BACK IN THE "good old days" before digital effects became the vogue for musical instruments, electric guitars were often used with "spring reverberation" to get the echo effect of a large concert hall. Not only did the reverberation sound great but it could also make an average performer sound a lot better. And judicious use of reverb could make a small venue sound much larger and more impressive.

The Spring Reverb Module is based on this compact 2-spring unit from Jaycar Electronics. It is much more compact than the spring modules used 20-30 years ago...

measuring just 264mm long x 52mm wide x 33mm deep. The two springs provide signal delay times of 22ms and 27ms.

But why bother with old technology when digital effects can be so much more flexible, more compact and not subject to any acoustic feedback? The answer is to that like trying to explain why Hammond organs are so popular in modern bands when digital key­boards are in so many ways superior. Spring reverb does have a particular "authentic" sound that isn’t quite duplicated by digital effects boxes. And anyhow, this little spring reverb module is cheaper than a digital effects box.

A spring reverb unit consists of a box containing two or three stretched springs which are driven at one end by a voice coil – just like a loudspeaker but without the paper or plastic cone. The audio signal travels down the springs and is reflected back and forth and then is picked up at the other end by another voice coil unit.

The echo signal can then be mixed with the original signal to produce a range of reverberation effects. For this project, we have arranged for Jaycar Electronics to import a compact 2-spring module which is much more compact than the spring modules used some 20 or 30 years ago. It measures just 264mm long x 52mm wide x 33mm deep.

The spring reverb module has two characteristics which determine its overall reverberation effect. The first of these is the signal delay time and this is determined by the springs themselves at 22ms and 27ms. Then there is the decay time and this is typically around 1.2 to 2 seconds, depending on the circuit settings.

We have a designed a PC board which fits over the metal chassis of the spring module and the complete assembly can then be suspended within your musical instrument amplifier, whether it is used for electric guitar, keyboard or any other musical in­strument.

The spring reverb unit requires an unusual drive circuit. This is because the driving voice coil is an inductor and it has an impedance which is directly proportional to frequency. For example, it has an impedance of 8Ω at 1kHz but at 10kHz it is 80Ω. Down at 100Hz, the impedance is only 0.8Ω.

To obtain a reasonably flat frequency response for signals fed through the module, we therefore need to apply ten times the signal level at 10kHz than at 1kHz and so on. And while the actual power levels are quite low, the drive current requirements are relatively large and so we have added a buffer stage which can do the job.