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Boss DD-20 Mods and Observations

Discussion in 'Effects [BG]' started by 12bass, Dec 13, 2009.

  1. Recently I became interested in improving the sound of my DD-20 Giga Delay. To that end I replaced the original cheap/low performance M5218 and NJM2100 op amps with modern low noise/high performance OPA1612 and OPA2365 respectively. Additionally, I added film bypass capacitors across the electrolytics which couple the various analog stages. Bypass capacitors help retain transparency in AC coupled circuits.

    The end result is that the DD-20 now sounds very close to the unaffected signal (when using a true bypass A/B switch for comparison). Before the mods it seemed to compromise the tone somewhat. It still loses a very slight amount of clarity, but the difference is unlikely to be noticed in a gig situation.

    I also did some experimentation with placing the DD-20 before the input of my Eden WT-500 and using it in the effects loop. When used before the amp, the DD-20 adds a noticeable amount of background hiss, even in bypass mode. This noise increases as you turn up the effect level. I was hoping to reduce the background noise with the low noise op amps, but I'm not sure how much effect they have had. At some point I would like to compare my modded DD-20 to another stock unit.

    What I found, however, was that placing the DD-20 in the Eden's effects loop reduced the hiss considerably, such that it is hardly noticeable. Thus, for lowest noise, I would recommend using this particular pedal in an effects loop, if possible.

    Another finding is that, when used in mono, the DD-20 sounds better using the "B" input and output. My suspicion is that the "A" input gets split and fed into the "B" input when used in mono, and that this reduces the input impedance by half, thus loading down the input and dampening the signal somewhat.

    For those who are curious, the A/D-D/A converter is an AK4552 (IIRC), which is an AKM 24-bit/96 kHz CODEC with decent, but not stellar, specifications. No idea of the sampling frequencies/bit depth used in this application. My suspicion is that most of the "sound" of this pedal comes from the analog circuitry, not the digital sections.
  2. After some more testing I found that the DD-20's -20 dBu setting is slightly overloaded on transients when placed in the Eden's effects loop. Switching to +4 dBu (to access: hold down on/off pedal at power up) cleans things up, with a slight increase in the noise floor.
  3. fretlessrock

    fretlessrock Supporting Member

    Aug 8, 2002
    I'm a big fan of the DD-20, and it was the heart of my looping rig for a long time (paired with a RC-20XL). I can say that I just played it the way it came out of the box and didn't worry about tweaking any hardware. I don't think I ever put it in the fx loop, but the difference in signal levels and impedance are probably causing most of the difference. Interesting note on the overloading. I always had some kind of buffer device in front of it and noticed that it didn't like anything too hot on the input. That sensitivity switch trick came in handy a few times. What I'm not sure of is if it alters the input sensitivity and output, or just one or the other. I've seen the settings described as both.

    Thanks for the post and keep posting any other findings as they come about.
    Driven Crane likes this.
  4. syndroam


    Sep 30, 2009
    Had a DD20, had to let it go because it messed up my signal!!! When it was in my chain I had to turn up my bass volume twice as much as i normally would.
  5. B.C.

    B.C. Supporting Member

    Jun 28, 2005
    I had one but sold it. Loved the sound, but just not the delay interface I was looking for. I think I'm better off with a DD-7 or something like it.

    Nice mods my friend. Typically when I am modding a pedal, the input op amp(usually used as an impedance buffer or something of that nature) is the first I go for, with the input caps a close second.

    Typically I would replace electrolytic with film if at all possible, for noise and lifetime improvement.

    You probably could, if you are very careful, swap A/D converters. Now that would be really interesting :smug:
  6. Quick update:

    Recently I was finally able to get a hold of the DD-20 schematic. This makes it much easier to see what is going on in the circuit and where best to make modifications. After noticing that the modded DD-20 was using around 230 mA with the new op amps (stock DD-20 uses around 175 mA), along with finding that the bypassed tone was still a bit veiled, I thought it might be worth having another look. A friend and I tried to see if we could find any obvious oscillation using an oscilloscope, but nothing was immediately apparent.

    In light of the high current draw, I replaced the three OPA1612s with OPA1642s, which have JFET inputs and use less quiescent current. The modded DD-20 now uses around 185 mA, or a little more than stock. Also, I think that the low noise FET op amps may have reduced the hiss slightly. Additionally, I reduced the values of the capacitors in the feedback loops of the op amps, which opened up the treble response somewhat. Now the pedal's buffered non-delayed audio path is quite flat up to 20 kHz and sounds only slightly different than true bypass. The delay path remains somewhat rolled off in the upper treble, but this appears to be due to design limitations.
  7. ShortBtwnHdset


    Jan 24, 2014
    Hi All!
    I'm hoping to resurrect this thread, as I'm very interested in modding this delay, and new info is available. First off, schematic now available here


    Also, Film caps have gotten much smaller, so I may be able to substitute these for the electrolytics in the direct signal path. c58,c59,c13,c34,c118,c129,c4,c8,c12,c18,c28,c6,c20. Higher voltage rating, but think they would work. Could also work for c31,c35

    MKS2B051001N00MSSD WIMA | Mouser

    So, a few questions. What cap is being used across the op-amp chips? Also, there are other caps that have one side touching the direct signal chain, or are going across signal chain devices. Could those be replaced (would there be a benefit)?c2,c16

    I know it will probably be hard to get 12Bass back onto this thread, but could really use some input. Thanks!!!
  8. First off, what is your goal in making modifications? It would be good to know that before deciding what to change in the circuit. Another thing to bear in mind when making modifications is that the PCB traces can be fragile, so one must be careful and keep reliability as a major priority. No sense making a slew of modifications and then having the pedal fail at a gig.

    Basically, I left the existing electrolytics* in the signal path and bypassed with polypropylene film, replaced some of the op amps with high quality parts and reduced the HF filtering to open up the frequency response. After modification, my DD-20 sounds more transparent when bypassed and perhaps a bit clearer when activated. There is still more background hiss than I would like, but there does not appear to be any easy way to address this given the design. For one, I suspect that the A/D-D/A converters are partly to blame.

    Not sure what you mean about caps across the op amps; I added 0.1 μF film across the supply pins.

    *I may have replaced some of the electrolytics with high quality Panasonics. I am unable to check at the moment.
  9. ShortBtwnHdset


    Jan 24, 2014
    Hi 12bass!

    Intent is to hopefully make the pedal less "dark" sounding, open up the transparency a little, both when bypassed and active. So, here are a few questions that might help me.

    1.When you replaced the op-amps, did you use desolder braid or did you have special hot-air station? I realize the traces are fragile, wondering what your methodology was for replacing them.

    2. The film caps that you piggybacked onto the op amps. Did you go from pin 4 to pin 8 (V- and V+)? Thanks for the value, by the way.

    3."Basically,... ...and reduced the HF filtering to open up the frequency response."

    I take it your talking about c1,c15 on IC1, c10,c30 on IC3, c11,c32 on IC5 (what about c119,c131), and c3,c17 on IC4. Do you remember which of these you changed, and what values?

    I am pretty handy with a soldering iron, but by no means have anything but a basic understanding of electronics. I spent many years selling and setting up high end audio systems, and am well familiar with the improvements that can be had sonically with replacing/bypassing electroytics with film caps. But I'm no electronics engineer, so what values to change is beyond my scope. I'll probably start with just changing the electrolytics out.
  10. Unfortunately, I'm at the road at the moment and it is not convenient to look at schematics.

    To remove SOICs, I quickly create a blob of solder on each side of the chip, then quickly alternate heating until both sides are molten, then slide the chip off of the board. This works fairly well, but takes some finesse. A proper desoldering station would be better....

    Yes, caps go across the power rails. Some recommend ceramics for this job.

    With regard to the low pass filtering, basically just follow the signal path and look at the capacitors in the feedback loops of the various op amps. For the bypass section, I used very small caps, perhaps 22 pF or less. I chose silver mica caps as they sound better to me than ceramics. Also, the delay section seems to require some low pass filtering to avoid aliasing and prevent the clock signal from passing to the output. This means that the delayed signal will always be somewhat darker than the straight signal. It may take some trial and error with values. As it stands, the modded DD-20 produces some aliasing artifacts when I pair it with my DIY A/DA flanger.
    alec likes this.