Converting
to run as a subwoofer amplifier is simple. Push in the switch from
“2ch LP” to “Mono LP.
has some added features. The RCA sockets “3” and “4”
are now another pair of inputs and so all four inputs can accept signals from the head
unit’s front and rear outputs. This allows for constant subwoofer fading. Some additional
circuitry is added to the mix (no pun intended). The four inputs are mixed to mono, routed
into a 24dB/octave Linkwitz high pass crossover (subsonic) and passed through a nifty
new low eq circuit. This allows for up to 12dB of boost with a centre frequency of 40Hz.
The signal then passes onto the low pass crossover where we only use one channel
(remember the signal is mono now) and then it flows onto the balance and level controls.
Please note that all FOUR inputs are required for the level control’s sensitivity to be
correct. The loss of each RCA input means a 6dB reduction in signal level
The final drive from the preamplifiers to the power amplifiers is done in the balanced
domain. Our amplifiers all have the preamplifiers running on their own 100% fully ground
isolated power supplies. This design guarantees no ground loops and thus no
engine/alternator noise.
The main class D amplifiers are a new design which incorporate the output filter within the
feedback loop. This method guarantees a flat response no matter the load impedance.
We use a pair of 65 amp TO-247 MOSFETs per channel. These are low gate charge
types which are utilized for their unique characteristics which are beneficial for class D full
range amplifiers.
The power supply is almost an all new design. Fully regulated, yes of course, as this is the
only way to build a good amplifier. A bad power supply is like putting bad fuel in your
auto’s gas tank. The PWM controller drives a brand new driver circuit which ensures rapid
turn and turn off of the eight high current MOSFETs. This is a four stage drive circuit
incorporating some novel features. The MOSFETs I have chosen are each rated at 110
amps....well in the real world I would never dream of running that kind of current through
the relatively skinny leads of each MOSFET. Rather I chose these devices because they
are fast, high current and are in TO-247 packages which allows them to dissipate heat
better than traditional TO-220 packages. A custom autoformer converts the switched
battery voltage to the higher supplies we require.
Each of the eight MOSFETs has its own bypass capacitor which helps to reduce the
current spikes due to PCB and busbar inductance. The MOSFETs are run in a unity gain
configuration thus their bandwidth is maximized.
When the amplifier is powered up, the power supply takes several seconds to initialize
itself for high current operation. The most dangerous time for a power supply is when it
powers on. Big old electrolytic capacitors must be charged and these present a virtual
dead short to the power supply. To reduce turn on current and stress to the power supply
an innovative circuit controls the ramp up of the supply to limit stresses to a minimum.
KRONOS
KRONOS
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