SSBM.V4 solid state buffer module manual

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About this manual

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As the V4 and its optional buffer module are 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, interfaces and operation of the optional ePotPi.V4 solid state buffer module (referred to as the “SSBM”, “SSBM.V4” or simply “buffer”  ) for use only with the ePotPi.V4 Max stepped attenuator and preamp controller board.

What is the SSBM.V4?

The SSBM.V4 is a solid state audio buffer module that plugs into the ePotPi.V4 Max board in order to convert the Max from a passive to an active preamp/attenuator.

The fundamental purpose of an audio buffer is to act as a current (energy) amplifier rather than the more common voltage amplifier that makes music louder. A buffer typically has a very high input impedance together with a very low output impedance. Thus any source device connected to a buffer’s input will not have to work hard to deliver its audio signal thereby preserving audio quality. Conversely, the output signal from a buffer will have any easier job delivering its audio energy (voltage & current) across the full frequency spectrum – typically to a power amplifier. The net benefit is a more robust and coherent audio signal with more authority, especially during high energy transient passages associated with striking drum heads, plucked strings, or other energetic signals with high initial attack. Simply put, music typically sounds better when properly buffered.

Key features

The features of the SSBM are summarized in the following list.

  • Plug-in board integrates fully with the V4 Max via the three sets of sockets
  • Power on/off and LDR calibration mode controlled directly by the V4 Max
  • Once installed, the LDR module plugs into the SSBM board’s own sockets
  • Direct audio input and output – no coupling capacitors
  • DC offset of outputs are manually adjustable to near zero and remain stable
  • Dual channel socketed 8-pin DIP op-amp input stage | can easily accommodate alternate plug-in op-amps
  • Adjustable gain from unity to +12 dB via plug-in resistor gain modules
  • High performance constant current JFET output stage
  • Powered by external 5 volts from the V4 Max board
  • Two stage internal SMPS DC-DC converter provides split voltages followed by second stage linear regulators for clean +/- 12 volts
  • Optional external higher performance power supply for 5 volts and +/- 12 volts into J3 header

Hardware versions

Version A of the Solid State Buffer Module was released on (date goes here) 2024. The layout of the module and its parts is shown below. Individual parts are identified by their reference designator (J3, U1, C4 etc.)

ePotPi.V4 buffer board prototype pcb
Solid State Buffer Module | Version A Board | PCB

Components on the SSBM board are functionally grouped into 3 hardware sections; 1) audio signal (Red), 2) LDR attenuator (Green); and, 3) power supply (Blue). Each are discussed in more detail below.

Buffer board stack-up

When the solid state buffer module is used, it creates a 3-layer hardware stack as approximated by the illustration below. The buffer module plugs into the V4 Max board and the LDR Module plugs into the buffer module.

V4 stack-up with optional buffer board

The 3 male headers J1A, J2A, & J8 on the underside of the SSBM board are for mounting the SSBM directly to the V4 board via sockets V4.J1, V4.J2 and V4.J8. Please note that the plug-in LDR Module must first be removed from the V4 board before inserting the SSBM board to the V4. Once the SSBM is mounted on the V4 board, insert the plug-in LDR Module into SSBM.J1 and SSBM.J2 same as you would with the V4 board.

LDR attenuator (green section)

The green section outlined above represents the series/shunt LDR module that is normally plugged into the V4 Max stepped attenuator & preamp controller. This is shown on the left side of the simplified schematic below.

SSBM.V4 buffer scheme with LDR attenuator
SSBM.V4 simplified design scheme with V4 LDR attenuator

During normal operation, the output from the plug-in LDR module becomes the input to the SSBM buffer board’s audio circuit as shown in the schematic below. However, during LDR calibration, control signal J8.Cal from the V4 controller, causes relays SSBM.K2 & K3 to isolate the LDR module from the SSBM board for the duration of the calibration process. This is fully automatic and handled by the V4 Max board.

Buffer circuit (red section)

Components within the red section represent the buffer module’s primary left and right channel audio circuits. A simplified circuit of a single channel is shown on the right side of the above schematic.

SSBM PCB Audio Section

The buffer employs a two-stage approach starting with a high impedance non-inverting plug-in op amp (U1) with optional gain. Gain is adjustable up to +12 dB via a plug-in resistor gain module (R2 & R4). The default gain of the SSBM is unity (no gain or 1x). Also, the dual op amp uses a standard 8-pin DIP socket, thus making op amp “rolling” is a real possibility searching for the ultimate edge in performance.

The output stage employs a high performance class A constant current LSK170 JFET discrete transistor together with a split voltage power supply such that the current can be manually trimmed to achieve a near zero volt DC offset at the audio output. The DC offset remains quite stable and rarely needs adjusting but can easily be re-zeroed when needed with a small screwdriver and a simple volt-ohm meter.

This 2 stage split voltage buffer design eliminates to need for coupling capacitors at both the buffer’s input and output thus simplifying the signal path and helping to maintain signal quality and integrity.

The positive and negative legs of the power supply are both separately bypassed using high quality metallized polypropylene film plus low impedance audio grade electrolytic capacitors.

Power supply (blue section)

The blue section of the SSBM buffer board contains most of the power supply components.

The buffer module can be powered two different ways – internally or externally. Selection of internal vs external power source is done manually using jumper header JP1. [ JP1 details needed here].

The “internal” power supply hardware is located in the lower right corner of the buffer board. The “internal” label is somewhat misleading insofar as the buffer still requires an external source of nominal 5 volt DC power which is supplied by default from the V4 Max board through the buffer power header J8. The switching regulator VR1 converts the +5 volts into split +/- 15 volts. These split voltages are then run through linear regulators VR2 and VR3 to produce the low-noise, low-ripple +12 and -12 volts respectively used by the buffers audio circuit.

SSBM PCB Power Section

Alternatively, the SSBM can also be powered entirely externally by setting the SSBM.JP1 jumpers to “External” and providing and external source of 5, +12 and -12 volts through the external power supply header SSBM.J3. Tortuga Audio intends to offer an optional hybrid linear power supply that can provide to very high quality split voltage power for the buffer’s audio circuit.

Physical dimensions

The SSBM board is 2.7 inches per side, the same width as the V4 Max. The overall height of the SSBM when plugged into a V4 Max including the plug-in LDR module is __________ but we recommend allocating ________ inches of total vertical space for the combined V4/SSBM assembly.

Ambient requirements

See V4 Max for requirements.

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