Digitech 2112/2120 Stereo Effects Loop Mod

Mark Gibson on the 2120-users mailing list brought up the idea of adding another FX loop to the 2120, after the distortion stages. After evaluating this idea and my 2120, it seems that it is doable quite easily.

Findings

• A/D = CS5335KS
• stereo inputs, 2 dual OPs (LF353) for anti aliasing LP filters
• install backdoor before dual LP filters
• adding SPDIF input != rocket science :-)

Applications

• pan tube and dist to the extreme left and right
• use dual or stereo effects, or even parallel paths to process 2 mono signals in stereo.
• morph between dual mono signals!
• use tube or SS dist on one channel and process it in stereo, insert external mono signal e.g. guitar synth on the other channel and process it in stereo, and mix both in the output mixer stage (wow!)
• create the pitch/arpeggio which I have been missing since I sold my SPX90, by using pitch/delay w/o feedback, and looping it into one or both inserts. This may require an external mixer or loop switcher.

Caveats

• no SW inside the 2120 to control the backdoor
• you have to use an use external switcher & DPDT relay / 4053.
• use 1 more dual OP to set line level to -10 dB or +4 dB (A/D converter takes 2V RMS for full scale)

Options

• we can build the Fx loop circuitry to allow for a 50/50 mix of the dist and external signals like the GSP2101.
• The Fx loop can also be located between the distortions and the normal Fx return, after the signal is split for the 2 distortion paths. This would allow to apply some distortion to a guitar synth or something. Actually, I prefer this idea in the meantime.
• If we manage to find the right traces, I'll make a small PCB (100x80 mm approx.) with 3 1/4" TRS jacks (Fx send 2-channel out, Fx rcv 2-channel in, 2 footswitches) and the -10/+4 dB level switch, pretty straightforward. The good news is if someone fails to appropriately cut the traces you can simply re-solder the cut traces and forget about it. I like to make reversible mods :-) The other good news is that the traces are very likely to be located on the component (i.e. upper) side of the PCB, i.e. they are easily accessible.

Please note that such a modification voids your warranty! You have been warned.