SEPTAR: Audio Breakout Circuit for Multichannel Guitar
SEPTAR V2 – 2015
In 2011, we set out to ‘hack’ MIDI guitar pickup systems and enable access to the audio outputs of the individual strings of a Guitar. These MIDI guitar pickup systems generally sense string activity with piezo transducers, which can be used as accurate microphones if amplified correctly.
There are many ways to create similar functionality (impedance buffering/conversion) as featured in SEPTAR, for instance two quad op-amps would suffice (e.g. TL074).
We chose JFET (as opposed to Op-amp) for several reasons:
1) They use very little current. This is good for battery longevity.
2) They offer the best tone versus some op-amp circuits and are comparable in many respects to vacuum tubes.
3) The signal path is as simple as possible – in comparison to Op-amp based circuits, which contain many hundreds or thousands of internal components. JFETs are simple, offer extremely high resolution, bring out more detail, and sound clear.
A: Resistors R1 and R2 divide the voltage in half. This sets the operating point of the JFET (C). This allows the maximum voltage swing before clipping can occur. These resistors also set the input impedance to a high value required (approx. 1.1 Megohm)
B: Resistor R3 and Capacitor C1 form a low pass filter at 194kHz. To limit the chance of picking up radio interference while just above the ‘studio standard’ of 192kHz.
C: This is the JFET. It carries out the impedance conversion duties.
D: Resistor R4 limits the amount of current that is drawn from the battery.
E: Capacitor C2 blocks the battery supply voltage from passing into the output.
F: Resistor Rz is a pseudo-balancing resistor. Its function is to present a resistance to ground that matches the output impedance of the main circuit. This allows better interfacing with balanced equipment that may follow.
SEPTAR V1 – Parts List Available Below – 2012
(Sold Out – No Longer Available)
The “Septar” board functions as a break out box for polyphonic audio pick-up systems for electric guitar (including those designed solely for MIDI application) to allow audio for each string. The following prototype has been developed with a newly designed circuit to improve audio output, specifically for piezoelectric based Graphtech Ghost Modular and RMC pick-ups, but also functions with the magnetic based Roland GK-3 pick-up system. Audio break out kits are available for users with Graphtech Ghost Modular, RMC, and Roland GK-3 pick-ups.
x 2 – Septar Boards (Drilled)
x 1 – 13 Pin DIN socket (Female)
x 14 – 2m2 Resistor 1 %
x 7 – 82pF Capacitor
x 7 – 6.35mm 3 Pole Jack Sockets
x 7 – Rz 560 ohm 1%
x 7 – 10K Resistor 5%
x 7 – 3K Resistor 5%
x 7 – J201 N Channel JFet
x 7 – 1uf Polyester Capacitor 5mm
x 1 – PP3 Battery Drawer
A step by step construction guide will be made available via email on purchase. The pdf is available here for download. The step by step guide includes instructions on how to wire appropriate power supply for listed Graphtech (9v) , RMC (9v) and Roland (+7v & -7v) pick-up systems.
A series of DSP abstractions created in the open-source object orientated programming environment, Pure Data, will be made available shortly. Multiple instances of each abstraction may be (re)called to attend to the audio of each string. It is suggested that each user downloads Pure Data Extended. Excellent Pd tutorials can be found here, here, and here.
The following DIY kit has been tested thoroughly with Graphtech’s Modular Hexpander, RMC, and Roland’s GK-3 pick-up systems only. We recommend that you have some soldering experience. Please ensure that the relevant pick-up system is compatible with the available kit before use, and that you follow our step by step construction guide carefully and correctly. We recommend the use of a voltage meter to ensure the voltage polarity is correct before attaching the relevant pick-up system. The breakout box should always be connected to a guitar with an appropriate pickup system, and never to any other equipment. We recommend that you have some soldering experience. We are not responsible for any damage caused.
I would encourage all users to contact me directly if you have any issues with the kits, given that this is a personal project. This is a very small operation, which serves the purpose of academic research. There is no money behind it. All design, parts and postage are covered by the time and money of the two individuals involved (myself and an engineer). The kits are sold as a beta project, not as a polished professional corporate product (although perhaps it may result in a more professional product, which is heavily dependent upon the feedback we receive at this stage of the research). In terms of pricing, we can assure you that we have made the kit as affordable as possible. We have to take into consideration the time spent on the construction of each circuit board. If you believe you are able to source parts at a cheaper price in your home country or elsewhere, you are welcome to source the parts by yourself and we may then arrange to send the parts which you require at a discounted price. Professional packaging is currently unaffordable, hence the reuse of existing packaging from parts. Please bear in mind the kits are coming from the U.K. The largest concern is that all parts and components should reach you safely and in good condition. If this has not been the case, you should contact me immediately. Please also note it does not list anywhere on this website that paper instructions will be provided. This was not advertised at any point. Instructions are made available for download and are sent via email to each individual who purchases a kit. So, please, for whatever reason, do not expect paper instructions.