status Draft
drc ok
erc ok
unconnected ok
Test: 5/5


Summe is a simple mixer utility module. It can be used as a mixer, attenuator or attenuverter for AC and DC signals.


This is a preview of the elektrophon website, the content may not be accurate and is subject to change without any notice.
Summe is a simple DC mixer that allows various applications. The input signals can be simply added. It is also possible to invert the input, which results in a subtraction of the signals. The first input has a default DC voltage. If nothing is connected to this input, a DC offset is applied to the output.


This is a work in progress module and might not work as expected. Feel free to file a bug report on the github page.


The function depends on the patching and settings of the potentiometers. When the potentiometer is at the centre position, the output signal is muted. No output signal is present. When turning to the right, the output will get louder until the output signal is equal to the input. When turning the potentiometer to the left, the output signal will be phase-shifted by 180°.

The first input is wired to a default DC voltage of +5V. With this input is not connected this voltage will be added as an offset to the mixed signal. With the potentiometer, the offset can be adjusted from -5V to +5V.

The output jacks are switched. When something is plugged into an output, this output is removed from the summed output. This channel is a simple attenuverter.

Figure 1: Summe Block Diagram

One interesting thing about the circuit is the attenuverter section. It uses an OPAMP in a differential configuration[1]. The resistors $R_3$ and $R_4$ are replaced with a potentiometer to make them adjustable.

Figure 2: Attenuverter Schematic

$$ \begin{array}{c} Vout = \Bigl(1 + \frac{R2}{R1}\Bigr) \Bigl(\frac{R4}{R3+R4}\Bigr) * V2 - \Bigl(\frac{R2}{R1}\Bigr) * V1 \end{array} $$

Above is the formula of the OPAMP differential amplifier. There are also the Resistors $R_5$ and $R_6$. These resistors make the response of the potentiometer logarithmic [2]. To calculate that these Resistors are in parallel to the Resistors $R_3$ and $R_4$.

$$ \begin{array}{c} Vout = \Bigl(1 + \frac{R2}{R1}\Bigr) \Bigl(\frac{\Bigl(\frac{R4*R6}{R3+R5}\Bigr)}{\Bigl(\frac{R3*R5}{R3+R5}\Bigr)+\Bigl(\frac{R4*R6}{R3+R5}\Bigr)}\Bigr) * V2 - \Bigl(\frac{R2}{R1}\Bigr) * V1 \end{array} $$

Figure 3: Attenuverter Potentiometer Sweep

The calculation shows the Result with a 5V input and a potentiometer sweep from 0-100%. The last curve shows the logarithmic result simulated with spice. It overlaps the calculated line.

It may be that the summing amplifier is not stable and self oscillates. In this case, small capacitors can be added to stabilize the signal.



2C1 C222uAluminium Electrolytic Capacitor, 35V, D=5 mm, LS=2 mm
12C3 C4 C5 C6 C7 C8 C11 C12 C13 C14 C15 C160.1uMultilayer Ceramic Capacitors MLCC
2C9 C10100pMultilayer Ceramic Capacitors MLCC
4D1 D2 D3 D4LED (Green/Yellow)LED 3mm bipolar
1D5LED (Red/Yellow)LED 3mm bipolar
1J1IDC HeaderPin Header IDC, 10 Pos, 2.54mm
4J2 J3 J4 J5IN3.5mm Eurorack Jacks
5J6 J7 J8 J9 J10OUT3.5mm Eurorack Jacks
2R1 R210Metal Film Resistors - Through Hole (L=3.6 mm, D=1.6 mm, 1%)
8R3 R4 R5 R6 R7 R8 R9 R1047kThick Film Resistors - SMD (0805)
15R11 R12 R13 R14 R15 R16 R17 R18 R24 R25 R26 R27 R31 R33 R36100kThick Film Resistors - SMD (0805)
5R19 R20 R21 R22 R391kThick Film Resistors - SMD (0805)
1R238.2kThick Film Resistors - SMD (0805)
5R28 R29 R30 R32 R34500Thick Film Resistors - SMD (0805)
4RV1 RV2 RV3 RV4100kAlpha 9 mm Pots – Vertical
1U1LM4040BIM3-5.0Precision Micropower Shunt Voltage Reference
6U2 U3 U4 U5 U6 U7TL072Low-Noise FET-Input Operational Amplifiers (SOIC-8 )




main panel

Figure 5: Opamp Failure


The first input jack is wired to 5 volts when nothing is connected.

All channels are mixed to the out jack. when something is connected to the channel out, this channel is removed from the overall mix.


  • connect the different channels from audio or cv sources to the in jacks.
  • connect the out jack to something
  • turn the pots clockwise to adjust the volume.


  • connect all or a single channel.
  • when the pot knob is turned counterclockwise, the signal is inverted.


212 Nov 2021Bipolar LED and redesign of PCB
11 Apr 2021initial commit of project


  • eCircuit Beginners' Guide to Potentiometers
  • ESP Beginners' Guide to Potentiometers