The Disappearing Act: Ducking Spring Reverb

[Ripthorn]

Hey everyone, I recently finished my Disappearing Act design. This is an all-analog spring reverb driver with optical compression/ducking. The spring reverb driver section isn't terribly novel, but combining it with a full-wave rectifier envelope follower and optical compression on the reverb signal makes it so that you can get the studio effect in a pedal. It is designed for an 800 Ohm tank, so there are tons of options for tank sizes and decay times available. I have found it to be a ton of fun. You can read my technical walkthrough and get the Gerbers, BOM, and build documents here: https://scientificguitarist.wixsite.com/home/disappearing-act.I hope you enjoy it!

[PRR]

No bias at IC1A?

R7's value seems odd.

[John Lyons]

Sounds great!

[Ripthorn]

No bias at IC1A?

R7's value seems odd.

It's dual supply. That makes the envelope follower configuration a lot easier to deal with.

R7 was determined through breadboarding. I was surprised as well, but that's what was needed to make it work right. I think it has to do with the fact that the 800 ohm tank is sort of in parallel with it. Smaller values resulted in distortion that I didn't want.

[Mark Hammer]

There was a project in ETI about 40 years back that incorporated an optical compressor just ahead of the spring-pan driver, to avoid hitting the springs too hard.  I keep meaning to build it but never got to it.

But kudos for bringing up the idea.  It is a deserving one.

[ElectricDruid]

I like the project a lot, but that sound of "hitting the springs too hard" is exactly why I like spring reverb. It's got that "twang" that you just can't get any other way. I've heard plenty of digital emulations, but none of them get close to a fairly crap spring hit by signal that's clearly too big. Imagine your reggae organ off-beat chord stabs without it - unthinkable!!

Sometimes (ok, quite often!) the failures of the technology become the features that we're trying to copy.

[Ripthorn]

I like the project a lot, but that sound of "hitting the springs too hard" is exactly why I like spring reverb. It's got that "twang" that you just can't get any other way. I've heard plenty of digital emulations, but none of them get close to a fairly crap spring hit by signal that's clearly too big. Imagine your reggae organ off-beat chord stabs without it - unthinkable!!

Sometimes (ok, quite often!) the failures of the technology become the features that we're trying to copy.

There are certainly cases where you want that big initial transient, but just about any real spring reverb can do that. I wanted something would do that and more. Very true about "flaws" becoming features.

[Mark Hammer]

The ETI project is here:  https://worldradiohistory.com/CANADA/ETI/80s/ETI-1983-07-Canada.pdf
Interestingly, it provides some feedback from the recovery stage back to the driver stage to essentially extend the decay of the reverb.  The limiter circuit (which is optional and has a variable limit control) detects the signal amplitude AFTER the feedback is incorporated, but applies the signal reduction before that stage.

The appreciation shown by others here for what happens when you smack the springs is well-founded.  But it is based on circuits that do not apply feedback from output to input.  I would imagine that smacking the springs AND feeding that back is not exactly the sort of runaway repeat one wants.  So applying some limiting when using more feedback strikes me as prudent.  If not applying any or much feedback, then slam away and leave the limiting off.

[PRR]

No bias at IC1A?....

It's dual supply. That makes the envelope follower configuration a lot easier to deal with....

I mean not even a resistor to a zero bias reference, ground. Few opamps will drift to zero naturally.

[patricks]

That's such a clever project!
I love the walkthrough on your website and the demo on YouTube, well done.
It's so easy to dial in the "sweet spot" where the reverb's working but not getting in the way of your playing, and really thoughtful touches using the second LED as a compression level indicator so your can dial it in visually

[Ripthorn]

No bias at IC1A?....

It's dual supply. That makes the envelope follower configuration a lot easier to deal with....

I mean not even a resistor to a zero bias reference, ground. Few opamps will drift to zero naturally.

Interesting, I didn't even think of that.

[PRR]

Since it works for you, I assume it is a drawing error, not a wiring error.

[diffeq]

Nice project. Couple of thoughts:

- can SENS and R8 be B100k and 10k respectively? since you have two b100k pots already
- D9 and D8 might have different Vf - wouldn't it benefit from series connection rather than parallel?
- R15 has an orphan value, but that's my OCD

[Ripthorn]

Nice project. Couple of thoughts:

- can SENS and R8 be B100k and 10k respectively? since you have two b100k pots already
- D9 and D8 might have different Vf - wouldn't it benefit from series connection rather than parallel?
- R15 has an orphan value, but that's my OCD

- SENS and R8 could probably be changed. I started with just a B10k pot for SENS, added R8, then changed the pot value and left it there when I was happy with performance.
- I specify both D8 and D9 as diffused red LED's, but you are right that someone could go rogue. Again, this was determined experimentally and I just left it where it was when I was happy
- See previous answer 

As with all circuits, there is an amount of flexibility over everything. I just got performance dialed in where I liked it and stayed with it.

[Toy Sun]

Oh yeah, Ripthorn does it again!
I've had great success with your spring reverb and this looks exciting. I love the fact that it's set up to work with any 800 Ohm tank, which gives us some choice there (form factor, cost, sound).
I'm intrigued by the ducking feature. I've been wanting something like this for delay for a long time (hint,hint, Ripthorn!). Ideally a standalone side-chain pedal with I/O that ducks the output of another pedal until I stop playing (ducked delay) OR mutes the output of the other pedal when I stop playing (noise gate-like).  Does that make sense?

Finally, I love how you provide Gerbers (and happy to help test them  ).

Thanks for the great work,

John

[Ripthorn]

Oh yeah, Ripthorn does it again!
I've had great success with your spring reverb and this looks exciting. I love the fact that it's set up to work with any 800 Ohm tank, which gives us some choice there (form factor, cost, sound).
I'm intrigued by the ducking feature. I've been wanting something like this for delay for a long time (hint,hint, Ripthorn!). Ideally a standalone side-chain pedal with I/O that ducks the output of another pedal until I stop playing (ducked delay) OR mutes the output of the other pedal when I stop playing (noise gate-like).  Does that make sense?

Finally, I love how you provide Gerbers (and happy to help test them  ).

Thanks for the great work,

John

Both of your suggestions are on my list and have already been mostly hashed out. I'm working on a dual voice double tracker with some fun bells and whistles right now, but those two are next on my list. You know, unless I get distracted, which never happens...