It was totally unintentional, but in the mess of old broken gear I stumbled upon the remains of my very first guitar amp. It was a Hohner SP55 I bought 2nd hand somewhere in the 90ties. A pretty unremarkable transistor based amp which never really tickled my pickle so to say, and on a day I fried the output amp and it ended on the pile of discarded gear. It had a useful reverb though so I scavenge the springs out of this thing and see if I could make myself a nice spring reverb for use in the studio and/or modular.
While doing some research on the topic there seemed a bit more to it then slapping a few opamps together. I found an excellent write-up on spring reverbs by Rod Elliott, which goes in detail about all things related to spring reverbs, and the implementations of the circuitry. It’s quite an educating read if you’re planning on building one yourself and don’t mind geeking out a bit on this stuff.
As I roamed around the internet looking for feature ideas I quickly released there wasn’t much you do to expand the basic capabilities. I settled for the following, rather sparse, feature list:
- Variable input gain so it can be used with modular signals as well as line levels.
- output mixer between reverb and dry signal.
- Basic tone controls to shape the reverb output.
- A feedback loop, because I saw it somewhere and why not, I like feedback.
- A limiter to prevent the drive transducer from overloading.
The input is handled by a non-inverting opamp configuration with variable gain. If you find you’re having too little gain to work with you can lower the value of R2 as needed or put a trimmer in there. C1 and R1 form a high pass filter with a cutoff around 150Hz, blocking any DC offset and getting rid of a chunk of sub bass. This prevents the input stage of choking on anything incoming that’s not really of any use anyways.
After the gain stage there’s the limiter. I copied Rod Eliott’s design for the limiter. It’s simple and effective and has a nice LED to give some visual feedback. If it’s good, it’s good, no need to re-invent the wheel. The only downside is it’s using a vactrol. I intended to build this spring reverb in a separate enclosure and since there is no light entering I just put a led next to a photosensitive resistor. Saves me the hassle of building a vactrol myself (or buying one).
And yet again there’s another high pass filter, this time a tad more extreme, with a cut off around 1.5KHz. This will get rid of a lot of the low frequency content of the signal since it will only muddy the reverb tail anyway and the spring transducers won’t like it either. You could mess around with this, make it variable or change the cutoff value if you’d like.
I’ve put one trimmer on the opamp driving the transducer so the final gain can be tweaked depending on the strings used. Different tanks have different impedances so you should trim this to suit your tank. I’ve tried different ways of driving those springs, but haven’t really found a lot of difference in sound with the ones I’m using. Off course that’s all measured in a totally unscientific, extremely amateur way. In short, it sounded well enough to my ears and didn’t caught fire.
The recovery amp is simpler, you just need lots of gain. I used 2 opamps to get the signal up again to a usable level. I’ve added a simple tone circuit here so the reverb output can be shaped a bit. It’s a rudimentary tone shaping tool based on bass & treble controls as found on many amps — It’s called a Baxandall tone stack apparently, who knew.
Finally a simple output mixer. I chose to have 2 pots instead of 1 pots for the mix because I was lazy and had room to spare on my front panel. The feedback is more of a novelty feature, I don’t think it will be of much use in the real world but it can an extra bit of ‘ringing’ if you hit the sweet spot. Or you can make it howl at the moon.
*Disclaimer: bla bla bla, don’t come complaining when it doesn’t work or you break something. Also never ever test your DIY builds using headphones at high volumes, just saying.
- Power supply isn’t included in the above circuit but a dual rail +/-12v is used. You could power from a Eurorack system, or build a PSU like I did.