13
Operating Manual - Powerflex Multi-Channel Power Amplifier
10.
DESIGN THEORY
Powerflex amplifiers are based on stereo driver
modules that use digital processing to generate a spread-
spectrum switching pattern between about 200KHz and
1.5MHz, depending on input signal amplitude and fre-
quency. This overcomes the self-limiting and inherent
weakness in traditional class D fixed-frequency PWM
amplifiers, and produces an output with THD+N numbers
comparable to class A and class AB linear amplifiers. (see
Specification Notes at end of this section)
In Powerflex amplifiers, the input signal is re-
ceived by a single operational amplifier configured as a
difference amplifier to reduce common-mode effects from
sources located at a distance. This difference amplifier has
a gain of 2.74 which allows the use of less gain later in the
system, improving overall system noise performance.
The amplified signal, based on the position of
various selector switches, either bypasses or is processed
by a two-pole high-pass filter with rolloff at 50Hz, and
then passes through the attenuating potentiometer. The
attenuated signal is buffered and passed to the amplifier
module along with a zero-volt reference.
The amplifier module receives the attenuated sig-
nal with a unity gain differential amplifier to eliminate
common-mode interference picked up within the ampli-
fier chassis. The signal is then added to a small DC offset
signal opposite in polarity to any DC offset on the
Powerflex output bridge and adjusted by the digital pro-
cessing module as described above. The digital process-
ing module, which also generates FET drive signals, has a
voltage gain of 11.7 for a total system voltage gain of 32.
The drive signals generated by the processing module are
fed to a pair of high-current MOSFET transistors, and the
switched output of these transistors is filtered and applied
to the speaker outputs. Fault conditions such as overvolt-
age, undervoltage, and output overcurrent are measured
by the module and therefore affect a stereo pair of chan-
nels regardless of whether the fault actually exists on both
channels processed in the module. Since every action ap-
plied to protect the digital processing module affects both
channels being processed, other fault conditions are not
separated by channel, but rather by amplifier module.
Specification Notes: Due to its spread spectrum out-
put switching pattern, the output signal of a Powerflex
ampli-
fier
contains significant dynamic frequency content* far outside
the audio band, which makes no difference to audible perfor-
mance, but which makes heavily bandwidth-limited** measure-
ment of amplifier noise and distortion characteristics mandatory.
*variable 200KHz to 1.5MHz
**greater than 48dB/octave above 22kHz
11.
TROUBLESHOOTING TIPS
11.1
No Audio Output
1.) Power LED not lit:
Line fuse is blown or power outlet is dead.
IF LINE FUSE IS BLOWN, REPLACE ONLY
WITH SAME TYPE AND RATING FUSE.
2.) Power LED is lit but Protect LEDs stay on:
Amp module is in protect mode. Speakers have
been disconnected from amplifier output and connected to
ground until the protect fault is corrected. See section 8
for a complete explanation of protect fault conditions. Out-
put overcurrent, overvoltage and undervoltage faults will
reset in about eight seconds once the fault condition is re-
moved, while excessive DC offset or high frequency faults
take about 30 seconds to reset.
Speaker Impedance:
The Powerflex amplifier
may go into self-protect at high output levels if the actual
speaker load impedance is much less than 4 ohms. To cal-
culate speaker impedance for a given combination of di-
rect-coupled speakers, use Ohm's law as applied to series
and/or parallel resistor networks, where each speaker (for
this purpose) can be thought of as a single resistor, using
DC resistance measurements.
Simply stated, speakers connected in series will add
together their impedance. Conversely, two speakers con-
nected in parallel will result in half the impedance, three
parallel speakers a third the impedance, four speakers a
fourth, and so on, assuming the speaker impedances are all
the same. Don't use mismatched impedances in parallel.
When using paralleled speakers, the available amplifier
power for that channel is evenly divided among speakers,
so 100 watts driving two parallel speakers of equal imped-
ance provides 50 watts to each speaker, etc. Combining
series/parallel speaker connections is common practice, but
if not sure, contact your dealer or Ashly tech support.
3.) Thermal LED and Protect LEDs stay on:
Amp module is in thermal protect mode and needs
to cool. The fan will continue to run while in thermal pro-
tect, and other modules will continue to function. See sec-
tion 8.4 for details.
4.) Power LED lit but no Signal LED activity
There is no input signal applied or input level con-
trols are turned down.
