This inexpensive, easy-to-build kit produces reverberation effects using analogue delay techniques in order to avoid the shortcomings of budget spring line devices. This month Paul White assembles the kit and comments on its ease of construction and sound quality.

It is only in recent years that reverberation has been electronically rather than mechanically generated, and that has been by using relatively expensive digital techniques.

This Digisound unit uses a special multi-tapped analogue delay line to produce the reverb effect at a fraction of the cost of digital units, but compromises in terms of flexibility and sound quality have had to be made.

In order to produce a usably long reverb time (three seconds maximum), the delay signal bandwidth has had to be restricted to 3-6kHz, but this is not as serious as it may first appear.

Natural reverberation contains very little high or low frequency energy, being predominantly mid range, but modern recording techniques tend to use a very bright reverb sound to such an extent that our perception of what natural reverb really sounds like has probably been severely distorted.

In theory then, this circuit should be able to produce an approximation to natural reverberation, so how exactly does it work?

Circuit

The essential ingredient for this design is the MN3011 analogue delay line. This differs from commonly available delay ICs in that it has six separate, non-harmonically related outputs and it is this feature that allows the random nature of reverberation to be imitated.

In order to prevent aliasing, the input passes through two 12dB per octave low pass filters so that any harmonics above 3-6kHz are removed. The delay IC is driven by an MN3101, which provides the necessary two phase clock, and the outputs from the six delay taps are summed into the output amplifiers which incorporate further filters in order to remove the clock frequency. A portion of the signal from the longest delay tap is fed back to the input in order to recycle some of the delayed signal, thus producing the decaying reverb 'tail'. An LED peak indicator is also incorporated so that the input level can be optimised.

The Kit

All the components for this device are provided in the Digisound kit. The PCB is beautifully made and has an earth plane on the component side on which is printed the component layout to ease construction. The PCB assembly is simplicity itself, providing that you can solder neatly and read the resistor colour code.

Most of the circuitry is contained on the PCB and it is only necessary to wire this to the pots, sockets and LED on the front panel using the wire provided in order to complete the task.

No power supply is provided with the kit and so a +/-15V supply is needed although +/-12V will suffice, and in this case, the E&MM TwinPack or RackPack power supply kits could be used (see Electronics & Music Maker July 84). Failing that, you can always use batteries.

The whole construction took around three hours and no unusual tools were required. All the ICs are fitted in sockets and, because they are CMOS devices, care must be taken when handling to prevent accidental damage by static electricity.

In Use

The circuit worked first time, which is always a good sign, and was subsequently connected to a recording mixer for evaluation.

The delay line is inherently noisy and so I turned down the effects return line until the noise was negligible at normal monitoring levels. Next, I connected various sound sources and increased the drive to the reverb unit until distortion was just audible, finally dropping the drive slightly to obtain a subjectively clean sound. To a large extent, this involved ignoring the overload light which was offering protest most of the time!

On vocals, the reverb effect the kit produced was very pleasing and noise was not really a problem if the treatment was used sparingly, the longer decay times being best suited to this application.

Some 'flutter' echo was noticeable on drums but again, if the effect is not used to excess, the result is very acceptable. Other instruments were tried, and all gave encouraging results, especially bearing in mind the cost of this unit.

Conclusions

The main limitations of this unit are its noise and bandwidth, although with careful use, neither present insurmountable problems.

If I were to use one of these units, I would be very tempted to incorporate a compander IC such as the NE571 or 572 in order to improve the signal-to-noise ratio and this would add little to the final cost.

The Digisound reverb kit is no substitute for a top-flight 'room simulator' but at a cost of less than the VAT on the nearest solid state alternative, it offers exceptional value for money. It doesn't suffer from the 'twangs' and 'boings' common to budget 'spring' units and it's both easy and fun to build.

Available exclusively from Digisound, the 80-17 Reverb Kit costs £36 including VAT/P&P. To order please contact: Digisound Ltd., (Contact Details).