LDR300.V25 Buffered Preamp


The solid state LDR300.V25 has been discontinued as part of our renewed strategic focus on our passive preamps and related tube based buffers

The LDR300.V25 Buffered Preamp is a remote controlled, singled-ended, solid state active preamplifier that combines Tortuga Audio’s popular LDR (photoresistor) attenuation technology together with a robust JFET current amplifier (output buffer) to deliver high performance audio that will appeal to the discriminating audio enthusiast.

The LDR300 is a unity gain preamp without the gain stage (amplification) that typically colors, clouds and adds noise to the signal path of most active preamps. The LDR300 embodies the “less is more” design philosophy.

Enthusiasts who value a dead quiet preamp with a deep dark background between the notes, exceptional transparency, and pleasing dynamics, will find the LDR300 to be a very satisfying preamp rivaling many high end peamps at a fraction of the cost. And it comes with a simple remote control and graphic display feedback so you can relax and enjoy your music in comfort from your favorite listening spot.

Comes equipped with a remote, external power supply,  3 inputs and 2 (parallel) outputs using high quality silver/rhodium plating over solid brass Cardas jacks.

Out of stock

SKU: LDR300.V25 Category:
Print Friendly, PDF & Email

Elegantly Simple Signal Path

The single greatest challenge for any line stage preamp is to remain true to the source material while delivering the attenuated audio signal to the amplifier honestly and without editorializing.  The LDR300 achieves this by combining Tortuga Audio’s proven LDR volume controller with a simple yet highly effective solid-state junction field effect transistor (JFET) current amplifier (output buffer) while eliminating the unnecessary voltage amplifier (gain stage) in between.

A 2 stage regulated split voltage power supply mitigates the need for any input coupling capacitor with the output operating a near zero volts DC offset.

This elegantly simple design keeps the signal path short and simple without compromising performance. So you can hear more of the music and less of the hardware.

LDR300 simplified schematic - March 2019 Revision

V25 LDR Preamp Controller

LDR V25 Preamp Controller Board (Rev A)The LDR300 is designed around our 4th generation remote controlled V25 preamp controller that uses digitally controlled analog light dependent resistors (LDRs) to provide smooth 100 step volume control over a 60 dB range. Features include ramped muting, channel balance adjustment, input switching, both  stereo and mono modes plus adjustable input impedance. Built-in calibration of the replaceable socketed LDRs ensures accurate attenuation, channel balance and optimal performance over the extended life of the preamp.

The LDR300 is easy to operate via an intuitive interactive menu driven control interface designed around  the 7-button Apple remote and a bright easy to see 256 x 64 pixel white-on-black graphic OLED (Organic Light Emitting Diode) display.

No Gain Stage

The reality with most of today’s line stage source components (DACs, CD transports, phono preamp etc. ) is you simply do not need your preamp to boost the signal voltage  in order to achieve acceptable volume levels. The LDR300 takes advantage of this by avoiding the complexity, noise and cost of a gain stage. The result is a better sounding preamp with less processing and manipulation of the audio signal.

Solid State Buffered Output

The proprietary Class A output stage is a robust current amplifier (buffer) designed around the highly respected  ultra low noise LSK170 junction field effect transistor (JFET).  The direct coupled JFET operates in combination with a constant current source between a split voltage power supply. The result is natural sounding audio with robust dynamics without the dryness often associated with solid state equipment. Impedance matching is never an issue so you can use longer interconnect cables without concern and feed both a main amp and a subwoofer from the same output.

Low Noise Split Voltage Power Supply

The LDR300 employs an inherently low-noise split voltage power supply topology that places the buffer’s JFET transistor almost exactly in between the positive and negative power rails where it operates near zero volts DC (minimal offset).

Just as the middle of a stretched rubber band remains nearly motionless when either end are vibrated randomly,  the majority of any noise present on the split voltage power rails will cancel out and have minimal impact on the output of the buffer that operates near zero volts DC. This results in a very high power supply rejection ration (PSRR) and a sound stage with an exceptionally deep and dark background.

In addition to the already inherently high PSRR, the independent positive and negative power supply rails each utilize 2 separate voltage regulators in series with a combined 100+ dB ripple rejection. Both the left and right channels each have their own separate filter/bypass capacitors at each rail. Premium components are used throughout including Vishay/Dale resistors, Elna Silmic II (silk) electrolytic filter capacitors, and Vishay polypropylene coupling capacitors in the signal path.

Optional Output Coupling Capacitor Upgrade

A good yet modest stock polypropylene output coupling capacitor can be upgraded to an excellent high performance V-Cap oil impregnated metallized polypropylene capacitor. More info on the V-Caps can be found here.


These specifications reasonably reflect the current model but are subject to updating and modification without notification.

Preamp OverviewSingle-ended, unity gain, remote controlled preamp with LDR type attenuator and Class A solid state JEFT buffer with 3 RCA inputs and 2 parallel RCA outputs.
Attenuator TypeDigitally controlled, optically isolated, Light Dependent photoResistors (LDRs) in a 100 step two channel attenuator with two (2) LDRs per channel in a series/shunt L-Pad voltage divider configuration.
Attenuation Range-60 db to 0 db in 99 steps of ~0.6 dB per step plus 1 additional muting step.
LDR ModulesEach individual LDR is packaged as a compact easily replaceable 4-pin socketed plug-in module that plugs into a 4 pin header. LDR modules are not mechanically keyed so attention must be paid to aligning white dot on module with corresponding white dot on printed circuit board when removing/replacing.
LDR Calibration The preamp controller includes a software driven calibration circuit that calibrates each attenuation LDR over 99 attenuation steps to compensate for any drift or aging, or in the event an LDR needs to be replaced and fully re-calibrated. Pre-matched LDRs are not required.
GainThe preamp has no gain stage (no built-in signal/voltage amplification) and as such is nominally unity gain.
Buffer TypeBuffer is an AC coupled (both input and output) JFET (junction field effect transistor) based current amplifier running Class A at a fixed current.
Buffer ModuleThe buffer proper is a separate small plug-in circuit board module that mounts on to the buffer board via a 4-pin header/socket. There are 2 buffer modules per preamp, one for each stereo channel. The modular approach is solely for convenience to allow alternative buffer components to be tried should superior performing parts become available in the future.
Input ImpedanceDefault input impedance is 20k ohms with ability of user to configure up to 9 additional input impedance settings each between 1k and 99k that user can switch between to determine potential optimal input impedance for user's system.
Output ImpedanceOutput impedance is fixed at ~**TBD** ohms
Number of Channels 2 independent channels configured for typical stereo operation
Channel Balance Adjustment+/- 20 step channel balance adjustment at ~0.6 dB per step for a nominal max channel balance range of +/- 12 dB
Channel Balance AccuracyWithin 0.5 db over full attenuation range
Channel IsolationEach channel signal is 100% isolated and independent from the other although audio signal grounds are tied to a common star ground point and there's an inevitable small degree of channel crosstalk of approximately ** TBD** dB.
Audio Inputs 3 switchable single-ended Cardas RCA stereo inputs
Signal Switching/IsolationAll input/output signal switching/isolation is done via LDRs. There are no mechanical relays in the signal path. Isolation resistance when LDRs are turned off is in excess of 10 megaohms.
Output IsolationOutput signals are disconnected when preamp is powered down, muted, or running LDR calibration.
Total Harmonic Distortion** TBD **
Power SupplyUS : Nominal 12 volts DC linear regulated "wall wart" rated at 0.5 amp
International: Nominal 12 volts DC switching "wall wart" rated at 1.0 amp
Power Regulation Attenuator: 2 stage regulation | 5V DC 1st stage via low noise switch mode regulator followed by 3.3 V low noise 2nd stage linear regulator

Standard - Dedicated low current linear regulator with minimum 80 dB ripple rejection
Optional - Belleson SuperRegulator with minimum 100+ dB ripple rejection and superior transient response
Control ProcessorARM M3 Cortex 32 bit microcontroller running at 72 MHz
Controls | RemoteSilver 7-button Apple infrared remote or equivalent using 38 kHz NEC protocol and Apple command/ID structure.
Controls | EncoderSubset of control functionality via front panel control knob attached to a rotary stepped encoder with integral push button switch
Controls | Display Graphic white-on-black OLED display provides an easy and intuitive user control experience through an interactive menu driven control interface designed around the 7-button Apple remote. Please refer to online documentation for detailed explanation of the preamp controls.
Mono ModeUser can switch between stereo and mono mode via the remote.
USB PortType A female USB port allows processor to communicate with external PC based bootloader application for updating firmware | USB cable provided
Trigger OutOutputs a 12 V DC signal when unit is turned on. Signal can be used to turn other devices on/off that are equipped with a Trigger Input.
EnclosureHeavy wall extruded aluminum case with black powder coat finish | Brushed anodized aluminum front and rear panel | Front panel with dark grey acrylic inlay in front of visual display | Rear panel with laser etched labeling | Black oxide socket screws with washers attach front and rear panel to case | no-slip urethane feet
Printed Circuit BoardHigh quality nickel immersion gold solder printed circuit boards with blue solder mask
SolderLead free solder with 3% or higher silver content
FusesNo internal or external fuses
ROHSCompliant or exempted
Ambient RequirementsPreamp must be operated in a stable, dry indoor environment only with stable ambient temperature within 60-90 degree F
Dimensions~ 3.25 inches tall | ~6.0 inches wide | ~ 9.0 inches deep
Weight~ 4.5 pounds | ~6 pounds as shipped
Scroll to Top