This is design for guitar reverb. Spring reverb units are most commonly used in
guitar amps, having been replaced by digital effects in most other areas.
The circuitry will always be somewhat experimental, and may change quite
dramatically depending on the type of spring reverb unit you can actually get
your hands on. The PCB is designed (available soon) to accommodate most
reverb tanks, with the optimum being those with a low impedance drive coil.
This is the figure of the complete schematic diagram of the Spring Reverb
circuit.
Since the P113 headphone amp is very easily modified for constant current drive, this used to be recommended because it works very well. However, wiring the two halves of the P113 board differently is somewhat clumsy, and while it works well it’s not ideal. The circuit shown here uses the same drive system, and has everything else that's needed for a complete reverb system. The circuit shown in Figure 2 is the latest incarnation of the reverb circuit. Unlike the others that are somewhat 'piecemeal', it's a complete reverb sub-system.
It can be used in an effects loop, or stand-alone. The reverb signal level is adjustable. You can choose to mix reverb with the 'dry' (original) signal, or keep the reverb completely separate. The basic spring reverb chamber is a simple affair (see Figure 1), with an input and output transducer, and one or more (usually three or four) springs lightly stretched between them. Each spring should have different characteristics, to ensure that the unit does not simply create 'boinging' noises. Stay well clear of single spring units, they are usually cheap Taiwanese and Chinese affairs and can often found in really cheap guitar amps. They sound awful, and nothing you do will ever change this. This is not to say that the Taiwanese or the Chinese don't or can't make decent spring reverb units too, I just haven't seen one yet. Really basic looking 2-spring units pop up on auction sites at regular intervals - I've not tried one, and I'm not about to waste any money to do so.The circuit uses an inverting buffer at the input. Although
shown set for unity gain, that can be increased or decreased to suit the signal
level you have available. The second stage is a small power amp to drive
the unit properly. You must be careful with the drive level, because
overdrive causes the small pole piece to become magnetically saturated, leading
to gross distortion that increases with decreasing frequency. The circuit
uses current drive, but with a defined impedance selected to suit the tank you
use. This usually improves frequency response, especially at higher
frequencies, but tends to reduce the bottom end response. This is not
always a bad thing, since low frequency reverberation in a typical room or
auditorium is rare, and generally sounds awful when it does exist.
An amplifier
with a high output impedance is used, and this is the approach taken in most
guitar amps. This might make the reverb a
bit 'toppy', with not much bass. Most players prefer the sound of a
modified current drive, where the output impedance is defined (rather than
'infinite') because you can tailor the sound to your liking much more easily. U1A
is the input preamp and buffer. It's inverting because the final mixer is
also inverting, and that preserves the signal polarity. The gain can be
changed by varying R2, and a higher resistance provides more gain. It's
unlikely to be necessary, but the option is there. The signal is then
split into two paths. The first goes to the reverb drive amplifier (U1B),
via VR1 which sets the drive level, followed by a 100nF cap which rolls off the
low frequencies (-3dB at 72Hz). The second path provides the 'dry' (no
reverb) signal to the output mixer. The level is fixed for an overall
unity gain for the dry signal. VR1 is followed by a trimpot so it can be
preset to the maximum level required. This can be omitted if you don't
include the limiter (join C3 directly to R3 with a link). The circuit is
configured for a nominal signal level of 1V RMS (0dBV).
The reverb drive amplifier uses U1B, along with a current booster
using Q1 and Q2. The output impedance is set by R9 (see Table 1), and the
transconductance (gain, in milliamps/ volt or mS - milli-Siemens) is set by
R4. The values shown for both resistors in Table 1 are known to work
well, but you can experiment. If more gain is needed for lower signal
levels, you can increase the value of R2. The gain is simply
R2 / R1. If R2 is made (say) 22k, R18 should also be 22k to
preserve unity gain for the 'dry' signal through the system. There's
provision for a link in series with R18, which lets you operate with no 'dry'
signal at all. This is useful if the unit is operated as an external effect
with a mixing console. Note that the 'earth' end of R4 must be connected
directly to the DC input earth (ground) point. This prevents the current
from generating a voltage drop across the circuit. If it's not done this
way, there may be a small (but measurable and possibly audible) 'dry' signal,
even if the 'dry' path is disabled by removing J2.
