Installation manual

MYTHS, MAGIC AND FACTS
What we have is a big old amplifier with a 0.04 ohm “resistor” in series with the output.
The DCR of the wire on the inductors is absolutely equal to a resistor with regards to
straight DCR. The output impedance of the amplifier is at best 0.04 ohms and this does
not take into account the output stage’s resistance which typically is at least 0.01 to 0.02
ohms. Rounding off let’s go for 0.05 ohms and this is at 20Hz. At 100Hz the value is 0.134
ohms.
Now I am not a big fan of the Damping Factor hoopla. What I object to is the claim of DF
values for these “mega watt” and other class D offerings of 300, 500 and 1000w etc. At
best the is 0.05 + 0.0188 (do not forget the AC impedance of the
coil) = 0.0688 ohms. Divide 4 by 0.0688 and we have a DF of 58 with a 4 ohm load, 29
with a 2 ohm load and just 14.5 with a 1 ohm load. at 4 ohm is 4/0.134 =
29.8, at 2 ohm it is 14.9 and at 1 ohm 7.46....mmm interesting is it not.
As the saying goes, “make my day”. Please read the specifications of all of these
amplifiers and I bet each and everyone will claim massive DF numbers and distortion in
the “double 00’s”. The THD of an uncompensated output filter is quite high.
These Far Eastern class D amplifiers typically use a switching frequency of 100KHz or
less. OK for sub woofer application but useless for full range (20Hz-20KHz) operation.
Unfortunately even at 100KHz one has to use a pretty aggressive filter to attenuate the
carrier. The inductors range from 80 to 200 micro Henry and the capacitors from 22mfd to
220 mfd.
Zed is not the first to use post filter feedback but it solves all of the above issues to a great
degree. Here is what we incorporate. The distortion of the filter is reduced by the feedback
factor and in our amplifiers is typically about 15 times. The DCR of our output inductors is
an order of magnitude lower than the above, even with Draconia having the skinniest wire
we use less turns as our inductors are in the 22-30uH range. We use switching
frequencies close to half a megahertz (500KHz) and so our filter parts are of lower values.
Now any DCR in the inductor is taken care of by the feedback network and so does not
appear as a series resistive element. Our DF values are modest even with reflected
inductor DCR of about 0.002 ohms.
Finally we have low THD as the imperfections of the filter components are almost
eliminated by the feedback being post filter. The frequency response of our class D
amplifiers is flat from less than 10Hz to 25KHz within 0.1dB and less than 2dB down at
55KHz..
DF with 4 ohm at 20Hz
At 100Hz the DF
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