This mod lets you use CE-300 for synths as the original design is for guitar impedance / levels and connecting a keyboard will easily overdrive the circuit.
Intro
This unit represents the classic Roland chorus sound in some way similar to Dimension series, yet little bit different in modulation. It is still true analog based design with BBDs. The problem however is the signal level and input impedance. This effect expects a low signal source (guitar) at the input and an amp (gain unit) at the output. In typical synth studio the setup is however a little bit different, or somehow reverse so to say. Because we want a unit to accept the full level signal of the synth (to maximize s/n ratio) and then add little or no amplification at the output (unity gain). If you connect a synth with max output gain to the CE-300, you’ll soon overload the input stage, while output will be quite silent forcing you to add gain (and noise) at the console. This mod solves both of these issues and converts CE-300 into a keyboard signal-level unit.
Modification
First thing we want to do is to reduce the input impedance. This will reduce the noise as well. Here is the schematic which shows a R2 1Mohm resistor defining the input impedance for the op amp. Replace it with 10k metalfilm resistor. A better look at the schematic reveals a high pass RC filter made with R2 and C1. To retain the properties of the input filter we need to replace the C1 as well. A value of 4.7uF is adequate. Since you will most likely use the electrolytic cap you need to pay attention to polarity of the cap. Since the signal goes from input to the op amp you should orient positive side of capacitor to the input and negative to the op amp. This finishes the impedance modification.
Next thing is the input gain which is defined by the R3 resistor across the op amp (and the input resistor R4) that takes signal into the op amp. On default unit, this gain is too strong, thus you’ll too easy overload the unit and lose precious dynamic range. By reducing R4 to 10k we are letting more voltage / less noise hit the op amp. And by reducing the R3, we are reducing the gain. Replace both resistors with metalfilm 10k.
Now it’s time to take care of the output stage. Default gain is little bit weak for average console. We will increase the gain by increasing values of resistors R105 and R106 from 47k to 200k. Use metalfilm resistors. Replace C72 and C73 (both 47p) with a 10pF. They are located in parallel to R105 and R106 (which you will be replacing, so it’s easy to locate them). The reason for this is to preserve the output frequency response curve as original. Without this the unit might sound a bit darker (at 10kHz you might already be loosing 1dB, reaching -3dB loss at around 18kHz).
This completes our modifications. Try CE-300 without the mod and then with the mod. Totally different unit. More dynamic range, way less noise. Here are the pictures that should help you locate points where these components are located. C1, R2, R3, R4:
and R105, R106:
