Specifications

AMP2-T assembly instructions. © 41Hz Audio / www.41hz.com Revision 1.0T boards
Consumer
audio
With +10 dB
pre-amp
Pro audio
+4dBU
Value in
AMP2 kit
Component in
AMP2
Nominal V
inRMS
0.316 0.775 1.23 1.23
AMP2 Gain 1 1 0.4 0.4
R
in
50 K 50 K 50 K 50 K R100, R200
R
feedback
50 K 50 K 20 K 20 K R101, R201,
(& R99 for bridging)
Headroom/margin 15 dB 5 dB 10 dB
The gain, G is calculated: G = -R
feedback
/R
in
The AMP2 kit is shipped with 50 K input resistors and 20 K feedback resistors for
a 0.4 gain. These are 0805 size low noise thin film type. You may want to adjust these values for
your own application. On request we can supply other resistor values than the ones included.
SUMMARY
• The input sensitivity with the AMP2 kit components are suitable for a pro audio signal
level of 1.23VRMS (+4 dBu)
• For home audio equipment you should probably increase the input stage gain by
replacing the 20 K feedback resistors with other, higher values.
Input capacitors
Some people have an almost religious in the belief of the influence on the sound quality from the
input capacitors. The AMP2 PCBs have space for RM5, RM10 and RM15 lead spacing for the
input capacitors so you can practice your belief. You can use many different types of
capacitors. Input capacitors are recommended as they block any DC from flowing between
equipment components. The inputs of the Tripath chips are biased by about 2.5V, so a capacitor
is usually required. However, if you hardwire the power amp to a preamplifier that has a
capacitor on the output, then the input capacitor to the power amplifier can be omitted and
replaced by a jumper. The input impedance resistor and the input capacitor form a high pass
filter. The corner frequency is
fc = 1/(2*Pi*R
in
*C
in
).
The value should be chosen so that the corner frequency is well below the used lowest
frequency. There will be a phase shift close to the corner frequency so a margin of at leas one
octave is desirable. A fc of lower than 10 Hz should usually be used for full range and woofer
amplifiers. Higher values can be used for mid range / tweeter amplifiers. Sometimes it is claimed
that input capacitor values should be about 5 uF or more. This may be true with older carbon
film input resistors where impedance had to be low to minimize noise. But with low noise thin
film resistors, fairly high impedance can be used and a corresponding low value for the
capacitor. The AMP2 kit is shipped with 50 K input resistors and 1 uF capacitors, giving a
corner frequency of 3.2 Hz.
It should be noted that two capacitors in series (in the pre amp output and power amp input) will