The short answer is a stepped attenuator is a type of volume control device.
The longer version takes a bit of introduction. Almost everyone has heard of, used or seen a potentiometer (or “pot”) which is the most common of all volume control devices. A key characteristic of a pot is that it’s smoothly continuous meaning that as you turn the pot the volume changes smoothly. There are no “steps” per se with a pot.
In contrast, a stepped attenuator changes volume in discrete steps. A stepped attenuator is really a big mechanical switch with many positions and each position gives you a different volume level. The image below shows a typical mechanical stepped attenuator with multiple switched resistor pairs.
Stepped attenuators exist for the simple reason that audio enthusiasts find that they sound better than potentiometers. And they do!
But stepped attenuators do have their drawbacks. Top of the list is that most are too coarse in that each volume step is often larger than ideal. So why not just add more steps? Because the mechanical complexity goes up dramatically as does the physical size not to mention the cost. Most do not have more than 30 steps.
Next on the list are sonic artifacts from the switching process such as clicks and pops as the volume is turned up or down. Some are better than others at mitigating this. In the end you are unavoidably left with dozens of moving/sliding mechanical contact points each of which is prone to corrosion, wear and misalignment over time.
Last but not least almost all mechanical stepped attenuators are manual devices. Forget using a remote control. You have to get up, walk to the panel, grab a big knob (you need the torque!) and turn it by hand to get the volume you want. Click, click, click, click.
The ePot.V3 (also referred to as either the ePot or V3) is an electronic alternative to a mechanical potentiometer or stepped attenuator that offers advantages that no mechanical attenuator can match. In addition to being a better stepped attenuator it’s also a complete preamp controller that can handle not only volume control, but also input switching, on/off power switching, muting, channel balance adjustment, and an interactive user interface that includes both remote control capability and a high contrast OLED display.
The ePot.V3 is an analog audio device with a software driven digital heart. It is the successor product to Tortuga Audio’s popular LDR.V25 Preamp Controller which traces its roots back to 2012 and our very first electronic stepped attenuator based on light dependent resistors (LDRs).
The new “ePot” designation is meaningful for 2 reasons.
First, it explicitly recognizes that, like its predecessors, the V3 is indeed an “e”lectronic “pot”entiometer and stepped attenuator.
Secondly, the ePot.V3 model we call the “Mini” is literally a direct physical replacement for a volume potentiometer or mechanical stepped attenuator insofar as it through-mounts directly into a typical front panel, is secured via a nut and washer, to which you then attach a conventional control knob just as you would to any pot or stepped attenuator.
There are two ePot.V3 models; the larger full featured ePot.V3 Max and the smaller ePot.V3 Mini with reduced capabilities but uniquely capable of direct panel-mounting and replacing a conventional volume pot in new or existing preamps.
Both models share a common hardware design and both utilize the same core software.
The full size Max is a 2.7 x 4.4 inch chassis mounted stepped attenuator and preamp controller. The Max can control volume, channel balance, provide on/off power command signal, mute output, switch up to 6 stereo input channels (no external relays required), handles both single-ended and balanced audio (2 boards needed for balanced), and has the built-in ability to calibrate any LDR Attenuation Module including adjustable input impedance. The Max is remote controlled via an Apple remote. The Max can also be controlled locally at the panel via a rotary encoder. While the Max can operate with only a single LED status feedback light it’s ideally suited to operate with an interactive menu driven 256 x 64 bit high contrast OLED display which expands its functionality and ease of use. The Max offers excellent performance as a passive attenuator or upstream of a gain/buffer stage in an active preamp.
At 1.35 by 2.9 inches, the Mini is 67% smaller than Max and is even smaller than a credit card. A key feature of the Mini is the board mounted rotary encoder that allows the Mini to be panel mounted similar to any potentiometer or stepped attenuator. The Mini has many of the same capabilities as the Max but in exchange for the smaller size does not offer input switching and does not have the built-in ability to calibrate its LDR Attenuation Module. However, it most definitely can also be remote controlled and also integrates with the same high contrast interactive OLED display as the Max.
The table below summarizes the similarities and differences between the the two ePot.V3 models.
2.7 x 4.4 inch
1.35 x 2.9 inch (67% smaller)
Yes - up to 6 stereo channels
No - single stereo channel
Chassis mounted w/ standoffs (not included)
Panel mounted via integral board mounted encoder (like a potentiometer)
Yes - requires 2 separate boards and a 2 wire data link
Not with current version
LDR Module Calibration
Yes - built-in
No - Uses replaceable plug-in LDR module or you can use a separate ePot.V3 Max to calibrate any LDR module for any Mini
Plug-in LDR or discrete resistor modules
* Rotary encoder (externally mounted)
* Apple remote control (infrared)
Same except the rotary encoder is mounted directly to the Mini board
* Status LED
* Optional OLED Display (with encoder and IR receiver interface)
Same except OLED and Status LED are mutually exclusive since they both use the same header
Auxiliary Control Header
6 bit signal for controlling external devices
100 pin STM32F2 microcontroller
64 pin STM32F2 microcontroller
Yes - via USB to 2-wire serial UART link
Yes - via Apple remote and externally mounted IR receiver
Like its predecessors, the ePot.V3 offers high performance attenuation using light dependent resistors (LDRs). LDRs offer a very pleasing clean, clear, unveiled, neutral, colorless, yet natural organic sounding volume control with remarkable bass performance. The reader is referred to the following link for a more in depth discussion about LDRs.
The V3 LDR Attenuation Module is a new LDR module design where all 4 LDRs are now mounted together on a single board along with an EEPROM memory chip that holds the impedance and attenuation setting data that are unique to each set of LDRs. The LDR Attenuation Modules plugs directly into either ePot.V3 model.
The LDR Attenuation Module provides 100 attenuation steps of ~ 0.6 dB/step over a -60 to 0 dB attenuation range. While we call them attenuation “steps” the transitions between steps are smoothly continuous since LDRs are themselves true analog devices. There are no audible stepping artifacts and the control is smooth and precise over the full range.
The ePot.V3 introduces a new alternative to Tortuga Audio’s LDR based attenuation module for users who looking for the sound and measured performance of stepped attenuators with more conventional low-distortion discrete resistor pairs.
Thin film resistors are precision low noise passive devices made from nickel-chromium metal deposited on to a ceramic substrate. Referred to as stepped logarithmic attenuation, 7 fixed resistor pairs per channel provide stepped attenuation in increments of 0.5 dB, 1 dB, 2 dB, 4 dB, 8 dB, 16 db, and 32 dB. These 7 stages provide 127 steps of super smooth attenuation in ~0.47 dB increments over a -63.5 to 0 dB attenuation range.
While each step is truly discrete requiring one or more resistor pairs to connect and/or disconnect from the attenuation circuit this switching is done via ultra-fast, and ultra low-distortion, break-before-make analog switches. The result is super smooth switching with no audible artifacts. Left and right channel switching is done independently as well so that channel balance adjustments are also possible; something unheard of with discrete mechanical stepped attenuators.
Both attenuation module types are available as interchangeable plug-in modules that plug directly into both ePot.V3 models via the same pair of sockets. You don’t have to tell the V3 what type of attenuation module you are using. Plug in either attenuation module type, restart the board, and it figures that out on its own.
As to which one to choose for your ePot.V3, why not try booth and decide for yourself which you prefer in your system.
The ePot.V3 introduces a significantly improved LDR control scheme compared to all prior models.
The light source inside a “light” dependent resistor is actually an LED. As with any LED, the light intensity emitted by the LED is directly proportional to the current running through it. And by controlling light intensity you control the resistance level of the LDR. Hence, precise current control is synonymous with precise resistance control. In the past we’ve accomplished this indirectly with voltage control.
With the V3 we have replaced voltage control with precision closed loop LDR current control. Precision current control decouples LDR performance from power supply voltage levels resulting in more predictable, stable and repeatable LDR performance over a wide range of current.
In addition to precision current control, the ePot.V3 also has an improved LDR calibration system that replaces the previous 12 bit system with a simpler yet more precise 16 bit approach. LDR calibration is a built-in feature of the ePot.V3 Max. The reader is referred to the following link for a more in depth discussion of LDR calibration.
As a result, the sonic performance of the ePot.V3 running with LDR attenuation is now at the highest levels we’ve achieved to date.
The ePot.V3 will operate under a reduced set of control functions without the need for any visual feedback. These basic functions include power on/off, volume control, channel balance control & reset, muting, input switching (Max only), and stereo/mono switching (Max only).
Minimal feedback is provided through the optional attachment of a single status LED which indicates on/off status and briefly flashes acknowledging a control input from the remote.
Full control functionality and feedback requires our graphical OLED display together with a rotary encoder and remote.
The following table summaries the differences in available control functionality and information with various feedback options for both the Mini and Max models.
Alphanumeric and/or graphical information
LED on, off or blinking
Calibration (LDR only)
Detailed status display
Periodic blink count indicating which LDR (1 - 4) is in calibration. Continually blinking indicated calibration failure (possible bad LDR).
The following table is a fairly comprehensive listing of the specifications of the V3 hardware. This table does not include details on the operation and control of the ePot.V3 or ePot.V3 Mini; such information will be posted once it becomes available.
Operates interchangeably with either a plug-in LDR Attenuation Module or a plug-in Discrete Resistor Attenuation Module
* Discrete Attenuation Module - 127 0.5 dB steps over a 0 to -63.5 dB range plus 1 additional muting step.
* LDR Attenuation Module - 100 0.6 dB steps over a 0 to 60 dB range plus 1 additional muting step
Unity (1x) gain | no amplification, processing or active manipulation of the audio signal | audio signal optically isolated from control system
No input or output stage buffering
* LDR Attenuation Module - 20k by default | may configure up to 9 additional impedance settings each adjustable between 1k and 99k and switch between the settings with live music to optimize input impedance for their system.
* Discrete Attenuation Module - 60k fixed
Number of Channels
2 independent channels (stereo)
Channel Balance Accuracy
Within 0.5 db over full attenuation range
Audio Signal Ground
No connection of signal grounds to power ground within controller, however, audio signal ground must share common ground with power supply via an external star ground point in order for LDR self-calibration feature to function properly.
Audio Inputs | Single Ended
ePot.V3 - 1 board accommodates up to 6 switchable single-ended stereo (2 channel) inputs
ePot.V3 Mini - 1 stereo input only
Audio Inputs | Balanced
ePot.V3 - 2 boards accommodate up to 6 switchable balanced stereo (2 channel) inputs. With 2 boards it's possible to configure a mix of single-ended and balanced inputs.
ePot.V3 Mini - not for balanced audio
Audio Input Switching/Isolation
ePot.V3 - Input switching via analog switches | typical 2 ohm resistance when on | in excess of 5 megaohms when off | no mechanical relays in signal path.
ePot.V3 Mini - no input switching
Audio Output Switching/Isolation
ePot.V3 - Output signals connected to ground when off or undergoing calibration
ePot.V3 Mini - No output isolation
Total Harmonic Distortion
LDR Attenuation Module - typically less than 0.5%
Discrete Attenuation Module - typically less than 0.1%
7-30 volts DC rated at 0.5 amp via J1
Switching DC-DC regulation followed by linear regulation for 3.3V digital power and split voltage +/-5 V for analog power
ARM STM32F2 32 bit microcontroller running at 120 MHz
Control Input | Remote
Accepts control inputs from infrared silver Apple remote or equivalent remote using 38 kHz NEC protocol and Apple commands | infrared commands received via infrared receiver module attached indirectly via display module or directly to board pins
Control Input | Encoder
Accepts control inputs from rotary stepped encoder with integral push button switch | encoder attached indirectly via display or can be attached directly to board pins
Control Input | UART
Accepts control inputs from external device using UART serial protocol and specific command codes (available upon request).
Display | OLED
Graphic OLED display with an intuitive, easy to use, menu based control scheme. Connects to the board via both J4 header. Requires display jumper across pins at J7.DISP pins.
Display | 7-Segment (legacy)
Original dual 2-digit numerical 7-segment LED display can be connected via individual J4 pins although overall functionality is not guaranteed.
Display | Status LED
Status LED light | connects to board via J7
Output switchable between stereo and mono mode via LDR switch | applies on to single-ended audio
Custom code written in the C language running on top of FreeRTOS Real Time Operating System.
Firmware can be updated by invoking a chip resident bootloader that communicates with an extern PC based application. Firmware interface from PC to ePot is via a USB->Serial (FTDI USB->UART) com connection. FTDI communication interface sold separately.
Future ability to plug in a simple WiFi module via J6 that will allow the V3 to be controlled via smartphone app.
Printed Circuit Board
High quality nickel immersion gold pads with green solder mask
Board must be operated in a stable, dry indoor environment only with ambient temperature within 60-90 degree F
Compliant or exempted
** Max only **
Length: 4.4 inches
Width: 2.7 inches
Corner Holes Center: 0.145 inches from edges
Corner Holes Diameter: 0.15 inches
Top clearance: 0.7 inches
Bottom clearance: 0.1 inches