- High Current Amplifiers:
- An amplifier designated as a
high current amplifier is generally a class A/B
amplifier which can drive a lower impedance load than the industry standard 2
ohms per channel. Some high current amplifiers
can drive loads as low .5 ohms stereo or even .67
ohms mono (yikes!). This page will give a review
of amplifier basics and will show some of the
differences between standard and high current
amplifiers.
- Note:
- The information below has links
which will take you back to an earlier page from
the site which will explain the technical terms
involved. If you don't understand something, go
back and read it (it doesn't cause any physical
pain... really)
- BACKGROUND INFORMATION:
-
- 1.The amplifier uses the power
supplied by the battery or charging
system.
-
- 2.The amp's switching
power supply
uses a transformer to boost the available voltage
(from the battery/charging system).
-
- 3.The amount that the voltage
is boosted is determined, in part, by the winding
ratio of the transformer.
-
- 4.The output of the transformer
is a high frequency AC voltage.
-
- 5.This AC voltage is rectified to a positive DC voltage (considered
to be the positive 'rail' voltage) and a negative
DC voltage (this is the negative rail voltage).
-
- 6.The rail voltages are both
higher than ground and lower than ground (plus
and minus voltages with respect to ground).
-
- 7.The audio output transistors
act as electronic valves to deliver varying
amounts of rail voltage to the speaker.
-
- 8.The audio output transistors cannot deliver more than the voltage
present on the rails.
-
- 9.The power that can be
delivered to the speaker is determined by the
amount of voltage that can be delivered to the
speaker.
-
- 10.If the amplifier has plus
and minus 30 volt rails (60 volts rail to rail),
it can deliver, at most, 30 volts to the speaker
at any given time. If we use the Ohm's law formula, P=E^2/R and we have a 4 ohm speaker:
-
- P=30^2/4
- P=900/4
- P=225 watts peak, 112.5 watts RMS
-
- *Keep in mind that the next
numbers don't take into account any
inefficiencies or losses.
-
- If we use a 2 ohm load and
pretend that the rail voltage won't sag, we get:
-
- P=900/2
- P=450 watts
peak, 225 RMS
- The example(s) above show how
lowering the impedance of a speaker load
increases the output power.
- 11. When you reduce the load
from 4 ohms to 2 ohms, the current through the
output transistors doubles (ignoring
inefficiencies). The transistors have a safe
operating area which, if exceeded, will likely
lead to their demise. High current amplifiers have to reduce the rail
voltage so that there won't be too much current
flowing through the outputs when driving a low
impedance load. You're probably thinking that
they could simply use more transistors and they
do... for their larger amplifiers.
-
- Common Practice:
- Many manufacturers try to get
the most bang for the buck. If they want to build
two amplifiers which will produce a given amount
of power (let's say 600 watts), they don't have
to build 2 completely different amplifiers. They
can just change a few parts and have another
amplifier (i.e. high current) to add to their
product line (with virtually no added R&D or
production cost). The difference between a 600
watt 2 ohm stable amp and a 600 watt high current
amp may be only the transformer winding ratio.
The example below shows the difference between 2
amplifiers and shows how they produce the same
power into 2 different loads.
Test example:
- High
Current Amplifier:
- No load rail voltage: +/- 36
volts
- RMS voltage delivered into a 1
ohm load per channel: 19.6 volts
- RMS power into each 1 ohm
stereo load: 384 watts
- Current draw at full power: 101
amps
-
- Standard
2 Ohm Stable Amplifier:
- No load rail voltage: +/- 47
volts
- RMS voltage delivered into a 2
ohm load per channel: 27.7 volts
- RMS power into each 2 ohm
stereo load: 384 watts
- Current draw at full power: 76
amps
-
- You can see that the 2 ohm
stable amp has a higher rail voltage and
therefore a higher output voltage. The HC amp has
36 volt rails the second amp has 47 volt rails
but they both produce the same amount or power.
- Note:
- From the previous example, you
will see that the high current amplifier draws
more current from the 12 volt source but produces
the same power output. If the alternator could not
maintain its target output voltage under the
tougher load from the high current amplifier, the
HC amplifier would not be
able to produce as much power as the non high
current amplifier.
- OAQ (once asked question):
- If you reduce the speaker's
impedance from 2 ohms to 1 ohms, will the current
draw double?
- Answer:
- Ignoring losses, the current
draw would double because the current through the
outputs (to the load) would double. In reality,
the rail voltage would sag, so the speakers
probably wouldn't get quite twice the voltage and
therefore the current through the outputs would
not quite be doubled but the losses in all of the
components (transistors, copper, resistors,
transformer) would be greater, so the current
draw from the 12 volt source would still be at
least double the 2 ohm current draw. In general,
all else being equal, a high current amplifier
will be less efficient than an amplifier
optimized for a higher impedance load.
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