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DOD R-875 Flanger/Doubler



Isn't technology wonderful? The range of effects on the market seems to grow daily and with this explosion has come the inevitable drop in price. It is now possible to buy quite respectable little foot-pedal effects for not much more than £25. With this trend, higher quality units - aimed essentially at small studios - have become correspondingly cheaper. The market must be seen to be profitable - most manufacturers seem to be very interested in this relatively new area of sales and have produced all manner of rack-mounting equipment in the sub-£1000 price bracket.

The DOD R-875 Flanger/Doubler is clearly aimed at this market. It is a genuine studio quality system (produced in America) retailing at around £420 in this country. It is based on an analogue delay line to keep the circuitry simple and the price low.

The housing is a 19", 1U high rack-mounting cabinet which is just 6" deep. There is plenty of room to spare inside and the unit could have been made even more compact if necessary. The finish is in satin black with clear white legends and the controls consist of collet-style knobs and pushbuttons which are logically laid out. All connections are made by means of ¼" jack plugs.

The unit has two main functions. It can either act as a flanger to produce a range of characteristic effects or it can act as a doubler (ie. ADT) to thicken up a signal, giving either mono or synthetic stereo from a mono source signal as desired. However, in addition to this the system can generate some quite respectable vibrato, chorus, slap back echo and reverberation effects which are discussed later.

Interior



The unit contains one single-sided PCB measuring 10" x 3" which houses everything apart from a small transformer. The pots and switches are mounted directly onto the board - always a good sign since it minimises loose wiring and hence reduces the chances of noise pickup. The circuit contains just 9 ICs, including the delay chips, which are sensibly seated in sockets. Three LEDs are mounted inside the case which are presumably intended for use in setting up before despatch or for maintenance since they are not visible from the outside.

The PCB itself is well made and sturdy. However, the quality of assembly on the review model was less than perfect. Some off the components (including the switches which protrude through the front panel) are mounted at slightly strange angles. This is not necessarily a problem in itself although it does perhaps suggest haste and a lack of attention to detail during production. Having said this the standard of construction seems perfectly good and, to give the manufacturers the benefit of the doubt, it is possible that the early model supplied for review is not typical of a normal production run.

The cabinet is reasonably sturdy although not as robust as some comparable equipment around at the moment. For a rack-mounting instrument the housing is more than sufficient.

Controls



The front panel houses the majority of the controls - seven pots and three push buttons. Working from left to right the pots are: Manual (for manual adjustment of the centre frequency around which the flanging occurs), Width (the excursion of the sweep), Speed (the rate at which the flanging occurs), Regen. (the internal feedback or regeneration fraction), Channel A Mix, Channel B Mix (the ratio of dry to processed signal in each channel), and Input (the gain of the input amplifier). The three push buttons are used to select the delay time (1-10 ms or 7.5-75 ms), Invert (to phase invert both channels) and effect On/Off. Each button has a corresponding LED to indicate the state of the system.

Two additional LEDs show the sweep speed of the flanger and indicate when the clipping level of the input amplifier has been reached. The former is achieved by modulating the brightness of the LED in proportion to the delay time. The clipping indicator comes on when the amplifier becomes saturated.

The rear panel contains two further pots to adjust the output level for each channel. These are placed at the back since they will generally be preset and not regularly disturbed.

Input and output connections are paralleled on both front and rear panels and either set may be used. This is a very good idea since it means that the rear-sockets may be used when the unit is 'hard-wired' into a recording set-up to keep the wiring out of harms way, but the front panel sockets may be used when frequent reconnection is anticipated. Finally a 'Delay Kill' socket is provided on the back panel to allow a footswitch to be added to the system.

For those interested the specifications are given briefly below. The input impedance is 100 kilohms (unbalanced) and the output impedance for each channel is 600 ohms. The claimed frequency response is 20Hz to 20kHz (+/- 1dB) for the dry signal and 40Hz to 12kHz (+/- 1db) for the delayed signal, at signal-to-noise ratios of 95dB and 90dB respectively. These figures, if accurate, speak for themselves. The unit can provide up to 20dB boost at the output to drive further effects and the clipping lamp flashes when the signal amplitude exceeds 5V.

The circuit employs two BBD delay lines. The first contains 512 stages and is used to produce delays of up to 10 ms. The second contains a further 4096 stages and may be connected in cascade with the first to allow the full delay to be produced.

Rear panel.


Effects



When used as a flanger some nice effects are possible. The system may be used as a normal mono flanger by using just one of the output channels, however, if both outputs are used a pseudo-stereo image is produced, with the channels giving a rich 'antiphase' effect. The channel A and B delay signals are identical but in antiphase with each other. The dry output components in each channel are identical and both in phase.

Careful adjustment of the controls is required to produce a good flanging effect. With the Width control set at minimum and a sample signal applied at the input the Manual control should be set to the desired centre frequency, ie. the frequency about which the sweep of the comb filter will occur. It is important to do this since there is no point in flanging a part of the audio spectrum which is outside the range of the source being processed. Alternatively, 'Width' may be set at maximum to ensure a very wide flange sweep although this is not generally a particularly successful technique. The centre frequency is relatively easy to locate aurally since a flanging effect is obtained when 'Manual' is moved around the correct point. Width and Speed may then be adjusted as required.

As the Regen control is advanced the flanging effect becomes more pronounced and, when set at maximum with no input applied, the characteristic 'whooshing' is present at the output, albeit at a fairly low level. With a delay of 75ms the higher settings of Regen. give strong colouration to the signal and are of limited use except for rather extreme special effects - a more sensible range of control settings could have been found. Such distortion does not occur when using a delay of 10ms.

The Invert switch influences the range of frequencies over which the flanger operates. When set to Invert the effect tends to act on the higher frequencies present in the input signal. This arrangement is ideal for processing a lead guitar. However, Manual operates over a wide range and gives the unit great flexibility, hence it may equally be used to process a bass.

Frequency modulation (or vibrato) may be produced using a low Width setting with low Regen. and fast modulation (Speed at maximum). This is a useful treatment to add a little subtle colouration and warmth to a sound. An interesting idea which came to mind whilst playing with the machine is to select a very narrow Width for the vibrato so that it acts on a narrow range of frequencies only whilst leaving the majority of the sound 'dry'. The Manual control may then be used to select the appropriate frequency range and hence it is possible to 'tune' the vibrato. For instance, one possibility is to arrange the system to apply vibrato to the higher notes of a guitar whilst leaving the bass range untouched. This results in a modulated melody line over dry bass notes coming simultaneously from just one instrument.

The unit may be used as a 'doubler' to thicken up a signal and, particularly in the stereo mode, this is very effective. The higher delay setting generally produces better effects, with Width and Regen. kept low. Again Manual should be varied to produce the best treatment for a given input signal, whilst the Invert switch may be used to vary the nature of the output slightly.

Various types of chorus may be generated depending on the delay selected and the modulation speed. The longer delay setting gives better results since it tends to produce a thicker sounding effect. Stereo chorus is becoming an increasingly used effect due to the radical improvement it can offer an instrument. Although the system is not primarily intended as a chorus generator I suspect it will be frequently employed as such.

Slapback echo may be produced when the unit is set close to its maximum delay although the results are not particularly striking. This is somewhat surprising since analogue echo units are available which perform perfectly well. Reverb may be produced in a similar manner to echo by using a slightly shorter delay with the addition of a little generation to give the complex decay required.

An important point to note when using the system to generate a stereo signal is to keep the two channels as separate as possible. If they are mixed at all, the effect tends to disappear and if they are simply added together following the unit to form a mono signal all effects vanish totally and a dry signal is again obtained. It may occasionally be more sensible to forego the stereo properties of the system altogether in order to allow a simple mono effect to get through.

Conclusions



Clearly a lot of thought has gone into the design of this unit. The duplication of connections at front and rear is very useful and the practice deserves to become more widespread. Good quality components have been used throughout and the choice has paid off since the unit adds little noise or (unintentional) colouration to a signal, especially if levels are set with care. All the controls operate noiselessly - even the bypass switch - which is vital for serious use. With slight reservations the possible control settings are sensible - some people prefer a unit capable of slightly 'over-the-top' effects anyway. A very wide flange sweep is possible with Width set at maximum and this gives the system a slight edge over some of its competitors.

One admittedly minor point which I noticed was the omission of rubber feet underneath the unit. Instead the case rests on bare screws. Obviously this is irrelevant if a rack is used but it is not inconceivable that the effect unit may be used freestanding. Perhaps the trivial extra investment by the manufacturers would pay off in terms of goodwill. I realised just in time that the unit was sitting on my favourite amp!

The system certainly generates some useful effects although I can't help feeling that there are other units available which represent better value for money. DOD tend to aim their products at the small studio rather than the individual and so generally deal with customers prepared to spend a little extra for a high quality system.

RRP £417.68 inc. VAT.

Distributed in the UK by Rank Strand Sound, (Contact Details).



Previous Article in this issue

Soundproofing Windows

Next article in this issue

Fostex B16 Tape Recorder


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

 

Home & Studio Recording - Jan 1984

Donated & scanned by: Mike Gorman

Gear in this article:

Studio FX > Dod > R-875 Flanger/Doubler


Gear Tags:

Flanger

Review by Ed Stenson

Previous article in this issue:

> Soundproofing Windows

Next article in this issue:

> Fostex B16 Tape Recorder


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