It is not uncommon for an unregulated "wall wart" type transformer to show much higher voltage with no load on it. If you checked the voltage with the wall wart plugged into the wall but with no load on it, your readings aren't very accurate during normal use.
One would literally have to have the device being powered under normal load in order to measure the working voltage of the wall wart / power supply in a more accurate manner. In order to to do this, you would have to remove the batteries from the circuit, fire the amp up, play music at normal volumes and then check the voltage on the circuit board as the amp sees it. Due to the efficiency of the amp and the lower than average power consumption, i would not doubt that the voltage will remain somewhat high. So long as it doesn't remain well above 16 volts ( as quoted by the manufacturer of the chip ), you shouldn't run into problems.
If the voltage isn't falling enough, one could very easily wire a few high powered resistors across the pos & neg inputs on the board. By increasing the amount of current draw via using lower impedance resistors in parallel with the amplifier draw, one can bring the voltage down to the desired level. There are also benefits to doing this as it will tend to reduce ripple voltage AND act as a broadband resistive shunt for RFI / EMI induced noises. Just make sure that the resistors that you use are NOT wire-wound as the stray inductance will reduce their bandwidth, negating their effectiveness as an RFI shunt.
The other alternative is to build a regulation circuit and / or a voltage divider. These are both more involved but can provide even better results than the resistive shunts mentioned above. That is, IF they are built and designed properly.
The first thing to do would be to check the voltage of the wall wart powering the amp under a normal load. Just make sure that you pull the batteries from the unit, so as to pull as much of a load from the wall wart as possible. In doing so, you might find that your voltage is fine without doing anything other than enjoying the music : )
As a side note, i've got three of these amps on order along with three 12 volt @ 2.4 amp wall warts. Both parts are currently out of stock from where i'm trying to get them, so i'll have to wait. I intend to use these in a vertically bi-amped mode for some actively crossed computer speakers that i'm going to build. The third will be for driving a subwoofer for the same system. By actively crossing the speakers prior to amplification, amplifier efficiency will be increased due to a reduction in bandwidth. This will further increase power availability, adding both more headroom and dynamics as needed. Due to the miniscule size of each amplifier board and having the power supplies outboard, all of the circuitry can be built into the cabinets with minimal loss in internal volume. On top of that, the signal path will be even shorter between the amplifier / driver interphase, which should result in improved sonics.
Given that i can do all of this for well under $150 total due to utilizing parts that i have laying around, doing my own woodwork and careful shopping in terms of the amplifiers / power supplies, i'm quite certain that the results will easily blow away anything that i can find on the market for many, many times the price. Sean
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