An electronic stepped attenuator
The ePot.V3 Max (the “V3” or “Max” ) is a chassis mounted electronic stepped attenuator for the DIYer, designer or OEM looking for 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:
- LDR (light dependent resistor) attenuation module
- 100 step attenuator
- -60 to 0 dB
- adjustable impedance from 1k to 100k (20k by default)
- integrated LDR calibration
- plug-in replaceable module
- Optional alternative discrete resistor attenuation module
- 127 step attenuator
- -63.5 to 0 dB
- 60k fixed impedance
- plug-in replaceable module
- Switch between 6 stereo sources
- Smooth stepping with no sonic artifacts
- No mechanical switches in the signal path
- Remote controlled (Apple infrared remote)
- Manual control via rotary encoder with integral push button
- Smooth muting/unmuting
- Left/right channel balance w/ reset (+/- 10 db)
- Optional status LED
- Optional high contrast menu driven OLED display with encoder & IR receiver
- Compact – 2.9″ wide x 4.4″ long x 1.2″ deep
- Chassis mounted with standoffs
- Serial (UART) and TTL communication & control interfaces for customization
- Object oriented C++ application software running on top of FreeRTOS real-time operating system
Software driven passive analog attenuator
The ePot.V3 Max is a passive analog volume control device with an active 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 Max is a larger version of its sibling the ePot.V3 Mini. The key differences from the Mini is the Max is chassis mounted instead of panel mounted, has built-in calibration for its plug-in LDR attenuation module, has built-in input switching, and has more communication/control interface options for customization.
While actively controlled, the audio signals passing through the V3 are not directly mixed with or manipulated by any power supply, there is no gain stage and there is no output/buffer stage. The audio signals only see pairs of resistors that control volume by reducing the incoming signal voltage. By this definition the V3 is inherently a passive volume controller.
By itself, the Max can be used as a passive preamp. When mated to a downstream gain and/or buffer stage the ePot.V3 Max becomes the attenuator/controller of a high performance active preamp.
The following items are included by default with each Max.
- ePot.V3 Max board with attached triple voltage power supply module
- IR receiver module with 6 inch 3-wire square pin connector cable
- Status LED with 6 inch 2-wire square pin connector cable
- Power connector plug with crimp pins
You can opt to purchase the Max board “bare” without any attenuation module at a reduced cost provided you already have an existing LDR or discrete attenuation module. To be clear, an attenuation module is absolutely needed for the Max to function.
What’s not included?
You will have to supply the following items:
- Source of DC power. The V3 can be powered from any DC voltage from 9 to 30 VDC and draws no more than 0.3 A of current (with OLED display).
- Apple remote (the silver metal model). You can operate the V3 with just the rotary encoder but you’ll wish you had the remote. Get the remote!
- A knob for the rotary encoder
What do you have to do?
The key steps to getting the Max fully functional include the following:
- Mount it into a chassis
- Connect it to DC power via the power plug
- Connect the left and right channel signal inputs, outputs and grounds via either solder pads or pin header
- Optional(recommended): Connect the IR receiver module to the Mini J7 header and mount the module somewhere on your front panel where it can “see” out into the room.
- Optional(recommended): Connect the status LED to the Mini J7 header and mount the LED somewhere on your front panel where it can be seen.
The following items can be purchased together with the Mini or purchased separately later on:
- Additional LDR attenuation module
- Alternative discrete resistor attenuation module – 60k fixed impedance
- OLED display with integrated IR receiver and connecting ribbon cable
- Apple remote
Attenuation module types
The ePot.V3 offers two types of interchangeable plug-in attenuations modules that use two very different types of volume control technology – light dependent resistors (LDR) and discrete resistor pairs.
Both attenuation module types are available as interchangeable plug-in modules that plug directly into V3 via a 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.
LDR attenuation module
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.
About LDR impedance and calibration
Due to its small size, the Mini is not capable of calibrating its own LDR attenuation module. Therefore is you need an LDR attenuation module with a difference impedance level or to replace a bad module you will need to purchase LDR modules from Tortuga Audio. Alternatively, if you also own an ePot.V3 Max you can use any Max to recalibrate any LDR module at whatever impedance level you wish. If you send in your existing V3 LDR attenuation module, we will recalibrate your module for you for a $5 fee plus return shipping.
For more general information, please refer to this link to read more about LDRs, their impedance and their calibration.
Discrete attenuation module
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.
Improved LDR control & calibration
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.
Visual feedback & display options
The ePot.V3 will operate without the need for any visual feedback. Basic status feedback is available 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 visual feedback requires our graphical OLED display and an Apple remote together with the Mini’s built-in encoder.
The following table summaries the key differences in control capabilities between the status LED light and the OLED Display options.
|No OLED Display|
|No OLED Display|
|With OLED Display|
|With OLED Display
|Power On||1) Remote menu button toggle|
2) Encoder press/release
|Power Off||1) Remote menu button toggle|
2) Encoder press/hold(5 sec)/release
|Same||1) Off Menu|
2) Encoder press/hold/(5 sec)/release
|Volume Change||1) Remote raise/lower buttons|
2) Encoder turn right/left
|Input Change||Encoder press/hold/turn||N/A||1) Input Menu|
2) Encoder press/hold/turn
|Mute Toggle||1) Remote mute button toggle|
2) Encoder press/release
|Channel Balance||Remote left/right buttons|
(Press/hold remote center button 5 secs to reset)
|LDR Calibration||No||N/A||Calibration Menu||N/A|
|Display Brightness||N/A||N/A||Display Menu||Same|
|Display Timeout||N/A||N/A||Display Menu||Same|
|Max Volume Upon Change||No||Same||Max Vol Menu||Same|
|# Inputs||No||N/A||#Inputs Menu||N/A|
|Menu Options||No||Same||Options Menu||Same|
|Version Info||No||Same||Version Menu||Same|
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 ePot.V3; such information will be posted once it becomes available.
|Attenuation type||Option #1 - LDR Attenuation Module (2 LDR modules (1 series & 1 shunt) per channel)
Option #2 - Discrete Thin-Film Resistor Module (7 stage logarithmic ladder)
|Attenuation Range||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
|Attenuation Modules||Option #1 - Plug-in stereo LDR module with 4 LDRs (6 pin & 10 pin sockets)
Option #2 - Plug-in stereo discrete resistor module with 30 thin film resistors (6 pin & 10 pin sockets)
|Gain||Unity (1x) gain | no amplification, processing or active manipulation of the audio signal | audio signal optically isolated from control system
|Buffering||No input or output stage buffering|
|Input impedance||LDR Attenuation Module - 80k 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)|
|Multi-channel Attenuation||2 or more V3 boards can be daisy-chained to provided mutl-channel attenuation with 2 channels per board|
|Channel Balance Accuracy||Within 0.5 db over full attenuation range|
|Channel Isolation||Each channel 100% isolated and independent from the other within the V25 board | audio signal grounds must to tied to a common star ground point external to the board|
|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%
|Power Input||7-30 volts DC rated at 0.5 amp via J1|
|Power Regulation||* Single stage switched DC-DC split voltage +/-5 V for analog
* 2nd stage linear 3.3 V regulator for digital
|Processor||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|
|Mono Mode||Output switchable between stereo and mono mode via LDR switch | applies on to single-ended audio|
|Firmware||Custom code written in the C language running on top of FreeRTOS Real Time Operating System.|
|Firmware Update||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.|
|WiFi Module||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 solder pads with green solder mask|
|Ambient Requirements||Board must be operated in a stable, dry indoor environment only with ambient temperature within 60-90 degree F|
|ROHS||Compliant or exempted|
|Physical Dimensions||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