We couldn't visit the theatre or go to concerts. The opera or the ballet houses were closed. But we could still listen to our records.
Long evenings set our minds as to how our range our Groove Sleuth Mk. II phono preamplifier might be enhanced.
The result is the Groove Sleuth Lockdown - our flagship preamplifier for listening and needle-drop recording.
Incorporating the features of the existing Groove Sleuth Mk. II, the Lockdown incorporates the following innovations.
We want our preamplifier to deliver a channel capacity substantially greater than that afforded by 192kHz, 24-bit audio since this is the maximum resolution with which we can make needle-drop recordings and we don't want the preamplifier to be the quality "bottle-neck".¹
A mono, 24-bit channel with 192kHz sampling has a channel capacilty of 2.3Mbits/s. (It's twice this figure for a stereo signal.)
The Phædrus Audio Groove Sleuth Lockdown has a superlative dynamic range (see Specification) and a bandwidth 20 times greater than the best digital channel.² The per-channel capacity is thereby greater than 30Mbits/s — an order of magnitude greater than a digital recording sampled at 192kHz with 24-bit resolution.
The Phædrus Audio Groove Sleuth Lockdown preamplifier will never be the limiting factor in a record-playing system for listening or making needle-drops.
The preamplifier is available for moving-coil and active PHLUX-II cartridges. For moving-magnet types, we recommend the Groove Sleuth Mk. II or the Groove Sleuth MICRO preamplifiers.
Virtually all small electronic devices nowadays incorporate switched-mode power supplies which provide significant interference - both as radio interference and conducted interference back into the mains house wiring. In some cases, interference is so pernicious and widespread that it seems to find its way into needle-drop recordings no matter how the equipment is configured.
As we developed the Groove Sleuth preamplifiers, we discovered that the best, simplest and most compact way to guarantee noiseless operation is for the external power-supply to feed an internal, rechargeable battery supply. In this way, the unit can be in recharge mode most of the time but runs on its internal battery during the all-important needle-drop recordings. Precision test equipment is often provided with a battery supply like this; thereby to isolate it entirely from mains power and interference which may affect precision measurements.
The internal battery in the Groove Sleuth Lockdown is a NiMH (nickel-metal hydride) type.3 These batteries, used routinely in computer and medical equipment. Nickel-metal hydride batteries are safe, green and abuse tolerant.4 These batteries have been used successfully in all-electric vehicles because of their low internal resistance. They therefore make an excellent basis for a low-impedance audio preamplifier power-supply.
The Groove Sleuth Lockdown preamplifier uses an intelligent NiMH charger circuit such that the external PSU provides an initial fast charge but applies charging current reductions as the charge progresses. This scheme continues until the battery is fully charged. Once the battery is fully charged, the rôle of the power-supply is reduced to trickle-charging the battery. This method of charging is known as the differential charge method and ensures the best life from the internal battery and the lowest noise from the power-supply. It also enables us to dispense with the requirement to switch manually between CHARGE and PLAY.
The high-bandwidth loop-thru (called iLOOP) is, once again, a feature of the Groove Sleuth Lockdown preamplifier.
iLOOP may optionally be replaced by a line-level RIAA equalised signal for monitoring or for high-quality listening. RIAA equalisation is performed with a high-accuracy, analogue, passive network with a class-A recovery-amplifier.
For the best possible results, we recommend RIAA correction via a real-time DSP application running on a third-party platform.
Specification of Groove Sleuth Lockdown
Size: 112 × 50 × 225 mm (W-H-L)
Preamplifier Supply: Low-noise +12V DC input on screw locking connector
Power: Compatible with European Eco-Consumption directives*
Inputs: UnbalancedAppendix MC cartridge or PHLUX-II active-cartridge
Sensitivity: 500µV RMS (-64dBu) or nominal 5mV (-44dBu) @ 5cm/s
Outputs: Gain (to computer output): × 500 or +54dB**
Gain (to iLOOP output): 0dB
Gain (RIAA equalised option): +58dB @ 1kHz
Frequency response: 2Hz to 2MHz (-3dB) to computer output and iLOOP
Distortion: Better than 0.0005% THD on 1kHz at 0.5mV RMS input
Equivalent input noise (MC): -140dBu†
Equivalent input noise (PHLUX): -133dBu‡
Max input: Greater than 6mV RMS (22dB above nominal level)***
Max output: 3V RMS (+12dBu)
* The unit is intended to remain energised all the time and still conforms with the European Union's Ecodesign Directive (Directive 2009/125/EC).
** × 50 (34dB) in PHLUX-II mode.
† Main output, RIAA equalised, 20kHz bandwidth, A-weighted, input shorted.
‡ Main output, RIAA equalised, 20kHz bandwidth, A-weighted, input shorted.
*** Greater than 60mV RMS in PHLUX-II mode.
Phædrus Audio reserves the right to alter specifications without notice.
Some say that a moving-coil phono cartridge is a balanced audio source like a microphone. Wouldn't it be better to treat phono cartridges like microphone and arrange a balanced preamplifier?
It's worth remembering what a balanced input preamplifier provides. It bestows gain to differential signals and cancellation to common-mode signals, that's to say, cancellation of signals which are common on both signal wires.
A microphone is a balanced source at the end of a long cable - often snaking amongst mains cables, guitar amplifiers and control cables. The wanted signal is generated differentially and any induced interference into the long cable run is induced equally into the two signal wires. At the balanced preamplifier, the wanted, differential signal is amplified and the induced interference is cancelled out. (Indeed, Phædrus Audio make a well respected range of microphone preamplifiers, so we know a thing or two about balanced circuits!)
But, the balanced preamplifier pays for this clever functionality with the penalty of greater noise.
Any balanced preamplifier circuit must have two inputs and that necessitates two input amplifiers instead of one – dual op-amps, dual transistors, or dual vacuum-tubes.
The noise sources in these two amplifying devices will have versions of the same physical noise-generating processes going on and will partially reinforce and partially cancel. All things being equal, the balanced preamplifier will be 3dB noisier than an unbalanced preamplifier.
....the balanced preamplifier pays for [its] functionality with the penalty of greater noise.
The phono cartridge is (almost always) at the end of a very short cable in a rather more controlled environment than the average microphone. There are few mechanisms for inducing interference into the connecting wires unless the signal leads are carelessly selected and/or are trailed next to mains cables or power transformers.
Our experience is that, with sensible equipment-positioning and cable dress, we have not experienced significant noise due to induction or coupling into the signal wires leaving a phono cartridge. Therefore, an unbalanced input is preferable. Indeed, it would not be possible to offer the best-in-class noise-performance of the Groove Sleuth LOCKDOWN if the input stage were balanced.
1. Channel capacity is calculated according to the Shannon–Hartley theorem which tells us the maximum rate at which information can be transmitted over a communications channel of a specified bandwidth in the presence of noise is,
C = B . log2 (1 + S/N)
where, C is the channel capacity expressed in bits per second, B is the bandwidth of the channel in Hertz, and S/N is the signal-to-noise ratio (expressed as a linear power ratio).
2. The extended low frequency response of the Groove Sleuth Lockdown preamplifier is engineered to be -3dB at 2Hz, which restricts loss at 20Hz to 0.1dB with a the phase-shift at 20Hz of only 4.5°.
3. The principle on which nickel-metal hydride (NiMH) cells operate is based on their ability to absorb, release, and transport hydrogen between the electrodes within the cell. The NiMH battery technology comes from the rare earth, hydrogen-absorbing alloys (commonly known as Misch metals) used in the negative electrode.
When a NiMH cell is charged, the positive electrode releases hydrogen into the electrolyte. The hydrogen in turn is absorbed and stored in the negative electrode. When a NiMH cell is discharged, hydrogen stored in the metal alloy of the negative electrode is released into the electrolyte to form water. This water then releases a hydrogen ion that is absorbed into the positive electrode to form nickel hydroxide.
4. Nickel-cadmium (Ni-Cd) batteries have been used for many years for industrial applications but the are now prohibited for consumer applications in the EU because of concerns about the toxicity of cadmium. NiMH batteries have no such issues.
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