Digital reverb vs spring reverb

Over on levnet, Philip was wishing he could have a spring reverb unit to use in between Gabby's mixer and power amp stages.

This was at a fortuitous time because I was working on comparing a digital reverb module that Antique Electronics sells and a spring reverb board. I designed 2 similar modules ... one digital, one spring.

They both provide about 1.44ms of delay when you put a sinewave through them. But the digital one does something a little unexpected. ... more on that next post.

Continuing ...

The spring reverb relies on standing waves building up on the springs when a more interesting signal is applied. The digital module, however, seems to be constantly doing both frequency and amplitude modulation ... that is, you see its output signal go back and forth compared to the input signal, AND it is going up and down in amplitude at the same time.

Maybe this is "intuitively obvious" to people more up to speed on digital audio than I am, but it was something I had not expected to see.

Sound-wise, the digital reverb module sounds more like a tape echo than spring reverb, and you have a wide range of effect just by controlling how much of it you blend with the uneffected input signal.

Conclusions:
1. If you are looking for the cheapest solution, use the digital module ... it's $4 cheaper than buying a reverb tank and it takes less other hardware as well.

2. If you want the "singing in the shower" sound, use the digital module.

3. If you don't mind going through 3-4 reverb tank units (at $18 each!) to find one that doesn't burn out, and want classic spring reverb sound, then you might consider using the classic reverb setup.

If anyone would like more detailed information, you can email me at djpb@pacbell.net

I too just discovered the magic of reverb. Just a bit will make any theremin sound better. Great for adding depth and eerieness.
I had a spring reverb tank from an old peavey amp and whiped this up over the holidays: Stage Center Reverb (http://www.generalguitargadgets.com/index.php?option=com_content&task=view&id=43&Itemid=26)
It was a pretty simple build. the op amp is even available at Radio Shack. I did the mods suggested by mark hammer: stage center mods (http://www.diystompboxes.com/smfforum/index.php?topic=75049.0)
For theremin or synth use, I would suggest adding the adjustable drive gain and bass roll-off switch (changing c5 to 4700pF.)

You should use a TL072 instead of the noisier TL084 you got from RS.

Not sure which tank Peavey uses, but if you want to drive the lower input impedance tanks like Fender uses, you'll need a bit more current drive. So I use a complementary-symmetry output stage for the driver portion and use +/-15V for power.

As you probably found out, there's lots of ways to build a reverb driver/recovery circuit.

I used a Tl074 from RS. isnt Tl072 a dual op amp with 8 pins? The SCR needs a quad with 14 pins... for the given layout that is... it could be reworked to use 2x TL072
here are some poor quality pictures of my build.

outside (http://www.flickr.com/photos/7996829@N02/4245358773/) inside (http://www.flickr.com/photos/7996829@N02/4245353937/)

Im using a extra power supply from a EM theremin to provide the +/- voltage. This provides +12v/-12v and the extra juice seems to improve performance, adding a little more headroom.
the TX is from RS. A 120v to 24 volt using the black and one yellow wire. This provides 12AC to the moog power supply.
Surprisingly, having the tx right next to the tank causes no noticeable hum or interference.

Don said: "The digital module, however, seems to be constantly doing both frequency and amplitude modulation"

I am not well up on DSP stuff.. But I think the "AM" you are seeing is due to phase cancellations from the delayed signal combined with the undelayed signals.. does the "AM" frequency change as the input frequency changes (with the delay time constant)? .. I think it may, effectively, be 'beat frequency' you are seeing.

There should also (I think) be something similar happening with an excited spring.. but it will be less observable due to the much more complex nature of a spring..

I prefer springs! ;-)

Sadly, time has not permitted me (yet) to try this setup out with my guitar, so I am not sure if the digital reverb will sound "better" than spring with actual instruments.

And, in answer to the previous post about opamps, I meant TL074. Radio shack only sells the "noisier" TL084 version. In line-level, unity-gain circuits you might not hear a huge difference, but if you have any gain at all the TL084s will have noticeably more hiss, and maybe "popcorn" noise (crackle sounds).

Regarding "AM'ing" on the springs ... if you don't perturb the signal (i.e. stay at a fixed frequency and don't bump the tank), you just get a delayed audio output. If you crank the freq dial back and forth on the oscillator, you do indeed see AM'ing. That's when standing waves start building up and interferring with each other. The digital module seems to "make it's own" standing waves ... with or without a varying input frequency.

I have heard it's not easy to create a natural sounding reverb algorithm. Delay is fairly straight forward, though.

Don

Took long enough, but I finally got a chance to play guitar through the amp I build with the digital reverb. It sounds great! Having a input pot to mix the uneffected signal into the module and an output reverb level pot gives you a very wide range.

At moderate settings it sounds like a good spring reverb tank. With everything wide open it sounds like a tape delay echo. Not bad at all!

Don

It would be very simple to adapt this digital reverb module for building into a theremin.

What I do, is a get the loudspeaker i took out of an amp i broke, and I place it on the back of my cello. I get a nice reverb, some harmonics, and my etherware goes from sounding scratchy to sounding natural and clean