LDR300x.V3 Passive or Active Preamp


The LDR300x.V3 has been discontinued as of June 1, 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 for up to 3 separate switchable single-ended (RCA) sources.

Remote controlled (Apple remote) or from the front panel (rotary encoder) with a high contrast, white-on-black, interactive menu driven OLED display.

The passive model utilizes our latest ePot.V3 stepped attenuator/controller with precision closed loop LDR current control for arguably the best sounding purely resistive passive attenuator available today.  The active model adds a minimalist solid state gain plus JFET buffer stage that delivers power when and where you need it with no impedance concerns or cable length limitations.

SKU: LDR300x.V3 Category:
Print Friendly, PDF & Email

A flexible passive or active preamp

The “x” in the LDR300x stands for fle”x”ible in that 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 single-ended line-level (RCA) sources.

If  you purchase the passive version you can opt later to convert to an active by adding a solid state buffer board.  If you have an active version you can bypass the active buffer and operate as a passive by swapping 2 internal jumpers.

A great sounding remote controlled preamp

The LDR300x delivers exceptional high fidelity the belies its small size and understated packaging.  Built around our latest ePot.V3 Max stepped attenuator & preamp controller, the LDR300x is arguably our best sounding preamp to date whose subjective performance will please even the most discriminating listener.

An affordable preamp

In redesigning our previous LDR3 into the LDR300x we sought to retain and improve upon elements that really mattered sonically while forgoing things that raised costs with dubious sonic benefits. As a result we are able to offer the LDR300x at a more aggressive price as our new flagship value line preamp product.

ePot.V3 “Max” electronic stepped attenuator & controller

The heart of the LDR300x is our latest preamp attenuator/controller, the ePot.V3 Max (the “V3” or “Max”  ). The V3 is the culmination of over a year of development and refinement of our evolving 10+ year LDR attenuation technology. The V3 combines exceptional sonic performance in an integrated, software driven, preamp controller that includes input switching, remote control and a menu driven interactive high contrast OLED display interface.

Features of the ePot.V3 Max include:

  • 100 step attenuator over 60 dB range
  • Smooth volume stepping with no sonic artifacts
  • No mechanical switches in the signal path
  • Menu driven visual control with high contrast 256 x 64 OLED display
  • LDR (light dependent resistor) attenuation
    • precision closed loop LDR current control
    • integrated offline self calibration of the LDRs
    • replaceable plug-in LDR module
  • Integrated input switching (3 single-ended stereo sources) via analog switches
  • Remote controlled (Apple infrared remote)
  • Manual control via rotary encoder with integral push button
  • Smooth muting/unmuting
  • Left/right channel balance  (+/- 10 db)
  • Updateable firmware

High performance SSPB.V2 solid state JFET buffer

Combined with the ePot.V3 front end, the active SSPB.V2 Solid State Preamp Buffer (the “SSPB” or “buffer”) delivers robust current drive to enhance the performance when a passive attenuator connected to sources with less robust drives may not satisfy.

The SSPB.V2 employs a high performance audio op amp input stage with just the right amount of gain to ensure ample volume control headroom without introducing noise. Gain can even be optimized through the use of plug-in gain modules including the ability to go with unity gain if desired. The op amp employs an 8-pin DIP socket so op amp enthusiasts who are so inclined can explore alternatives.

The buffer output stage employs a discrete high performance LSK170 JFET transistor that provides robust current amplification with low output impedance and minimal DC offset.

A dedicated 2 stage split voltage power supply powers both the op amp input stage JFET output stage. This power supply is separate from the power supply for the ePot.V3 attenuator/controller.

The split voltage power supply makes it possible to run the active version of the LDR300x with direct coupled output – no AC coupling capacitors. This feature was not initially available in earlier production units but can be enabled with only minor modifications to existing units. It is now a standard feature with the LDR300x.V3

sspb.v2 solid state preamp buffer simplified schematic
(the JFET in the above image is mislabeled – it’s an LSK170)

Operating Instructions

The operation of the LDR300x.V3 is indistinguishable from the underlying ePot.V3 Max Controller/Stepped Attenuator at the heart of the LDR300x. The operating instructions for the V3 Max are available in our online product documentation and can be found here.


Preamp OverviewSingle-ended, remote controlled 2 channel stereo preamp with 3 RCA inputs and 2 parallel RCA outputs. The LDR300x is available as a passive or as an active type preamp.
ControllerProprietary ePot.V3 Max LDR (light dependent resistor) digitally controlled preamp controller and electronic stepped attenuator with precision closed loop LDR current control driven by a 16 bit command DAC. Analog switches are employed for input switching and output isolation. The controller uses an embedded 32 bit STM32F2 ARM M3 Cortex 100 MHz microcontroller.
AttenuationStereo (2 channel) LDR electronic stepped attenuation with 100 smooth steps over -60 to 0 dB range with two (2) LDRs per channel in a series/shunt L-Pad voltage divider configuration.
Attenuation ModuleSingle plug-in replaceable LDR type attenuation module with 4 embedded LDRs plus EEPROM memory chip for storing attenuation tables derived from in-situ calibration of the LDRs
LDR Calibration Embedded closed-loop offline calibration circuit which normalizes the behavior of each of the 4 attenuation LDRs over 100 steps of attenuation each corresponding to a specific targeted resistance value. Calibration can be run to compensate for performance drift, aging, or in the event of LDR Module replacement. Calibration is user initiated. Pre-matched LDRs are not required.
Gain/Buffer BoardThe active version of the preamp has an additional active gain/buffer board with an op amp input buffer/gain stage (voltage amplification) and a class A JFET buffer output stage (current amplification). Input gain is set at 0 dB (no gain) by default but can be raised to +6 or +12 dB by replacing the plug-in gain module.
No Coupling CapsThe input and output are both direct coupled with no AC blocking capacitors in the signal path.
DC OffsetThe DC offset of the output on the active version is controlled manually on the gain/buffer board by adjusting the bias current of the class A JFET buffer output. The DC offset is relatively stable and should rarely require adjustment after initial break-in. If needed, this adjustment is easy to do with a small screwdriver and a simple volt-ohm meter.
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 ~50 ohms
Channel Balance Adjustment+/- 20 step channel balance adjustment at ~0.6 dB per step for a nominal max channel balance range of +/- 12 dB
Mono ModeUser can switch between stereo and mono mode via the remote.
External 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
Internal Power Regulation ePot.V3 Max Controller/Attenuator - 2 stage switch mode plus linear DC-DC regulation producing primary 3.3V plus +5V and -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 MenuHigh contrast 3.12 inch white-on-black 256x64 pixel graphical OLED display providing an intuitive interactive menu driven control. Control Menu details are available online as part of the product documentation for the ePot.V3 Max controller at the following link:
Remote ControlInteraction with the Display/Control Menu is done with a simple 7-button Apple infrared remote (or equivalent remote using 38 kHz NEC protocol and conforming with the Apple command/ID structure)
Encoder Control A subset of the controls are available through a front panel control knob attached to a rotary stepped encoder with integral push button switch
EnclosureBlack brushed anodized aluminum clam-shell case with dark acrylic front panel, anodized aluminum rear panel with laser etched labeling, and no-slip urethan feet. Black oxide socket screws with washers attach front and rear panel to case.
Circuit Boards/Solder High quality nickel immersion gold printed circuit boards with lead free solder having 3% or higher silver content.
ROHSCompliant or exempted
FusesNo internal or external fuses
Ambient RequirementsPreamp must be operated in a stable, dry indoor environment only with stable ambient temperature within 60-90 degree F
Dimensions2.5 inches tall | 6 inches wide | 9.0 inches deep
Weight~4 pounds as shipped
Scroll to Top