Groove Sleuth
Non-equalising phono preamplifiers

flat phono preamplifier Phædrus Audio's phono preamplifiers were developed in conjuction with Pspatial Audio's Stereo Lab software.

For the vinyl enthusiast, Stereo Lab offers a suite of tools for capturing the highest quality needle-drops from both modern, LP records and historic disc recordings.

Laboratory quality solution....

RIAA equalisation is performed within Stereo Lab with digital accuracy. Optional rumble-filtering is implemented with a phase-linear filter; something impossible in the analogue domain.

In addition, Stereo Lab includes: distortion cancellation algorithms (which compensate for the way the stylus tracks the physical groove) and cartridge-compensation algorithms to compensate for the shortcomings of practical phono cartridges.

Stereo Lab also support half-speed needle-drops and thereby offers the ability to extract the maximum information possible from a record. Also included is a unique, world-class "click and pop" filter to reduce the perceptible effects of surface damage.

These techniques delivers a performance from disc replay which needs to be heard to be believed.

For the record collector...

For the record collector and sound archivist, Stereo Lab also includes a Historic Equalisation mode which implements a comprehensive collection of gramophone-record equalisation curves; enabling the correct de-emphasis of 33, 45 and 78 RPM recordings made from 1880 to today!

Stereo Lab also includes a unique de-crackle process intended for this problem with 78 records. The technique is automatically adaptive and very effective. It is a unique process developed by Pspatial Audio and, unlike so many other, competitive techniques, the crackle-reduction has no effect on the high-frequencies so that the sound is not "dulled" when it is cleaned-up.

Support for older, encoded surround-sound formats is also included with Quadraphonic and Ambisonics decoding to a modern 5.1 system.

Our preamplifiers

During development of the Stereo Lab software, Pspatial Audio searched for hardware solutions with irreproachable performance for recording non-equalised needle-drops. The resolution of 24-bit high-resolution recordings requires preamplifiers which guarantee extremely low-noise, exemplary headroom, excellent linearity, and an accurate and extended frequency-response.

Few exist! And, in fact, in the end, Pspatial Audio were forced to develop their own test equipment which they nicknamed, the Groove Sleuth.

Phædrus Audio retained this name in these attractive commercial realisations of Pspatial Audio's original laboratory test-set.

How Groove Sleuth preamplifiers differ from other phono amplifiers

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 range

non-equalising phono preamp

The larger Groove Sleuth-1 preamplifiers are available in three base configurations: GS-1-high, GS-1-low and GS-1-active:

Each base unit is equipped with a high-quality looping path called iLOOP, and the GS-1 preamplifiers have an option for RIAA equalisation.

Groove Sleuth MINI

The miniature, lower-cost GS-MINI preamplifier is suitable for HMC (high-output moving-coil) and MM cartridges only and does not have an RIAA option. However, the GS-MINI does support the Phædrus Audio PHLUX active moving-magnet cartridge.

To order your Groove Sleuth, or for more information, email:

Product Description

In the base options, all Groove Sleuth preamplifiers 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 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. This amplified signal is sent to an external audio interface for digitisation and subsequent recording equalisation in Stereo Lab.


The GS-1-low (MC) Groove Sleuth 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 preamplifier are of the highest quality: they possess high primary inductance, very low leakage and are fully mu-metal screened.

Signal loop (iLOOP)

All Groove Sleuth preamplifiers feature the iLOOP; a wide-bandwidth, low-noise and distortion buffered loop-through. This is especially convenient for enthusiasts who already have an excellent LP replay solution but who want to introduce computer audio into their system. This is because the Groove Sleuth integrates transparently into a pre-existing setup and provides new functionality without upsetting what's there already.

iLOOP always outputs the phono signal at high-output HMC (or MM) signal levels. In fact, if a moving-coil cartridge is used (with the GS-1-low) and the iLOOP output is fed to a moving-magnet input, the Groove Sleuth will function as a first-rate, wide-bandwidth, low-noise MC preamplifier.


In the case of the GS-1 preamplifiers, a high-quality RIAA stage (GS-RIAA) may be included so that the "loop" signal is RIAA equalised and emerges at line level on the phonos marked iLOOP/RIAA. The RIAA filter is a high-precision, passive network. This is followed by slightly more make-up gain (+24dB) than the mid-band loss in the RIAA network because the signal now has the crest-factor of the original music signal. (The RIAA option is not available in the GS-MINI.)

Warm up

The Groove Sleuth preamplifiers are designed so that the analogue sections remain energised all the time since this is consistent with the best audio practice. By means of an advanced power-supply design, this condition is achieved whilst remaining compatible with European Eco-Consumption directives. If the unit is unplugged or the power is removed or interrupted, the analogue circuitry is held in a muted state for about 5 seconds until the various bias voltages are stabilised. Therefore do not be alarmed if the unit does not produce an audio output immediately after connecting power.

Connecting the Groove Sleuth to your sound card

The Groove Sleuth is connected to your sound-card's line inputs (as shown above). Levels should be set on the interface in the normal way and you will find that the Groove Sleuth delivers audio at a sufficiently high level for all commercial sound-cards, both internal and external.

The Groove Sleuth MINI

The Groove Sleuth-MINI is a cost-effective non-equalising preamplifier for audiophiles on a budget. Just like its big brothers, the GS-MINI provides iLOOP and a computer output. The only limitation is that it is for HMC and MM cartridges only, and it does not support a RIAA equalisation option.

Whilst the GS-MINI may be operated with the RESOLVE power-supply (as described later), it incorporates a rechargeable battery and a front panel switch which permits total disconnection of the PSU, By this means, the unit runs solely on low-noise, low-impedance battery power and the mains power-supply cannot influence the audio quality.

The GS-MINI may be thus used with a low-cost plug-top power supply without any compromise in audio excellence.

The GS-MINI has about 24 hours operational charge, but we recommend battery play be limited to approximately 10 hours. For parties, or very long listening sessions, the unit may be kept in charge mode when listening with only a minute loss of quality.

The Groove Sleuth-MINI does support the Phædrus Audio PHLUX active moving-magnet cartridge. This is selectable with internal links (see manual).

The Groove Sleuth power supplies

All Phædrus Audio Groove Sleuth preamplifiers require a high-quality, external power-supply. There are two options of PSU to power the Groove Sleuth preamplifiers. The basic PSU is an external, high-quality, universal power-supply unit (GS-PSU). As an upgrade, the external power supply may be upgraded to the GS-RESOLVE-PSU. Housed in an identical chassis to the preamplifier, the RESOLVE power supply is of the very highest audio quality, but both supplies are manufactured with the highest audio-quality in mind and ensure that the power provided to the preamplifier is very low noise and very low impedance.

All PSUs connect to the preamplifiers unit via a cable with screw-lock connectors.

Groove Sleuth preamplifiers are designed to be compatible with European Eco-Consumption directives. Consumption is less than 4 Watts in all configurations and active. The unit is intended to remain energised all the time. The design is thereby compatible with the best audio practice of leaving the unit energised all the time whilst respecting the environment, and your electricity bill!

To order your Groove Sleuth, or for more information, email:

Groove Sleuth preamplifier manual

The Groove Sleuth preamplifier manual is here.


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