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POLYKIT SYNTH DIY

Eurorack Spring Reverb Driver

Eurorack Modular Spring Reverb Effect Through-Hole Kit
Table of Contents

This spring reverb module is part of my Eurorack series. It drives a reverb tank and has two inputs, control for dry/wet to blend between original and processed signal, and a resonance control for feedback.

  Buy kit or PCBs in my store

Key features
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  • spring reverb driver
  • two inputs
  • dry/wet control
  • resonance control
  • PCBs/panel and full DIY kits available
  • easy to assemble
  • 6 HP

How it works
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Two input signals are mixed together, and there is a dry/wet control to blend between the original and processed signal. The reverb output signal is fed back to the input and can be controlled by the resonance pot.

Two op-amps are connected in parallel to drive the reverb tank. I’m using a NE5532 op-amp, which has a higher output power than the TL074 to deliver enough current.

Using two electrolytic capacitors back-to-back (C2/C3 and C7/C8) is to have a non-polarized capacitor with half the value, around 10uF in this case.

You need a separate reverb tank; it is not part of the kit. This module is well tested with the Accutronics AMC2BF3/Intellijel small reverb tank but should work with any tank that has an input impedance of 150 ohms or higher. Make sure that the input coil is not grounded to the tanks casing, or it won’t work. There are two 2-pin Molex connectors to connect the tank to the module.

R9 can be adjusted to change the gain of the amplifier’s recovery circuit, a lower value give a lower gain and resonance.

Assembly
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The build is pretty straight-forward. The module only consists of through-hole components and is easy to assemble. There are three boards: a front panel, a board for controls/jacks, and a board for the circuit.

First, solder all parts on the component board. Keep the jacks and potentiometers for the end. Begin with the components with the lowest profile and work your way up. Screw the jacks and potentiometer to the front panel before soldering them. All components have a fixed designator and a component value printed on the board. Additional information on the parts can be found in the BOM below.

See the Component Basics guide for help identifying parts.

BOM
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Link to interactive BOM of the component board

ReferenceValueFootprintQtyDescription
C1100nCapacitor_THT:C_Rect_L7.2mm_W2.5mm_P5.00mm_FKS2_FKP2_MKS2_MKP21Film
C2,C3,C7,C822uCapacitor_THT:CP_Radial_D5.0mm_P2.50mm4Electrolytic
C41nCapacitor_THT:C_Rect_L7.2mm_W2.5mm_P5.00mm_FKS2_FKP2_MKS2_MKP21Film
C5,C10100nCapacitor_THT:C_Disc_D5.0mm_W2.5mm_P2.50mm2Ceramic
C6,C1110uCapacitor_THT:CP_Radial_D5.0mm_P2.50mm2Electrolytic
C91nCapacitor_THT:C_Disc_D5.0mm_W2.5mm_P2.50mm1Ceramic
CONN1,CONN2Conn_01x04Connector_PinSocket_2.54mm:PinSocket_1x04_P2.54mm_Vertical24-pin socket, 2.54mm
POWER1Conn_02x05_Odd_EvenConnector_IDC:IDC-Header_2x05_P2.54mm_Vertical1IDC 10-pin power connector
R12k2Resistor_THT:R_Axial_DIN0207_L6.3mm_D2.5mm_P10.16mm_Horizontal11/4W 1% resistor
R2,R5,R10,R1110Resistor_THT:R_Axial_DIN0207_L6.3mm_D2.5mm_P10.16mm_Horizontal41/4W 1% resistor
R3,R7,R8,R17,R18100kResistor_THT:R_Axial_DIN0207_L6.3mm_D2.5mm_P10.16mm_Horizontal51/4W 1% resistor
R4,R1310kResistor_THT:R_Axial_DIN0207_L6.3mm_D2.5mm_P10.16mm_Horizontal21/4W 1% resistor
R6,R14220Resistor_THT:R_Axial_DIN0207_L6.3mm_D2.5mm_P10.16mm_Horizontal21/4W 1% resistor
R922kResistor_THT:R_Axial_DIN0207_L6.3mm_D2.5mm_P10.16mm_Horizontal11/4W 1% resistor
REVERB_IN1,REVERB_OUT1Conn_01x02Connector_Molex:Molex_KK-254_AE-6410-02A_1x02_P2.54mm_Vertical22-pin Molex connector, 2.54mm
U1TL074Package_DIP:DIP-14_W7.62mm_Socket_LongPads1Quad operational amplifier, DIP-14
U2NE5532Package_DIP:DIP-8_W7.62mm_Socket_LongPads1Dual operational amplifier, DIP-8

Link to interactive BOM of the control board

ReferenceValueFootprintQtyDescription
CONN1,CONN2Conn_01x04Connector_PinHeader_2.54mm:PinHeader_1x04_P2.54mm_Vertical24-pin header, 2.54mm
DRY_WET1,RES1100kPotentiometer_THT:Potentiometer_Alpha_RD901F-40-00D_Single_Vertical2Linear, vertical, threaded, 6mm
J1OUTConnector_Audio:Jack_3.5mm_QingPu_WQP-PJ398SM_Vertical_CircularHoles1Mono Thonkiconn 3.5mm jack sockets
J2IN1Connector_Audio:Jack_3.5mm_QingPu_WQP-PJ398SM_Vertical_CircularHoles1Mono Thonkiconn 3.5mm jack sockets
J3IN2Connector_Audio:Jack_3.5mm_QingPu_WQP-PJ398SM_Vertical_CircularHoles1Mono Thonkiconn 3.5mm jack sockets
R11kResistor_THT:R_Axial_DIN0207_L6.3mm_D2.5mm_P10.16mm_Horizontal11/4W 1% resistor
R2,R3,R4100kResistor_THT:R_Axial_DIN0207_L6.3mm_D2.5mm_P10.16mm_Horizontal31/4W 1% resistor

Schematics
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Changelog
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rev2: Change R9 from 10k to 22k

References
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https://sound-au.com/articles/reverb.htm

https://kassu2000.blogspot.com/2015/10/spring-reverb.html

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