Zed Audio Draconia Instruction & Installation Manual - page 15
Our power supplies use massive amounts of capacitance on the 12 volt side and this
capacitance is spread over 19 capacitors in parallel. A total of 41,800mfd is used.
Fourteen (14) high current low Rds on Mosfets are used as the main switching elements.
Each is rated at 110 amps but we derate them to 50 amps for safety and each has an “on”
resistance of 0.01 ohms at 80 deg C. With seven Mosfets in parallel on each half of the
supply the total on resistance is 0.0014 Ohm (One point four thousandths of an ohm!)
We further temperature derate this for a final on resistance of 0.002 ohms.
With 14
Mosfets each rated at a theoretical 110 amps we have a reserve of 1,540 amps. Well this
number is wishful thinking and the figure we work with is 50 amps per Mosfet for a total
current capability of 700 amps in a circuit limited to about 240 amps. So we have a large
safety factor built in. This does not tell the whole story about how the switching Mosfets
behave. We must turn them on and off quickly, minimize the overshoot and make sure
their junction temperature remains in the safety zone. We achieve some of these
parameters by using the Mosfets as source followers, which means that they operate
without any voltage gain and only current gain. Pretty much all car amplifiers use their
Mosfets with voltage gain which causes various ills. Our Mosfets operate with maximum
bandwidth and due to the circuit design the main power transformer (of which there are
two in
Megalith) has it’s design greatly simplified. Less turns are required, which means
lower copper losses and we can minimize leakage inductance in the transformer(s) by
better winding methods.
The above describes what happens on the primary side of the power supply. The
secondary side has some equally impressive specs. Two pairs of high current fast
recovery diodes are used to rectify each transformer’s secondary voltages to the required
rails used by the output stages of the amplifier. A total of 22,400 mfd of capacitance is
used for the main rails and this stores 71 Joules of energy. A pair of fully regulated +/- 14
volt supplies feeds all the small signal stages of the PWM amplifier.
A separate transformer supplies the isolated supplies for the pre-amplifiers. These are
again fully regulated. The grounding system for the pre-amplifiers is 100% floating from
the rest of the circuits. This ensures that no ground loops can be formed which in turn will
reject alternator whine. A fully balanced differential drive circuit is used to couple the pre-
amplifier’s outputs to the inputs of the PWM amplifier section.
The protection circuits are extensive and incorporate DC sensors on the speaker
terminals, a thermal shut down should the heatsink reach 80 deg C, a muting circuit which
ensures quiet turn on/off of the PWM amplifier. The PWM section incorporates the short
circuit protection. Lastly a battery level monitor shuts down the amplifier if the battery
voltage exceeds 15 volts.
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