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Effect pedal for the voice


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Aélis is a DIY effect pedal for processing voice (microphone input) but it can be easily adapted to electric instruments like guitar or bass. This page contains the building instructions.

Originally, it was a birthday gift for Alice, the singer from the SheWolf band. Because I couldn’t find schematics for a decent reverb for the voice, I designed one a we built it with the other band members.

Technically, the circuit is based on a Spin FV-1 processor. We could add other effects since it is programmable. The microphone preamplifier is simple but efficient. However it is lacking phantom power.

Above, the prototype version with a few patches on the circuit. Artwork by Diane Aberdam.


Bill of materials

NameQtyValueSpecific contraints
or model
C5, C13218pCeramic, pitch 2.5mm
C1111nFilm, 7.2×4.5mm² pitch 5mm
C1912n2Film, 7.2×4.5mm² pitch 5mm
C1, C15, C16, C30, C42, C446100nCeramic, pitch 2.5mm
C4, C10, C12, C1843u3Electrolytic, 16V, ⌀ 5mm, pitch 2mm
C2, C3, C14, C17, C45510uElectrolytic, 16V, ⌀ 5mm, pitch 2mm
C41147uElectrolytic, 25V, ⌀ 5mm, pitch 2mm
C40, C432220uElectrolytic, 16V, ⌀ 6.3mm, pitch 2.5mm
C01470uElectrolytic, 10V, ⌀ 8mm, pitch 3.5mm
Q1, Q22PNP transistorPinout EBC, 2N4403
Q3, Q4, Q53NPN transistorPinout CBE, high gain, BC550C
Y1132768Hz crystal⌀ 3.2mm, pitch 1.1mm
RV1110k potInverted logarithmic (C type), ⌀ 16mm, vertical on PCB, 16mm min pin length. Alpha 16mm type 4
RV2, RV3, RV4350k potLinear, ⌀ 16mm, vertical on PCB, 16mm min pin length. Alpha 16mm type 4
SW1112-position switchLorlin CK1049
D10, D222LED5mm, rated current 20–40mA
D11–D199Standard diode1N4148 or 1N4448
D20, D21, D303Schottky diode1N5817
JP1, JP222×1 pin header
R81221%, ¼W
R21, R23, R29, R40, R4151001%, ¼W
R2, R326801%, ¼W
R15, R20, R25, R42, R4351k1%, ¼W
R4, R522k21%, ¼W
R0, R1, R4533k31%, ¼W
R6, R724k71%, ¼W
R22, R24, R26, R27, R28, R50610k1%, ¼W
R10, R11222k1%, ¼W
R9, R12, R13, R14, R445100k1%, ¼W
U1, U42Operational amplifierDual, DIL 8, NE5532
U21DSPSpin FV-1
U31EEPROMI²C interface, DIL 8, 24LC32A min
U51Negative voltage converterLT1054, DIL 8
U6 or U71LDO 3.3VL4931ABD33 (SOIC-8), L4931CZ33 (TO-92) or even LP2950CZ33 (TO-92)
U1, U3, U43DIL 8 socketHollow body in order to put a 100nF capacitor inside
Footswitch13PDT20mm max
Enclosure11590BBMin ext size 94×119.5×34mm³, 2mm thickness. To be drilled.
WireAbout 2mInsulated
Power input12.1mm male jack socketInsulated from the enclosure
Audio input1XLR female socket26×30mm², max 20mm internal depth including the solder lugs. Neutrik NC3FD-L-1
Audio output1XLR male socket26×30mm², max 20mm internal depth including the solder lugs. Neutrik NC3MD-L-1
Knobs for the potentiometers4
Knob for the switch1⌀ 6mm shaft, with screw
PCB1See below

Important notes:

Count about 60€ of good quality components (2017).

Tools: soldering material, cross-head screwdriver, cutting pliers, drill, drill bits from 3 to 12mm for the metal, round file for the metal, a set of wrenches or adjustable wrench, tools to strip wires, insulated tape, metal saw.

Schematics and PCB

Electronic schematics, Gerber files for the print, layouts, FV-1 programs and KiCAD source files are located in the following archive: aelis-2020-06-14.zip (1214kB).

I can provide PCBs at cost price + shipping fees. Please contact me if you are interested. Sorry, all boards are sold out now.



The pedal is put together pretty tight in a 1590BB enclosure. There is no room for a battery, power supply must be external. Holes must be placed as accurately as possible. The XLR sockets must be located behind the enclosure. They pack tightly vertically, make sure they fit well. Use a round file to finish drilling their complex hole.

Dimensions to locate holes for the potentiometers, LEDs, and switches are given on the layout picture.

First solder the SMD components: the FV-1 and possibly the L4931. Solder the remaining components on the top layer of the PCB, sorted by height. Carefully cut the legs of components that will overlap with the potentiometers. Cut the wires to the right length and solder them to the PCB.

Solder the wires to the connectors mounted by the outside (like the XLR). Now the enclosure is assumed finished (drilling, painting, …), because the PCB will be attached permanently.

With cutting pliers, cut the anti-rotation bits from the potentiometers and the rotative switch. With the saw, cut the switch shaft. Adjust its ring in order to limit the range to 8 positions. Fasten all these components to the enclosure. Put insulated tape at the back of RV2 and RV4 to make sure they won’t make contact with the soldered leg ends on the PCB.

Put the LEDs then the PCB. First solder the rotary switch, then the potentiometers and the LEDs. Put the footswitch and solder the remaining wires.

When everything is ready, screw the knobs on the shafts.

EEPROM programming

Because I don’t own a dedicated EEPROM programmer, I made a very simple board using the I²C interface on the Raspberry Pi 3 GPIO (works with any Pi model).

Pull-up resistors are 10kΩ.

Effects were designed with SpinCAD Designer on Windows. They were exported as HEX files and copied on the Pi. From there, I used hextobin, a program that can be found in the archive, to convert the HEX file into a raw binary file. The resulting file is tansferred to the EEPROM with eeprog.

If EEPROM programming is not possible (or for testing purpose), you can still use the FV-1 factory presets by adding a jumper to JP1.

Modification for guitar and bass

The pedal can be adapted to instruments impedance and levels. The PCB is the same, there is only a few components (mainly the preamp section) to remove or change. Check all the pages of the schematics for a complete description of the modifications. XLR sockets should be replaced with female mono jack connectors.


First, adjust the input gain (top right knob). Set it as high as possible without lighting the clipping LED (keep some margin tho).

The middle knob selects the effect. The other knobs have different roles depending on the current effect. Generally, the bottom right knob is a dry/wet mix. For reverb effects, the top left knob is the reverberation time, and the bottom left knob controls the filter.

The effect is lacking proper phantom power support. It is not recommended to provide a phantom power on its output, however this should not damage it.


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