Electronic Stepped Attenuator | ePot.V3 Mini

An electronic stepped attenuator

The ePot.V3 Mini (the “Mini”  ) is a panel mounted electronic stepped attenuator that offers exceptional sonic performance with advantages that no mechanical attenuator can match. The “Mini” is literally a direct physical replacement for any volume potentiometer or mechanical stepped attenuator that through-hole mounts directly into a typical front panel, is secured by a nut and washer, to which you then attach a conventional volume control knob. The ePot.V3 Mini is intended for any DIYer, designer or OEM who is looking for a high performance volume control alternative to conventional potentiometers or mechanical stepped attenuators. Features of the ePot.V3 Mini include:

• LDR (light dependent resistor) attenuation module
  • 100 step attenuator
  • -60 to 0 dB
  • programmable impedance | 20k default but any value 1k to 100k can be ordered
  • 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
• Smooth stepping with no sonic artifacts
• No mechanical parts in the signal path
• Remote controlled (Apple infrared remote)
• Manual control via rotary encoder
• Panel mounted via encoder w/ nut and washer – just like any pot or stepped attenuator
• 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
• Smaller than a credit card – 2.9″ long x 1.35″ wide  x  1.2″ deep
• Can also be chassis mounted with standoffs
• Screw terminal interface for power and audio input/output

Software driven passive analog attenuator

The ePot.V3 Mini 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). While actively controlled, the audio signals passing through the Mini 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 Mini is inherently a passive volume controller. By itself can be considered a passive preamp. When mated to a downstream gain and/or buffer stage the ePot.V3 Mini can becomes a core part of a high performance active preamp.

What’s included?

The following items are included by default with each Mini.

• ePot.V3 Mini board with attached triple voltage power supply module
• * 20k LDR attenuation module fully calibrated & tested (you can order any impedance from 1k to 100k)
• IR receiver module with 6 inch 3-wire square pin connector cable
• Status LED with 6 inch 2-wire square pin connector cable

*  You can opt to purchase the Mini board “bare” without any attenuation module at a reduced cost but we pre-select the LDR attenuator module option by default. You can opt for no module, an LDR module only, a discrete module only, or both!  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 Mini 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). Power connection is via screw terminals.
• Apple remote (the silver metal model). You can operate the Mini with just the rotary encoder but you’ll wish you had the remote. Get the remote!
• A knob for the rotary encoder

Optional Items The following items can be purchased together with the Mini or purchased separately later on:

• Additional LDR attenuation module with nominal impedance from 1k to 100k
• 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 Mini 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.

Minimal control and visual feedback options

The ePot.V3 should is designed to be operated with an Apple remote together with Mini’s built-in encoder. However, you can operate the Mini with only the encoder with a few limitations. The ePot.V3 Mini will operate without the need for any visual feedback. However, we recommend you employ a status LED which indicates on/off status and briefly flashes acknowledging a control input from the remote. For optimal control and enjoyment we recommend the visual feedback provided by our menu driven graphical OLED display.

Hardware, operation and control manuals

A more comprehensive description of the Mini’s hardware, installation, operation, and controls can be found in the following online documentation: ePot.V3 Mini hardware manual  ePot.V3 Mini & Max operation and controls manual

ePot.V3 specifications

The following table is a fairly comprehensive listing of the specifications of the V3 hardware is NOT specific to the ePot.V3 Mini; such information will be posted once it becomes available.

Criteria
Attenuation	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
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 - 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%
Power Input	7-30 volts DC rated at 0.5 amp via J1
Power Regulation	Switching DC-DC regulation followed by linear regulation for 3.3V digital power and split voltage +/-5 V for analog power
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 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	** 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