Installation manual

MYTHS, MAGIC AND FACTS
Headroom - continued.
So a 100w/ch amplifier running into 4 ohms must develop 20 volts across the speaker
terminal. This requires a net (under load) rail voltage of about +/- 33 volts. Now for it to
have 3dB of headroom it must be capable of delivering 28.28 volts across the speaker
terminal. This requires a rail voltage of +/- 43 volts. So the above power supply will droop
a total of +/- 10 volts (a 23% droop!). This puts additional stress on the output devices
(Mosfets or Bipolars) because they still have to deal with this higher rail voltage . To us
this kind of power supply sounds like the amplifier is “breathing” and not the kind of
amplifier we want to listen to. Regulated power supplies are more expensive to
manufacture, are less efficient but we feel those are tradeoffs we can live with!
If one examines the specifications of an amplifier, it is relatively easy to tell apart those
With well regulated power supplies and those with sloppy unregulated power supplies.
The ratio of 4 ohm as to 2 ohm power will readily inform us of the quality of that power
supply. Typically if the amplifier can double or almost double it’s continuous power rating
from 4 ohm to 2 ohm at ALL battery voltages this is indicative of a well regulated power
supply. There are a few manufacturers who manipulate the rail voltages at lower speaker
impedances so that the 4,2 and 1 ohm power specs are the same. We believe that this is
a cop out to save putting in a beefy power supply which is capable of the higher currents
needed for these low impedance loads.
Fusing of amplifiers. Of all the amplifiers which we see at Zed, less than “5%” have
correctly rated fuses installed. These are non Zed manufactured amplifiers. The result of
this over fusing practice is that the power devices and printed circuit card burn up and
save the fuse (Ever heard of Murphy’s Law)?
During the past year we have done quite a bit of testing with regards to how large a fuse is
required. The rating of the fuse is dependent on two things, the impedance of the
speakers and how long and how loud the music is to be played. The average power is
about 10-15% of the maximum peaks. So add up all the channels and multiply by the
power per channel. Divide this by 0.3 for an average efficiency of 30%. Let’s divide this by
say 5 (20% average to peak ratio for some fudge factor) and this number divide by 12.
Example: A 100 watt x 4 at 4 ohms amplifier. We will assume that we will be playing it so
musical peaks reach 100w/ch. Okay so 100 x 4 = 400. Divide by 0.3 = 1333 Divide by 5 =
266 and divide this by 12 = 22 amps. Use a 25 or 30 amp fuse.
Putting a 100 or 150 amp fuse * on an amplifier is useless. That guy Murphy says the
amplifier will burn up to protect the fuse..... And it will.
* Of course very powerful amplifiers >2Kw require large fuses.
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