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 The A34.2 is a 2
x 150 watt stereo power amplifier
utilising proprietary UFPD
technology. It is designed to
provide high power output with very
low distortion for Primare’s PRE32
audiophile preamplifier and all
other preamps, as well as
multi-channel audio processors.
UFPD’s instantaneous rise time
results in a naturally fast, clean
and agile sound over a much wider
frequency range and with exceptional
headroom. Ecologically the A34.2 is
far superior to conventional Class
A/B designs being extremely
efficient without generating
excessive heat.
Audiophile Topology
The A34.2 houses two discrete
UFPD amplifiers housed in a heavy
gauge alloy steel chassis, which
provides strength, rigidity, and
screening, while being effective at
damping vibrations from external
sources. Each channel incorporates
unbalanced (RCA) and balanced (XLR)
inputs, as well as high quality gold
plated speaker terminals, trigger
and RS232 control inputs.
The A34.2 can easily be switched to
bridged mode and will then provide
one fully balanced mono channel
capable of delivering up to 550W
into 8ohms.
Balanced signal transmission means
that two identical signal lines are
used to carry the same signal with
opposite phase. Any noise is common
to both lines and is present in
equal amounts with identical phase.
At the receiving end a (differential)
receiver retains the opposite phase
signals (music) and rejects the
common phase ones (noise), leaving
only the pure original signal.
Balanced circuits therefore keep the
signal as free as possible from
interference. All audio signal
switching between bridged or stereo
mode, unbalanced (RCA) or balanced (XLR)
inputs is carried out using high
performance signal relays.
The A34.2 incorporates a preamp
stage comprising Burr-Brown OPA2134
and Analogue Devices AD8512 OP-amps
that automatically maintains a
constant gain of 30dB (unbalanced)
and 24dB (balanced), while
stabilising input impedance at
15kohm in stereo or bridged
configuration.
Ultra Fast Power Device (UFPD)
Class-D Amplifier
The use of switch mode power
electronics is gaining in popularity
as the result of its lower energy
consumption and as a way to squeeze
more amplifier channels into smaller
spaces. Unfortunately Class D
amplifiers and their switch mode
power supplies have traditionally
deserved a reputation for poor audio
quality because it is very difficult
to produce a full range ‘hi-fi’
signal through them. A major source
of distortion is the demodulation
filter on the output which becomes
unstable with variations in
loudspeaker impedance, unless it’s
controlled by sufficient feedback.
The failure to provide enough
feedback to cope with dynamic
changes in loudspeaker impedance
across the audio band results in the
‘classic’ Class D sound of rising
THD with frequency. While very
dynamic
and vivid initially, it can sound
tiring and uncontrolled in the long
term, especially when driving
complex loads like multi-driver
speakers.
Primare’s UFPD
(Ultra Fast Power Device) provides
for the possibilities of an ‘audiophile’
Class D design. It is a Class D
technology which has a consistent
26dB feedback loop gain across the
entire audio range and is stable way
beyond the audio band. This is quite
easy to achieve in conventional
linear ‘continuous signal’
amplifiers, but much more difficult
in ‘non-continuous’ high speed
switching amplifiers.
Rather than have the amplifier and
then the filter as discrete stages,
the UFPD design integrates the two,
making control with feedback much
more immediate and accurate. The
UFPD amplifier actively adapts the
loop gain to keep the total loop
stable during start up, clipping and
current limit. It senses the changes
to the filter output and compensates
by applying the precise amount of
feedback. This adaptive pole control
allows for several more dBs of
constant loop gain across the audio
band and maintains performance
irrespective of load (impedance)
variations. Primare’s UFPD treats
all signals equally regardless of
frequency or slew rate and has the
ability to suppress the filter
resonance entirely. Consequently THD
is kept very low at all frequencies.
With a very wide ‘load independent’
frequency response UFPD is able to
drive any speaker while maintaining
control and accuracy. Primare has
optimised the performance of its
innovative UFPD design with the
precise selection of circuit
component values and quality,
verifying the design with extensive
measurement and listening.
UFPD notes
- UFPD switching frequency is above
300kHz.
- The output filter cut-off is
higher than the audio band but is
well below 300kHz.
UFPD displays
- Wide bandwidth
- Flat frequency response
- Load independant frequency
response
- Low output impedance in the entire
audio band
- Low THD in the entire audio band
- Low noise
Most Class D technologies display
- Limited bandwidth
- Peaking frequency response
- Load dependant frequency response
- High output impedance at high
frequencies
- High THD at high frequencies
- High noise
PFC Power Supply
Although switch mode power supplies
have gained a reputation for noise
and unreliability, the theoretical
advantages of the design are well
known. The rails can be regulated
with precision and current demand
from the mains is lower as the
result of high efficiency and the
absence of current spikes: energy is
taken from the mains over a larger
period of the sine wave.
In conjunction with UFPD, Primare
uses an isolated PFC (Power Factor
Control) technology in the power
supply, which controls the current
from the mains voltage so that it is
a pure sine wave with the same
frequency and phase as the mains
voltage. This means that even if
1000W is taken from the mains, other
equipment in the room will not be
affected. Its presence becomes
virtually invisible to the mains
voltage! The isolating stage of the
converter works in a ZVS mode and as
a result, the switch flanks contain
a lower quantity of harmonics,
providing lower EMI and a clean
environment for the amplifiers to
work in.
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