Modification of module ARP1016 Behringer

Yves Usson (yusynth), feb.2022

Herein I explain how to modify its Behringer 1016 module to correct the design bug of the ARP1016.

Required level :

    To understand the principle: good level in electronics
    Practical realization: accessible for anyone who knows how to weld (braze)

1-Foreword

A friend of mine is a lover of the ARP2500. Given the rarity of the instrument and the astronomical price he would have to pay to acquire an authentic one, my friend turned to Behringer's series of Eurorack modules cloning the ARP2500 modules.

Upon receipt of each module (acquired on a monthly basis) he asked me to assess the quality of these clones. The result is very convincing but for the ARP 1016 module (Dual noise generator/Random) I was appauled by what happened as soon as one activated one or the other of the 4 output level adjustment potentiometers (noise or random voltage). For example for white noise, the output sound disappears as soon one turns the potentiometer to reappear at the new level a quarter of a second after you have stopped turning the potentiometer. This creates an unpleasant "pumping" sound effect when one turns the potentiometer. This being valid for the two noise generators and the 2 random voltage generators.

Very surprised by this behavior, I began to wonder about the quality of the cloning work of the company in question. The behavior observed led me to think that the problem would come from a capacitor in the audio path which would be subject to a variation of a DC component when the potentiometer is operated. I therefore studied the original diagrams as drawn by the company Tonus Inc. (the B. clone conforms with these diagrams) and I found there a design error explaining this behavior. What surprises me the most is that no one among the owners of the original ARP2500 has complained about this defect even though it is indeed present. As for Behringer, they perfectly cloned the module with its original defects!…

So I identified the problem and developed a modification to fix it. I describe below the practical realization of this modification on the Behringer 1016 module. This modification requires only a few additional components (4 resistors and 4 capacitors) and "light surgery" on each of the potentiometers.

2 -Analysis and diagnosis of the problem


Figure 1: Original schematic of the 1016 module by Tonus Inc. Only a noise and random voltage generator is shown.

The second generator is identical to the first one.

Analysis of the diagram (figure 1) reveals a "gross" design error: if we look at the different parts of the circuit surrounding the volume adjustment potentiometer R1, we see upstream a transistor amplifier circuit (Q3 and Q4 ) at very high gain and downstream a coupling capacitor C7 of 1µF routing the signal to the output operational amplifiers (Z1 and Z2). The problem lies in the fact that, probably wanting to save on components, the engineer who designed the circuit used potentiometer R1 both as the bias and load resistor of transistor Q4 and as a voltage divider to adjust the volume of the signal. The problem is that on the transmitter of Q4 we find the amplified signal (with an amplitude of around 100mV) on which is superimposed a strong DC component (about 10V approximately!). So on the slider of R1 we find this DC component more or less attenuated depending on the position of the potentiometer and superimposed on the much weaker signal. When the potentiometer is turned, the DC voltage on the potentiometer cursor will vary during the rotational movement applied to potentiometer R1. One could then believe (and this must have been the designer's reflection) that capacitor C7 will eliminate this DC component before transmitting the signal to Z1 for amplification. The problem lies in the fact that if this configuration works as expected in the static phase (when the button is not turned), it poses a real problem in the dynamic phase (when the button is turned). As soon as R1 is turned, the DC component which arrives at C7 varies and therefore C7 must rebalance the electrical charges at the level of its electrodes, this creates the appearance of a variable DC component downstream of C7 which will saturate the stages of amplification that follow and make the signal disappear until the charges are again balanced on the electrodes of C7 and therefore the DC component downstream of C7 disappears.

3-A solution

If the movement of the potentiometer R1 influences the load of C7, it is necessary in some way to block the DC component before arriving at R1. The solution is simple, for that one must insert a capacitor of 10µF between the emitter of Q4 and the potentiometer R1. However, doing so removes the Q4 emitter bias-and-load resistor, but this alone would cause Q4 to malfunction! To avoid this pitfall, one must add a 10K resistor between the emitter of Q4 and the 0V (ground) of the circuit. The "pumping" effect during the rotation of the potentiometer has therefore disappeared because R1 and C7 no longer see a DC component. Figure 2 shows the modified circuit diagram for module 1016. Added components and wiring changes are outlined in pink/magenta.


Figure 2: Modified diagram of module 1016: the areas in pink indicate the elements added to the original diagram.

It now remains to physically carry out this modification on the module itself.

4-Practical realization

The implementation of this modification is relatively simple to carry out, however it requires dismantling the module from its facade, cutting in half the right leg of the four potentiometers (see figures 3 and 4, the printed circuit before modification) and soldering components (4 electrochemical capacitors and 4 resistors).

WARNING: by carrying out the following operations you cancel the guarantee which covers your module. So if you make this modification, it is your responsibility and only yours that is engaged. If you are not experienced in electronics or unsure of yourself, do not do it, or ask a competent professional to make it .

Figure 3: Top face of the Behringer circuit board (with the faceplate removed)

Figure 4: Underside of the Behringer circuit board
The modification is carried out both on the upper face of the printed circuit (fig.3) on which the additional capacitors will be inserted, and on its lower face (figure 4) on which the additional resistors will be soldered.

4-1. List of necessary components and tools:

  •     4 x 10µF/35V Radial Electrochemical Capacitor (5mm diameter max, 15mm height max)
  •     4 x 10K resistor (brown, black, orange — gold; or brown, black, black, red — brown)
  •     1 small cutting pliers or a Dremel milling cutter
  •     1 soldering iron for electronic circuits
  •     1 small screwdriver

4-2. First step, identification and dismantling:

  •     Remove the potentiometer buttons (4)
  •     Unscrew the 4 fixing screws holding the printed circuit to the front spacers
  •     Store buttons, screws, and front in a box for the time needed for the modification
  •     Locate the modification points: the 4 potentiometers (fig. 3) on the upper face and the solder pads of the potentiometers on the lower face of the printed circuit (fig.4).
Figure 5: View of a potentiometer before modification: we will operate on the leg on the right of the potentiometer.


4-3. Second step, inserting the capacitors:

  • Locate a first potentiometer on the upper face of the printed circuit (figure 5).
  • With small size side cutters cut its right leg (when the potentiometer is seen from above with the legs facing down) into two equal pieces, one still attached to the body of the potentiometer, the other still attached to the printed circuit board (see figure 6).
  • Using a flat screwdriver, bend the 2 half tabs outwards (fig. 6).
  • Using the soldering iron, tin the free ends of the 2 half-legs.
  • Shorten the legs of the capacitor by 10µF with the cutting pliers so as to leave a leg length of 1 cm. (fig.6).
  • Tin the free ends of the capacitor leads.
  • Place the capacitor to solder it on the half-legs, choose an orientation perpendicular to the 2 half-pins so that the body of the capacitor is in a space with little clutter in components (see figure 8), with the negative pole of the capacitor turned upwards and the positive pole facing the printed circuit.
  • Solder the legs of the capacitor to the 2 half legs, the negative pole of the capacitor (marked by a white or gray band) to the half leg connected to the potentiometer and the positive pole on the half leg connected to the printed circuit (see figures 6 and 7).
  • There you have inserted the first capacitor (fig.7) you must now repeat the previous operations to insert the other three capacitors on the other three potentiometers (fig.8).
Figure 6: Insertion of the 10µF capacitor at the right leg of the potentiometer.
– Step 1: split the right leg of the potentiometer in two (with cutting pliers or with a Dremel cutter).
– Step 2: fold the two leg halves outwards.
– Step 3: cut the legs of the capacitor to a length of 1 cm.
– Step 4: solder the capacitor, negative pin connected to the leg half connected to the potentiometer and positive pin connected to the leg half connected to the PCB.


Figure 7: Detail of the insertion of the electrochemical capacitor at the level of the right leg of the potentiometer.


Figure 8: Photos showing the modification made for two potentiometers by trying to optimize the position of the capacitors in relation to the space available on the plate.

4-4. Third step, adding resistors.

This step is relatively simple and easy to perform.

Prepare the 10K 1/4W resistors, cut the legs to a length of 1 cm and bend these legs at 90° in relation to the axis of the resistor body.
On the underside of the board, locate the potentiometer solder pads (groups of three pads, see Figure 9).
Do the same for the other three resistors (fig. 10).


Figure 9: Underside of the plate: soldering of the 10K resistors on the right and left soldering pads of the potentiometers. In red: symbols of the locations where to solder the other 3 resistors.

Figure 10: Two resistances in place, it remains to solder the last 2 (location indicated in red) on the pads of the other two potentiometers.
That's it, all you have to do is screw the plate back on the front panel, put the potentiometer buttons back in place.