Installation manual

MYTHS, MAGIC AND FACTS
Class A and class B amplifiers do not have output filters and so the feedback network is
always taken off from the output node of the amplifier. Class D amplifiers have a “monkey
on their backs” in that they ALL have to have an output demodulator filter to get rid of the
high frequency carrier. Class D amplifiers are simply Pulse Width Modulated power
supplies where the modulation is the audio signal. How good or bad they sound depends
on how the whole design is implemented.
Our class D amplifiers are of the self oscillating type which has proved to be simpler and
better sounding than driven types (Where a fixed clock at some high frequency is used as
the carrier). Analogous to FM or AM radio where the signal is transmitted at some high
frequency, your radio picks up this signal (Which contains both the carrier and the audio)
and then finally demodulates ie. Filters out the carrier leaving just the audio.
The absolute vast majority of sub woofer class D amplifiers sold today come from Asia
and they typically use designs from one or two different companies. They all make one
error, their feedback is taken BEFORE the output filter. What does this mean? Quite
simply the deficiencies of the inductor and capacitor in the filter will contaminate the signal
and the response will vary with frequency.
The reactance (AC resistance) of the coil which after all is in series with the speaker is
calculated from this formula Xl = 2 x Pi x F x L.
Xl = Reactance = AC resistance
Pi = 3.14
F = Frequency at which we want to calculate Xl
L = Inductance of the coil.
Simple example: F is 100KHz and L is 150uH (150 micto Henry) typical of these amps.
Xl = 94.2 ohms at 100KHz
Xl = 0.0942 ohms at 100Hz
Xl = 0.0188 ohms at 20Hz.
The 94.2 ohms is great because at 100KHz we want it has high as possible for maximum
rejection of the carrier. However we see in the narrow range from 20Hz to 100Hz is has
varied by a ratio of 5 as to 1. We have not included the simple DC resistance of the coil
which must be added to the 0.0942 and 0.0188 numbers. A typical coil in one of these
amplifiers uses a mean turn length of about 50mm (2”) per turn and may have between 70
to 100 turns of wire. #15 wire (whether made from a single strand or many strands if finer
wire) has a DC resistance of 3.18 ohms per 307 metres (1000’). So we will have about 70
x 2” of wire = 140” = 11.66 feet call it 12 feet for round numbers. Well 12 feet have a DCR
of 0.038 ohms and this does not include the second series coil used in everyone of these
Far Eastern made amplifiers. So giving the benefit of the doubt to the second coil lets give
it a DCR of only 0.002 ohms so our total DCR of wire in SERIES with the speaker is 0.04
ohm. Well now what do we have here?
Page 4