• Plate Reverberator Project
  • Plate Reverberator Project
  • Plate Reverberator Project

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Plate Reverberator Project

Literally our biggest project to date!


Have you ever wondered how to obtain clear, bright reverb in huge quantities? Sick of the spring effect? Well, read on for a design that will not go 'boing' and can be built very easily.

Figure 1. 1¼
(Click image for higher resolution version)


A plate reverb is an electro-mechanical device for simulating the impression of space around a sound. The effect of dimension or depth is conveyed to the listener by the apparent reflections of a sound source, a sort of 'cathedral' effect. The most sophisticated plate units cost thousands and use rare metals activated by complex drive systems. Here is a straightforward design using a stainless steel plate driven by a simple loudspeaker.

However you choose to mount the plate depends on the space available, but the plate itself must be able to vibrate quite freely. Suspension can be from hooks, springs, rubber bands or threaded tensioners, which permits fine adjustment of the resonant frequency of the plate. Don't tighten them too much or you'll break the corner of the steel frame. A rule of thumb: when the plate is at the correct tension, tapping it will produce a burst of the type of reverb so beloved by the world's top recording engineers. Stainless steel gives a bright, toppy reverb which is superb on drums and voices.

Pre-amp Circuit for Piezo pickups. This should be placed as close as possible to the transducer, at most 30cm away to avoid high frequency losses and mains hum at 50Hz. Output impedance is around 470 ohms but this can be changed to 330 ohms (if a long cable to the mixer is used) by replacing the output load resistor with a 330 ohm value.
(Click image for higher resolution version)


The Driver



The simplest way to activate the plate is by placing it near the sound source. This is often difficult so we use a loudspeaker driven by a power amplifier. This means that the plate can be put somewhere well away from the control room (where it would pick up all kinds of noises). It is possible to use an ordinary hi-fi amp to drive the loudspeaker, but keep the amp well away from the plate to reduce hum pickup from the mains transformer.

The loudspeaker is best placed near the centre of the plate, so that bending waves induced in the steel will spread out evenly towards the edges. Placing the driver elsewhere will produce a different burst of reverb which will be less useful for many sounds. If the driver is placed closer to the edge of the plate, the reflections from the nearest edge will be denser, producing a slight delay before the main reverb is heard. This effect is called pre-delay and is obtained more easily by putting the reverb return signals through an electronic delay line. The loudspeaker can be placed about 1 inch from the plate without harm as long as it is not driven too hard, otherwise unpleasant distortion will result. As with all parts of a plate, nothing must rattle, so make sure all nuts and bolts are thoroughly tightened and all wires secured.

Experimental Plate Reverb unit. Welded steel frame is mounted on box with PSU, amplifier and 2 pre-amps, with piezo-type transducers. Note: tightening tool inserted in holes in box (top near corner).
(Click image for higher resolution version)


Transducers



The pickup transducers are piezo devices chosen for their immunity to magnetic interference and ability to be directly bonded to the metal plate.

Piezo devices have a high impedance and need to be driven into a FET circuit which will have a sufficiently high matching impedance; The LF 351 is a good choice. The pre-amp should be placed as close as possible to the pickup transducer to reduce mains hum and noise, and a properly grounded metal case should screen each pre-amp separately. The output of the pre-amp circuit is not balanced so you'll need a balancing transformer to match the input of balanced mixing desks; or tie the anti-phase desk input to ground as a second best. The preamp should be powered by a thoroughly smoothed and regulated supply, as line variations will cause unwanted noises to be amplified.

Figure 2. Schematic of plate reverb - suggested stereo version.
(Click image for higher resolution version)


Moving The Plate



Experimenters will use an old hi-fi amp to drive the plate via a loudspeaker; indeed this can give great results. Or you can build the driver amp from one of a variety of commercial amp boards available. Note that plenty of reserve power is needed for the system as a heavy metal plate needs lots of energy to move it at higher frequencies. A fuse placed strategically in the speaker circuit will help protect the loudspeaker from overloading by enthusiastic engineers in the distant control room! And distant it should be - because the plate is sensitive to outside noises, so don't locate it in the control room. It's best to place it in a store room or cupboard out of the way, in an upright position with the plate vertically suspended. Take great care when moving the plate because it is very heavy and the transducers must not be ripped off their mountings.

The Sound



"Aaaaaaaaaah" said the engineer, "I want reverb, gallons and gallons of it." And the client was happy to hear his voice sparkle, the toms throb, the strings sing, without any wobbling or funny 'boing' sounds. But that was not all done in a day. It took a long time finding the 'sweet spot' for the piezos, learning not to blast the plate with 200 watts of bass guitar, and curing the hum from earth loops.

Figure 3. Experimenter's plate schematic.


People think that there is some secret to piezo placement. But realise for a moment that the plate works by inducing bending waves in a piece of metal. The waves radiate outwards from the induction point just like ringlets from a drop of water. As the waves hit the edges of the plate, they are reflected back to the metal. At certain points on the plate, nodes and antinodes will occur where many waves meet in-phase and produce a large standing wave or trough if they meet out-of-phase. These spots are to be avoided, but you'll find them by trial and error.

A general rule is that the transducers in a stereo system should be placed asymmetrically to produce a more realistic 'width' effect.

Close-up of plate electronics showing amp board, pre-amp boxes, one pickup, decoupling capacitor (PSU is remote) and sub-PSU to the pre-amps (to bring 33V down to 9V). Note: inside of access door is covered with grounded aluminium foil to improve hum characteristics
(Click image for higher resolution version)


The first thing that you notice when using the plate is the brightness and smoothness of the reverb. To cut down the delay time, damp the plate with a very soft cloth over a small portion of the metal. Remember that this will alter the frequency characteristics of the device. Heavier damping will cause more delay reduction but will mute the sound drastically.

A welded steel frame with 24 swg plate (mounted), is available for £150 (not including electronics) from: (Contact Details)

Note: See also an update on this project here: Plate Reverb Project - Supplementary Information



Previous Article in this issue

Home Recording Equipment Guide


Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.

 

Home & Studio Recording - Mar 1984

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Feature by P.J. Kunzler

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