Building a Spring Reverb Unit...

[architeq]

I'm looking to build my own Spring Reverb unit for use as an auxiliary effect...

I am going to use a Ac@#$%ronics Type 4 Spring Tank to construct the unit around but I have no idea what i need to drive the unit. I presume I will need some sort of pre/post amplifier with the Spring Tank so that signal actually goes in and comes out of the unit at a reasonable level, but I cannot find any information on the web about what I need for this?

If you haven't noticed, I am a complete novice in this topic (I'm a producer with a basic basic knowledge of electronics) so I'd preferably be pointed towards something I can buy or find that I can wire in-front of the spring tank to do this task. Rather than build from scratch.

Can anyone help?

Many thanks,

Sam

[Ashurbanipal]

You need a circuit to drive it, and then one after for recovery. I experimented a lot and my best results were with an small amp with a 386 IC to drive it, and a simple recovery circuit with a ne5534 IC. I have an input capcitor switchable between .001mF and 470pf on the 386 amp to cut the low frequencies going to the springs and brighten up the overall sound. I built these from scratch however.

I don't know of anything that you can buy that doesn't already include the tank. There are used spring reverbs and even a couple of new ones out there for $100-$300. I'm at work and don't have any of the links to give you. There all saved at home.

[Mark Hammer]

The transducer on the input of the spring unit is essentially a miniature speaker, which means a low input impedance.  In most instances, you can pretty much guarantee that the input impedance will be less than 2.5k, and often much much less.  This means that whatever you drive it with has to be comfortable around low-impedance loads, and is able to supply enough drive current under those circumstances.

The 386 lowpower amplifier is always mentioned in such contexts as a candidate for the driver, because it is designed to be able to produce a half watt or so into low-impedance loads typical of small speakers and headphones.  It's not exactly a high-fidelity device, but then neither are springs.  I may be wrong, but my sense is that when the impedance of the load starts to increase noticeably beyond typical speaker loads (and the Accutronics Type 4 *could* be as high as 1.5k - http://www.accutronicsreverb.com/prodspec.htm), you probably want to revert to a different driver than the 386.

Other recommended drivers include a few select op-amps like the NE5532 and LM833 that are quite comfortable handling low-impedance loads.  These are often used for headphone amps in mixers, but are also quite comfortable looking at higher-impedance loads as well.  A common strategy you'll see involves paralleling the outputs of several op-amps to provide the combined current drive needed to make those springs wiggle appropriately.  Alternatively, you'll see fairly mundane circuits with matching transformers between the driver circuit and the input transducer on the reverb pan.  Forrest Cook's designs make use of the different strategies described here: http://www.solorb.com/elect/musiccirc/index.html

Craig Anderton's old Stage Center Reverb project, that appeared in Guitar Player before many folks here were born, is available as a project over at www.generalguitargadgets.com (you can buy a board for that).  Craig's Hot Springs reverb is also availale in kit form from PAiA electronics (http://www.paia.com/hotsprgs.htm).  Note that both these designs may be picky enough about the reverb pan impedance that your pan is a bad match.  I guess we'd need the full part number and specs to know.

Whether you build or buy pre-made, one of the things to think about is "spectral management" of the signal going into and coming out of the springs.  In a sense, springs want something sort of like an RIAA equalization curve adaptor.  For the young folk among us, vinyl's limited capacity to fit lots of music with wide dynamic range on a 12" side, resulted in a recording/pressing process where the bass would be trimmed back and treble boosted - so reducing overall amplitude and improving S/N, sort of like Dolby - and a playback EQ curve where bass would be boosted and treble attenuated in complementary manner to restore "flat" frequency response.  Springs are sort of the same way.  Feeding them too much bass gets too much flogging of the springs and "sproing".  Given that you have to provide a fair amount of gain in the recovery stage, cutting back on the treble at that point can improve hiss characteristics, and provide for a more natural-sounding ambience.  Once you decide what you are going to get or build, let us know and we can guide you to tailoring the unit for most pleasing tone and best performance.

In the meantime, here's an interesting older article about spring reverb to get you thinking: http://img.photobucket.com/albums/v474/mhammer/Reverb1.png
http://img.photobucket.com/albums/v474/mhammer/Reverb2.png