Australian Hi-Fi 39Questyle Reference System Golden Edition
Despite the emphasis on the words ‘Cur-
rent Mode Amplification’ in all Questyle’s
literature and model numbers, Questyle’s
products are not ‘current source’ (a.k.a.
transconductance) amplifiers—they are con-
ventional voltage source amplifiers, but inter-
nally use a circuit concept similar to that used
by Krell in its ‘CAST’ (Current Audio Signal
Transmission) amplifiers, where after a volt-
age has been applied to the input terminals,
it’s then amplified via current mode, then
converted back to voltage source before being
delivered to the output terminals. Or, to use
Questyle’s own terminology exactly: ‘voltage
input and output, while the core amplification
takes place in the current domain operating in a
pure Class-A state, and allowing the output stage
to be either Class-A or Class-AB.’ According to
Wang, the result is ‘extremely wide bandwidth,
ultra-low distortion and a total lack of transient
intermodulation distortion.’
Also, despite the fact that Questyle makes
much of the fact it’s using balanced amplifiers
to drive the headphone outputs, and claims
this is superior to using unbalanced (sin-
gle-ended) amplifiers to drive headphones,
all headphones are electrically ‘balanced’
irrespective of whether they’re being driven
by a balanced amplifier or an unbalanced
amplifier. The drivers inside headphones have
only two wires and since the current into and
out of any two-wire network is always equal
and opposite, this means headphones cannot
‘tell’ whether the audio signal comes from
a single-ended voltage source or a balanced
voltage source.
Notable on the ‘Golden Edition’ series
is Questyle’s use of Rogers 4350 ceramic
printed circuit boards (PCBs), which are only
0.8mm thick, yet have a dielectric constant
of 3.48 compared to just 2.55 for, say, Teflon
PCBs. Ceramic PCBs are used in high-insu-
lation, high-frequency, high-temperature
applications and are particularly suited for
low-volume electronic products. However
it’s important to note that these ceramic
PCBs are used only in the Reference ‘Golden
Edition’ Series components, not the ‘Silver
Edition’ components.
Another point of difference is that
whereas the PCBs of the ‘Silver Edition’
components are populated by high-qual-
ity electrical components, Questyle uses
even-higher-quality hand-selected compo-
nents in its Golden Edition components.
Questyle’s CAS192D DAC is unusual be-
cause whereas most high-end DACs that of-
fer DSD use a digital-to-analogue conversion
technique known as DoP (DSD over PCM),
the CAS192D processes
the DSD bit-stream
directly—it even by-
passes the CAS192D’s
own onboard (and
switchable) digital
filters to provide
true DSD. (Note that
computer playback via
Windows—everything
from XP up—requires
installation of driver software that Questyle
provides on an optical disc provided with
the CAS192D.)
There is certainly no shortage of digital
filters selectable on the CAS192D, because
Questyle has activated all the optional filters
that are already resident inside the Cirrus
Logic Wolfson Microelectronics WM8741
DAC that it’s using as its digital ‘engine’.
(The WM8741 natively also provides the
options of direct DSD, or DSD via PCM.)The filters available for use vary depending
on the conversion mode you choose (that
is, whether you choose to use oversampling
or no oversampling) and the bit-rate of the
digital input signal, but altogether you can
select between FIR Brickwall, FIR Apodising,
FIR Soft-knee, FIR Half-band, IIR Apodis-
ing, IIR Soft-knee and IIR Half-band. A full
explanation of the differences between these
filters is beyond the scope of this review, but
there is a fundamental difference between
an FIR (Finite Impulse Response) and IIR
(Infinite Impulse Response) which is that an
FIR filter has time-reversed ringing, whereas
an IIR filter doesn’t (a result of having to
have a higher Q than an IIR filter to meet
the same specification regarding stop band,
ripple, and roll-off.) This is best illustrat-
ed visually, as per the three shots from an
oscilloscope that are shown on the following
page. The first image shows an analogue
pulse, the second is what that pulse looks
like after passing through one of the Que-
style’s FIR filters, and the third after it’s been
passed through one of the Questyle’s IIR fil-
ters. Although the IIR filter has more ringing
after the pulse, there is no pre-ringing. (And,
just in case you were wondering, the reasons
for the names are that a pulse processed by
an IIR filter response never falls to zero, but
continues indefinitely, whereas one pro-
cessed via an FIR filter eventually does.)ÄȭɁǜǒƬʁɡɁǔȭǜɁnjƞǔǏƬʁƬȭƋƬǔʊǜǒŘǜ˹ǒƬʁƬŘʊǜǒƬñ,*ʊɁnjǜǒƬɽăǔǚ˸ƬʁFƞǔǜǔɁȭɻƋɁȧɡɁȭƬȭǜʊŘʁƬɡɁɡˁǚŘǜƬƞŽ˿
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