@andrewsrea which mods did you make to your DOD? Sounds like exactly something I would like to do to my board
For reference, I have an early 1980's grey-box DoD 280 compressor. As I understand it, the DoD250 had tweaks in its evolution. I acquired this in 1992 (disassembled), from a friend and revived it / modified in the past few years.
My Mods:
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R5 is the resistor in parallel with the photoresistor of the optoisolator which provides negative feedback to the gain stage, when its internal led gets brighter, from playing harder. I changed this from the stock 3M9 value to a
910K, which tamed it by providing more practical / usable travel of the 'compression' control.
-
C6 is the cap which affects both the attack and release timing, which connects the collector of Q1 & Q2 (tied together) and drains it to ground. The lower the value, the faster it gets and the less it overreacts to bass frequencies which have more energy than higher frequencies. I changed this 47uF -16v cap to a
33uF - 25v. I tried a 22uF and it was nice on the attack, but too fast for me on the sustain decay. I've read some folks going as low as 4.7uF. For reference, R10, the 4K7 which controls the led brightness, connecting the led legs of the optoisolator to Q1, will change the attack, but will also mess with the intensity of the compression. Optoisolators are inherently only so fast in response, in which they let more fast transients in than other types of compressors. Given these two conditions, I found it better to mod R5 & C6 and leave R10 be.
- Changed
IC1 from a TL022 (was extremely noisy, IMHO damaged) to a
TL082 via socket, which was the next low-noise-low voltage evolution of the TL022. The TL082 is FET based and is faster with a higher slew rate, than the bi-polar transistor based TL022. IMHO and in this application, the TL082 is clearer, but not too HiFi.
- Added a
47pF cap connecting pins
#5 & #6 of IC1b. This noticeable reduced the hiss compressors have and the reduced harshness. Still plenty of sweet treble on tap. Note that I only did it on the first gain stage, where the hiss here gets multiplied in gain stage 2, in IC1a.
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Power connector / battery hack. IDK if the original DC power jack was 1/8" or the one that is on it now, which is a switched 5mm x 2.3mm DC jack that mounts into a 3/16"hole. The 2.3mm center pin is not often used, especially in the pedal supply world where 2.1mm is standard, so I built a small lead cable which has a 5mm x 2.3mm plug on one end and a 5mm x 2.1mm DC jack on the other. This way, I can use my pedal supplies (preferred), an external 9v battery or an internal battery (by unplugging the hack).
Hope this helps.
