Mod for reverb FX - Removing the "dry" signal entirely

[The Bathroom]

So I wasn't sure where to post this, so I opted for the open chat. Recently a friend of mine has been coming to me asking for a "Reverse Reverb" effect, which I guess is a reverb that sort of "swells" when you play a note. But since there is no reverse reverb layout, I told them I could only make a regular verb.

I'd like to build one of the reverb effects from the blog using a Long-decay Belton BTDR-2h brick, but I'd like to be able to make the necessary modification that will give me a 100% wet reverb signal, and no dry signal. If you start the video at 6:18 ish you'll know what I mean.
https://www.youtube.com/watch?v=zvnEcRWwlXY

To my ears, the Box of Hall Reverb on the blog is the best sounding, and the simplest verb to make, but I'd like to add a pot or a small switch that can keep the dry guitar signal from going through the circuit, and maybe have part of the wet signal loop back in so it can 'build upon itself', so to speak. Can anybody help me?

Box o' Hall link: http://tagboardeffects.blogspot.com/2012/04/box-of-hall-reverb-culturejam.html

[M. Spencer]

I'm also interested in a "wet only" mod. For the looper, perhaps the Beavis True Bypass with Feedback Loop could be incorporated?

[induction]

To kill the dry signal, replace the link that goes from row 3 to row 4 counting from the bottom (its just to the lower left of the TL074) with an SPST.

If you want feedback from the reverb output to its own input, you should consider a different circuit. The Tenebrion may suit you because it's essentially the updated Box of Hall with feedback added, and maybe a few more mods. The updates were designed by Culturejam, the original designer of the Box of Hall. I haven't played it, so I can't comment on how it sounds, but it's very popular. If you like the Box of Hall, you'll probably also like the Tenebrion.

Of course there's also the All-Star Reverb by (ahem) induction. It has the feedback (dwell control) built in. There's a vero in the linked FSB thread (you'll need to have a (free) account at FSB to see the images), as well as instructions for an optional switch to kill the dry signal, a true bypass version, a buffered version with tails, a switchable TB/tails version, and a demo. There's also at least one thread on DIYSB about it, with another demo. Is it the single greatest digital reverb pedal known to man? Yes, in my (very humble) opinion, it is. I may be biased, though.

[Bugsi]

There is a mod to the Box of Hall that adds "Dwell" by feeding back an adjustable amount of the "wet" reverb signal back into the Belton brick.  I've done it and it's great.  I use this instead of the "Damp" control, which does nothing for me.  
Box of Hall with Dwell
Do remove the Damp control just replace it and its cap with a 10K resistor.

I've done that build and it's my favorite reverb.
For "wet only" I'd need to mod the last op-amp bit in the Box of Hall, which acts as a mixer for the dry and wet portions of the signal.

[The Bathroom]

Why thank you induction :) I appreciate your advice, and your suggestions. Did you check out that video I linked? If so, would making the modifications you suggested give me that sort of "swelling" ambient tone I'm looking for?

[The Bathroom]

Great suggestion Bugsi. I'm not sure if I like the dampen pot either, I mean I've never been a fan of tone knobs on effects pedals like chorus's and such. I never use them, thought they were pretty useless. But the Dwell would be cool to control the amount of feedback going into the reverb circuit. I'll take up both yours and inductions advice.

My plan is to have this reverb and a delay circuit in the same box with 4PDT switch wired to the in's and out's so it's just a stomp away from reversing the order in which the effects are paired.

[induction]

The Bathroom wrote

Did you check out that video I linked? If so, would making the modifications you suggested give me that sort of "swelling" ambient tone I'm looking for?

Yes, I think it will. For that particular sound, the dry kill will get you started, and the dwell control will add the icing. For what it's worth, the dwell mod for the Box of Hall (at least on the schematic I've seen) has the dwell signal tapped after the last op-amp, which means you are feeding back both the dry and the wet signal. This won't matter if the dry kill is engaged, but when the dry signal is present I find that the effect of the dwell is mostly just like turning up the reverb. When the dwell signal is wet only, you get a different, more ambient flavor of reverb. Based on the video you linked, I predict you will prefer that. The All-Star Reverb has the dwell tapped from the reverb brick before the mixing op-amp, so it is purely wet feedback.

As for the damp knob, I felt the same way you do, at first. I spent months breadboarding until I finalized the All-Star design, and I expected not to include the damp knob because it seemed like a subtle detail that wouldn't make much difference. I was wrong. I tried it out just to make sure I wasn't missing anything, and I can't emphasize enough how happy I am that I did. Crank the treble and you get an unnaturally bright-sounding reverb that sounds great for ambient stuff with the dwell cranked up. With the dwell at min and the reverb cranked up, you get a nice, surfy, spring reverb sound. But for normal rhythm and lead playing, too much treble can get in the way. When you reduce the treble, you get a reverb that provides a sense of space around the music, without overpowering it. That's how I keep mine set most of the time (unless I'm using the reverb as a special effect). I sometimes can't tell it's turned on, until I turn it off just to check that's functioning properly, and it's a night and day difference. It's hard to describe, because it's both very subtle and absolutely obvious at the same time. Reduced treble also minimizes the modulation from the brick, which I find useful. Note that the damp knob affects the wet signal only, it does not change the tone of your dry signal at all.

I've heard reports of the damp knob not working in several Box of Hall builds, but rest assured it works very well in the All-Star. If you haven't followed up on the FSB thread, here's a link to my demo. You should be able to hear the damp knob doing it's thing, but to get a real feel for the value of the damp knob, you have to try it for yourself. I'd never build another belton-style reverb without one.

[tabbycat]

induction wrote

To kill the dry signal, replace the link that goes from row 3 to row 4 counting from the bottom (its just to the lower left of the TL074) with an SPST.

hey induction, i have a ghost echo belton reverb which takes a TL074, as does the hall of reverb, and i have been considering putting in a dry kill switch for a while. have worked out where i think it should go but wanted to double check with a wise head.

i noticed you suggested above to put the dry kill spdt for the hall of reverb between pin 1 (output 1) and pin 10 (non-inverting output 3) of the TL074, instead of the jumper.
the same situation on the ghost echo seems to be between pin 7 (output 2) and pin 10 (non-inverting output 3), which is where i was thinking of putting my dry kill.
i have also looked at the madbean moodring (TL072 rather than TL074, but same set up for input). that has a 22k resistor between the output and input pins (as does the ghost, 22k) with the dry kill spdt before the resistor. is that where is should put it? before the resistor not after.

ghost echo schematic (spectre clone by fuzzdog, which is what i have) here:
http://pedalparts.co.uk/docs/SpectreVerb.pdf

moodring here.
http://www.madbeanpedals.com/projects/Moodring/Moodring2015.pdf

would appreciate your approval before diving in as i quite like my ghost and would rather not trash it on a reckless mod. it seems straightforward to me, which always starts alarm bells ringing...

[induction]

Hi tabbycat. You are correct about the SpectreVerb layout. Just like the Moodring, simply disconnecting the output of the first op-amp from the input of the last op-amp will achieve the dry kill you seek. It doesn't matter whether the switch comes before or after the 22k resistor, either way will work. Go with whichever is easier to work into the layout.

Op-amps are op-amps, so one layout may use pins 1-3 for the input buffer while another layout uses pins 5-7 for the same function. Make sure your layout matches the schematic with respect to which op-amp is used for which function.

Let us know how it works out.

tabbycat wrote

induction wrote

To kill the dry signal, replace the link that goes from row 3 to row 4 counting from the bottom (its just to the lower left of the TL074) with an SPST.

...

i noticed you suggested above to put the dry kill spdt for the hall of reverb between pin 1 (output 1) and pin 10 (non-inverting output 3) of the TL074, instead of the jumper.
the same situation on the ghost echo seems to be between pin 7 (output 2) and pin 10 (non-inverting output 3), which is where i was thinking of putting my dry kill.

Just for clarity, note the bolded sections. My recommendation is to replace the jumper between pin 7 and the 10k inline resistor that leads to pin 12. The confusion seems to be whether you count rows 3 and 4 from the top or from the bottom. Count from the bottom in this case. Disconnecting pin 1 from pin 10 would misbias the op-amp feeding the brick, which will kill (or maybe distort) the wet signal. So you'll get a wet-kill/mutilate switch instead of a dry-kill switch.

Also, you can do this with a simple SPST, though an SPDT will also work if you ignore the second throw.

[tabbycat]

many thanks for the clarification, induction. definitely puts my mind at rest to get the nod from you.

this diy bug is awful. i can't leave anything alone. i used to think i would only build a handful of effects i couldn't afford or find and then i'll stop. but you can't. once you get a little understanding of the science, so many possibilities open up that you end up in a permanent state of option paralysis. so many options open to you at any point in time that you are frozen in a state of permanent indecision. when to stop.

anyway, i will dive into the spectre and report back when i get a good result.

btw, were you aware that you are fuzzdog famous, or infamous, now?

http://pedalparts.co.uk/docs/GraphicFuzz.pdf

[The Bathroom]

induction wrote

The Bathroom wrote

Did you check out that video I linked? If so, would making the modifications you suggested give me that sort of "swelling" ambient tone I'm looking for?

Yes, I think it will. For that particular sound, the dry kill will get you started, and the dwell control will add the icing. For what it's worth, the dwell mod for the Box of Hall (at least on the schematic I've seen) has the dwell signal tapped after the last op-amp, which means you are feeding back both the dry and the wet signal. This won't matter if the dry kill is engaged, but when the dry signal is present I find that the effect of the dwell is mostly just like turning up the reverb. When the dwell signal is wet only, you get a different, more ambient flavor of reverb. Based on the video you linked, I predict you will prefer that. The All-Star Reverb has the dwell tapped from the reverb brick before the mixing op-amp, so it is purely wet feedback.

Alright induction, I'm swaying :) I'll build your All-Star reverb, but since you're the evil genius behind it all, I'd like to get some help with the mods.

So if I wanted to build the All-Star with this layout here, and I wanted to add the Dry Kill SPST switch, would I put the switch at the rail (with a cut) just above the 89K and the TL074 (Or for the other people looking at the thread, R10 on the schematic) or do I half to ninja in the switch on the other side of the opamp? Also, I wasn't sure about the difference between the true bypass and the buffered bypass with tails. Could you explain the latter for me?

Lastly, regarding the damp knob, I see you have a 47nf cap for the low pass. If I adjust the 10k before that, will I be adjusting the "treble headroom" at all? Not doubting your ingenuity at all, I'm just trying to get ideas for a possible mod.

And finally, I wanted to thank you for all your help. You gave a lot of input and helpful info in your responses.

[induction]

tabbycat wrote

btw, were you aware that you are fuzzdog famous, or infamous, now?

That's awesome. I had no idea. As I recall, someone else posted a similar schematic on DIYSB before I posted mine on FSB. I'll gladly take credit, though.

Have you heard of Stigler's Law? "No scientific discovery is named after its original discoverer." Published in 1980 by Stephen Stigler, who credited the discovery of his law to Robert K. Merton.

Who am I to meddle with scientific tradition? (Ok, it's not really a scientific discovery, but why let facts get in the way of a good excuse?)

[induction]

The Bathroom wrote

So if I wanted to build the All-Star with this layout here, and I wanted to add the Dry Kill SPST switch, would I put the switch at the rail (with a cut) just above the 89K and the TL074 (Or for the other people looking at the thread, R10 on the schematic) or do I half to ninja in the switch on the other side of the opamp?

To me, it looks like the easiest thing to do would be to omit the jumper that touches pin 1 of the TL074, and put one of the leads of the SPST in the hole in the 4th row from the top that is now vacated by the jumper (just above pin 1). Then put the other lead two holes to the right of point A (one hole below the red 1M resistor, between the 100n cap and the 10k resistor).

Also, I wasn't sure about the difference between the true bypass and the buffered bypass with tails. Could you explain the latter for me?

True bypass, I'm sure you know, completely disconnects the signal path from the circuit. Buffered bypass means that the bypass signal passes through the input buffer and the output mixer. The bypass is achieved similarly to the dry-kill switch: we disconnect the input buffer from the reverb sidechain at point A. This means that the reverb sidechain still contributes to the output signal, but since the reverb sidechain has no input signal, all you hear is the dry signal. This method has the potential to add noise to your bypass signal, so I A/B'd it carefully to make sure I couldn't hear any difference. I was very happy with the result, and I built my pedal with buffered bypass, but you may have better ears than me, so I'd consider playing with it on the breadboard if you're not sure. Some folks are fanatical about true bypass, and some prefer at least one buffer in their chain. You'll have to decide what works best for you. Besides the buffer, the upside of the buffered bypass is the resulting tails. 'Tails' refers to reverbs and delays that don't cut off the effect instantly when bypassed. Any signal that is present when you switch to bypass will continue to reverberate and decay naturally, but no new signal will reach the reverb sidechain. This is especially useful for delays, but can also be useful if you turn your reverb on and off within a song. Reverb that suddenly stops cold can sound jarring and unnatural. If you leave your reverb on all the time like I do, it won't make any difference.

If you can't choose between true and buffered bypass, the FSB thread has instructions to add a switch to select between them.

Lastly, regarding the damp knob, I see you have a 47nf cap for the low pass. If I adjust the 10k before that, will I be adjusting the "treble headroom" at all? Not doubting your ingenuity at all, I'm just trying to get ideas for a possible mod.

I stole the idea for the Damp knob from the Box of Hall, but I increased the cap value to allow for more treble cut. I can detect no difference between the maximum treble setting and removing the cap to ground entirely, so there should be no treble headroom issue to deal with. Increasing the pot value will increase the maximum treble attenuation, and decreasing the pot value will reduce the maximum treble attenuation. Either way, the brightest setting should not be affected.

However, the gain applied to the wet signal in the mixing amp does depend on the value of the damp pot (just the value, not the setting), so I wouldn't stray too far from 10k without verifying it on the breadboard.

I'll also point out that I have thoroughly tested the original version of the circuit, but I have never tried the 'wetter' mod. At the request of a builder, I rejigged the gains of the mixing amp to allow for twice as much wet signal. They said they were happy with the mod, but I haven't tried it myself.

And finally, I wanted to thank you for all your help. You gave a lot of input and helpful info in your responses.

I'm always glad to help, and I'm honored that you want to build the circuit. If you have any more questions, please ask. And let us know how it turns out.