Plate Reverb Project (Update)
Further details on this unusual project.
Judging by the number of readers' letters received following the publication of our Plate Reverb project (HSR March 84), it is apparent that there is a lot of interest in the subject. This month we shall be answering some of your questions as well as passing on a few practical tips for those who are going to have a go at building one.
The circuit diagram for the pre-amp is correct as it stands, but the value of the output capacitor (which was omitted from the circuit diagram) should be 10uF and the working voltage for all the electrolytics should be at least 12 volts and preferably 16 volts.
The recommended size for the stainless steel plate is six feet by three feet and it is fabricated from 24 S.W.G. stainless steel. It is possible to deviate from these dimensions and indeed, we have had several letters enquiring as to the feasibility of building a smaller plate. The answer is yes you can, if you bear the following points in mind.
(1) A smaller plate is likely to have a higher resonant frequency and so will add some colouration to the sound whilst also producing a slightly shorter decay time.
(2) It is probably not worth going below three feet square and it would be worth trying a thinner plate material in order to reduce undesirable colouration.
On the subject of transducers and their source of supply, piezo-electric microphone inserts are certainly the cheapest at around one pound each. These may be glued to the plate (Figure 1) but the more adventurous amongst you may prefer to dismantle the insert and remove the piezo-electric wafer which may be Araldited directly onto the plate surface.
Once your plate is operating in a satisfactory manner and you feel it worthwhile to upgrade the transducers, they may be replaced by the more expensive piezo-plastic type which offer better noise performance and sound quality (see Figure 2). These are available from P. J. Kunzler (address below) at an all inclusive price of £15 a pair. This type of transducer must be mounted in electrical contact with the plate and so should be taped directly to the surface or fixed with electrically conductive glue. Conductive glue is difficult to obtain and very expensive so we would recommend using tape.
The tension applied to the plate affects its resonant frequency (and hence the sound colouration) and the best results are produced by ensuring that the tension is as even as possible.
There have also been questions concerning the choice of plate material and it must be said that there is room for experimentation here. Different materials will produce differences in decay time, colouration, and frequency characteristics, and it must be borne in mind that stainless steel is a tried and tested material for this application. The wrong choice of material may cause the high frequencies to decay too quickly, giving a muddy and lifeless reverb sound.
At this point it is worth stressing that the EQ applied to the drive signal has a profound effect on the overall sound and it is a good idea to experiment with a graphic equaliser if one is available.
Finally, remember to keep your signal cables away from mains leads and transformers to avoid hum pick up.
In the event that you make a major breakthrough in reverb technology, let us know and we'll pass it on.
We hope that this information clears up all of the queries we've received and helps shed more light on the subject of plate reverb. Now you can all return to building your own device. Good luck!
Piezo - plastic mics are available from: (Contact Details). Price £15 per pair.
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