I did a copy and paste in case the page gets taken down:
with Robert Keeley
Howdy Folks! This issue we will look at a few ideas related to modifying digital delay circuits. Many of you have asked for ways to tame the brilliance or "coldness" of the delayed/repeated signal. Let¹s look at a couple of very simple mods that are no real stretch of the imagination, yet are quite fun. Take tone in your own hands; don¹t let designers usurp your tone! Raise that soldering iron defiantly!
The basic idea is simple; soften or affect the output so that it has a different response, possibly sounding more like an analog delay. The no-brainer is just attaching a cap to a feedback loop. On the DD3 by BOSS you can tack the capacitor across the outside terminals of the E. LEVEL control (VR3 on the schematic). I have heard people using a 1uF (use a non-polarized variety) across the pot. The effect of this is very dramatic. It severely limits the repeat time and frequency response of the delays since this signal gets regenerated by feedback. The smaller the cap value you use, the more you return to a stock sound and regeneration increases. Armed with that knowledge, we can come up with mods to please those looking for real tone as well as the artists looking for a low-fidelity, art-noise machine!
One cautionary note: these pedals have static sensitive integrated circuits in them. So watch out combing your hair and rubbing balloons while you work.
Below, Figure 1 shows the above mentioned simplest approach, giving you 3 tones. By using a three-way toggle (On-Off-On), you have the stock DD-3 sound in the center position. The outside two positions give you a response that limits the amount of repeats as well as the frequency response.
A variation on the theme is seen in Figure 2. In this case we have a de-emphasis circuit with an angular frequency w = wo = 1/RC or a cutoff frequency of 3.4 kHz if solely considered. The values I came up with are rather arbitrary and not intended to be a final judgment on what the output should sound like. It¹s a starting point in creating a low pass RC network to attach to VR3 for some high frequency roll-off. Since there are a couple of low-pass filters in the circuit with a cutoff frequency of 7 kHz already, choose a starting point even lower for an obvious effect. What is a low pass filter? At high frequencies, the capacitor 'shorts out' the treble, but hardly affects low frequencies. So, this sound is less 'bright'.
The other variation seen in Figure 2 demonstrates the use of clipping diodes. In this branch of the circuit there are a group of diodes to suggest slightly distorting the signal. There is a blend of silicon and germanium diodes to keep open the idea of experimenting with different flavors and voltage levels that would be clipped. Try just a pair of germanium diodes like the 1N34A to ground for a lot more distortion on the output and let the noise begin! Or, you can combine the two branches in parallel for a tape compression simulation. You can even turn R1 into a 10K or 100K pot and C3 into a 0.47uF or 1uF cap for a simple tone control circuit.
Another idea for those wanting to have it both ways, both higher fidelity in one area and Lo-Fi in another, you can upgrade some of the caps to metal film if you can get them to fit. Examples include C2, C3, C11, C26, C28, C29, etc.
Lastly, for those that might want a different sound for the entire pedal, you can tinker with the De-Emphasis circuit, in particular C10 and R16. Again, this will affect even your original signal when the unit is in delay mode, ON. An expression pedal is possible if you have a volume pedal available and can change the pot in it. You can disconnect VR3 and install a stereo jack on both units, change the pot in the volume pedal to a 50K and connect with a stereo cable to control the effect level. This is helpful if you switch between clean and distorted sounds into the delay pedal; with distortion the repeats can seem much louder than desired and an expression pedal can be helpful.
Hopefully this helps with your investigation into digital delays that can often be found cheap and are ripe for experimentation.
Robert Keeley Electronics Inc.