Boss DE-200 gain mod for keyboards

To anyone who wanted to use DE-200 with synths, he most likely encountered two major problems which relate to signal levels and input impedance. Those are: output of the effect unit is very low in the volume while the input would overload at slightest increase of the input knob. The unit can be quite difficult (and frustrating) to operated under these conditions!

Here is why. This effect expects a low signal source such as the guitar at its input. Further more, it expects a guitar amplifier at the output. In typical synth based studio the setup is reversed actually. Because we want a unit to accept the full level signal of the synth (synth set at max volume) to achieve maximal s/n ratio. We also want effect unit to add a little or no amplification at the output (also known as Unity Gain). If you connect a synth with max output gain to the DE-200, you’ll soon overload the input stage, while output will be very silent forcing you to add a lot of gain at the console. This mod solves all these issues and converts DE-200 into a keyboard signal-level accepting unit with approximately -10dB level signal at the output (typical for average synth).

Gain Modification
First thing we want to do is to reduce the input impedance and gain. This will reduce the noise as well. Here is the schematic which shows a R10 1Mohm resistor which should be replaced with 10k metalfilm resistor. A better look at the schematic reveals a high pass RC filter made with R10 and C3. To retain the properties of the input filter we need to replace the C3 as well. A value of 4.7uF is adequate. This so far effectively reduces input gain stage from the current +12dB to around +6dB of gain at 10kHz. We will further reduce it across the op amp by changing R2 to 4.7k. At that point we should have input stage set at around +3dB gain which is just fine.

Important: Since you will most likely use the electrolytic cap you need to pay attention to polarity of the cap. Polarity must follow the signal, and as we all know the signal goes from the input jack to the op amp thus you should orient positive side of capacitor to the R1 and negative to the op amp. This finishes the input buffer and impedance modification.

Now we come to the Roland’s pre-emphasis part of the machine built around IC 2b where additional gain is built. Pre emphasis is basically a high pass filter with gain defined by R6, R7 and R8 resistors and corner frequency defined by C5. Our goal is to have near 0 dB of gain of the filtered signal and amplify only the non filtered part. We will achieve that by replacing resistor R8 with 110k ohm value. This effectively reduces gain from 19dB to around 13dB at 10kHz. Or globally, we reduced gain of the input stage for a total of 15dB.

Now it’s time to take care of the mixer / de-emphasis stage. Default gain is a little bit weak for average console, since it is expected a guitar player will us an amp, rather than line mixer. We will increase the gain replacing R159 with 47k and R158 with 220k ohms. It is necessary to replace C109 as well, since we’re working on a low pass filter in here made of R158, R159 and C109 (ignore the C110, it’s purpose is something different and unchanged value isn’t that critical unless you live right across FM transmitter). Same applies to C7. Both can be left at existing values.

This completes our modifications. Try DE-200 without the mod and then with the mod. Totally different units! More dynamic range, much much less noise. If you want more, please continue to read.

Crazy Ivan modification (addendum for experienced)
While calculating component values for the de-emphasis part i came with one idea to try. How would machine sound if we would have the signal partially pre-emphasized and passed thru. The rest was easy. In fact this is uber-simple yet very effective modification for all of you experimenters and le sonique bizarre aficionados. The best part is that it includes one of the components that you already need to replace during the previous gain modification.

In the image above, points A and B mark the place where existing C109 capacitor is, which you need to replace with 1nF as detailed earlier. However, while replacing it, you can solder just one of its legs, then use a wire and lead it to the switch, which can be placed on the front panel. From the other side of the PCB, in the same place you can solder another capacitor, this time 100pF value and again lead it with a wire to the switch. Then you add another wire, from the center point of the switch and lead it to point B.

There’s enough space on the front panel to do the drilling right next to the Power on/off switch. Just measure everything carefully. I strongly advise you to start drilling from the front side, with the plastic panel on. Once you get the small hole done, you can remove the plastic, and use correct width for metal drill so that switch perfectly fits in. The goal of the hole in the plastic is to make it as small as possible yet enough space to move the switch up/down. Please do not place nut on the front plastic. You will make the unit look ugly for no reason. Because there’s a metal plate right below the plastic where a nut can be placed and tightened.

In the image above you can see the Crazy Ivan ON/OFF switch added right next to the POWER switch. The goal of such modification is to make it non destructive (a click of a switch resets the unit to original sound) and of course to maintain the sleek design of the unit.

What this modification does is that it shifts the de-emphasis curve (low pass) from the 600Hz up to the 6kHz. Result are really exotic and make the unit cut through the mix like a razor. Use carefully though, since harsh delays can irritate the listener after a while. Flanges and Choruses with Crazy Ivan are interesting as well and give some extra edge to the sound!! Unit can be used for processing drum beats as well (feedback set to 0) particularly because the signal path contains some exotic elements like a compression, dynamic expansion and the emphasis. The work detailed in this article can be performed under one hour.

Leave a Reply

Your email address will not be published. Required fields are marked *