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Dod Mod II: Chorus

The DOD 690 is one of the most affordable stereo chorus units on the market (for a complete review of the 690, see my "Notes" column in the December 1980 issue of Modern Recording & Music); so, it seems fitting that this should be one of the least expensive mods ever published in Polyphony. How expensive? Well, by spending 5 cents or less for a single resistor, you can expand the 690's chorus capabilities to include flanging and flanging/chorusing effects. If that sounds interesting, read on....


The 690 uses an SAD512 delay line that is initially set for slightly more than 20 ms of time delay. A triangle wave LFO modulates the delay time to create a chorusing effect; a width control determines the extent of this modulation. With the width control fully counterclockwise, there is no modulation and the delay is fixed at about 23 ms. Turning the width control clockwise allows the LFO to smoothly vary the time delay over the following ranges:

max counterclockwise 23 ms 23 ms
9 o'clock 22 ms 24 ms
12 o'clock 17 ms 25 ms
max clockwise 14 ms 26 ms

This time delay variation creates the actual chorusing effect. As the above table shows, the greatest delay line variation occurs with the width control all the way clockwise.


The idea behind this modification is to change the delay time range variation into the flanging range, namely 0 to about 15 ms. While I was not able to get this wide a variation, the end result was close enough to give some reasonably good flanging effects.

The mod itself is so simple it doesn't even really need a diagram. Open the case (with the line cord unplugged, of course), and locate the LM324 IC. The closest resistor to the left-hand side of this IC (pins 1-7) is a 470k (yellow-violet-yellow) resistor... in fact, it's the only 470k resistor that's even remotely close to 324. Solder a 33k (orange-orange-orange) resistor in parallel with this part, and the modification is complete. If you want, you can drill a hole in the back of the box for a toggle switch to switch this resistor either in parallel with the 470k, or out of the circuit. By adding this switch, you'll be able to get the stock chorus settings when desired.

So what do you get for your efforts? You get a change in the way the width control acts, as given in the new chart below:

max counterclockwise 23 ms 23 ms
9 o'clock 14 ms 21 ms
12 o'clock 7 ms 16 ms
max clockwise 4 ms 12 ms

What all this means is that at the maximum clockwise setting, you're getting right into the heart of the flanging range. Thanks to the compansion included in the 690 circuitry, the flanging sound is real quiet; and it sounds GREAT in stereo! What's more, even with the modification, you can still get pretty good chorusing sound with the control set at 9 o'clock (however, the stock 690 does sweeps over a wider range when chorusing). In between these settings, you get a mix of chorusing and flanging that sounds quite good.


With one resistor, you've added several new effects to the 690. An added bonus is that at shorter delay times, the SAD512 is inherently quieter so that noise levels at the flanging and the flanging/chorusing settings of the width control are excellent - even with low level input signals.

Many people will be interested in the 690 simply because it is an affordable way to get that wonderful chorusing sound; but add this mod, and you'll have more than a nice chorus box on your hands.


Here's some additional information for those of you who like to keep up with analog delay line techniques. The SAD512 does not require a traditional clock driver, since one is included on the chip. As a result, there is only a single clock pulse input to the 512; you can monitor this clock signal on pin 8 of the 4001. If you have an oscilloscope with a calibrated time base, the delay time is almost exactly equal to the clock period (in microseconds) X 1000. So, a clock period of 20 microseconds means a delay time of 20 milliseconds.

Previous Article in this issue

Dod Mod I: Envelope Follower

Polyphony - Copyright: Polyphony Publishing Company


Polyphony - Jan/Feb 1981

Donated & scanned by: Mike Gorman


Maintenance / Repair / Modification

Gear in this article:

Guitar FX > Dod > 690 Chorus

Gear Tags:


Feature by Craig Anderton

Previous article in this issue:

> Dod Mod I: Envelope Follower...

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> Build a 50 Watt/Channel Ster...

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