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
1kHz but at 10kHz it is 80Ω. Down at 100Hz, the impedance is only
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.
- 2-spring reverb unit
- Input level control
- Reverb depth control
- Reverb in/out switching
- Wide frequency response