Chapter 4
–
The Groove Sleuth
Mk.II Preamplifier
The signal from a gramophone pickup is too small to
apply to the line input of most sound-cards or audio
interfaces. What is required is a dedicated phono
preamplifier to bring the small signal from the cartridge
up to a healthy level for input to the computer sound-
card.
Many phono preamplifiers exist at all price levels
from a host of manufacturers. But, the vast
majority of the phono preamplifiers apply RIAA
equalisation in the hardware of the unit and that is
not what is required here. We need a preamplifier
which provides gain and impedance conversion,
without applying the equalisation. These are
unfortunately very rare, especially models which
guarantee
extremely
low-noise,
exemplary
headroom, excellent linearity, and an accurate and
extended frequency-response. It is to fill this need
that the
Groove Sleuth
preamplifiers were
developed.
The Groove Sleuth Mk.II
The Groove Sleuth Mk.II is a second generation
product which replaces all the models in the
original Groove Sleuth range.
Please refer to this
block diagram of the
Groove Sleuth Mk.II
preamplifier in the following description.
The
Groove Sleuth Mk.II
preamplifier is suitable
for high-output moving-coil (HMC) and moving-
magnet
(MM)
cartridges,
and
optional
transformers may be fitted for low-output moving-
coil (MC) cartridges. The
Groove Sleuth Mk.II
is
also suitable for Phaedrus Audio
PHLUX (PHLUX II)
active moving-magnet cartridges. (See Chapter 5
for more information about
PHLUX
active
cartridges.)
Each base unit is equipped with a high-quality
looping path called
iLOOP
, and the
Groove Sleuth
Mk.II
preamplifier has an option for RIAA and
limited alternative equalisations.
In the base options, the unit simply presents the
cartridge with the correct impedance at the IN
phonos and amplifies the signal. The output is
presented on the phonos marked OUTPUT on the
rear of the
Groove Sleuth Mk.II
unit.
The gain of this preamplifier is a little lower than
the typical mid-band gain of an RIAA preamplifier
because, without RIAA equalisation, the crest-
factor of the signal is somewhat greater than
with a typical music signal (by about 4dB). The
signal amplification is designed for the lowest
possible distortion and noise-floor; a discrete
transistor
stage
with
multiple,
paralleled
transistors being necessary for the latter. This
amplified signal is sent to an external audio
interface for digitisation and subsequent recording
equalisation in
Stereo Lab
.
Transformer option
When ordered with the MC option (
GS-MC
), the
Groove Sleuth Mk.II
preamplifier includes a step-
up transformer stage. Transformers are heavy and
expensive, but they sound great! And they do offer
the lowest possible noise-floor. The transformers
selected for the
Groove Sleuth Mk.II
preamplifier
are of the highest quality: they possess high
primary inductance, very low leakage and are fully
mu-metal screened.
