with
my CD player, iPod, and electric guitar. I added an
inexpensive line filter to reduce common mode noise. In
addition, I replaced the 1K resistors at R6 and R14 with
2K resistors to provide 2x gain. As with most op-amps, the
gain of the AD8610 is proportional to R6/R5 in the left
channel and R14/R16 in the right channel. You can create
a variable amplification option by replacing R6 and R14
with on-board eight-position switches and surface-mount
resistors. Another option is to replace R6 and R14 with
10K pots. NV
■ Custom engraved front panels. Front Panel Express.
■ Step attenuators, audio connectors, capacitors, and
cables. DIYCable.com. www.diycable.com
■ Step attenuators, knobs, and shaft couplers. GoldPoint.
■ FIGURE 14. Frequency response with 50K
stepped attenuator input.