Triple Tank Spring Reverberation Unit

Triple Channel Spring Reverberation Unit Introduction

This spring reverberation unit comprises three identical main sections each of which contains an Accutronics type 9 spring reverb tank with short, medium, and long decay times. Controls are provided to adjust the amount of signal arriving at the output from the input directly and from the spring tank. Conventionally these signals are called "dry" and "wet." A feedback control allows the tank output to be returned to the tank input to produce oscillation effects. The amplified output from the spring tank is available on a socket to send to other effects units and an insert input socket is provided for the return from that effect. The insert socket overrides the signal from the tank output. This return is prior to the feedback control so that the effects unit is within the tank feedback loop. In this way a frequency shifter could be used in the send-return loop to provide rising or falling reverb effects. The Accutronics type 9 is the best unit made by that manufacturer and is considered to be a high quality spring reverb used in many respected products.

Controls and Inputs For Each Section

Main Signal I/O

mono source input 1/4 inch jack unbalanced mono
tank output send 1/4 inch jack unbalanced mono
effect return insert 1/4 inch jack unbalanced mono
main output 1/4 inch jack unbalanced mono

Inputs are insert-switch daisy chained to allow input 1 to feed inputs 2 and 3 if there are no plugs inserted in inputs 2 and 3.

Secondary I/O

One additional SuperVerb output which is a sum of the three main outputs for extra rich reverb.

Controls For Each Section

dry gain manual control ordinary pot
wet gain manual control ordinary pot
tank feedback manual control ordinary pot

Main power switch

Extras

+/-13.8V supplies on rear panel screw terminals to supply power to other synth units.

Other simple synth modules have been installed in this case to use up space on the front panel.

2 simple six-way splitters
2 buffered attenuators with gain

Triple Spring Reverb Tank Drive Circuits Schematic spring_reverb01-01 PDF



This sheet shows the three tank drive circuits. These are simple class B drivers driven from the TL074 outputs. The ground side of the tank input transducers returns to ground via an 8.2Ohm resistor. This provides the first stage of frequency response shaping by making the mainly inductive tank input transducer look more resistive. The tank has an impedance of approximately 10 Ohms at 1KHz which is mostly inductive. With the 8.2 Ohm resistors the current in the tank driver will be about -3dB at 1KHz and reducing at approximately 6dB per octave above that frequency. Taking the feedback for the op-amp/driver combination from the top of this resistor makes the driver a constant current circuit, so the output swing will increase as the impedance of the tank transducer rises at higher frequencies. This continues up to a certain frequency defined by the capacitor in the feedback circuit. The current drive is rolled off above about 7kHz as the tank mechanical response rolls off rapidly much above 6kHz. Extending the drive bandwidth higher would be useless and result in lots of unnecessary current circulating around the circuit at 15 or more kHz. Stability of the circuit is very good despite the phase shift created by the inductive load above 1kHz. This circuit requires the use of the tank version with isolated input transducers. Using this circuit with the sense resistor grounded also allows for easy single-ended measurement of the drive current.

Spring Reverb Tank Signal Recovery Amplifiers spring_reverb01-02 PDF



This sheet shows the low noise pre-amp circuits for the tank output transducers, voltage gain stages, and the output socket drivers and summers. The dual PNP transistor differential input stages with the op-amp feedback are a standard circuit with the operational currents set to minimise noise at the nominal input impedance of 2K Ohms. The 100nF input cap and the dominant 22K Ohm bias resistor form a high pass filter to eliminate deep bass rumble from vibrations in the spring tank. The 2.2nF input cap reacts with the tank impedance to roll off the frequency response above 8kHz and to limit the noise bandwidth. Gain is set by the op-amp feedback divider which is low impedance to reduce noise in the feedback transistor. The input amplifiers are AC coupled into a further op-amp stage which brings the signal back to the level at which it entered the front panel input sockets. This is presented at the effect send sockets.

While using the unit I found that the reverb feedback controls produced a useful effect but it was very easy for the feedback to build up to a point where the oscillation level was very high and clipping excessively. This was risking possible damage to the tank input transducers and so subsequently I have added simple diode limiters after each of the feedback control pots. With a small adjustment to the subsequent gain setting feedback resistors, there is still enough gain round the loop to ensure a swift build-up to a feedback oscillation, but the excessive signal levels are avoided.

Spring Reverb PSU spring_reverb01-03 PDF



These are standard regulated power supplies, with quite a large transformer, reservoir capacitors and heatsinks to allow enough power for the tank drive circuits and some external supplies to other synth modules in the stack.

Spring Reverb Front Panel Wiring spring_reverb_wd01-01 PDF



On the main signal inputs, wires from socket 3 connects to input 2 when there is no plug inserted in 3. Input wires from socket 2 connects to socket 1 when there is no plug inserted in 2. In this way a plug in socket 1 connects to all three inputs if there is nothing plugged into the others. This can be useful to save patch leads when it is desired to send the same signal to all three sections. The effect send outputs connect back to the switched terminal on effect return when there is no plug inserted in the effect return. This is the main signal path when an external effects unit is not being used.

Auxilliary Front Panel Wiring Diagram spring_reverbwb01-02 PDF

This shows the wiring for the additional extras not directly related to the spring tank operation.

Spring Reverb Pictures

Most of the wiring loom to the front panel is disconnected here, with just the tank drives and returns plugged in. It would appear that I paralleled up two diodes in the bridge rectifier circuit to get 2 Amp capability.



Top Inside View Showing The Extensive Wiring Loom

This is the whole box from the top showing the three tanks bolted together and the front panel wiring disconnected. The mains wiring and ground bonding is incomplete in this picture. Note the deliberate use of cheap phono plugs for the input drives on the right and the screened outputs on the left. Cheap phono plugs are often a better, tighter fit for cheap or older phono sockets than the expensive higher quality items.



Tank 1 Springs And Unit Under Construction On The Bench

Here is a view with the open side of Tank 1 showing the three springs and the piece of foam stuck behind them to stop them banging around when the tank is dropped. These tanks will only just fit in a 19" rack case.
 


Triple Tank Reverb Under Initial Test

Here you can see the unit under test without labels and the later auxiliary additions used for different functions in the general modular synth set-up.

Later Additions And 2026 Page Updates

while using my modular synthesiser. To this end, two simple six-way splitters were added in the form of six standard jack sockets commoned up inside. I also found that I often needed some attenuation or a bit of gain on some control or audio signal. This is provided by two buffered amplifiers with front panel gain controls. Various pictures follow which again show how far digital imaging has progressed over the years.

Main Circuit Board

Here you can see the main circuit board, the coiled ground connection for the tank shells, and the six self adhesive cable tie bases used to secure the bolted together tanks. This is surprisingly effective, and they are still stuck down firmly after 20 years.



Front Panel From The Rear

Ground and signal single ended twisted pair wiring is used for most of the higher level signals. The tank drive signals have a drive and return sense twisted pair. Only the low level tank outputs employ fully screened cables.



Original Triple Spring Tank Foam Padding

Originally I used foam padding and cable ties around the circuit board and cable tie bases to hold the spring tanks in place. One of the cable ties heads had broken.



Somewhat Improved Tank Mounting Scheme

Here the main part of the foam is at the bottom and the sides, and the cable ties no longer go around the main board. This arrangement is surprisingly secure.



Close Up Of The Insides Of Spring Tank 1

Here you can see the three springs and the transducers at each end. The different spring lengths, tensions, and the links are deliberate to create a better overall reverberation effect



Rear Panel Of The Rack Case And Spring Tank Input Outputs

It looks like I never did fit an auxiliary power output to the rear panel. There's the standard IEC inlet with a built in 20mm fuseholder.



Spring Tank Input Output Phono Connectors And Full Part Numbers

You can see the rubber grommets where you would more usually have a mounting bolt for your single reverb tank. Tank 1 has a short decay time, Tank 2 has a medium decay time, and Tank 3 has a long decay time.



Spring Tank Input Signal Transducers

Here is the transmit side showing the input winding and the laminated iron stampings. They look like they've got a bit bent in this case. You can also see the supporting springs which isolate the tank from general environmental vibration.

It looks like that lower mounting spring has had a bit of a knock as well. The transducer creates a rotational turn in the spring rather than a longitudinal compression wave or a side-to-side transverse wave motion. This makes the effective length much bigger.



Spring Tank Low Level Output Transducer

The signals here are quite low level, so this needs to be treated much like a dynamic microphone output.

Spring Tank Reverberation Sounds

And Finally

Can you still get the tanks from the USA? I've not checked. I have three, one of each in the three available decay time variants with 600 Ohm input and output coils, so you could put those in a rack box fitted with XLR panel connectors and drive them direct from a proper mixing desk, with the returns coming back to balanced microphone inputs. There would be no power supply required, and all the feedback and EQ would be done in the studio desk. As with every guest on Antiques Roadshow, they are not for sale... until someone makes me a good enough offer:)
 
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