Elliott Sound Products

Project 34

Spring Reverb Unit For Guitar or Keyboards
Rod
Elliott (ESP) - Updated 17 November 2006

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Introduction

Well, its not really just for guitar or keyboards, you can use it for
anything that you want. Spring reverb units are most commonly used in guitar
amps, having been replaced by digital effects in most other areas. This cannot
really be classed as a "real" project, because the circuitry is somewhat
experimental, and may change quite dramatically depending on the type of spring
reverb unit you can actually get your hands on.

The one I have is an Accutronics (they are still going, so check out their
web site - see below), but you might already have one, or can get something
different, so you will have to experiment.

Most of the possibilities are discussed here, so with a small amount of
mucking about you should be able to create a reverb unit tailored to your exact
needs. For additional
information, see the end of this page.

Since the P113 headphone amp is very easily modified for constant current
drive, this is recommended. The circuit diagram in Figure 3 is the original, but
it does have limitations. The main limit is the allowable voltage swing, but
this is overcome by using the P113 board with appropriate modifications (all described in
the construction article).

Description

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 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.

Figure 1 -
Traditional Spring Reverb Unit

Many reverb units appear to have only two springs, but you will see that
there are joins in the middle. This is where two springs are joined, and each
spring should be very slightly different. Ultimately it doesn't matter how many
springs they really have, a spring reverb always sounds like what it is. This is
not a criticism, merely a description of the sound.

Of the units around, most of the newer ones have a low impedance (about 8
Ohms) input transducer, and are well suited to being driven with a small power
amp IC. The one I have has a relatively high input impedance (173 Ohms DC
resistance, and according to the specs, about 1700 Ohms impedance), but the
principles are still pretty much the same.

Another common type of reverb tank (common terminology, BTW), was the folded
spring type. These had the springs arranged in a Z pattern and sounded quite
good. They have been used by some very well known guitar amp manufacturers, but
do not appear to be available any more other than the occasional one that pops
up on eBay. Made by O.C. Electronics Inc, they were "MANUFACTURED BY
BEAUTIFUL GIRLS IN MILTON, WIS, IN CONTROLLED ATMOSPHERE CONDITIONS"
according to the label, but I'm unable to verify that  .

Transducer Drive

In all cases you will need a small power amp to be able to drive the unit
properly, but you must be very careful, because overdrive causes the small pole
piece to become magnetically saturated, leading to gross distortion that
increases with decreasing frequency. One solution to this is to use a series
resistor to reduce the drive and give a higher output impedance from the amp.
This usually improves frequency response, especially at higher frequencies, but
tends to disappear the bottom end. This is not always a bad thing, since in
reality low frequency reverberation in a typical room or auditorium is rare, and
generally sounds awful when it does exist.

Another possibility is to use an amplifier with a high output impedance, but
this is not necessary because of the very low power handling of the input
transducer. One method of obtaining at least some degree of current drive is to
use a series resistor. This is the easiest to implement, and helps to protect
the transducer from gross overloads. The basic scheme is shown in Figure 2, and
has the added advantage that modification to the reverb unit is not needed (many
have the earth of both input and output transducers connected to the chassis -
to use a current amp, this would need to be changed).

Using current drive is explained (see additional info, below)
and I have used it and it works well. The problem is that it makes the reverb
very "toppy", with very little bass at all. While this might suit some players,
I prefer 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. This is a little tricky with the small power amp ICs though.

Figure 2 - Reverb
Input Transducer Drive Amp

I have seen quite a few reverb drive amps used in other circuits, including
just an opamp. Few opamps have sufficient current capability to drive the input
transducer properly, and even some of the small power amp ICs are a pain. The
circuit shown has good drive, low current drain and works well. Many of the
circuits I have seen also do not make any attempt to obtain current drive, and
use the low impedance output from the drive amp. This is not the best way to
drive these transducers, and the method shown works much better.

The resistor marked S.O.T. (Select On Test) will need to be selected to
provide the best reverb sound, with the minimum voltage loss. I suggest a
starting value of about 47 Ohms (depends on the input transducer's impedance),
and experiment from there. The positive voltage needs to be not less than 15V
(18V is the rated maximum) to be able to get good drive levels with a high
enough value of series resistor. In a pinch you might be able to get away with
9V, but you will not have much drive level and will need more gain at the
output. This increases noise and the possibility of feedback. This circuit is
only really suitable for an 8 ohm input transducer, as it doesn't have enough
voltage swing for the higher impedance coils.

Reverb Preamp

The output transducer will have an output of (typically) about 10mV, and a
gain of 10 (20dB) is usually enough to match the output of the guitar - for use
with an amplifier insert or a mixer, an output of around 1V is preferred. This
requires a gain of 100. The circuit shown uses 1/2 of a NE5532 low noise opamp
(a TL072 can also be used, but with a noise penalty) - this is quite adequate
for what we need here.

With the values shown, the gain is variable from unity up to a maximum of
about 40dB (100 times), which should be enough for anyone. ("640k of RAM should
be enough for anyone" - Bill Gates ). If your application requires less
gain, simply increase the value of R7 (1k). With 2.2k, maximum gain will be 46
(33dB).

Complete Circuit

The complete circuit is shown in Figure 3, with a reverb mute switch and
level controls. The drive control (VR1) can be a trimpot (or even fixed), since
once you have determined the maximum level this will not need to be changed.
There is no gain control for the guitar input, as the circuit has unity gain, so
amp settings are unaffected by using the reverb.

The capacitor marked S.O.T. will need to be selected to give the sound you
want. High values (above 100nF) will give quite a lot of bottom end, which tends
to sound boomy and very indistinct. You will probably find that a value
somewhere between 1.5nF and 10nF will sound the best - try 4.7nF as a starting
point. Like the guitar amp itself, a reverb unit has its own sound, and it is
only reasonable that you should be able to change it to suit your own taste.

Figure 3 - Complete
Circuit

The power for the opamp is, Pin 4 -ve, Pin 8 +ve. Note that the opamp
requires a dual supply - +/- 15V is fine, or for battery operation (not really
recommended) you could get away with ±9V.

The unit could also be installed inside the amp head, and wired into the
circuit. I will have to leave it to you to determine the gain needed for the
various stages, since it is currently designed for "typical" guitar levels. Make
sure that you provide proper isolation between the input and output of the
reverb tank. I have seen circuits where this was not done, and the whole reverb
circuit goes into feedback. Isolation is provided in this circuit by the virtual
earth mixer (pin 2 of U1 is at 0 Volts at AC and DC).

Most reverb units use RCA sockets for input and output, and be careful with
mounting. The springs will clang most alarmingly if moved about while playing,
and acoustic feedback can also be a problem, especially if the low frequency
gain is too high.

Alternative Version

While the circuit shown above can give good results, it has a couple of
potential issues. Firstly, it has limited drive capability due to the single low
voltage power supply for the LM386. Some reverb tanks require a significant
voltage at higher frequencies, so the circuit of Figure 3 can only be used with
an 8 ohm input coil. Secondly, there is little equalisation capability, and not
enough voltage swing to handle it anyway. A far more elegant solution is to use
a buffered opamp, which can drive reverb tanks having a drive coil impedance of
up to 250 ohms or even 600 ohms at a pinch. See Project 113
for the details of the project PCB upon which this version is based.

For a great deal more information about how to drive reverb tanks properly,
including measurements and tables with optimum values, see Care and
Feeding of Spring Reverb Tanks. The circuit below is easily configured to
suit the tank you have (or can get), and has very good drive capabilities, both
for voltage and current. Note that it is not suitable for the high impedance
drive coil - an amplifier for those coils needs ~±35V supplies to get sufficient
voltage swing.

Figure 4 - Modified
Version, Using P113 Headphone Amp PCB

The version shown in Figure 4 uses the drive amp configured for high output
impedance. Maximum input level depends on the gain of the drive amp, which is
controlled by R3L. The optimum value depends on the impedance of the reverb
tank's input impedance. As shown, it is optimum for a (nominal) 160 Ohm coil.
Note that the input transducer must not connect to the tank chassis. Reverb
units are available with isolated inputs for just this purpose.

The recovery amp has a gain of 100 as shown (40dB), but this can be changed
by using a different value for R3R (higher value, lower gain). The remaining
section can use the Project 94 universal preamp mixer board, which includes
tone controls to allow tailoring of the reverb effect.

Additional Information

Project 113 -
Headphone amplifier that is easily converted for reverb tank drive and
recovery.
Care and
Feeding of Spring Reverb Tanks - Lots of additional information, some of
which is unavailable anywhere else.
Torres Engineering - Supply and information on spring reverb
tanks.
Roy's
"Accutron" Page - Some potentially useful info.

Almost all the reverb tanks that you will see are Accutronics (aka
Sound Enhancements). They are made by:

Sound Enhancements, Inc.
185 Detroit St.
Cary, IL
60013

Note: This is not a specific endorsement of their products or services, but a
reader service.

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Copyright Notice. This article,
including but not limited to all text and diagrams, is the intellectual
property of Rod Elliott, and is Copyright (c) 1999. Reproduction or
re-publication by any means whatsoever, whether electronic, mechanical or
electro- mechanical, is strictly prohibited under International Copyright
laws. The author (Rod Elliott) grants the reader the right to use this
information for personal use only, and further allows that one (1) copy
may be made for reference while constructing the project. Commercial use
is prohibited without express written authorisation from Rod
Elliott.

Page Created and Copyright (c) 18 Oct 1999./ Updated 19 Oct
1999 - added some additional info and links and fixed a couple of
errors.