Installation manual

MYTHS, MAGIC AND FACTS
Bridging two channels of an amplifier is not a magical thing. Most are mystified by the
power figures quoted under the “bridge” column. It is actually very simple. When two
channels are driving a common load, one channel is out of phase with the other by 180
degrees. So when one channel swings positive the other swings negative. There is a
catch however. Each channel “sees” fifty percent of the common load and that means that
each channel of the bridged pair must be capable of delivering current to this lower load
impedance. Thus a 4 ohm bridged load presents a 2 ohm load to each of the bridged
channels. The power into a 4 ohm load in bridged mode is twice the rated 2 ohm power
per channel, the reason is that the power supply voltage is effectively doubled when two
channels are bridged.
Harmonic Distortion - This specification has for years been a benchmark with which to
compare one amplifier to another. This is all fine on the test bench where pure resistive
loads are used and sinewaves are amplified. Unfortunately it tells us very little about the
audible performance of an amplifier. Today it is relatively easy to build an amplifier with
THD figures in the “triple oh” region, but what do they sound like. Normally not very good.
To obtain these low THD numbers all we do is design an amplifier with high open loop
gain. That is before negative feedback is applied. Once we apply a lot of global feedback,
we improve all measured parameters such as THD, Noise, Frequency response and
Damping factor. Our class B amplifiers are designed a little differently. We use very little
global feedback but rather optimize each stage with local feedback. This allows us to
design an amplifier with lower open loop gain and thus we only have to apply about 8dB of
global feedback. Ultra low THD was not our goal but rather an amplifier which sounds the
way we want it to. Other factors affect THD such as PCB layout, grounding and power
distribution to the amplifier channels. Our class B amplifiers do however achieve very low
distortion due to the fact that we follow the “rules” and their circuit design is conducive to
low distortion. Class D amplifiers operate somewhat differently to Class B types.
Headroom
This term does not refer to how much room there is above your head! Rather it is a
specification that signifies how good or bad the power supply is. Zed Audio has NEVER
quoted a headroom specification. Why you may ask? Simple our amplifiers have no
headroom, zero dB, zip dB, nada dB however you say it. A regulated power supply does
not allow the amplifier to have any headroom. A quote from a well respected designer who
said that amplifiers with many dB of headroom simply have poorly designed power
supplies, either through ignorance or to save costs. When one sees a specification of an
amplifier quoting a headroom figure of 3dB this means that the droop of the power supply
is such that when unloaded it is capable of twice the power as compared to it’s loaded
condition.
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