Elliott Sound Products   Project 34

Spring Reverb Unit For Guitar

Rod Elliott (ESP)


Introduction

Well, its not really just for guitar, you can use it for anything that you want, but spring reverb units are most commonly used in guitar amps.  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.


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 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 don't make decent spring reverb units too, I just haven't seen one yet.

Figure 1
Figure 1 - Basic 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), is the folded spring type.  These have the springs arranged in a Z pattern and sound quite good.  They have been used by some very well known guitar amp manufacturers.



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.  As a result, I suggest the series resistor method, as 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 (most?) 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.  If you want more information on this, send me an e-mail - if I get enough interest I will work something out.

Figure 2
Figure 2 - Reverb Input Transducer Drive Amp

I have seen quite a few reverb drive amps used in other circuits, including just an opamp.  Opamps do not 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.  Most 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.



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.  It is an easy matter to increase this if you want to.  The circuit shown uses 1/2 of a TL072 opamp - 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 (22 times), which should be enough for anyone.  ("640k of RAM should be enough for anyone" - Bill Gates  :-)



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
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.



Additional Information

Torres Engineering - Supply and information on spring reverb tanks
Roy's "Accutron"  Page - Some more useful info

Almost all the reverb tanks that you will see are Accutronics. 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.