LDR300x.V3 Passive or Active Preamp
Price: $849 (base)
This is a legacy product and is no longer produced or sold by Tortuga Audio.
| The LDR300x.V3 was discontinued as of June 2022. |
The LDR300x.V3 is an affordable high performance LDR (light dependent resistor) preamp available as either a passive or active type, for single-ended (RCA) 2-channel stereo applications with up to 3 separate switchable RCA sources. It’s remote controlled (Apple remote) or operated from the front panel (rotary encoder), with a high contrast, white-on-black, interactive menu-driven OLED display.
The passive model uses the ePot.V3 Max stepped attenuator/controller with precision closed-loop LDR current control. The active model adds a minimalist solid state gain plus JFET buffer stage that delivers power when and where it’s needed, with no impedance concerns or cable-length limitations.
A flexible passive or active preamp
The "x" in LDR300x stands for fle*x*ible: the LDR300x is available as either a passive or active, remote-controlled preamp for single-ended (RCA) 2-channel stereo applications able to switch between 3 line-level sources.
Buy the passive version and you can opt later to convert to active by adding a solid state buffer board. Have the active version and you can bypass the buffer and operate as a passive by swapping 2 internal jumpers.
In redesigning the earlier LDR3 into the LDR300x, Tortuga Audio sought to retain and improve upon what mattered sonically while forgoing things that raised costs with dubious sonic benefit — positioning the LDR300x as a more aggressively priced value-line preamp.
ePot.V3 Max electronic stepped attenuator & controller
The heart of the LDR300x is the ePot.V3 Max stepped attenuator/controller. Features include:
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100-step attenuator over a 60 dB range
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Smooth volume stepping with no sonic artifacts
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No mechanical switches in the signal path
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Menu-driven visual control with a high-contrast 256x64 OLED display
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LDR attenuation with precision closed-loop current control, integrated offline self-calibration, and a replaceable plug-in LDR module
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Integrated input switching (3 single-ended stereo sources) via analog switches
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Remote controlled (Apple infrared remote), with manual control via rotary encoder
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Smooth muting/unmuting; left/right channel balance (+/- 10 dB)
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Updateable firmware
High performance SSPB.V2 solid state JFET buffer
The active version pairs the ePot.V3 front end with the SSPB.V2 Solid State Preamp Buffer to deliver robust current drive where a passive attenuator alone may not satisfy. The SSPB.V2 uses a high performance audio op-amp input stage with just enough gain for headroom without introducing noise — gain is adjustable via plug-in gain modules, including unity gain if desired, and the op-amp sits in an 8-pin DIP socket for experimentation.
The buffer output stage uses a discrete high performance LSK170 JFET transistor for robust current amplification with low output impedance and minimal DC offset. A dedicated 2-stage split-voltage power supply, separate from the ePot.V3’s own supply, powers both stages and makes direct-coupled output possible — no AC coupling capacitors.
Specifications
Preamp overview |
Single-ended, remote controlled 2 channel stereo preamp with 3 RCA inputs and 2 parallel RCA outputs. Available as a passive or active type. |
Controller |
Proprietary ePot.V3 Max LDR digitally controlled preamp controller and electronic stepped attenuator with precision closed-loop LDR current control driven by a 16-bit command DAC. Analog switches handle input switching and output isolation. Embedded 32-bit STM32F2 ARM M3 Cortex 100 MHz microcontroller. |
Attenuation |
Stereo LDR electronic stepped attenuation with 100 smooth steps over -60 to 0 dB, two LDRs per channel in a series/shunt L-Pad voltage divider configuration. |
Attenuation module |
Single plug-in replaceable LDR module with 4 embedded LDRs plus an EEPROM memory chip storing attenuation tables from in-situ calibration. |
LDR calibration |
Embedded closed-loop offline calibration circuit normalizing each of the 4 attenuation LDRs over 100 steps. User-initiated; pre-matched LDRs not required. |
Gain/buffer board |
Active version adds a gain/buffer board with an op-amp input buffer/gain stage and a Class A JFET output stage. Input gain defaults to 0 dB, raisable to +6 or +12 dB by replacing the plug-in gain module. |
Coupling |
Both input and output are direct coupled, with no AC blocking capacitors in the signal path. |
DC offset |
On the active version, manually controlled by adjusting the bias current of the Class A JFET output. Rarely requires adjustment after initial break-in. |
Input impedance |
Default 20k ohms; up to 9 additional configurable settings between 1k and 99k. |
Output impedance |
~50 ohms |
Channel balance adjustment |
+/- 20 steps at ~0.6 dB per step (max +/- 12 dB) |
Mono mode |
User can switch between stereo and mono via the remote. |
External power supply |
US: nominal 12 VDC linear regulated "wall wart," 0.5 A. International: nominal 12 VDC switching "wall wart," 1.0 A |
Internal power regulation |
ePot.V3 Max: 2-stage switch-mode plus linear DC-DC regulation producing 3.3V plus +5V/-5V split voltage. Active gain/buffer board: switch-mode split voltage +/-15V followed by second-stage +/-12V linear regulators with high quality polypropylene and Elna silk bypass capacitors. |
Display/control menu |
High-contrast 3.12" white-on-black 256x64 pixel graphical OLED with intuitive interactive menu-driven control. |
Remote control |
Simple 7-button Apple infrared remote (or equivalent using 38 kHz NEC protocol and the Apple command/ID structure) |
Encoder control |
A subset of controls is available via a front panel control knob attached to a rotary stepped encoder with integral push-button switch |
Enclosure |
Black brushed anodized aluminum clam-shell case with dark acrylic front panel, anodized aluminum rear panel with laser-etched labeling, and non-slip urethane feet |
Circuit boards/solder |
High quality nickel immersion gold PCBs with lead-free solder, 3% or higher silver content |
RoHS |
Compliant or exempted |
Fuses |
No internal or external fuses |
Ambient requirements |
Stable, dry indoor environment only, ambient temperature within 60-90 degrees F |
Dimensions |
2.5" tall x 6" wide x 9.0" deep |
Weight |
~4 lbs as shipped |