Specifications

AMP2-T assembly instructions. © 41Hz Audio / www.41hz.com Revision 1.0T boards
APPENDIX 2: Fine tuning the AMP2
The components included in the AMP2 kit work well for many users. For optimal performance
the AMP2 board may be fine tuned to each application. Here is a summary of what to think of.
Input impedance, input gain
The TK2350 can be described as a two stage amplifier. The first is an operational amplifier while
the second is the power stage / modulator.
So first we have to select input and gain resistors for the input operational amplifier. The input
impedance has an effect on any equipment upstream of the amplifier, such as pre amplifiers.
Modern pre-amplifiers are usually capable of driving fairly high currents / low impedance loads.
But usually, the THD+N is better if the impedance is not to low. On the other hand very high
impedance in the power amplifier will introduce current noise so a compromise has to be found.
Values of between 10k  and 100K are typical while 20 K to 50 K are most common.
Some people prefer carbon thick film resistors for the input and feedback resistors while others
prefer thin film or metal film resistors which usually have lower noise. With thick film carbon
resistors we could use lower values, say 20 K for low noise while higher values can be used
with thin film or metal film, often 50 K.
Most consumer audio equipment like CD players etc have a signal level output of -10dBV which
corresponds to 0.316 V RMS. Portable devices like MP3 players, MD and CD players often
have lower output to extend battery life. A preamplifier would usually have a gain of about +10
dB giving an output of 0dBV or 0.775 V RMS maximum. Similar levels could be expected from
computer soundcards. Professional equipment usually has a nominal +4 dBU maximum output
signal which corresponds to 1.23 V RMS.
The TK2350 amplifier chip modulator stage has a maximum input level of about 4V peak to
peak, equivalent to 1.41 V RMS. For best performance we should select the gain for the input
stage so that at maximum volume (from the preamp) the voltage in the input stage is close to the
maximum 4Vptp. Some headroom margin for transient peaks is however required.
The following table gives some examples. For a consumer audio level signal of 0.316 V and
select a gain of one (1) for the power amplifier input stage, we would have headroom of 15dB
which is a bit much and means the amplifier can not reach its maximum power. If we add a
+10dB gain preamplifier we would have headroom of 5 dB which is reasonable. With a pro
audio input of 1.23 V and an amplifier gain setting of -0.4 we would have 10dB headroom,
which again is reasonable.