The dwell circuit is modified version of the BYOC dwell circuit, but it takes the output directly from the brick instead of from the reverb pot. This decouples the reverb control from the dwell control, allowing you to crank the dwell at low reverb levels, adding some versatility. The gain of the dwell op-amp is also modified: it goes as high as possible without oscillating. Finally, the dwell path gets its own output from the brick, instead of being split off of the output path. Unlike some of the other dwell controls (Ghost Echo, Tenebrion) the dwell path diverts before the differential amplifier stage that combines the wet and dry signals. Thus the dwell path has no dry signal in it. I find that purely wet dwell paths have a very different character than combined wet/dry dwell paths. Wet-only dwell sounds more ambient and effect-like, and extends the decay time considerably without increasing the perceived size of the room. Mixed dwell paths seem to mostly make the reverb louder. They increase the perceived size of the room, and I find they sound more jumbled and less focused, but more realistic. Both sounds are valid, but I prefer the wet-only dwell because it gives a useful sound that isn't available from the mixed-dwell circuits. The sound is more artificial, but if you want realistic spring reverb you just turn the dwell control to zero.
The damp control is modified ever-so-slightly from the box of hall. It affects the output path only, not the dwell path, so it is fed by only one of the brick outputs instead of both of them. The result is nearly identical at minimum damping, but gives around twice the treble roll-off at maximum damping. When I breadboarded the box of hall, I wasn't sure I needed a damp control. The more I used it, the more obvious it became that the damp control was essential. Reducing treble from the reverb signal is a brilliant way of adding depth and space without calling attention to the reverb. Like a good compressor, you might not even realize it's turned on, but you definitely notice if you turn it off. Brighter sounds are also useful, but are more noticeable as an effect.
The gains of the last op-amp stage were chosen by trial-and-error. I initially chose gains that gave unity to both the wet and dry signal, and found that the dry signal was above unity. So I aimed at a slight gain reduction for the dry signal and got unity. I don't understand why the gains came out wrong, but I accept it. I suggest you breadboard it first, just to be sure.
I built mine with buffered bypass and tails. The layout will accomodate true bypass if you prefer, or you can add a true bypass/tails switch if you want both.
The only down-side I've found is that the circuit doesn't behave well after my amp. I run a Vox AC4TV through a resistive dummy load/line out box, into a LM386-based power amp with an input buffer (like a ruby, but without the bass attenuation), and into a 12" cabinet. I like to set the Vox to edge-of-breakup or higher, and would prefer to put the reverb between the Vox and the power amp, but when I do this the reverb has a strange ascending pitch as it decays. (Maybe I should try buffering Vref?) It behaves normally in front of the amp, but I'd prefer to put it after.
I used a medium brick in my build.
Anyway, I hope someone finds it useful. Sorry about the blurry photos. Comments and suggestions are welcome.
I still have the 100R bypassed, however.
Consider it verified 