PowerCube basics and
example graphs
To understand the need of the PowerCube test, you must be aware
of how a loudspeaker appears to the amplifier during dynamic
conditions.
Why the PowerCube test?
It’s well known that an 8 ohms speaker for example, is
nothing but 8 ohm at most frequencies. How far down against 1
ohm it goes depends on the specific loudspeaker. And we are not
talking about the static impedance curve, but how the
loudspeaker appears in a dynamic point of view.
These examples does not concern the loudspeaker, but the
amplifier.
It’s assumed that you know that loudspeakers can produce severe
load cases to an amplifier, which can be difficult to handle for
the amplifier.
Example graphs and how to interpret them
This will bring you up to speed in how to interpret all the
information that the graphs contains. The PowerCube test is a
dynamic test – it tests the amplifier’s dynamic output power
capabilities.
Technical background information
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The two axes at the bottom of the cube
represents the load conditions, and the height of the cube
shows the amplifier’s output voltage capability for the
different conditions. |
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• Test is done at 1kHz
• Test signal* is 20 periods of 1kHz full power, 480 periods
attenuated 20dB.
• Output power is measured at 1% THD for 20 different load
conditions
• Loads used are 8,4,2,1 ohm, Resistive, +/-30 degrees, +/-60
degrees
* According to standard EIA RS-490 (formerly known as IHF
A-202)
Example graphs
(click images to enlarge)
Example 1: The Perfect amplifier.
- The perfect amplifier would be a pure voltage generator that
does not care at all about the load, hence the cube would be
perfectly flat.
- In the versions of the PowerCube graphs you find here, a floor
indicating the rated output power is overlaid.
- This amp for example, is rated to 70 watts @ 4 ohm (RMS). That
equals 16.7 Volts, hence, the overlaid floor is at the level of
16.7 Volts.
- The space in-between the floor and the top of the cube, is
called dynamic headroom (which with some amplifiers will take
negative values).
Example 2: A good example, a well designed amplifier.
- You will find some losses at lower impedances, but those are
acceptable.
- The dynamic headroom is positive for all loads – This means
that the cube is above the red floor (indicating rated
continuous output power), at all load conditions.
- The cube has a slight V-form, but that is quite normal – it’s
tough for the amp to produce high output levels when the current
and voltage are in phase – output devices will get heated fast.
Example 3: Poor power supply.
- This amplifier works well at 8 ohm, but at lower
impedances the output power decreases rapidly.
- If you test this amplifier with a classic 8 ohm resistor, it
will appear quite normal. But if you connect it to a loudspeaker,
it will quite soon reach voltage clipping.
- Dynamic headroom is negative at low impedances, and maybe this
amp shouldn’t be marketed as a 70 W amplifier?
Example 4: Bad design of current limiting.
- This amplifier will act normal at all loads in 8 ohm, and also
in 4 ohm resistive load.
- In all other load cases, the amplifier’s current protection
scheme shuts it down way to early. The result is voltage
clipping when trying to drive a nonresistive loudspeaker. That
would be all reallife loudspeakers.
Example 5: Oscillating at inductive loads.
- This amplifier oscillates at a certain inductive load, causing
the THD to hit levels over 40% and hence makes it impossible to
measure any output power at all.
- This result is a dip to zero watts at that load, and probably
burned tweeters if this would have been a real life test…
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