About this manual
As the V4 is still under active development, information in this document is preliminary and subject to both major and minor changes without notice.
This manual describes the physical hardware and interfaces of the ePot.V4 Max (“V4” or “Max” ) remote controlled preamp controller & electronic stepped attenuator.
The detailed operation and control of the V4 are covered in a different manual which can be found _____________________.
What is the ePot.V4
The Pot.V4 is a software driven preamp controller and electronic stepped attenuator, that when equipped with appropriate power supply and OLED display can function as a unity gain passive preamplifier, and when equipped with an optional solid state buffer board, can function as an active preamplifier with adjustable gain.
Like all Tortuga Audio preamp controllers, the V4 employs light dependent resistors (LDRs) as the sole means for for audio attenuation (volume control). No mechanical potentiometers or switched mechanical attenuators are used.
The features of the V4 are summarized in the following list.
- Volume controller for 2-channel stereo audio systems (a stereo attenuator)
- Applicable for single-ended or balanced audio (balanced audio requires an additional expansion board)
- Fully independent channels including independent channel grounds
- Employs analog light dependent resistors (LDRs) for attenuation in lieu of potentiometer or discrete resistor pairs
- Uses a single replaceable plug-in LDR attenuation module (4 LDRs per module)
- Built-in self-calibration of the LDRs | no pre-matching of LDRs required
- 100 discrete attenuation steps over 0 to -60 dB attenuation range (~0.6 dB per step) plus a mute step
- Smooth, quiet transitions between each attenuation step (no mechanical switching)
- Controlled by a 32 bit ARM microcontroller utilizing 16 bit resolution control & measurement
- Optional solid state buffer board converts passive V4 into an active preamp platform with adjustable gain (buffer board sold separately)
- Fully controllable with via commercially available infrared remote
- Partially controllable with a rotary encoder with integral pushbutton
- Channel balance adjustable over a +/- 20 steps (+/- 12 dB) range
- No electrical connection within the board between the audio signals and control power (optically isolated)
- Input switching between up to 4 stereo inputs via miniature electromechanical relays
- Audio signal interface via either a pin header or solder lugs
- Muting with optional smooth ramping of volume up/down
- Adjustable input impedance | default 50k plus 9 additional optional user determined settings between 1 and 100k
- OLED graphical display with 3 inch 256×64 pixel 8-bit white-on-black grayscale (sold seperately)
- Interactive menu driven control interface via the OLED display
- Auxiliary outputs include panel LED and on/off trigger relay (5V trigger output requires an external boost device for 12V trigger logic level)
- Communications via UART, I2C and USB
- Firmware updating via USB connection to PC, Mac or Linux using free 3rd party STM32CubeProgrammer application
- Powered by any 5 volt DC external power supply with nominal 0.5 amp capacity
- Optional external linear power supply board to power the V4 plus optional buffer board (sold separately)
The V4 Max builds on 13 years of LDR preamp controller evolution that began in 2010. The board is intended for use by DIY audio enthusiasts and audio OEMs. Tortuga Audio has used similar hardware in building its own line of finished preamp products and may do so from time to time in the future.
Hardware versions
Version A of the V4 Max board was released on (date goes here) 2024. The layout of the V4 board and its parts is shown below. Individual parts on the V4 board are identified by their reference designator (J2, U4, C18 etc.)
In addition the the primary V4 board shown below, there’s a similar V4 expansion board (not shown) that is used in combination with the primary V4 board for balanced audio applications.
Physical dimensions
The V4 board is the same width as the V3 at 2.7 inches wide but at 4.25 inches is slightly shorter than the V3. The overall height including the plug-in LDR module is slightly greater than 1 inch but we recommend allocating 1.5 inches of vertical space above the board; 2 inches if you opt for the solid state buffer board addition. There are no parts mounted on the underside of the board but a minimum clearance of 0.125 inch is recommended underneath the board to avoid making contact with any through-hole soldered parts and to allow for adequate air circulation.
Ambient requirements
The V4 is designed to operate within nominal room temperature conditions that are typical for home stereo equipment. LDRs are known to be temperature sensitive. Therefore large departures from nominal home room temperature conditions may cause the LDR attenuation module to operate poorly. Operating an LDR attenuator module inside of equipment that gets very warm may still work but you may have to run calibration after board has warmed up and stabilized. We have not found this to be a problem within our own preamp products but the caution is valid.
32 bit ARM microcontroller | U1
Overall control of the V4 is handled by a proven 80 pin STM32G4 embedded 32 bit ARM Cortex microcontroller chip. The V4 application code (firmware) runs on this microcontroller chip. The firmware can be updated by the user via the USB interface provided on the V4 board together with a free STM32CubeProgrammer application which can run on the users PC, Mac or Linux machine. Detailed firmware updating instructions can be found [__________________].
Single-ended vs. balanced audio
While the V4 is designed for both single-ended and balanced audio, the primary V4 board itself can only handle single-ended audio. A secondary V4 expansion board is required for balanced audio.
In balanced audio applications the primary V4 board serves as the right channel controller for both the positive and negative right channel phases. Conversely, the secondary expansion board handles the two phases for the left balanced channel. Within each board, there’s a Right(+ phase) channel and a Left(- phase) channel. Thus for balanced audio the V4 actually provides 4 separately attenuated audio signals, 2 for the right stereo channel and 2 for the left.
LDRs – light dependent resistors
Light dependent resistors (LDRs) are at the core of Tortuga Audio’s preamp controller & stepped attenuators including the V4.
LDRs are variable resistors that are employed in pairs as voltage dividers to attenuate volume. LDRs combine two distinct electronic devices – a light emitting diode (LED) and a photoresistor, These two devices are integrated into a sealed package approximately the size of an M&M candy. LEDs do what their name says – they emit light. Photoresistors are made with a photoreactive semiconductor material that changes resistance based on the intensity of light impinging on the photoreactive material. The intensity of the light from the LED is controlled by regulating the current running through the LED. As the current increases, the light intensity increases and the LDR resistance decreases.
When employed in an audio preamp, LDRs can literally control volume using light! However, due to the complex and variable relationship between current, light intensity and the resulting resistance, the application of LDRs for volume control is technically challenging. LDRs also exhibit higher distortion than conventional volume control devices like potentiometers or discrete resistors which many designers deem to be a disqualifying characteristic. Nevertheless, we use LDRs for volume control for one very simple reason – music sounds better to our ears with LDR volume control. If music didn’t sound better with LDRs, then what would be the point?
More information on LDRs and how LDR volume control works can be found here.
Electronic stepped attenuation
The V4 is a true electronic stepped attenuator that controls volume over a 100 step range of ~0.6 decibels per step from -60 to 0 decibels. At this level of granularity, volume change between each step is smooth and continuous with no audible artifacts. There are no noisy mechanical switches or other moving parts involved as compared to conventional mechanical volume control devices like rotary potentiometers or switched stepped attenuators.
Each step change in volume can be made using either a handheld remote control or by using a rotary encoder (control knob) attached to the V4.
LDR calibration
The resistance of each LDR at each step is achieved by precisely regulating their control current. More current equals lower resistance. Less current equals greater resistance. However, the relationship of current vs. resistance of each LDR is not identical between any two LDRs and thus cannot be assumed. Instead this relationship must be empirically measured for each LDR. The V4 employs a proprietary software driven calibration process to do this measurement. A calibration cycle takes roughly 10 minutes to complete and can be initiated at any time by the user. The measured current vs. resistance data for each of the 100 volume steps for each of the 4 LDRs are stored in an EEPROM memory chip located on the plug-in LDR module. The V4 microcontroller uses this stored calibration data to set the resistance level of each LDR to achieve the volume level associated with each volume step.
LDR module | J1 & J2
Tortuga Audio uses a proprietary LDR module which incorporates 2 pairs LDRs plus a memory chip as shown below on the left. This removeable and replaceable LDR module plugs directly into matching sockets on the V4 board. Each pair of LDRs controls the volume of one stereo channel. The LDR pairs are configured in a series/shunt arrangement creating a voltage divider that essentially emulates how a potentiometer controls volume. The two topmost LDRs (RSeries and RShunt) are the right channel LDRs and the two bottom LDRs (LSeries and LShunt) are the left channel LDRs. If an LDR ever fails or goes out of specification, it’s a straightforward matter to replace a plug-in LDR module.
The LDR module has 2 pin headers, J1 and J2. These male pin headers correspond to the J1 and J2 female sockets on the V4 board.
The 6-pin J1 header carries the audio input and output signals plus signal ground for both the left and right channels. Each of these two channels are fully independent of the other including audio signal ground. The J1 pinouts are as follows:
- RI – right input
- RO – right output
- RG – right ground
- LG – left ground
- LO – left output
- LI – left input
The 12-pin dual row J2 header provides the power, LDR control currents, and SPI serial communications signals to the module’s EEPROM memory chip. The J2 pinouts starting at the top row and left pin #1 are as follows:
- #1 – 3.3V power
- #2 – no connection
- #3 – ground
- #4 – no connection
- #5 – right shunt LDR current
- #6 – right serial LDR current
- #7 – SPI chip select
- #8 – SPI MISO (serial data out)
- #9 – SPI clock
- #10 – SPI MOSI (serial data in)
- #11 – left shunt LDR current
- #12 – left series LDR current
To insert the LDR module into the V4 sockets, first align the pins on the 12-pin J2 header into the J2 socket with the LDR module held tilted up lengthwise at a slight angle (J1 end up, J2 end down), then lower the J1 header pins into the J1 socket making sure the pins align. Then gently press the module into both sockets. DO NOT FORCE IT! If the pins are not perfectly aligned into their socket holes, do not expect the module to go in. Re-align the pins on both ends first before attempting to press the module into the sockets. If the module won’t align and go in, visually inspect the pins on the LDR module to see if any pins are slightly bent or out of alignment. The pins can usually be easily manually manipulated back into alignment.
16 bit LDR calibration & control | U2, U3 & U5
The resistance of each of the 4 LDRs is maintained by a dedicated current controller. A simplified schematic of a single LDR current controller is shown below. Each consists of a fixed voltage source, an LDR, a current control transistor, a current measuring resistor, and a precision current control op amp. The op amp receives a filtered analog voltage setpoint from a 16 bit pulse width modulated (PWM) signal generated by the microcontroller. Each of these 4 precision closed loop controllers run independent of variations or noise in the voltage source. During calibration, the microcontroller adjusts the 16 bit PWM setpoint to achieve the target LDR resistance level at each of the 100 volume control steps. The microcontroller also measures the resistance level of each LDR using an ADC with oversampling to achieve an effective 16 bit measurement resolution. The 100 set point levels for each of the four LDRs are stored for subsequent use by the microcontroller during normal volume control.
More information on the calibration process can be found here.
Power supply | J8 header
The V4 requires an external 5 volt DC power supply with a nominal capacity of 0.5 amps although actual current demand is typically no more than 0.3 amps. The V4 also uses 3.3 volts that it generates internally using a low noise on board linear voltage regulator powered by the external 5 volt source.
The external 5 volt DC enters the V4 board through the 4-pin J8 header on the V4 board. J8 is a 4-pin 0.1 inch pitch shrouded JST male header with the following pinout:
- J8.5V – 5 volt DC regulated
- J8.G – ground
- J8.Cal – calibration enable signal from the V4 to optional buffer board
- J8.Pwr – power enable signal from the V4 to optional buffer board
“JST” type connectors are either made by the Japan Solderless Terminal company, or by other manufacturers using the same connector design.
Only the J8.G (ground) and J8.5V (+power) pins are needed for powering the V4 board. The J8.Cal and J8.Pwr pins are only used when the optional solid state buffer board is present. When a solid state buffer board is installed on top of the V4, power to the V4 is provided by the buffer board through the J8 header.
Audio signal inputs & outputs | J4
The V4 Max can accommodate up to 4 different stereo audio inputs. There is only a single stereo output. All audio signal switching is done with miniature electromechanical relays. When the V4 is muted, the audio outputs (RO & LO) are switched to audio signal grounds (RG & LG), effectively killing the output. During calibration, both the inputs and the output are disconnected by relays.
Audio input, output and signal grounds to the Max board can be terminated via either the 12-pin 0.1 inch pitch J4 header or a separate collection of larger solder pads. These connections are identified as follows:
- J4.RI4 | right inputs #4 | relay K7
- J4.RI3 | right inputs #3 | relay K5
- J4.RI2 | right inputs #2 | relay K4
- J4.RI1 | right inputs #1 | relay K3
- J4.RO | right output | muting relay K6
- J4.RG | right signal ground
- J4.LG | left signal ground
- J4.LO | left output | muting relay K6
- J4.LI1| left inputs #1 | relay K8
- J4.LI2 left inputs #2 | relay K8
- J4.LI3| left inputs #3 | relay K8
- J4.LI4| left inputs #4 | relay K8
The left and right signal grounds (LG & RG) are fully independent and DO NOT connect within the Max board to the power supply ground.
OLED display header | J5
J5 is a 14-pin dual row (2 x 7) header that carries the power and control signals for the OLED display module, the IR receiver module, and the rotary encoder. The IR receiver and the encoder are both part of the OLED display module.
The OLED display module is an essential part of the the V4 board design providing an interactive menu driven visual interface for control and operation of the V4. More information on the OLED display module can be found here.
The J5 header pins are labeled as follows in the clockwise direction:
- J5.3V3 – 3.3 V DC power
- J5.G – ground
- J5.SC – SPI serial data clock
- J5.CS – SPI OLED chip select
- J5.SW – encoder switch
- J5.EB – encoder leg B
- J5.RES – OLED reset
- J5.G – ground
- J5.G – ground
- J5.DC – OLED data/command select
- J5.EA – encoder leg A
- J5.IR – IR receiver signal
- J5.SO – SPI serial data
- J5.__ – no connection
Expansion board header – balanced audio | J6
For balanced audio applications, the V4 Max requires connection to a separate expansion board that connects to the primary V4 board via a 14-pin ribbon cable connected between the 14 pin (2×7) J6 headers on each board. The primary V4 board controls the expansion board directly through the J6 header as the expansion board has no microcontroller of its own.
[image of expansion board goes here]In balanced audio applications the primary V4 board serves as the right channel controller for both the positive and negative right channel phases. Conversely, the secondary expansion board handles the two phases for the left balanced channel. Within each board, there’s a Right(+ phase) channel and a Left(- phase) channel. Thus for balanced audio the V4 actually provides 4 separately attenuated audio signals, 2 for the right stereo channel and 2 for the left.
The J6 pins are labeled clockwise from top-left as follows:
- J6.DA -I2C serial data
- J6.CL – I2C serial clock
- J6.RSe – right series PWM output to expansion board
- J6.LSe – left series PWM output to expansion board
- J6.SO – SPI serial communication MOSI output
- J6.SC – SPI serial communication clock
- J6.RHi – right expansion channel high side calibration voltage
- J6.RLo – right expansion channel low side calibration voltage
- J6.LLo – left expansion channel low side calibration voltage
- J6.LHi – left expansion channel high side calibration voltage
- J6.SI – SPI serial communication MISO input
- J6.CS – SPI serial communications chip select
- J6.LSh – left shunt PWM output to expansion board
- J6.RSh – right shunt PWM output to expansion board
Auxiliary status & communication header | J7
The J7 auxiliary header is not necessary for the operation of the V4 but provides additional status and communication options for the user’s designs.
J7 is an 10-pin dual row (2×5) male pin header that provides several logic level status output signals, a 5 volt trigger out signal, plus both UART and I2C serial communications ports for communication with 3rd party external devices.
J7 status outputs are logic level (3.3V) signals only and should NOT be relied on to source or sink more than a few milliamps of current. The J7.LED signal can directly drive a typical LED equipped with an appropriate current limiting resistor. The J7.TRIG signal is a robust 5 volt but requires an external 5 to 12 volt boost to achieve a true 12 volt trigger output.
The J7 pinouts are labeled clockwise from upper left as follows:
- J7.G – ground
- J7.Trig – 5 volt trigger output (relay switched)
- J7.LED – unit on/off status signal for panel LED
- J7.RX – UART serial communication receive
- J7.CL – I2C serial communication clock
- J7.S1 – unassigned spare output #1
- J7.S2 – unassigned spare output #2
- J7.DA – I2C serial communication data
- J7.TX – UART serial communication transmit
- J7.G – ground
Solid state buffer board (optional)
The V4 Max can accommodate an optional solid state buffer board (sold separately) that mounts directly on to the V4 board using the same sockets as the LDR Module. The LDR Module is then plugged into the solid state buffer board.
The buffer board turns the V4 Max into an active solid state preamp with adjustable gain.
The manual for the solid state buffer board can be found in a separate section [** TBD **].
Firmware updating
The firmware updating procedure for both the Max and Mini can be found in a separate section [ ** TBD **].
Operation and control
The detailed procedures for operating and controlling the V4 can be found in a separate section [** TBD **].
Specifications
Click on the sections below to expand and view.