Elliott Sound Products

Project 125

Care and Feeding of Spring Reverb Tanks

Rod Elliott (ESP)

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Introduction

Reverb is one of those effects that simply will not go away. While there are
some excellent DSP (digital signal processor) based reverb systems that really
do sound very natural, the unique sound of spring reverb tanks is still
preferred by a great many guitarists and many electric/electronic organ players
as well. It becomes obvious that the sound of a spring reverb must be a classic,
when it becomes available as a software plug-in for computer based recording
systems.

For detailed info on the history of reverb, it's hard to go past the Accutronics website.
There is also a lot of information provided showing various drive methods, a
simple recovery amplifier, overload characteristics, and much more.

The problem for the hobbyist or DIY builder is that the information is too
detailed and the circuits are too generalised. This makes component selection
difficult, and makes it almost impossible for the average enthusiast to work out
what is needed for their application. While it's nice to have so many choices
(they make a lot of different tanks), it's extremely difficult to work out the
combination of the most suitable tank, optimum drive circuit, and the ideal
recovery amplifier.

With this in mind, and given that I have a reverb design amongst the
projects, there is actually nothing specified as to the best tank to use to
ensure good performance. I have an old 4FB2A1A tank that was used for some of
the tests described. This tank is a Type 4, and has drive coil impedance of
1,475 ohms (~1.5k), pickup coil impedance of 2,250 ohms (2.2k) and is designed
for long reverb time (2.75-4 seconds). All this information is available from
the Accutronics website.

It should be mentioned that the info provided is often at variance with
reality. A measurement of inductance (for example) gives a very different value
from that calculated, but an impedance scan shows that the quoted figures are
fairly close. Inductance measurements on transducers often give incorrect
results, because the coil resistance is high enough to trick the meter into
claiming the inductance is much higher than it really is.

Figure 1 -
Accutronics Reverb Tank

Figure 1 shows a complete spring reverb tank. I was originally going to show
a photo of one I already have, but it looked a bit too gruesome, so I got a new
one to do some further experiments with. While the old one has seen better days
(it's well over 20 years old) it still works perfectly. The single biggest
problem with it is that the input coil has a very high impedance, making it
rather difficult to drive.

Figure 2 -
Drive Transducer Details

Above, you can see a close-up of the drive coil. All coils are colour coded
to show their impedance. This info was not included in the table below, but is
available from the Accutronics website. This can be helpful if the model number
is missing or can't be read.

Figure 3 -
Pickup Transducer Detail

Here's a close-up of the pickup coil. The basic design of these tanks dates
from around 1960, and has changed very little in all the time since then. As a
result, it is possible to replace even extremely old tanks if necessary, and a
direct replacement is almost always available. The transducers of this tank are
virtually identical to those in my 20-odd year old tank.

Figure 3a -
Pickup Transducer Magnet

Here is a picture that you won't see very often. This is photo of one of the
transducer magnets, taken from a broken reverb spring. To get an idea of the
size, the magnet is sitting on a piece of 5mm grid graph paper. You can see the
ends of identical magnets in the two photos above of the drive and pickup
transducers, but it's hard to gauge the size from those other photos.

Figure 4 -
Accutronics Reverb Tank Drive Coil Impedance

The above scan was taken of an Accutronics 4FB2A1A reverb tank (my old one).
I used a woofer tester (normally used for measuring Thiele-Small parameters).
The impedance was also verified using a vector impedance meter, and gave
virtually identical results. The spikes at the high end of the sweep are caused
by the measurement signal causing a disturbance in the springs, and confusing
the reading. Although the impedance is different from the claimed or calculated
value at various frequencies, the difference is inconsequential. In theory, the
impedance at 6kHz should be around 8.8k, but is closer to 6.5k - while this
might seem like a large error, it makes little or no difference to the way the
tank behaves.

Reverb Tank Drive

The drive circuit for any spring reverb tank is critical - by far the most
critical part of the final system. The drive coil has a nominal impedance
specified at 1kHz, but it also has considerable inductance. It is actually quite
difficult to drive properly. It is necessary to know the optimum drive level,
but this is specified as a current, not a voltage.

In order to produce a constant drive level into a coil as the frequency
varies, it is necessary to drive the coil with an amplifier that produces
constant current, rather than the much more familiar constant voltage. This can
be done with equalisation, but it is preferable to use a dedicated amplifier
with a high output impedance. This approach is easier, and it automatically
adapts itself to the actual (as opposed to nominal) value of impedance at any
frequency of interest. However, current drive accentuates the upper frequencies,
and in some cases this may well be excessive.

It is common to include a high pass filter as well, because the spring reverb
effect doesn't work well at low frequencies. While it is possible to get good
low frequency performance, it's generally undesirable because it tends to muddy
the sound too much. Accutronics provides a table of impedance and RMS drive
current at 1kHz, but some of the information that one really needs is missing.
To rectify that, I have added a column giving the approximate inductance and
deleted the columns that are unimportant. The following table applies to the
Type 4 tank - the 425mm long, 4-spring version as used by Fender and many
others.

Type 4 Input

Coil Impedance

DC Resistance

RMS Current

Inductance

A

8

0.81

28mA

1.27mH

B

150

26

6.5mA

24mH

C

200

27

5.8mA

32mH

D

250

36

5.0mA

39mH

E

600

58

3.1mA

95mH

F

1,475

200

2.0mA

234mH

Table 1 - Type 4 Accutronics Input Coil
Data

There are several different tanks available, most of which are somewhat
smaller than the Type 4. Being smaller, this means fewer (or shorter) springs,
and different reverb characteristics. The Type 4 has been the unit of choice for
many guitarists for a very long time, although some do prefer the other types.
The details in this article are equally applicable to any reverb tank, but some
small changes may be needed to account for different impedances.

The first thing that the intending user should look at is the 1kHz impedance.
For example, a coil with an impedance of 1,475 ohms at 1kHz requires a voltage
of 2.95V RMS to produce 2mA coil current. At 10kHz, this rises to 29.5V because
the coil impedance rises to 14.75k. While this voltage and current are certainly
achievable, the drive amp ideally needs a supply voltage of over ±40V - often,
this is simply not available.

For use with an opamp or small chip amplifiers, we must use a lower supply
voltage, and with these the high impedance drive coil is of no use. All in all,
the 8 ohm coil is the most attractive, although theoretical current is higher
than we might like at 28mA RMS (about ±40mA peak). The optimum impedance for
opamp drive is 150 ohms ('B' input coil), and even at 10kHz when the impedance
has risen to 1500 ohms, the voltage remains below 10V RMS. With a peak current
of 9mA, an opamp will require a couple of small transistors to boost the output
current, and we end up with a circuit such as that shown in Figure 1. For opamp
drive, the 150 ohm coil doesn't allow much headroom though - it would be nice if
there were an intermediate impedance available. Something around 50 ohms would
be perfect.

An 8 ohm coil is a good choice if the power supply is adequate, and a boosted
opamp (or a chip amp such as an LM1875) will be needed to drive the coil. The
maximum voltage needed is 2.24V RMS (at 10kHz) to be able to provide the full
28mA needed for maximum output. While the chip amp seems like a good choice and
is very easy to do, the cost is considerably higher than a boosted opamp.
Current drive is a little harder too, because most IC power amplifiers are not
stable at unity gain, adding a small amount of additional complexity to achieve
unconditional stability. The following circuit will drive 8 to 250 ohm coils
well, without any changes.

Figure 5 -
Basic Drive Circuit For Low Impedance Coil

The circuit shown above requires that the input coil be isolated from the
reverb tank chassis. The input voltage required for full drive is determined by
the coil impedance and the value of R2, and with a 150 ohm coil and R2 set to
150 ohms as shown provides 6.5mA/Volt. Note that the value of R7 must be
selected based on the coil impedance. The following table gives the suggested
values for R2 and R7, based on the coil impedance. Some experimentation may be
needed, but only the values of R2 and R7 need to be changed.

After getting a new 8 ohm tank for some experiments and to take a few
measurements, it turns out that the coil can be driven somewhat harder than
claimed. I was able to drive the 8 ohm coil to 250mA at 1kHz before saturation
(almost 10 times the current claimed). The saturation current remains roughly
the same at all frequencies from around 300Hz and up, and at 1kHz the voltage
was measured at 2V RMS. This rises to 8V RMS at 5.8kHz, the highest frequency
where useful output was measured. I drove the input transducer from an LM1875
amplifier, feeding the coil via a 10 ohm resistor. Amp output at 5.8kHz is a
little over 8V RMS, and it was the resistor alone that reduced the drive voltage
at lower frequencies. However, I don't recommend that you drive the coil to the
maximum, because it may shorten the life of the unit. Somewhat surprisingly,
using almost 10 times the rated coil current does not produce almost 10 times
the output level - you will be lucky to get even twice the output. On that
basis, using a much higher drive current should not be attempted (other than for
experiments of course).

The Figure 5 circuit is not capable of driving an 8 ohm coil to the full
250mA that it can handle. This is of little consequence though, because the
maximum current gives no real benefits, but could easily lead to premature
failure of the tank.

Coil Impedance

R2

C2, C3

R7

Current

Voltage at 6kHz

8 ohms

33 ohms

47uF

150 ohms

28mA RMS

1.34 V RMS

150 ohms

150 ohms

10uF

3.3 k

6.5mA RMS

5.85 V RMS

200 ohms

180 ohms

10uF

3.9 k

5.8mA RMS

6.96 V RMS

250 ohms

220 ohms

10uF

5.6 k

5.0mA RMS

7.50 V RMS

600 ohms

330 ohms

10uF

12 k

3.1mA RMS

11.2 V RMS

1,475 ohms *

n/a

n/a

n/a

2.0mA RMS

17.7 V RMS

Table 2 - Suggested R2, C2 &
R7 Values For 1V RMS Input

* Note that the Figure 5 circuit is not suitable for the 1,475 ohm coil, as
it can't provide a high enough voltage to get good results. The circuit will run
out of drive voltage at about 3kHz, and a high voltage drive circuit such as
that shown in Figure 5 is needed.

In general, an input voltage of about 1V is pretty close to ideal for most
applications. Be aware that if the coil is overdriven you'll get some
distortion, and excess overdrive can damage the coil due to overheating if the
available voltage and current is grossly excessive. This is actually fairly
unlikely - even with 250mA in the 8 ohm coil the dissipation is negligible. It
is best to avoid clipping the drive amplifier, so some headroom is needed. Amp
clipping may be worse than core saturation, and the level needs to be monitored
for best results.

The circuit of Figure 5 is not suitable for the high impedance coil - see
below for more on that topic. R2 sets the sensitivity, and can be determined as
R2 = VIN / ICOIL. For example, for an
8 ohm coil and 2V input, R2 becomes
2 / 0.028 = 71 ohms. A 68 ohm resistor is fine in this
case. R7 is based on an estimation, where the resistor value is roughly 20 times
the coil's 1kHz impedance. Reduce the value of R7 for less treble response and
vice versa. With R7 set at 20 times the coil impedance, high frequency response
is 3dB down at about 5.5kHz. The resistor sets the amplifier's output impedance,
so it can't keep rising with increasing frequency. The effect is identical to
using a voltage amp with a series resistance. The choice of C2 and C3 is
somewhat personal, but they should ideally be bipolar electrolytics as
indicated. The values shown will give a fairly good drive level down to about
200Hz with all coil impedances, and is less bass response is desired it's better
to reduce the value of C1. As shown the -3dB frequency is 159Hz. A smaller value
will give more aggressive bass rolloff and vice versa.

We also need to consider the maximum voltage needed to provide the required
current into the coil at high frequencies. At 10kHz, we need almost 10V RMS,
which is the maximum possible from an opamp. Fortunately, response to 10kHz is
not only unnecessary, but especially for guitar is undesirable (and it won't be
reproduced by the tank anyway). An upper limit of ~6kHz is usually more than
enough, so there is some headroom, although it is marginal with the 600 ohm coil
(I suggest the high voltage circuit for that).

For an 8 ohm coil, R2 needs to be reduced to about 33 ohms for a 1V RMS input
voltage. The dissipation of Q1 and Q2 is about 160mW at full level and 1kHz, and
remains relatively constant with frequency. Peak dissipation is below 500mW -
well within the ratings of the BC639/640. Feel free to use BD139/140 if you
prefer. They will run cooler because they are considerably larger than the TO92
devices. The power supply demands are naturally noticeably higher than would be
the case with a higher impedance coil, but are still easily handled by P05 or
similar.

As the drive coil impedance rises further, the voltage needed to drive the
coil exceeds that available from any opamp. The current is low, but this doesn't
help a great deal if there is no easy way to get the voltage needed. The 1,475
ohm coil requires a voltage of just under 15V RMS at 5kHz, and almost 30V RMS at
10kHz. Allowing for a maximum sensible frequency of 7kHz, you'll need 21V RMS to
drive it. This can be obtained easily enough using the ±35V supply for a typical
100W solid-state guitar amp (such as Project 27. The drive circuit
must be discrete though, because no opamps are rated for such a high voltage.
While a pair of opamps in bridge would work, obtaining stable current drive in
this configuration is not easy. Even in that configuration, the maximum voltage
available without distortion is ~20V RMS - not enough for the high impedance
coil, especially since some headroom is desirable.

Figure 6 -
Basic Discrete Drive Circuit For High Impedance Coil

The discrete amplifier is not designed for outstanding performance, because
it's not needed. It will drive the high impedance Accutronics coil to the full
2mA RMS required though, and the circuit as shown should satisfy anyone who has
a high impedance tank. Since I'm one of those (since I have my old high-Z tank
as well as the new one), I shall built the circuit to verify that the simulation
is correct and because I want to be able to use the tank I have. It should work
exactly as described, and is certainly not a difficult or expensive circuit to
construct.

VR1 is an adjustment to enable the output voltage to be adjusted to zero.
This is important to ensure maximum headroom. C4 (10uF bipolar electrolytic) is
included to ensure that no DC flows in the drive coil. DC causes the magnetic
circuit to saturate, and this reduces sensitivity and greatly increases
distortion. It is also important that the circuit is driven from a low
impedance. In the interests of simplicity there is no additional decoupling in
the network of R1, R3, D3 and VR1, so a high impedance source may allow some hum
and noise from the power supply to enter the amp's input. A low impedance source
lets C1 act as a coupling cap and also decouples any noise. C1 is chosen to
provide the desired low frequency response. With 68nF as shown, the -3dB
frequency is about 160Hz. Reduce the value of C1 to reduce the amount of bass
and vice versa - this is often a very personal choice.

This simple circuit has a deliberately limited output impedance, and the
constant current characteristic only extends to about 6.5kHz, after which there
is a 6dB/octave rolloff. This is the equivalent of using a resistance in
parallel with the coil as shown in Figure 4 - all drive circuits require a high
frequency limit. The response of any spring reverb tank is very limited above
~5kHz anyway, and there is little point trying to get very high frequencies.
Even if the tank could provide good HF response, it would sound unnatural
because natural reverb at high frequencies is very uncommon. Despite the high
operating voltage, this circuit will still struggle if you drive it a bit harder
than normal. With 2V input at 5kHz, the amp will clip - this is unlikely to be a
problem though, since the energy at this frequency is usually much less than at
lower frequencies.

Recovery Circuits

Recovering the signal is every bit as important as driving the coil properly.
The recovery circuit given by Accutronics is barely adequate, and will be rather
noisy. It is important that the opamp used gives its best performance with low
to moderate source impedances, and maintaining a high load impedance is
essential for optimising the signal level. Both high and low frequency response
should be tailored to suit the expected response of the tank itself. I would
suggest that a range from 200Hz to about 6kHz is about right. Output above 7kHz
is almost nil, so a wide bandwidth pickup amplifier is not needed. The
relatively low bandwidth maximises signal to noise ratio - essential since the
output level is generally about 10mV at maximum drive level.

Type 4 Input

Coil Impedance

DC Resistance

Output Voltage (Typ)

A

500

42

3.0mV

B

2,250

200

6.5mV

C

12,000

800

15mV

Table 3 - Type 4 Accutronics Output Coil
Data

Several opamps are well suited to the task, and of these the dual NE5532 (or
NE5534 for a single version) is one of the better choices. These opamps are
reasonably fast, have low noise and excellent drive capability for low impedance
loads. They have rather uninspiring DC offset figures, but that's not an issue
in this application. You could also use the OPA2134 dual opamp - very quiet, but
(IMO) too expensive and overkill for this application. With a typical output
level of around 6mV (which is rather frequency dependent), a total recovery gain
of about 150 (43dB) is needed to obtain a 1V output. Although this can be
obtained from a single opamp, the result may not be satisfactory.

Accutronics recommend adding a capacitor in parallel with their 'B' (2.25k)
coil, for "improved high frequency response". While this is a good idea, the Q
of the tuned circuit may be found to be too high. If this proves to be a
problem, adding a resistor in series with the capacitor tames this nicely,
reducing the peak amplitude and spreading the HF boost over a wider range. This
is included in the circuit below, but the resistor value may need to be tweaked
to get the sound you want.

Figure 7 -
Generalised Recovery Amplifier Circuit

The circuit shows the basis for the recovery amp. Gain is 40dB (x100), and
may be reduced if necessary. I do not recommend attempting more gain from a
single stage. Vary the value of R1 to adjust the degree of high frequency
peaking created by C1. R2 prevents the opamp from swinging to the supply rail if
the reverb tank is unplugged. Gain will need to be increased (by adding another
stage) for the 500 ohm coil and reduced for the 12k coil. The latter is probably
a poor choice, and I suggest the 'B' coil if you have the opportunity for be
choosy. Gain is varied with R4 - lower values give less gain. A pot can be used
at the output for level control. Although the NE5532 can drive low impedances
easily, the pot should be 10k (audio taper is preferred). The disadvantage of
this arrangement is that all the gain is concentrated in the one opamp, and if
the signal level is higher than expected (which reverb tanks can do fairly
easily), there is a risk of clipping. However, for this to occur, the tank's
output would have to exceed 80mV peak. I have tried, but was unable to get
anywhere near that much.

For maximum flexibility a two stage amplifier can be used with the level
control between the two, but it's unlikely to be needed in reality, unless you
have the 500 ohm coil and need additional gain. The second stage will typically
only need a gain of 2-3, and this helps keep noise low.

Overload Protection/Indication

The final step is to decide if you want to add a clipping indicator or level
meter to the drive amp. Having some form of metering allows the drive level to
be set to the optimum, maximising output level. While generally not included in
guitar amps because of the added complexity (and marginal usefulness), for a
studio or PA application it's essential. Provided the drive amp has sufficient
headroom, I wouldn't recommend any form of compression or limiting, but a meter
or indicator is a relatively simple addition.

The Project
60 LED level display is ideal. It's small enough to make it easy to fit into
a small chassis, and is easily calibrated to indicate the maximum allowable
drive level. The schematic (with values amended to provide about 1V RMS input
sensitivity) is shown below. The meter is usually connected in parallel with the
input to the drive amp, but there's no real reason that it can't be reconfigured
to measure the signal level from the drive amplifier. See the project article
for the required values of R3 and R4.

Figure 8 - LED
Meter For Drive Level Monitoring

The meter should be operated in dot mode, because it is likely to be too
irksome to provide the various different supplies needed to allow the unit to
operated in bargraph mode, which increases the IC dissipation dramatically. The
input sensitivity as shown is 1.25V with VR1 at maximum - this means that the
sensitivity is pretty close to perfect for a nominal 1V input. LED current is
set to about 11mA with R3 at 1.2k but it is easily reduced if needed - if R3 is
increased to 1.5k the LED current is just under 9mA.

In many cases, a simple clipping indicator will be sufficient. It's actually
harder to do than the LED meter though, because there are no PCBs available for
a suitable circuit. If you don't mind some Veroboard wiring, you may use the
circuit below.

Figure 9 -
Clipping Indicator For Drive Level Monitoring

It's a pretty simple circuit, and will work well with almost any opamp. The
suggested opamp is cheap and has limited performance, but is perfect for this
circuit. The second stage is a comparator, and is used to "stretch" the overload
peak so the LED is on for long enough for you to see. While the circuit might
seem like overkill, really simple circuits just don't work well enough to be
useful. It's important that the meter doesn't load the input or drive signal -
depending on where you choose to connect the indicator - so a high input
impedance is essential. The 100k pot is used to set the circuit gain so that a
signal that just exceeds the coil current will trigger the LED.

If connected to the high impedance coil driver's output, the signal level
applied to the circuit must be reduced because it's too high for the opamp.
Input impedance needs to be as high as possible, and if used with the high
impedance drive circuit, replace R1 with 100k. D2 is used to protect the LED
against reverse voltage. You may use as many of these circuits as you need, but
most constructors will just use one for the drive amp.

Input And Mixing Stages

In most cases, reverb units are designed to allow a "dry" signal (no reverb),
and use a pot to adjust the reverb level to the output. Sometimes (for example
if used with a PA mixer), only the "wet" signal (reverb only) is needed. To be
really useful, the support circuitry should allow both modes of operation. Some
guitarists might like to experiment with using a second small amp and speaker
just for the reverb - it's an interesting sound.

The input stage can be balanced if desired, and likewise the output(s),
although I've only shown an unbalanced version. It's useful to provide a high
input impedance, but unless you plan to plug the guitar straight into the reverb
circuit (not really recommended), there's not much point in having an input
impedance above 22k or so.

Figure 10 -
Unbalanced Input, Mixer & Output Stages

The direct (dry) path has unity gain, so 1V input gives 1V output with the
level control at maximum. The wet (Rvb Out) output has a level that's fixed, as
it's intended to be fed to another amplifier or mixer. The mix of dry and wet
signals is set by the Reverb control - it's not really possible to give a gain
figure, because it will vary widely, depending on the input source, selected
reverb tank, etc.

While there are many more possibilities, the purpose of this article is to
give ideas, rather than complete details of a defined project. Using ESP boards,
there is a wide range of additional possibilities. Using a P94 "universal"
preamp/mixer allows the addition of tone controls, as well as full mixing
capabilities. The P113 headphone amplifier is ideal as a driver for low
impedance tanks (8 ohms is no problem), and the second channel can be configured
as the recovery amplifier. The only things missing are the simple clipping
indicator and discrete high impedance drive circuit.

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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 © 2009. 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 © Rod Elliott, 25 September
2009.